nuclear physics volume 13/no. 2 newstwo-neutron halo nuclei such as 6he have been dubbed...

Contents

Editorial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Laboratory PortraitNuclear Theory at the University of Surrey

by J. S. Al-Khalili et al. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Meeting Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Feature ArticlesNew Magnetic Dipole Phenomena in Atomic Nuclei

by Norbert Pietralla and Krzysztof Starosta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Gravitation at a Micron and Mixing of Quarksby H. Abele . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Nuclear Exchange Currentsby Dan-Olof Riska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Facilities and MethodsCYCLONE44 and ARES: New Tools for Nuclear Astrophysics

by Pierre Leleux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

News from NuPECC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Calendar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . pg. 44 and Inside Back Cover

NuclearPhysicsNews

Volume 13/No. 2

Cover illustration: The University of Surrey in Guildford hosts the Centre for Nuclear and RadiationPhysics, home of the largest nuclear physics research activity in the UK. Photo courtesy of Steve Heritage,University of Surrey.

Vol. 13, No. 2, 2003, Nuclear Physics News 1

editorial

2 Nuclear Physics News, Vol. 13, No. 2, 2003

In recent years nuclear physicshas developed into a broad field ofscience covering all aspects of sub-atomic physics and a range of appli-cations in energy, biomedical, andmaterial sciences. Experimental andtheoretical findings and methodsdeveloped in nuclear physics arehaving important impacts on otherfundamental sciences as well. Theopposite is also true. We are clearlyarriving at an era where nuclearphysics has to be seen as a part ofglobal science, and therefore it is ofgreat importance to all of us to keepit that way.

Activities on advancing basic nu-clear sciences, their education, andpublic awareness have also been ofconcern to the community. The ef-fort in these areas is organized in Eu-rope under the umbrella of Nu-PECC, the expert committee of theEuropean Science Foundation, al-lowing for broad participation of thewhole community in these importantactivities. Current work on the newLong Range Plan of NuPECC is abeautiful example of coherent effortsto plan the future of the field in Eu-rope and worldwide. This plan isbeing prepared under the conduc-tance of nearly 100 leading experts,and it aims at producing a coherentdocument for guiding our common

research efforts over the time frameof 5 to 15 years to come. This exer-cise is carried out as an openprocess, allowing the whole scien-tific community, from small andlarge countries to participate. Suchan open process carried out as a bot-tom-up procedure is also importantfor successful realization of futurelarge-scale projects identified anddiscussed in the Long Range Plan.The plan will offer an excellent doc-umentary on modern nuclear sci-ences. It will offer a true challengefor funding of an internationally co-herent plan within the EuropeanUnion as well as in all countries con-cerned, and with a long-term per-spective. The first true test will beexercised in the context of the SixthFramework Programme, where twoIntegrated Infrastructure Initiativeproposals EURONS for nuclearstructure physics and I3HP forhadron physics, will be presented.

It is important that the uniquestrength of nuclear physics derivingfrom broad international collabora-tions also be brought to the atten-tion of those responsible for nationalresearch and education efforts. Thisis particularly important for smallercountries where the field often is in aminority position. It goes withoutdoubt that the university environ-

ment offers the best forum for com-munication between nuclear phy-sicists and other scientists and forcontributing towards creating futuregenerations of scientists and teachersfor the field. Open academic compe-tition and communication is the onlyway for nuclear physics to perma-nently leave behind its reputation asa closed field with a glorious pasthistory only.

After serving European nuclearphysics as a NuPECC chair for threeyears, I have great trust in this com-munity and its ability to develop thefield towards a New Era that is char-acterized by important large interna-tional projects in coexistence withnational efforts on research and edu-cation.

JUHA ÄYSTÖ

Jyväskylän Yliopisto

Title to be Supplied?

The views expressed here do not represent the views and policies of NuPECC except where explicitly identified.

QY:Author: Supply title?

Nuclear Theory at the University of Surrey

laboratory portrait


IntroductionThe Centre for Nuclear and Ra-

diation Physics (CNRP) of the De-partment of Physics at the Universityof Surrey hosts the largest nucleartheory activity in the UK. The Uni-versity is situated in the town ofGuildford, 48km SW of London.Theoretical research includes boththe structure and reactions of lightand exotic—particularly halo—nu-clei, as well as the structure of heavynuclei. Together with the experimen-tal activity, the theory group is partof the newly formed CNRP, whichalso includes applied nuclear physicsfor medicine, imaging, and materialscharacterisation.

The focus of theoretical researchat Surrey is on the structure and re-actions of extreme states of nuclei.Surrey theorists have pioneered tech-niques for nuclear reactions involv-ing loosely bound projectiles. Thesetechniques are highly applicable atthe incident energies of great currentexperimental interest.

Few-body structure phenomenareveal great sensitivity to nucleon-nucleon interaction effects beyondthe mean-field. Experimental workhas shown that the mean field andeffective interactions deduced fromstudies of near-stable nuclei fail tohave predictive power near the neu-tron drip line. In particular, knowl-edge of the essential structure ofhalo nuclei comes largely from de-tailed reaction studies, but their in-terpretation depends crucially on theavailability of good reaction theory.Surrey has been in the forefront ofthe development of realistic few-body models of nuclear reactions.

Surrey has a tradition of sup-porting researchers on extended vis-

its from outside the UK. It also hostsa Marie Curie Training Site, fundedby the EU Fifth Framework HumanPotential Programme for the trainingof European doctoral students whovisit Surrey for specific nuclear the-ory training. This is the only suchsite for nuclear physics in the UK.We are currently organising an EUFramework 6 theory network.

In January, 2003 Surrey hostedan international workshop, NuclearStructure at the Limits of Stability, tofocus on recent advances in nucleartheory. The 3rd International Work-shop on Direct Reactions with Ex-otic Beams will be held at Surrey inJuly 2003.

HistoryThe origins of nuclear theory at

Surrey can be traced to 1958 whenDaphne Jackson was invited by L. R.B. Elton to join him as a researchstudent. Together, Elton and Jackson

laid the foundations of an interna-tionally respected group studying thetheory of nuclear reactions and nu-clear structure. In 1966 DaphneJackson was appointed Head of theNuclear Physics Group. During thisperiod, the group also appointedRon Johnson (1964) and Roger Bar-rett (1967). In 1971 Jackson was ap-pointed as Head of the Physics De-partment, becoming the first womanprofessor of physics in the UK.

Johnson became Head of theNuclear Physics Group in 1972, aposition he held until 1992, when hehanded over the role to Phil Walker,the first experimentalist to be ap-pointed by the group (in 1987).Under Johnson’s leadership the Nu-clear Group grew to 5 theorists and4 experimentalists and became theNuclear Physics Group with thestrongest Research Council Supportin the UK. In 2002 the group amal-gamated with applied nuclear physi-

The Surrey Nuclear Physics Group—both theorists and experimentalists—at the January 2003 International Workshop on Nuclear Structure at theLimits of Stability.

QY:Author: Please cite Figs. 1,2, 3, and 4

NOTE:This is revised e-mailed file.

cists in the department to form theCNRP under the directorship of IanThompson.

The Surrey EthosEarly research at Surrey featured

studies of nuclear sizes. Importantwork from that period included twowidely cited books by Elton [1] andby Barrett and Jackson [2]; but sincethe early days of the group there hasbeen an emphasis on nuclear reac-tion studies. This effort has had twostrands:

(i) The development of approxima-tions to direct reaction theoriesthat have provided insights intothe physics of the processes in-volved. Examples include: theadiabatic approximation fordeuteron stripping and elasticscattering [3]; the projectile ex-citation model of polarizationeffects in Li scattering [4]; theconcept of tidal symmetry inheavy-ion scattering [4]; andthe core-recoil model for halo-nucleus scattering [5].

(ii) The development of state-of-the-art computer codes thatwere designed to be useful tothe experimental community aswell as breaking new groundtheoretically. Examples of theseare the deuteron stripping codeof Santos and Johnson [6],which was the first DWBA codeto include the effects of the deu-teron D-state and was widelyused in the 1980’s to analyseexperiments with polarized deu-terons, and the code FRESCOof I. J. Thompson, which solvesthe differential equations forcoupled two-body channels of avariety of direct reaction theo-ries [7]. This code has been ex-ported to many groups all overthe world.

Recent work in this area has fo-cussed on few-body models of thestructure and reactions of halo-nu-clei [8]. However, the group has re-cently expanded its interests to in-clude many-body nuclear structureresearch and hadron physics.

Few-Body Methods in Scattering and Reactions

Nuclear reactions play a crucialrole in the study of nuclei, and halonuclei are no exception; well-devel-oped perturbative theories of nuclearreactions already exist. The interest-ing question is whether these theo-ries will work for reactions involvinghalo nuclei, and if not, how theyshould be modified. The weak bind-ing of halo nuclei makes a positiveanswer to the first question unlikely.The weak binding of the valenceneutron in 11Be, for example, meansthat the halo degree of freedom iseasily excited by the nuclear andCoulomb fields of a target nucleus.The weak binding also means thateven a small transfer of momentumfrom the relative motion of the twonuclei will excite the halo nucleusinto the continuum of unboundstates from which fragments maypropagate to large distances. Thechallenge for theory is to find meth-ods that treat such configurations re-alistically, while at the same timelending themselves to the practicalanalysis of experimental data. Casesof interest typically involve manystrongly coupled channels, in addi-tion to those involving halo excita-tion and a large range of angularmomenta. This challenge is espe-cially difficult when the fragmentsare charged.

The Adiabatic ApproximationOne way of meeting the chal-

lenge of nuclear reactions withloosely bound projectiles is to usethe adiabatic approximation, as de-veloped by Johnson and Soper [3].This permits a significant simplifi-cation of the few-body scatteringproblem while treating coupling tobound and continuum channels onthe same footing. In an experimentin which a beam of projectiles con-

laboratory portrait


Figure 1. Two-neutron halo nuclei such as 6He have been dubbed “Bor-romean” due to the nature of their three-body structure. Since no two-bodysubsystems are bound, such nuclei behave like the interlocked Borromeanrings, whereby if any one is removed, the other two separate. The Surreygroup has made significant contributions to the study of the structure and re-actions of such nuclei.

sisting of halo nuclei is scatteredfrom a stationary target, the adia-batic approximation assumes thatthe motion of the halo nucleons rel-ative to the projectile’s centre-of-mass is much slower than the projec-tile’s motion relative to the target.Note that this does not assume thatthe projectile remains in its groundstate during the collision, but ratherthat the spatial coordinates describ-ing the halo degrees of freedomwithin the projectile are frozen. Thescattering amplitude is then calcu-lated separately for all relevant val-ues of the halo coordinates.

The adiabatic approximationhas been widely used for reactionsinduced by deuterons [8–11] at in-termediate and high-projectile ener-gies, where simple analytical consid-erations suggest it should be valid.However, the method is also welladapted to the case of halo nuclei forwhich few-body models are a goodstarting point, and this has been thefocus of much work at Surrey

With the current interest in ra-dioactive nuclear beams operating atenergies in the region of tens ofMeV, it is important to investigatehow far the adiabatic method can beused at these energies. The adiabaticmethod is usually thought of as ahigh-energy approximation. For theelastic scattering of strongly ab-sorbed systems, we find [12], in fact,that the adiabatic model is accurateat much lower energies than onewould expect from qualitative esti-mates.

It was shown in [5] that whenthe interaction between the halo par-ticles and the target is absent, theadiabatic scattering wave functionhas a simple form. This result has ledto new insights into the role of thehalo in elastic scattering and has alsobeen used as the basis of a new non-perturbative quantum mechanical

theory of the Coulomb break-up ofneutron halo nuclei. The same ideahas recently been used in a new ap-proach to the theory of transfer re-actions involving a halo nucleus ininitial and final channels [13].

The Glauber Model and Its Development

Glauber’s microscopic theory ofthe scattering of composite systemshas the adiabatic approximation asits starting point. In addition, it as-sumes that each constituent of theprojectile follows a straight line pathand the scattering wavefunction isobtained by calculating the phasechange produced by the interactionpotential along this path (semi-classical eikonal approximation).Over the past decade the Surreygroup has exploited Glauber’s the-ory to study the scattering of looselybound few-body systems, such ashalo nuclei, to include fully corre-lated few-body wave functions forthe projectiles and spin-dependentinteractions between the projectileconstituents and the target. Whentreating the scattering of nonradio-active nuclei, it is common to makethe additional assumption (opticallimit approximation) that only theground state density of the projectileis relevant, but this assumption wasshown by the group [15] to have se-rious shortcomings when looselybound neutron halo nuclei are in-volved. When applied to the inter-pretation of fragmentation cross sec-tions, Surrey’s improved calculationsrequired halo radii that were signifi-cantly larger than those deducedusing the static density approxima-tion. This effect has recently beenshown to have a very general conse-quence of the static density approxi-mation [16].

There now exist procedures forcalculating the scattering wave func-

tion corresponding to projectileswith two [11] or three [16] con-stituents whose relative coordinatesare treated adiabatically but whoseremaining degrees of freedom aretreated quantum mechanically. Usingeikonal methods as an additionalapproximation, the case of five con-stituents (8He) has also been evalu-ated using the best available projec-tile wave functions. These methodshave been successfully applied tothe analysis of the elastic scatteringand reaction cross sections of halonuclei.

For practical calculations of re-action observables such as differen-tial and total cross sections at lowenergies where the eikonal assump-tion is no longer valid, we have de-

laboratory portrait


Figure 2. Model calculations showthe total reaction cross section of abound two-cluster projectile againstthe assumed rms separation betweenthe clusters. The three configura-tions all have the same overall one-body density (hence the decrease ofintrinsic cluster sizes with increasingintercluster separation). In the opti-cal limit (OL) of the Glauber model,they all predict the same cross sec-tion, but within a few-body (FB) pic-ture, the cross section is reducedwith increasing cluster separation.The OL and FB cross sections onlycoincide when the two constituentsoverlap (no clusterisation). The insetshows the assumed projectile density.

veloped a new method [8] based onthe use of Exact Continued cluster-target S-matrix elements. This ap-proach retains many of the simplici-ties of implementation of the Glaubermethod but without the restriction toextreme forward scattering. This re-places eikonal phase shifts for the in-dividual cluster-target scattering bythe physical ones. This approachtherefore gives a powerful and nu-merically simple means of extendingmany-body multiple scattering theo-ries with no more than an adiabaticassumption. Such an approach,which has been shown in specificcases to be reliable for elastic scat-tering down to ~10 MeV/A of pro-jectile energy, does not require solv-ing a full coupled channels problem.

Spectroscopy Using Nucleon Knock-Out Reactions

The ordering and distribution ofsingle-particle strength and the occu-

pation of nucleonic single-particlestates in nuclei are fundamental totheir structure and stability. Experi-mental verification of such struc-tures is also vital to test shell modeland other many-body theoreticalstructure predictions away from thestable nuclei. At the high beam mo-menta delivered by fragmentationfacilities, it is possible to use thesemiclassical eikonal and adiabaticmethods discussed elsewhere in thisreport and, for reactions on light tar-gets, we have shown that these areindeed very accurate. Most impor-tantly, these methods allow both thestructure and reaction dynamics as-pects of the problem to be treatedwith comparable rigour.

In an ongoing Surrey/MSU col-laboration, one- and two-nucleonknockout reactions, which selec-tively excite a minimal number ofnucleonic degrees of freedom, arebeing developed as a novel single-

particle spectroscopic tool for usewith rare exotic nuclei [17]. Recentimportant results include the pos-sibility of determining not only rela-tive, but also absolute spectroscopicfactors from single-nucleon knock-out and the identification of two-proton knockout on very neutron-rich systems as proceeding via adirect two-proton removal mecha-nism [17].

Transfer ReactionsThe group has a long-standing

interest in reactions in which one ormore nucleons are transferred be-tween the projectile and target toleave the final nuclei in low-lyingstates. The reaction A(d,p)B is a typ-ical example.

Normally these reactions aretreated as one-step processes inwhich, in the (d,p) example, the neu-tron is transferred directly to the tar-get nucleus in one step. This is calledthe Distorted Wave Born Approxi-mation (DWBA). The unique contri-bution of the group was to explainhow the contribution to the transferfrom paths in which the deuteron isexcited—i.e., broken up into a con-tinuum state since the deuteron hasno bound excited states—by tidalforces generated by the target couldbe relatively easily incorporated intothe theory. In the early 1970s theseeffects were treated in the adiabaticapproximation. The ideas generatedat that time still influence the analy-sis of modern experimental data.

A key modern development fromthe adiabatic approximation was theContinuum Discretised CoupledChannels Method (CDCC), first pio-neered in Japan and the U.S. Whereasthe adiabatic approximation replacesthe continuum of deuteron break-upchannels by a single channel degener-ate in energy with the deuteronchannel, the CDCC approach treats

laboratory portrait


Figure 3. The cross section as a function of the momenta of 14C fragments(in their ground state) emerging, at various laboratory angle intervals, for theremoval of a single neutron from 15C. The angular distributions reveal un-precedented details of the reaction mechanism. The coupled channels few-body break-up theory, using the CDCC method, is in excellent agreementwith the data.

the nondegeneracy of the continuumdirectly by replacing it in an unam-biguous way by a set of discretechannels. The theory can be furthergeneralised to include couplings be-tween different rearrangement chan-nels [7].

Recent applications at Surrey aremainly to the one- or two-halo neu-tron transfer in reactions with ra-dioactive beams. In particular, thereis a special interest in reactions fromwhich astrophysically important in-formation can be extracted. Infor-mation deduced by comparing trans-fer cross sections with experiment(“asymptotic normalisation con-stants”) can be used to predict crosssections for A(p,γ)B reactions at thevery low energies needed for astro-physical studies of nuclear synthesis.In parallel, theoretical studies ofANCs and other quantities associ-ated with transfer reactions, such asoverlap integrals and spectroscopicfactors, are also carried out with theaim of relating them to the structureof the nuclei involved.

Multiple ScatteringAn important problem in nuclear

theory is to show how nucleon–nucleus and nucleon–nucleon scat-tering cross sections can be related.Attempts to do this by expandingthe nucleon–nucleus scattering am-plitude in a series in which the inci-dent nucleon scatters off more andmore target nucleons in successiveterms in the series is known as mul-tiple scattering and has been a long-standing interest of the group. De-tailed studies in momentum space ofelastic proton scattering including fi-nite nucleus medium effects werecarried out in the 1990s. Recent ap-plications have been to halo nuclei.

Recent work on p + 11Li inelasticscattering at 68 MeV/u leading tocontinuum states in 11Li [18] obtains

good agreement with the peaks seenin the experimental cross sectionbelow 3 MeV excitation, but no evi-dence for an interpretation in termsof a dipole resonance is deduced [19].

Halo Break-up and Fusion StudiesIn break-up studies of exotic nu-

clei, we can calculate the most gen-eral observables at low- and high-incident energies, by the CDCC ap-proach. In selected observables thereis interference between the break-upamplitudes from different spin-par-ity excitations of the projectile. Theresulting fragment angle and energydistributions reveal the importanceof higher order continuum state cou-plings.

The break-up of weakly boundlight projectiles on heavy targets atenergies near the Coulomb barrierwas also studied in the adiabatictwo-centre shell model. The effect of

continuum couplings in the fusion ofthe halo nucleus 11Be on 208Pbaround the Coulomb barrier wasstudied using a three-body modelwithin the CDCC formalism. We in-vestigated in particular the role ofcontinuum–continuum couplings.These are found to hinder total,complete, and incomplete fusionprocesses. Results show that contin-uum–continuum couplings enhancethe irreversibility of break-up andreduce the flux that penetrates theCoulomb barrier. Converged totalfusion cross sections agree with theexperimental ones for energiesaround the Coulomb barrier, but un-derestimate those for energies wellabove the Coulomb barrier.

Photonuclear ReactionsA new probe of the halo struc-

ture is through the use of mesonphoto-production reactions, reach-

laboratory portrait


Figure 4. Two proton decay: width or half-life as a function of resonance en-ergy for 19Mg and 48Ni. Solid curves represent three-body calculations. Starscorrespond to diproton estimates. Dashed and dash-dotted curves are for di-rect decay to continuum, with, respectively, l = 0, and the l-value from theshell model.

ing the nuclei of interest through anelectromagnetic charge exchangingprocess. Work by the group has al-ready shown that reactions will pro-vide an interesting probe of the halo,particularly for those short-livedstates, such as the first excited stateof 17F, that cannot be studied in ra-dioactive beam experiments. In addi-tion, while fragmentation reactionstend to probe the long-range tail ofthe halo wave function, pion photo-production reactions are also sensi-tive to the interior, and as such cangive new insights into their structure,such as the role of antisymmetrisa-tion. The group is currently studyingthe importance of including clustermodel wave functions in the calcula-tions, to investigate the sensitivity todetails of the nuclear structure. Reac-tions of interest include 6Li(γ,π+)6He.

Few-Body Structure TheoryThe subtle interplay between

quantum mechanical 3-body effectsand nucleon–nucleon correlationsmake the study of two-neutron halonuclei of particular interest to the-ory. Bound states of such systemshave been studied at Surrey by manymethods. Some of the most interest-ing challenges arise when the systemis in the continuum of unboundstates, and this is the focus of muchcurrent research. For example, thehyperspherical harmonics method isbeing used to investigate low-lyingresonances and the soft dipole modein the two-neutron halo nucleus 6He.A unique structure for true three-body resonances has been revealed.The method of many-body hyper-spherical functions has also beendeveloped and used to calculate thebinding energies of light neutron-rich isotopes of hydrogen and he-lium, and to search for multi-neu-trons.

Recently the group applied itsexpertise in three-body structuremodels to the study of two-protondecay. Emission of two protons fromnuclear states has been studied since1960 when Goldansky predictedtwo-proton radioactivity for nucleibeyond the proton dripline. Two-proton decay may occur throughthree possible mechanisms: (i) se-quential emission of protons via anintermediate state, (ii) simultaneousemission of protons (direct decay tocontinuum), and (iii) di-proton emis-sion, i.e., emission of a strongly cor-related 2He cluster. Experimentally,only the first two mechanisms havebeen identified. The third case is tra-ditionally associated with two-pro-ton radioactivity but has not beenobserved. Surrey calculations [20]show that the pp pairing interactionstrongly influences penetrationthrough the complicated multi-dimensional Coulomb and centrifu-gal barriers. Semiclassical models forthe penetration probability give onlyupper and lower limits, which differby orders of magnitude and can giveresults which differ substantiallyfrom the more exact 3-body meth-ods used at Surrey.

Hadron PhysicsRecently, the group has pursued

an interest in nuclear medium effectson nucleon resonances via mesonphotoproduction reactions and de-veloped a quark model approach formeson photo and electroproduction[21]. The advantage of this QCD-in-spired phenomenology is that with alimited number of parameters, allthe resonances can be self-consis-tently included and analysed. Wehave also extended our quark modelapproach to the study of mesonphoto-production on nuclei, wherethe interest is to learn how a nucleon

and its resonances behave in the nu-clear medium.

Nuclear Structure TheoryThe shape of the nucleus seems

at first glance to be one of the sim-plest of its macroscopic properties. Itturns out, however, that it can as-sume many possible shapes. It can bea spheroid or a prolate, oblate, octo-pole, etc. ellipsoid. Moreover, wefind many examples of nuclei inwhich states or groups of states withdifferent shapes exist. This, togetherwith the fact that single particle ex-citations, vibrations, and rotationsin nuclei have comparable excitationenergies, makes nuclear structureand its associated nuclear spec-troscopy complex and difficult to re-produce theoretically.

Among the many intriguing as-pects of nuclear structure, we findthat nuclei with oblate shapes arerare compared to those with prolatedeformations. In simple terms this isto be expected. If we consider thecollective rotation of an axially sym-metric nucleus, around an axis per-pendicular to the symmetry axis, themoment-of-inertia is larger for theprolate than for the oblate shape.Since the excitation energies of arotor are inversely proportional tothe moment-of-inertia, they arelower for the prolate shapes for agiven angular momentum.

Experimentalists at Surrey arekeenly interested in the properties ofneutron-rich nuclei, which are be-coming more accessible to study asbeams of radioactive nuclei are de-veloped, as well as the spectrometersthat allow us to identify individualnuclear species produced in the deepinelastic collisions of heavy ions.Theoretically, two distinct regions ofthe nuclear chart have been foundwhere well-deformed oblate rotors

laboratory portrait


are expected to be favoured in en-ergy over prolate rotors.

This occurs when there are sev-eral proton- and neutron-holes rela-tive to the doubly-magic 208Pb and132Sn nuclei. The nuclear Fermi sur-face then lies among single (Nilsson)particle orbitals with high j valuesaligned close to the axis of rotation.This results in a strong Coriolis forcethat stabilises the oblate shapes.Both of these regions where oblatedeformation is favoured lie suffi-ciently far from stability on the neu-tron-rich side that it has not yet beenpossible to verify these predictions.

Competition between oblate andprolate shapes is also sensitive to thestrength of the spin-orbit interac-tion, and it is generally consideredthat this is a key factor in decidinghow nuclear structure will evolve aswe move away from stability. Ourpredictions are part of meeting thechallenge of finding mean fieldmodels to describe this evolution.

Shapes and IsomersAt Surrey, shape calculations

based on the TRS macroscopic/

microscopic shell correction ap-proach, with the Woods-Saxon po-tential and Lipkin-Nogami pairing,have been carried out. One of thefavoured oblate regions is found inneutron-rich hafnium isotopes, asfirst reported by Hilton and Mangfor 180Hf. They predicted giant back-bending at a spin of 26 h, with a dra-matic switch from prolate to oblateshape. We have found that the effectbecomes stronger, and the backbend-ing spin lower, when isotopes with afew more neutrons are considered.

One intriguing aspect of our re-sults is that these nuclei, at least theneutron-rich hafnium isotopes, carryangular momentum in such a waythat the structure that competes withoblate collective rotation at highspin is not prolate collective rota-tion, but prolate multi-quasi-particlestates. Such states are well known toform long-lived isomers in ordinaryhafnium isotopes, and they are ex-pected to do likewise in the neutron-rich isotopes, with perhaps even moreretarded decays [22]. The level order-ing in calculations with a Woods-Saxon potential provides an intuitive

explanation of this oblate collective/prolate-non-collective competition.For the oblate shapes to come lowestin energy, a key feature is that theFermi levels for both protons andneutrons lie close to the tops of theshells they are in.

All of these calculations awaitthe test of experiment.

Tilted Rotations with Triaxial Symmetry

In nuclear physics, an emergenceof the triaxial symmetry allows so-called wobbling motion at high spinand chiral rotation at medium spin.We have been studying in detailthese exotic dynamical modes bymeans of microscopic (tilted-axiscranked HFB method) and quantalapproaches (generator coordinatemethod and quantum number pro-jection techniques). In particular, nu-merical investigations were carriedout for wobbling motion in 182Os asa coupling mode between low andhigh-K states [23,25]; and for a pos-sibility of nuclear chirality in 134Ce[26]. Figure 5 shows the energy sur-faces of 134Ce (J = 26 and 28) with

laboratory portrait


Figure 5. Energy surfaces of high spin states (I = 26 h and 30 h) for 134Ce. The polar angles θ and ϕ are tilt anglesof the total angular momentum vector.

QY:Author: lower case “h” ok here (3 times)?

several minima, corresponding to1d, 2d, and 3d-rotations. Theta andphi denote tilt angles of the total an-gular momentum vector.

SummarySurrey theorists are actively de-

veloping and contributing nucleartheory expertise to reactions andstructure studies explored with ra-dioactive nuclear beams, topics thatare currently providing the maindrive towards new physics and newnuclear physics facilities around theworld. The description of nuclei farfrom stability in terms of the under-lying forces is one of the most im-portant current areas of scientific re-search. Studies of nuclei far from thevalley of stability probe nuclearstructure at lower than normal den-sities, in contrast with ultra-relativis-tic heavy-ion collisions, which seekto create nuclear matter at high tem-perature and density. This area willcontinue to be a main part of Sur-rey’s effort. We will also continueour tradition of fostering close linkswith experimental groups all overthe world, through both the promul-gation of new ideas and the provi-sion of computer codes.

The recent appointment of twonew members of the group reflectsSurrey’s confidence in the future offundamental nuclear physics. Theseyoung theorists will bring an extradimension to the Group’s research.Oi has developed a theory of nuclearrotation of direct relevance to exper-imental work in the group [23].Stevenson has developed a new ef-fective nucleon–nucleon interactionfor use in mean-field theories andbeyond [24]. These novel contribu-tions they have already made to thefield auger well for the future.

Finally, the group is proud of itsmany activities in promoting itswork to a wider audience of physi-cists, and nuclear physics in generalto the wider public, through lec-tures, magazines articles and eventhe first ever coffee-table glossybook on nuclear physics [27].

References1. L. R. B. Elton, Nuclear Sizes,

Clarendon Press, Oxford, 1961.2. R. C. Barrett and D. F. Jackson, Nu-

clear Sizes and Structure, ClarendonPress, Oxford, 1977.

3. R. C. Johnson and P. J. R. Soper,Phys. Rev. C1 (1970), 976–990; J.D. Harvey and R. C. Johnson, Phys.Rev. C3 (1971), 636–645.

4. J. Gomez-Camacho and R. C. John-son, Polarisation in Nuclear Reac-tions, in Scattering, Edited by P.Sabatier and E. R. Pike, AcademicPress, London and San Diego, 2002,Chapter 3.1.5, pp. 1414–1432.

5. R. C. Johnson, J. S. Al-Khalili, andJ. A. Tostevin, Phys. Rev. Lett. 79(1997), 2771–2774.

6. R. C. Johnson and F. D. Santos,Phys. Rev. Lett. 19 (1967), 364–366.

7. I. J. Thompson, Methods of DirectReaction Theories, in Scattering,ibid., Chapter 3.1.2, pp. 1360–1372.

8. J. S. Al-Khalili and J. A. Tostevin,Few-Body Models of Nuclear Reac-tions, in Scattering, ibid., Chapter3.1.3, pp. 1373–1392.

9. J. S. Al-Khalili and R. C. Johnson,Nucl. Phys. A546 (1992), 622.

10. R. C. Johnson, E. J. Stephenson, andJ. A. Tostevin, Nucl. Phys. A505(1989), 26–66.

11. I. J. Thompson, Computer Pro-gramme ADIA, Daresbury Labora-tory Report, 1984.

12. N. C. Summers, J. S. Al-Khalili, andR. C. Johnson, Phys. Rev. C66(2002), 014614.

13. N. K. Timofeyuk and R. C. Johnson,Phys. Rev. C59 (1999), 1337.

14. J. S. Al-Khalili and J. A. Tostevin,Phys. Rev. Lett. 76 (1996), 3903.

15. R. C. Johnson. and C. J. Goebel,Phys. Rev. C62 (2000), 027603.

16. J. A. Christley, J. S. Al-Khalili, J. A.Tostevin, and R. C. Johnson, Nucl.Phys. A624 (1997), 275.

17. P. G. Hansen and J. A. Tostevin,Ann. Rev. Nucl. Part. Sci. 53 (2003),in press.

18. R. Crespo and R. C. Johnson, Phys.Rev. C60 (1999), 034007.

19. R. Crespo, I. J. Thompson, and A.A. Korsheninnikov, Phys. Rev. C66(2002), 0210023.

20. L. V. Grigorenko, R. C. Johnson, I.G. Mukha, I. J. Thompson, and M.V. Zhukov, Phys. Rev. Lett. 85(2000), 22.

21. Q. Zhao, J. S. Al-Khalili, Z. P. Li,and R. L. Workman, Phys. Rev. C65(2002), 065204.

22. P. M. Walker and G. D. Dracoulis,Nature 399 (1999), 35.

23. M. Oi, P. M. Walker, and A. Ansari,Phys. Lett. B525 (2002), 255.

24. P. Stevenson, M. R. Strayer, and J.Rikovska Stone, Phys. Rev. C63(2001), 054309.

25. M. Oi, A. Ansari, T. Horibata, andN. Onishi, Phys. Lett. B480 (2000),53.

26. M. Oi and P. M. Walker, submittedto Phys. Lett. B.

27. R. Mackintosh, J. S. Al-Khalili, B.Jonson, and T. Pena, NUCLEUS: ATrip into the Heart of Matter, Cano-pus publishing, 2002.

Contributors to thisarticle include

J. S. AL-KHALILI, W. GELLETLY, R. C. JOHNSON, M. OI,

P. D. STEVENSON, I. J. THOMPSON, N. TIMOFEYUK, J. A. TOSTEVIN,

P. M. WALKER, AND Q. ZHAO

laboratory portrait


QY:Author: Ref. No’s.24, 25, 26, 27, ok?There were two “27’s” in file.

The 17th International NuclearPhysics Divisional Conference of theEuropean Physical Society, “NuclearPhysics in Astrophysics,” was heldin Debrecen, Hungary from Septem-ber 30 to October 4, 2002. This con-ference was originally planned totake place in Eilat, Israel in 2001,but was moved to Debrecen becauseof security considerations. Many ofthe topics and much of the programwere similar to those planned for theEilat meeting.

The main objective of the con-ference was to deal with all thosesubjects of nuclear physics that im-pact astrophysics and are an essen-tial input in the understanding of as-trophysical processes. The emphasiswas on the recent theoretical and ex-perimental developments in nuclearphysics that are of relevance to as-trophysics. The topics discussed atthe conference included: cross-sec-tion measurements and nuclear datafor astrophysics, stellar and big bangnucleosynthesis, the application ofnuclear structure far from the stabil-ity line to astrophysics, neutrinophysics, nuclear reactions pertainingto astrophysics, rare ion beam facili-ties and experiments, and a fewother diverse topics.

The program of the conferenceincluded 26 invited lectures and 30contributed ones delivered by speak-ers representing 22 countries. In ad-dition, a poster session was organ-ized. There were a number of talksrelated to neutrino physics includingsome of the most recent experimentalresults (for example, those of SNO,

LSND, neutrino mass measure-ments), results in the field of nuclearreactions of astrophysics relevance,and measurements of radiation fromextinct, rare, and very short-livednuclei that may be a window into abetter understanding of nucleosyn-thesis. The importance of studyingexotic nuclei in order to understandastrophysical processes was widelystressed. The activities of variouslarge laboratories in the world (andin particular in Europe) in the pres-ent and future were presented at themeeting. There were also a numberof talks of more general interestabout dark matter, relativistic heavyion collisions, and cosmology as wellas a talk about public awareness ofnuclear science. The talks have led tovery lively and lengthy discussionsand interaction among the partici-pants. The proceedings of the con-ference will be published in NuclearPhysics A.

The breadth of the European ac-tivity in these fields of nuclear astro-physics, as revealed at the confer-ence, is truly impressive. This seemsto be one of the most important fu-ture directions of research in Europeand countries in America and Asiarepresented at the meeting. Particu-larly pleasing was the the large par-ticipation of young researchers.Among the 100 conference partici-pants, 35% were scientists under theage of 35 years. It is obvious thatthis subfield of nuclear physics has abright future. (The Nuclear PhysicsBoard of the EPS is considering thepossibility of having this type of con-

ference, “Nuclear Physics in Astro-physics,” on a regular basis, everyfew years.)

During the meeting Prof. ClausRolfs (Ruhr University, Bochum,Germany) was awarded an Hon-orary Membership in the RolandEötvös Physical Society of Hungaryfor his contributions to the field ofnuclear astrophysics and for his helpin the development of this field inHungary. The short ceremony wasfollowed by a lecture presented bythe recipient of the award.

The conference was organizedby the Nuclear Physics Board of theEPS and by the Institute of NuclearResearch (ATOMKI) in Debrecen.The conference was sponsored bythe European Commission, HumanPotential Program, the EPS YoungPhysicist Fund, Hungarian Academyof Sciences, Hungarian Ministry ofEducation, and the Hungarian Na-tional Fund for Scientific Research.In addition, the initial stages of or-ganization were sponsored by theBen-Gurion, Hebrew, and Tel-AvivUniversities and the Weizmann Insti-tute in Israel.

N. AUERBACH

School of Physics and AstronomyTel-Aviv University, Israel

ZS. FÜLÖP

ATOMKIDebrecen, Hungary

meeting reports


XVIIth Nuclear Physics Divisional Conference on“Nuclear Physics in Astrophysics”

The XXXIII European Cyclo-tron Progress Meeting, held Septem-ber 17–21, 2002, was organizedjointly by the Heavy Ion Laboratoryof the Warsaw University andNiewodniczanski Institute of Nu-clear Physics, respectively. The eventwas held at both locations, a newfeature in the history of ECPMs.Also, for the first time, the proceed-ings will be published in Interna-tional Journal of Nuclear ResearchNukleonika.

The conference gathered almost100 participants from Europe, USA,Canada, and Japan. The reportswere devoted not only to the cyclo-tron techniques, but also to applica-tions—medical and industrial—aswell as to the hottest subjects in nu-clear science, namely the use of cy-clotrons as radioactive beam sources.Much of the time was spent on dis-cussions about novel designs of thehighly proficient ion sources and as-sociated transport systems. The sub-ject of the ion sources was coveredby Santo Gammino (LNS Catania),Claude Bieth (PANTECHIK, Caen),Hannu Koivisto (Jyväskylä), andVladimir Loginov (Dubna). Jean-Loup Belmont (Grenoble) and Wolf-gang Pelzer (HMI) addressed theproblems associated with the trans-port of low-energy beams from thesource to the accelerating structure

of the cyclotron. New ideas concern-ing the acceleration of the hadronbeams were presented by YvesJongen (IBA), Michael Schillo(ACCEL), Anne Paans (Groningen),and Leonid Onischenko (Dubna).Willem Kleeven (IBA), Helge Jung-wirth (IKF Jülich), and Jarosl/awChoinski (HIL Warsaw) discussedthe different modes of beam extrac-tion. Finally, Marc Loiselet (Lou-vain), Marcel Lieuvin (GANIL),Grigorij Gulbekian (Dubna), Ger-ardo Dutto (TRIUMF), and SytzeBrandenburg (KVI) talked about thedevelopments in radioactive beamfacilities. Of course this classifica-tion, following the concluding re-marks of Heinrich Homeyer (HMI),is not showing the real scope of thetalks of the keynote speakers. All theproblems discussed interweave, sothe meeting was an excellent oppor-tunity to bring together scientistsspecializing in sometimes narrowareas.

Besides the participants repre-senting academic and research labo-ratories, commercial companies werealso present, stressing the increasingrole of applications of acceleratortechniques. ACCEL InstrumentsGmbH, General Electric MedicalSystems and Ion Beam Applications(IBA) presented their offers duringpermanent exhibitions.

The working visits to the War-saw heavy-ion cyclotron, the onlyoperating installation of its kind inCentral Europe, and to the CracowAIC-144 stimulated an agitated ex-change of ideas on the basic techni-cal level. Social events were alsohelpful to give an opportunity foreye-to-eye discussions not onlyabout the achievements, but alsoproblems. Although welcome recep-tion at the Warsaw cyclotron, fol-lowed by the the concert performedby Chamber Choir of the WarsawChamber Opera as well as the con-ference dinner at the Royal WawelCastle in Cracow did not seem to bean opportunity to argue about space-charge effects, field imperfectionsand so on, but scientists are alwaysscientists . . .

The organizers of the XXXIIIECPM are thankful for the engage-ment of the Polish State Committeefor Scientific Research, Ministry ofEducation and Sport and NationalAtomic Energy Agency, as well as tothe industrial companies involved,for the support which, to a great ex-tent, made the event successful andfruitful.

TOMASZ CZOSNYKA

Heavy Ion LaboratoryWarsaw University

meeting reports


XXXIII European Cyclotron Progress Meeting

The 3rd IBSNucPhys was held inthe auditorium of the Central Li-brary of the Aristotle University ofThessaloniki, Greece on September18–24, 2002, continuing the tradi-tion started in Istanbul, Turkey(1998) and Bodrum, Turkey (2000).

The school was under the aus-pices of the Faculty of Physics of theAristotle University and the BalkanPhysics Union. It was sponsored bythe Ministries of Education, Cultureand Economics, as well as by theSchool of Phyiscs and the Researchcommittee of the Aristotle University,the Hellenic Physical Society (Cen-tral-Western Macedonia branch), theHellenic Nuclear Physics Society, theMunicipality of Thessaloniki, andthe National Bank of Greece.

The school was attended bymore than 90 participants from 22countries all over the world. Thelarge participation of young col-leagues at the M.Sc. and Ph.D. levelsshould be particularly noticed. Theirinterest and enthusiasm was re-flected in the 31 contributions theymade during the special evening

seminars. The titles and short ab-stracts of the contributions, alongwith the viewgraphs of the lecturers,can be found on the school’s website:http://nuclear.physics.auth.gr/bschool/index.html.

The program included six one-hour lectures per day. There werealso 30-minute seminars and studentcontributions. The school was fo-cused on hot subjects connected withnuclear structure and reactions ofexotic nuclei and their influence onnuclear astrophysics. The representa-tives of several major experimentallaboratories—G. Muenzenberg (GSIDarmstadt), M. Lewitowicz (GANIL),G. De Angelis (Legnaro), P. Butler(Liverpool), M. Thoennessen (MSU),R. Casten (Yale), M. L. Aliotta (Ed-inburgh), S. Harissopoulos (N.C.S.R.Demokritos), A. Youngclaus (Ma-drid)—presented their latest achieve-ments as well as their future plans.From the theory point of view, manyof the state-of-the-art theoretical ap-proaches were presented and dis-cussed in detail by P. Ring (Munich),T. Otsuka (Tokyo), J. Dobaczewski

(Warsaw), J. Tostevin (Surrey), D.Vretenar (Zagreb), S. Goriely (Brus-sels), J. L. Egido (Madrid), L. Fer-reira (Lisbon). The volume with theproceedings of the school is inpreparation and will be publishedsoon.

During the school the HellenicNuclear Physics Society honoredProfessor G. Muenzenberg for hisleading role in the discovery of newsuperheavy elements at GSI Darm-stadt. Finally, the participants hadthe chance to visit the recently dis-covered tombs of Macedonian kings(including the one of Philip II, thefather of Alexander the Great) inVergina, the ruins and the museumof the ancient city of Dion, the cru-saders’ castle of Platamona, as wellas Mount Olympos, the traditionalabode of the ancient Greek gods.

GEORGIOS A. LALAZISSIS

Department of Theoretical PhysicsAristotle University of

Thessaloniki, Greece

meeting reports


Report on the 3rd International Balkan Schoolon Nuclear Physics (3rd IBSNucPhys)


IntroductionIn recent years a number of nuclear phenomena was

discovered that are accompanied by strong magnetic di-pole (M1) transitions. Measurements of M1 matrix ele-ments enabled nuclear physicists to deduce new structureinformation in a model-independent way, since the elec-tromagnetic interaction is well understood. Due to thedominantly isovector character of the electromagneticM1 transition operator (see Appendix A), informationon M1 matrix elements helps to clarify various aspects ofthe nuclear isospin degree of freedom (see Appendix B).This is of particular importance as a prerequisite formaking full use of newly established or planned radio-active ion beam (RIB) facilities probing nuclei at extremevalues of isospin.

M1 transitions are fundamental in nuclear physics.The nucleon with spin and parity Jπ = 1/2+ is not ele-mentary itself and can be excited to the Jπ = 3/2+ ∆-reso-nance by an M1 transition. The corresponding matrix el-ement [1] amounts to 3 nuclear magnetons (µN =eh- /2mpc in Gaussian units) and sets the scale for M1 phe-nomena in nuclear physics. The anomalous magneticmoments of the free nucleons, µp = +2.8 µN and µn = –1.9µN, are of a similar absolute size. Their deviation fromthe values expected for elementary J = 1/2 Dirac particles[µD(p) = +1.0 µN and µD(n) = 0 µN] underpins the impor-tance of the subnucleonic degrees of freedom. In studiesof nuclear structure, Schmidt’s successful prediction ofmagnetic dipole moments of many (near closed-shell)nuclei using the single particle approximation stronglysupported the microscopic shell model ansatz and repre-sented a conceptual breakthrough.

Out of the variety of M1 phenomena, the currentpaper presents a brief overview of those that have re-cently attracted attention because of new experimentalevidence or approaches. Certainly, all the details cannot

be discussed here and we apologize for the personal biasregarding the covered topics and necessarily incompletereference list; for discussions of specific M1 phenomenawe recommend the recent review articles by Clark andMacchiavelli [2], Frauendorf [3], and Heyde and Richter[4].

This overview is organized according to increasingcomplexity and collectivity (not necessarily increasingM1 strength) of the covered M1 phenomena. We first ad-dress the observation of remarkably pure quasideuteronconfigurations in the level schemes of heavy self-conju-gate (N = Z) odd-odd nuclei formed by the coupling ofthe unpaired proton and neutron in the same j-orbital.Subsequent alignment of more complex several-particleconfigurations leads in heavier nuclei to the phenome-non of magnetic rotation through the development ofregular shears bands. This alignment causes in triaxialnuclei the recently discovered chiral bands and the dou-bling of states in nearly degenerate level sequences. Fi-nally, recently discovered proton-neutron mixed-symme-try multi-quadrupole phonon vibrational structures willbe discussed. All these M1 phenomena are related to thecoupling of proton and neutron subsystems with finiteangular momenta.

Quasideuteron ConfigurationsThe most elementary source of isovector M1 transi-

tions are two-particle so-called quasideuteron configura-tions (QDCs) in odd-odd N = Z nuclei. These nuclei arespecial because of the coexistence of states with isospinquantum numbers T< = 0 and T> = 1 at low energies,which leads in some cases to anomalous ground stateswith isospin T>. The deuteron is the simplest odd-odd N= Z nucleus. Its Jπ = 1+, T = 0 ground state and its Jπ =0+, T = 1 excited state at ~2.24 MeV are dominantlyformed by the coupling of the 1s1/2 proton and neutron.

feature article

New Magnetic Dipole Phenomena in Atomic NucleiNORBERT PIETRALLA

Institut für Kernphysik, Universität zu Köln, Köln, Germany

KRZYSZTOF STAROSTA

Department of Physics and Astronomy, State University of New York at Stony Brook, Stony Brook, New York, USA


Analogous configurations can be formed in the heavierodd-odd N = Z nuclei by an unpaired proton and neu-tron in the same j-orbital coupled to the 0+, T = 0,ground state of the neighboring even-even N = Z nu-cleus. Such simple two-particle configurations:

J+, T ⟩ = [ jp ⊗ jn]TJ+ ⊗ Ψ 0+

core,T=0

are uniquely specified by the total angular momentumquantum number J = 0, 1, . . . , 2j with isospin T = [1 +(–1)J]/2 and are called QDCs due to their formal analogyto the states of the deuteron. The QDCs with isospin T= 1 possess corresponding configurations in the neigh-boring isobaric nuclei while those with T = 0 do not.

For the QDC wave functions it is straightforward toanalytically derive the ∆J =1 M1 transition rates in afashion similar to the derivation of the Schmidt-valuesfor magnetic moments of odd-mass nuclei. Such transi-tions are purely isovector and are expected to be en-hanced due to the isovector character of the M1 transi-tion operator (see Appendix A). This task has beencarried out by Lisetskiy et al. [5] for cases with sphericaland axially deformed cores. For spherical QDCs, the M1transition strengths are given by

where G(l, j,J) = (2j + 2 + J)(2j – J)(J + 1)/(l + 1/2)2 is ageometrical factor and gl(s)

V are defined in Appendix A.A destructive interference occurs between orbital and spinparts for j = l –1/2 cases while for j = l + 1/2 constructiveinterference can lead to particularly large, up to 20 µN

2,values of B(M1), as observed for the B(M1; 0+, T = 1 →1+, T = 0) value for the 1041-keV 0+ → 1+ transition in18F, where a d5/2 proton and neutron couple to the spher-ical 16O-core. Deformation of the even-even core intro-duces further dependence of the M1 strength on the Kquantum number and on the size of the deformation.Since the QDC scheme represents one of the most effi-cient ways to produce strong M1 transitions, largeB(M1) values can serve as good signatures for QDCs.

Sizeable B(M1) values (>1 µN2) were known in odd-

odd N = Z nuclei up to 42Sc. At larger masses the valley

of stability departs from the N = Z line and the accuratemeasurement of transition rates becomes difficult. It is,however, an intriguing problem to study whether theQDC coupling scheme holds true for heavier nuclei orthose with large deformation. An ideal testing ground isprovided by the f7/2 shell isolated from other shells by thegaps at Z, N ∈ {20,28}; the f7/2 shell includes deformed(48Cr) and spherical (40Ca, 56Ni) even-even N = Z corenuclei. The f7/2 orbital has j = l + 1/2 and, hence, largeB(M1) values are expected.

An experimental program for the investigation of thelow-spin structures of odd-odd N = Z nuclei in the f7/2

shell has been initiated at the γ-ray detector arrays inCologne and at Yale by Brentano. The previously poorlyknown low-spin structures of 46V, 50Mn, and 54Co havebeen considerably extended [6–8] in Cologne with(p,nγγ) reactions. The high-spin parts of the levelschemes were intensively studied, predominantly at Leg-naro and at the Gammasphere array; see e.g., recent ref-erences [9–11]. Precise measurements of E2/M1 multi-pole mixing ratios were possible in Cologne fromγγ-angular correlation data.

Information on E2/M1 branching ratios in compari-son to known E2 transition rates between correspondingT = 1 states in the even-even partners provided evidencefor large B(M1) values and, thus, for the existence ofconsiderably pure QDCs in 46V, 50Mn, and 54Co. Directmeasurements of crucial M1 transition rates were per-formed [12] at Yale University in “cold” heavy-ion in-duced reactions on 40Ca at the Coulomb barrier andDoppler-shift analyses of the observed γγ-coincidencedata. Marked Doppler shifts were found in the sequenceof QDC candidates owing to the high transition rates.Figure 1 compares the QDC structure found in 50Mnwith the full level scheme below 3.3 MeV of the T = 1isobaric partner 50Cr.

The relative excitation energies R(J) = Ex(J)/Ex(21+)

of the T = 1 partner states with Jπ = 4+ and 6+ agree in50Mn and 50Cr within 1%. The experimental B(M1; 31

+ → 21+) = 2.9+1.0

–0.7 µN2 value coincides within the

errors with the analytical result, B(M1; 31+ → 21

+) = 3.1µN

2, obtained [5] considering an appropriately deformed48Cr-core. This agreement proves the existence of quitepure QDCs in f7/2-shell nuclei. The structure in 50Mn,missing only the T = 0, 7+ state, represents the most ex-tensive sequence of QDCs observed so far.

An interesting question is to what extent the isospinsymmetry is broken in N = Z nuclei; significant ex-perimental effort is currently devoted to this problem.

feature article


Coulomb energy displacements in isobaric mirror nucleirevealed valuable information on the alignment mecha-nism; e.g., see [11]. Recently this information was usedto extract isospin mixing from a comparison to largescale shell model calculations. The properties of robuststrong M1 transitions from QDCs have also been shownto be useful for a direct determination of isospin mixing,e.g., in 54Co [13], where mixing matrix elements of ~10keV were found.

Shears Bands and Magnetic RotationDiscovery of the shears mechanism is one of the

most exciting recent achievements in high spin γ-rayspectroscopy. Unexpectedly, it was observed that config-urations based on high-j intruder orbitals in weakly de-formed nuclei result in very regular γ-ray cascades [14]as shown in Figure 2a for 199Pb. The surprise was relatedto the fact that these cascades involve a sequence of ∆I =1 transitions in contrast to already well-established rota-tional bands in deformed and superdeformed nucleicomprised of stretched E2 transitions. It was shortly re-alized [15] that angular momentum in these novel dipolebands is generated by a mechanism significantly differentfrom collective rotation.

In weakly deformed nuclei near closed shells, or-bitals of either particle or hole character can be occupiedby nucleons of the opposite isospin projection (for thebands first discovered in the lead region, protons occupyh9/2 and i13/2 particle states, while neutrons occupy i13/2

hole states). For the lowest energy state the angular mo-menta of the resulting particle and hole configurationsare oriented in perpendicular directions since this orien-tation minimizes the interaction energy; this has beenverified with the g-factor measurement of the isomericband head in 193Pb (see [2] for the reference and details).

feature article

Figure 1. Comparison between the QDC structure found in50Mn with the full level scheme below 3.3 MeV of the T = 1 iso-baric partner 50Cr. The numbers below the level bars denote rel-ative excitation energies R(J), those next to 50Mn-levels showmeasured lifetimes in picoseconds. Deduced B(M1)↓ values aregiven in units of µN

2 at the transition arrows. The correspon-ding B(M1) predictions of the quasideuteron scheme coupled toa deformed rotor are displayed in the column above “QDC.”Data are from [12].

Figure 2. (a) Experimental spectrum showing M1 transitions inone of the ∆I = 1 bands in 199Pb. (b) Geometry of angular mo-mentum coupling for particle and hole states in shears configu-rations near the band head (top) and at higher angular mo-menta (bottom). (c) Reduced M1 transition probability B(M1)as a function of angular momentum for the band in 199Pbshown in panel (a). Data are from [2].


These long angular momentum vectors are said to form“shears blades” with the shears open at the band head.Due to small deformations near closed shells, it is morefavorable to generate higher angular momentum statesby closing the shears blades rather than by collective ro-tation, as shown in Figure 2b.

The increasing energy of the states formed as theshears blades are closing reflects the energy of the repul-sive interaction between the particles and holes formingthe blades; multipole expansion of the effective forceyields rotational-like behavior with energies propor-tional to the square of total angular momentum [2]. Theabove scenario, known as the “shears mechanism” forgenerating total angular momentum, inspired a numberof important theoretical developments, including thesemi-classical approach and the microscopic Tilted AxisCranking model reviewed in [2] and [3], respectively.

Multiplets of levels generated by the closing bladesform ∆I = 1 bands with states connected predominantlyby M1 transitions. The magnetic dipole character ofthese transitions was established from angular correla-tion, linear polarization, and electron conversion meas-urements (see references in [2]). The E2/M1 admixturesfor ∆I = 1 transitions deduced from angular correlationsimply, in the lead region, deformations of oblate shape.The relatively low magnitude of this deformation is in-dicated by the small intensities of ∆I = 2 E2 cross-overtransitions observed in a number of cases; the B(M1)/B(E2) ratio of transition strengths extracted from thebranching ratios exceeds 20 µN

2/e2b2 in the lead region.The M1 strength is related to the component of the

magnetic moment perpendicular to the total angular mo-mentum. The fact that this component is reduced as theshears blades close (see Figure 2b), results in an impor-tant prediction, namely that the B(M1) strength in shearsbands decreases as a function of increasing angular mo-mentum. The B(M1) values have been addressed by anumber of lifetime measurements [2] which result for thelead region in B(M1) ~ 2 µN

2 and B(E2) ~ 0.1e2b2 ingood agreement with the expectations. Moreover, the de-creasing trend for the B(M1) as a function of spin hasclearly been identified, as shown in Figure 2c.

Another prediction of the shears mechanism is theband termination expected when the shears are fullyclosed. This has been seldom observed, since specificconfigurations are hard to trace up to the non-yrast ter-minating state; crossings with bands involving a largernumber of aligned particles and holes determine theyrast line accessible to high-spin spectroscopy. Suchcrossings, however, correspond in the above terminology

to a “reopening of the shears,” and thus should yield acorrelated increase in the B(M1) strength. The effect hasbeen indeed confirmed experimentally in the recent,highly sensitive, lifetime measurement [16].

An appealing interpretation of the shears mechanismnamed “magnetic rotation” is discussed in detail in [3].It is noted there that the current loops associated withhigh-j particles and holes embedded in the near sphericalmass distribution of the nucleus, as well as associatedtransverse magnetic moment, allow one to specify theangle of rotation with respect to the axis defined by thetotal angular momentum. This anisotropy in space re-sults in a rotational-like behavior, in analogy to well-known rotational bands in deformed nuclei resultingfrom the anisotropy of the mass distribution.

The shears mechanism is currently well establishedin a number of mass regions near closed shells; more spe-cific experimental information can be found in the datatables of [17], while [2], [3], and [17] should be con-sulted for references to original work.

Nuclear Chirality and M1 PropertiesTriaxial deformation defines in the intrinsic, body-

fixed frame three mutually perpendicular directionsalong the principal axes of the mass distribution andthree principal planes spanned by these axes. Valenceparticles and holes in a triaxially deformed potentialminimize their energies by aligning their angular mo-menta with the short or long axis, respectively, while thecollective core rotation aligns with the intermediate axiswhich, for irrotational flow-like moments of inertia, is apreferred axis of rotation [18]. These three mutuallyperpendicular angular momenta couple to form a totalangular momentum vector which is tilted away from anyprincipal plane, and thus they can be arranged [19] intoa right-handed or a left-handed system, as shown in Fig-ure 3a.

As a consequence of the two possible couplings, dou-blet states of the same spin/parity and nearly identicalexcitation energy are formed for a given single particleconfiguration. Indeed, intriguing ∆I = 1 doublet bandstructures have been observed systematically for theπh11/2νh11/2 configuration in the triaxial A ~ 130 region[20–24] (see Figure 3b) and for the πg9/2νh11/2 [25] con-figuration in 104Rh. The best examples of level degener-acy are provided by 134Pr shown in Figure 3c) with lev-els at spin 15+ and 16+ separated by less than 60 keV andby 104Rh with levels between spin 15- and 17- separatedby less than 90 keV but with remarkably small ~2 keVseparation at spin 17-. The separation between doublet

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bands in other nuclei varies between ~150 and ~350keV. An explanation [20] for this energy displacement isrelated to the fact that the triaxial deformation in thesecases may not be stable, but perhaps more soft, resultingin an average moment of inertia along the intermediateaxis which is reduced closer to the values along the othertwo axes. In this situation, an angular momentum vectorof collective rotation is not constrained to positive/negative orientation along the intermediate axis: insteadit can oscillate from one chiral system to the other over(or tunnel through) the saddle-point energy barrier acrossthe short-long plane. In these “chiral vibrations,” how-ever, the remnants of the chiral doublets are retained.

The properties of the electromagnetic M1 and E2operators play a crucial role in proving the doubling ofstates within the same configuration. The single particlestructure of the ∆I = 1 main bands (these bands are mostoften yrast in the nuclei of interest) is established as com-prising the unique parity high-j intruder orbitals fromsystematics [26] and g-factor measurements for isomericband heads. The linking transitions connecting the sidebands to the main bands are of mixed M1/E2 multipo-larities based on uniquely conclusive angular correlationstudies [27], as well as polarization measurements [22].The selection rules for the one-body M1 and E2 opera-

tors allow for the transitions between configurationswith single particles of the same parity only; this com-bined with the unique parity for orbitals forming themain band implies the same unique parity configurationsfor the main and the side bands.

Chirality in nuclear rotation is currently an activearea of research with the main experimental effort con-centrated on identifying nuclei with nearly degeneratedoublet bands. The M1 properties, however, provide animportant tool for studies of nuclear chirality due to thefact that each of the three angular momentum vectorsforming the total angular momentum has an associatedmagnetic moment. The geometry of angular momentumcoupling is reflected in the properties of M1 matrix ele-ments. Consequently, g-factors of high spin states candistinguish between chiral or planar coupling [28].

Moreover, the structure of the wave function im-posed by chiral geometry has important consequencesfor M1 and E2 transition rates. The right- and left-handed systems of angular momenta are related by theTRy(π) operator which combines time reversal and rota-tion by 180° around one of the principal axes (conven-tionally, the y axis is chosen). The wave functions for thedoublet states in the laboratory frame are those combi-nations of the right- and left-handed systems which areinvariant under the TRy(π) operator:

+⟩ = 1/��2 ( R⟩ + L⟩),

–⟩ = i /��2 ( R⟩ – L⟩).

In the high spin limit electromagnetic transitions be-tween the right- and left-handed systems are not al-lowed. This results in specific selection rules for transi-tions between physical +⟩ and –⟩ states in thelaboratory frame [25]:

• stretched +⟩ → +⟩ and –⟩ → –⟩ M1 transitionsare enhanced;

• stretched +⟩ → –⟩ and –⟩ → +⟩ M1 transitionsare hindered;

• stretched +⟩ → –⟩ and –⟩ → +⟩ E2 transitionsare enhanced;

• stretched +⟩ → +⟩ and –⟩ → –⟩ E2 transitionsare hindered.

These selection rules manifest chirality in the staggeringof relative reduced transition rates as a function of spinfor B(M1)/B(E2) ratios in the yrast band and B(M1)in/B(M1)out ratios in the side band. This has been observed

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Figure 3. (a) Two possible couplings of angular momenta in atriaxial odd-odd nucleus to produce the total angular momen-tum. (b) πh11/2νh11/2 doublet bands in 128-132Cs isotopes in En-ergy-vs. Angular momentum plot [25]. (c) πh11/2νh11/2 doubletbands [28] in 134Pr. (d) Staggering in relative reduced transitionrates for πh11/2νh11/2 doublet band in 128Cs, B(M1)/B(E2) in theyrast band (top) and B(M1)in/B(M1)out in the side band (bot-tom). Data are from [25].


experimentally for chiral band candidates in 128,130Cs[25] and shown in Figure 3d. Similar behavior has beenobserved in 104Rh. Absolute M1 and E2 strengths are ofsignificant interest and several experiments are under-way or planned.

Mixed-Symmetry Multi-Phonon StructuresAnother M1 phenomenon of recent interest is the

formation of multi-quadrupole phonon structures withmixed proton-neutron symmetry in heavy quadrupole-collective nuclei. Isovector quadrupole-surface vibra-tions have been anticipated in terms of collective models.Static quadrupole deformation can give rise to orbitalout-of-phase oscillations of the deformed proton- andneutron-bodies forming a so-called scissors mode [29]with large M1 excitation strength from the ground state.The scissors mode has subsequently been discovered indeformed even-even [30] and odd-mass nuclei [31] andhas been extensively studied, predominantly in electron-scattering and photon-scattering experiments at Darm-stadt and Stuttgart, see, e.g., [4, 32]. In deformed nucleithe scissors mode appears at about 3 MeV excitation en-ergy as a somewhat fragmented concentration of M1strength with a total value of about B(M1)↑ ~ 3 µN

2.Within the framework of the interacting boson

model (IBM-2) the Jπ = 1+ scissors mode is only one par-ticular member of a more general class of quadrupole-collective states with non symmetric coupling with re-spect to the proton-neutron degree of freedom [33]. TheIBM-2 considers pairs of valence protons and neutronsoutside of an appropriately chosen closed-shell core andapproximates them by proton and neutron bosons thatinteract via two-body forces. The formalism of isospincan be applied on the boson level, where the “elemen-tary” bosons form an “isospin doublet” with projections+1/2 (proton boson) and -1/2 (neutron boson). This“boson isospin” is called F-spin and allows one to quan-tify the proton boson–neutron boson symmetry charac-ter of the IBM-2 wave functions formed by Nπ protonbosons and Nν neutron bosons. Symmetric wave func-tions have the maximum F-spin quantum number Fmax =(Nπ + Nν)/2 and correspond to IBM-1 states where nodistinction between proton bosons and neutron bosonsis made.

The new feature emerging in going from the IBM-1to the IBM-2 is the appearance of whole new classes ofcollective mixed-symmetry states (MSSs) with quantumnumbers F < Fmax of which the scissors mode is one ex-ample. The boson-boson residual interactions are suchthat states with symmetric wave functions lie lowest in

energy followed by states with quantum numbers Fmax –1, Fmax – 2, . . . . The top of Figure 4 shows the low-lyingparts of the spectrum of a schematic IBM-2 Hamilton-ian. The operator nd = ndπ+ ndν counts the number of d-bosons and M = [Fmax(Fmax + 1) – F(F + 1)]/2 shiftsmixed-symmetry states to higher energies. This Hamil-tonian has U(5) symmetry and supports a vibrationalspectrum with symmetric and mixed-symmetry one-quadrupole phonon states and two-phonon multiplets.The one-phonon state’s wave functions can be generatedfrom the ground state by applying the symmetric (Qs =Qπ + Qν) and mixed-symmetric (Qm = Qπ/Nπ– Qν/Nν)

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Figure 4. Top: Spectrum of a simple IBM-2 Hamiltonian forthe harmonic vibrator. Mixed-symmetry states with quantumnumbers Fmax – 1 form multiphonon structures. Bottom: Par-tial level scheme of the nucleus 94Mo. Mixed-symmetry stateshave been identified on the solid basis of sizeable M1 matrixelements of about 1 µN. The structure of 94Mo deviates some-what from the U(5) symmetry.


quadrupole phonon operators, respectively. Multi-phonon states are obtained by a successive action of theone-phonon operators. The most distinct feature ofMSSs is the existence of allowed F-vector M1 transitionsto symmetric states with large matrix elements of ~1 µN.M1 transitions between symmetric states are forbiddenwithin the sd-IBM-2 which considers only monopole (s)and quadrupole (d) bosons. Further selection rules forM1 transitions exist, e.g., with respect to the number ofd-bosons. In particular, the 1+ scissors mode cannot bedirectly excited from the ground state by an M1 transi-tion in pure vibrators.

One-phonon and two-phonon MSSs have recentlybeen studied in a couple of weakly deformed nuclei; see,e.g., [34–36], and there have even been claims for MSSswith quantum numbers down to F ≤ Fmax – 2 [37]. Atpresent, the best studied example for a one-phonon andtwo-phonon mixed-symmetry structure has been found[38, 39] in the level scheme of 94Mo with 52 neutrons inthe vicinity of the N = 50 neutron shell closure. The low-spin level scheme of 94Mo has been studied towards thefar-off-yrast region at Stuttgart, Lexington, and Colognewith a variety of γ-ray spectroscopic tools including in-elastic photon-scattering, inelastic neutron-scattering,and γγ-coincidence spectroscopy in (α,n) fusion reactionsand β+-decay from 94Tcm. Comprehensive data on thelevel scheme, spin quantum numbers, γ-ray multipole-mixing ratios, and level lifetimes made the identificationof MSSs possible and allowed for the measurement oftheir fragmentation. The bottom of Figure 4 shows theMSSs of 94Mo uniquely identified by their large M1 tran-sition strengths with matrix elements of about 1 µN. TheMS two-phonon multiplet occurs close to the sum-en-ergy of the corresponding one-phonon energies indicat-ing rather harmonic phonon coupling. Its energy split-ting as a function of spin is less than 10%. The observedMSSs are almost unfragmented in 94Mo. This can beconcluded from the comparably small M1 strengthsmeasured for surrounding levels with the same spin andparity quantum numbers. Besides the identifying M1transitions, also E2 transitions to symmetric states andeven to the MS one-phonon state were observed. Thesedata [38] give the first direct evidence that the MSSs in-deed form a class of quadrupole-collective states withsimilar structure. The discovery of the two-phonon 2+

2,ms

state by Fransen et al. [39] is significant because it is thefirst demonstration that even off-yrast states in themixed-symmetry sector can survive relatively purely in anuclear level scheme.

Similar structures have subsequently been observedin various parts of the nuclear chart. Of particular ex-perimental interest is the successful identification of theone-phonon 2+

1,ms state in 96Ru done in inverse-kinemat-ics Coulomb excitation of a stable 96Ru-beam at Yale [40].The technique of Coulomb excitation in inverse kinemat-ics develops into a major spectroscopic method for theinvestigation of neutron-rich nuclei at high-intensity RIBfacilities. Corresponding experiments on neutron-richRIBs of N = 52 isotones to search for mixed-symmetrystates are already scheduled within the RISING cam-paign at GSI. Information on isovector valence shell ex-citations, such as MSSs, will help to conclude on theisospin dependence of the proton-neutron restoring forceand, finally, on the formation of neutron skins or evendecoupled neutron matter in exotic neutron-rich nuclei.

Summary and OutlookExamples presented in the current overview show

that, because of a number of recent theoretical and ex-perimental results, M1 transitions have taken on a newrole for the investigation of nuclear structure. No longerare E2 transitions alone the signature of collective modesin nuclei. In view of the proposed and planned advancesin nuclear structure experiments, further progress is ex-pected in the near future; new radioactive beam facilitieswill provide an opportunity to access nuclei far from thestability line, while segmented Ge detector technologyholds the potential of revolutionizing the field. New andexciting physics information, as well as an extension ofthe studies presented above, is anticipated from investi-gations of magnetic dipole excitations due to their fun-damental character and sensitivity to the details of nu-clear many-body wave functions.

AcknowledgmentsWe thank all those who have collaborated with us on

the discussed topics. Help with the preparation of thisarticle by R. F. Casten, R. M. Clark, D. B. Fossan, andA. F. Lisetskiy is gratefully acknowledged. This workwas supported by the Emmy Noether-Program of theDeutsche Forschungsgemeinschaft under support No. Pi393/1-2 and the U.S. National Science Foundationaward number 0098793.

Appendix A: M1 operatorIn terms of the nuclear shell model the M1 operator

can phenomenologically be written as

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where gl(s)p(n) is the effective orbital (spin) g-factor for

protons (neutrons) and l(s) is the corresponding orbital(spin) angular momentum operator in units of h-. The ap-propriate values for the effective g-factors in nuclei candeviate from those of free nucleons because of in-medium modifications due to sub-nucleonic degrees offreedom or model space truncation. For the discussion ofM1 transitions it is convenient to decompose the M1 op-erator into isoscalar and isovector components and athird part proportional to the total angular momentumoperator

where J = Lp + Ln + Sp + Sn is the total vector sum of allproton and neutron orbital and spin angular momenta,L(S)p(n) = Σp(n)l(s)p(n) is the total orbital (spin) angularmomentum of the protons (neutrons), and TIS(IV) = Lp ±Ln + cs(Sp ± Sn) denotes the remaining isoscalar (isovec-tor) part of the M1 transition operator. The parametersgJ, gIS, gIV, and cs are rational functions of the factorsgl(s)

p(n). Since nuclear states have good total angular mo-mentum, the term proportional to J, which is diagonal inthe set of eigenstates, does not generate M1 transitions.The ratio gIS/gIV turns out to be small, (<0.1) if one usesg-factors for free nucleons. The M1 operator, conse-quently, has predominantly isovector character and,hence, M1 transitions can serve us as a “magnifyingglass” for isovector nuclear properties. This is in impor-tant contrast to E2 transitions that usually enhanceisoscalar features. Using bare values for the nucleon g-factors, the relevant isovector orbital- and spin- g-factorsbecome of gl(s)

V = (gl(s)n – gl(s)

p)/2 =–1/2 and –4.7, respec-tively.

Appendix B: IsospinThe discussion of enhanced M1 transitions in nuclei

is intimately connected to the isospin degree of freedomdue to the predominantly isovector character of the M1transition operator (see Appendix A). The concept ofisospin represents a convenient classification scheme fornuclear properties that enables us to comprehend nu-clear states, i.e., product states of protons and neutrons,as quantum states of many indistinguishable fermionicnucleons times a spin-like factor to the wave function

which keeps track of the proton-neutron degree of free-dom (isospin d.o.f.). The isospin-concept was introducedby Heisenberg in 1932 and has been an important clas-sification scheme for nuclear and hadronic states eversince.

In the nuclear case the isospin concept makes use ofthe charge independence of nuclear forces, i.e., the ex-perimental fact that protons and neutrons have the samemass and react identically to nuclear forces in good ap-proximation. Protons and neutrons are, therefore, con-sidered to be identical particles with only a differentvalue of an internal spin-1/2-like degree of freedom, theisospin projection Tz. Conventionally, neutrons are as-signed isospin projection Tz(n) = +1/2 and protons Tz(p)= –1/2. Nuclear states formed by N neutrons and Z pro-tons have isospin projection Tz = (N – Z)/2 and can takevalues for the isospin quantum number |N – Z|/2 ≤ T ≤|N + Z|/2. In most nuclei the lowest-lying states haveisospin quantum numbers T = T< = Tz, while states withT ≥ T> = T< + 1 are usually unbound or at least highlyexcited. Nuclear states in neighboring isobars with |Tz�| > |Tz| form isospin multiplets with T > T< and shareidentical nucleonic structure (although different isospinprojection). Considerable similarities between isospinmultiplet partner states in different nuclei offer a valu-able source of information on nuclear properties. Mostimportant is the existence of selection rules for variousnuclear reactions or decay processes with respect to theisospin quantum numbers. One example is the suppres-sion of (T = 0) → (T = 0) M1 transitions due to the pre-dominantly isovector character of the M1 transitionoperator. Isospin symmetry is, however, weakly brokenby the small proton/neutron mass difference, by theCoulomb force, and by small isospin-breaking parts ofthe nuclear forces.

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siefen et al., Phys. Lett B275, 252 (1992).15. H. Hübel, Nuovo Cim. 111A, 709 (1998).16. J. R. Cooper et al. Phys. Rev. Lett 87, 132503 (2001).17. Amita, A. K. Jain, and B. Singh, At. Data Nucl. Data Tables

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Norbert Pietralla

Krzysztof Starosta

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IntroductionGalilei Galileo would be somewhat surprised. When

neutrons become ultra-cold, his famous fall experimentshows quantum aspects of the subtle gravity force in thesense that neutrons do not fall as larger objects do. In afree-fall experiment they don’t fall continuously. We findthem on particular levels, when they come close to a re-flecting mirror for neutrons. Of course, such boundstates with discrete energy levels are expected when thegravitational potential is larger than the energy of theparticle. Here, the quantum states have pico-eV energy,a value that is smaller by many orders of magnitudecompared with an electromagnetically bound electron ina hydrogen atom, opening the way to a new techniquefor gravity experiments and measurements of fundamen-tal properties. New motivations for gravity experimentscome from frameworks where the fundamental Planckscale (the scale about 10-44 s after the big bang wheregravity becomes comparable in strength to the other in-teractions) is taken to the weak scale, the energy scale ofthe Standard Model at 10-10 s after the big bang. Con-sidering this very early stage of our universe, we have thestrong feeling, that a Standard Model description is in-complete, and many new observables pointing to physicsbeyond the Standard Model emerge from superstringtheory, supersymmetry or other Grand Unified Theories(GUT). For example, in theories with submillimeter di-mensions, gauge fields can mediate repulsive gravity-likeforces ~1010 times stronger than gravity in submillimeterdistances. Furthermore, the quark-mixing Cabibbo-Kobayashi-Maskawa (CKM) matrix remains unex-plained in the Standard Model as well as CP-violation,which might explain the baryon-antibaryon asymmetryof the universe. Some observables of these theories re-quire neutron physics, for others the neutron providesone of several possible ingredients.

Experiments in the field of particle physics usuallymake use of highest beam energies. In the sub-field ofparticle physics with neutrons however, physicists use“low energy” neutrons, neutrons that are much colderthan the molecules around us. Some experiments profit

from high intense cold neutron beams, other experi-ments need even colder neutrons, so called ultra-coldneutrons. These neutrons are reflected from surfaces andcan be stored in “neutron bottles.”

This articles discusses two aspects of neutron physicswith cold and ultra-cold neutrons. First, a recent gravityexperiment at the Institut Laue-Langevin demonstratesquantum states in the gravitational potential of the earthwith ultra-cold neutrons [1] and places limits on gravity-like forces in the range between 1 µm and 10 µm [2].Second, measurements by various international groupsof researchers determine the strength of the weak inter-action of the neutron, which gives us unique informationon the question of the quark mixing. Neutron β-decayexperiments now challenge the Standard Model of ele-mentary particle physics with a deviation, 3 times thestated error [3].

Cold and Ultra-Cold NeutronsNeutrons are produced in a spallation source or a re-

search reactor. At production, these neutrons are veryhot; the energy is about 2 MeV corresponding to 1010

degrees centigrade. On the other side of the scale, thegravity experiment uses neutrons having 1018 times lessenergy in the pico-eV range (see Table 1). In a first step,spallation or fission neutrons thermalize in a heavywater tank at a temperature of 300 K. The thermalfluxes are distributed in energy according to Maxwellianlaw. At the Institut Laue-Langevin (ILL), cold neutronsare obtained in a second moderator stage from a 25 Kliquid deuterium cold moderator near the core of the 57MW uranium reactor. These cold neutrons have a veloc-ity spectrum in the milli-eV energy range. For particlephysics, a new beam line with a flux of more than 1010

cm-1s-1 over a cross section of 6 cm x 20 cm is available[4]. An overview of many observables in the rich field ofneutron particle physics and of related physical ques-tions are taken from [5] and can be found in Table 2.

Ultra-cold neutrons are taken from the low energytail of the cold Maxwellian spectrum. They are guidedvertically upwards by a neutron guide. The curved guide,

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Gravitation at a Micron and Mixing of QuarksH. ABELE

Physikalisches Institut der Universität Heidelberg, Heidelberg, Germany


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Table 1. From hot to ultracold: neutrons at the ILL.

Fission Thermal Cold Ultracold Gravityneutrons neutrons neutrons neutrons experiment

Energy 2 MeV 25 meV 3 meV 100 neV 1.4 peVTemperature 1010 K 300 K 40 K 1 mK —Velocity 2200 m/s 800 m/s 5 m/s υ⊥ ~ 2 cm/s

Neutron particle properties

Mass: Bound states in gravitational field Limits on gravity-like forces expected from large extra-dimensionsmn/np, h/mn Value of electromagnetic interaction strength α

Charge Charge quantization, GUT’sMagnetic dipole moment Quark modelsElectric dipole moment Time reversal violation, GUT’sElectric polarizability Quark confinement potential

Neutron β-decay

Lifetime Weak lepton-quark interaction, as input for cosmology and astrophysics, quark models, Standard Model tests

Correlation coefficients Unitarity of quark mixingβ-asymmetry Right-handed currentsNeutrino asymmetry Conservation of weak vector currentβ neutrino correlation Flavour symmetryβ-helicity Limits on scalar and tensor admixturestriple neutron spin-correlations Gut’s, time reversal violationtriple electron spin-correlations Baryon asymmetry of the universe

Energy spectra:of electrons, protons Weak magnetism in electroweak interactionof various correlation coefficients Second class currentsof inner bremsstrahlung Radiative correctionsNeutron decay into hydrogen Yes/no experiment on right-handed currents

Neutron interaction

Scattering length: Neutron intrinsic charge distributionNeutron-electron Quark modelsNeutron-proton Isospin invarianceNeutron-Neutron

Parity violating effects:Spin rotation in nonmagnetic mediumNeutron polarizating action of

nonmagnetic mediumn-p γ-asymmetryn-p circular polarization Quark-quark electroweak interaction

Table 2. Observables in neutron-particle physics and related physical questions [5].


which absorbs neutrons above a threshold energy, acts asa low-velocity filter for neutrons. Neutrons with a ve-locity of up to 50 m/s arrive at a rotating nickel turbine.Colliding with the moving blades of the turbine, ultra-cold neutrons exit the turbine with a velocity of severalmeters per second. They are then guided to several ex-perimental areas.

A mirror for neutrons uses the strong interaction be-tween nuclei and a neutron, resulting in an effective re-pulsive force: neutrons propagate in condensed matter ina manner similar to the propagation of light but with aneutron refractive index less than unity. Thus, one con-siders the surface of matter as constituting a potentialstep of height V. Neutrons with transversal energy E⊥ <V will be totally reflected. Ultra-cold neutrons (UCN)are neutrons that, in contrast to faster neutrons, areretro-reflected from surfaces at all angles of incidence.When the surface roughness of the mirror is smallenough, the UCN reflection is specular. Neutron mirrorsare interesting because they can be used to store neu-trons, to focus neutrons, or to build a Fabry Perot inter-ferometer for neutron de Broglie waves. UCN storagebottle experiments have improved our knowledge aboutthe neutron lifetime significantly and, together with theRamsey method of separated oscillating fields, they havebeen used for a search for an electric dipole moment ofthe neutron.

Quantum States in the Gravitational Field of the Earth

The idea of observing quantum effects in the gravi-tational potential occurring when ultracold neutron arestored on a plane was discussed long ago by V. I.Lushikov and A. I. Frank [6]. An experiment similar insome aspects was discussed by H. Wallis et al. [7] in thecontext of trapping atoms in a gravitational cavity.Quantum theory and gravitation affect each other, and,when neutrons become ultra-cold, we find them on dif-ferent levels, when they come close to a reflecting mirrorfor neutrons. These quantum states have been observedin a collaboration between the ILL (Grenoble), PNPI(Gatchina), CERN (Geneva), and our group at Heidel-berg University [1]. The population of the ground stateand the lowest states follows the quantum mechanicalprediction. An efficient neutron absorber removes thehigher, unwanted states. At the entrance of the experi-ment, a collimator absorber system cuts down on theneutrons to a adjustable transversal energy E⊥ in thepico-eV range. Of course, such bound states with dis-

crete energy levels are expected when the gravitationalpotential is larger than the energy of the particle.

A side effect of this experiment is its sensitivity forgravity-like forces at length scales below 10 µm. In lightof recent theoretical developments in higher dimensionalfield theory [8], gauge fields can mediate forces that are108 to 1012 times stronger than gravity at submillimeterdistances, exactly in the interesting range of this experi-ment and might give a signal in an improved setup.

Figure 1 shows the schematic view of the setup: Neu-trons pass through a mirror absorber system and theyare eventually detected by a 3He-counter. Signatures ofquantum states in the gravitational field of the earth areobserved in the following way: the 3He counter measuresthe total neutron transmission T, when neutrons are tra-versing the mirror absorber-system. The transmission ismeasured as a function of the absorber height h and thusas a function of neutron energy since the height acts as aselector for the vertical energy component E⊥ . Above anabsorber height of about 80 µm, the measured transmis-sion is in close agreement with the classical expectationbut below 50 µm, a deviation is clearly visible. Fromquantum mechanics, we easily understand this behavior:Ideally, we expect a stepwise dependence of T as a func-tion of h. If h is smaller than the spatial width of the low-est quantum state, then T will be zero. When h is equalto the spatial width of the lowest quantum state then Twill increase sharply. A further increase in h should not

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Figure 1. Sketch of the setup: (a) classical view: neutron trajec-tories; (b) quantum view: plane waves and Airy functions [24].


increase T as long as h is smaller than the spatial widthof the second quantum state. Then again, T should in-crease stepwise. At sufficiently high slit width one ap-proaches the classical dependence and the stepwise in-crease is washed out (see Figure 2). The measurementdoes agree well with a simple quantum mechanical de-scription of quantum states in the earth’s gravitationalfield.

Limits for Non-Newtonian Interaction below 10 microns

A quantum mechanical consistent theory of gravityremains a big challenge to physicists. However, new the-oretical efforts are underway and superstring theory is apromising attempt to unify general relativity and quan-tum theory. Discouraging in the past was the fact, thatthe Planck length, the scale of quantum fluctuation ofspace time geometry, is twenty orders of magnitudesmaller than a neutron with diameter of 10-15 m. Butnew theories on string theory and multidimensions causegravity to reach Planck length well above 10-35 m. Cor-rections to the gravitational inverse square law are dueto compactifed extra dimensions. On the assumptionthat the form of the non-Newtonian potential is given bythe Yukawa expression, for masses mi and mj and dis-tance r the modified Newtonian potential V(r) takes theform

V(r) = –G (1 + α • e-r/λ), (1)mi • mj––

r

where λ is the Yukawa distance over which the corre-sponding force acts and α is a strength factor in com-parison with Newtonian gravity. G is the gravitationalconstant. In theories with submillimeter dimensions, anumber of gravity-like phenomena emerge. For example,a hypothetical gauge field can naturally have minisculegauge couplings, independent of the number of extra di-mensions [8]. If these gauge fields couple to a neutronwith mass mn, these gauge fields can result in repulsiveforces of million or trillion times stronger than gravity atmicrometer distances, exactly in the range of interest.The results of a fit to the measured data (see section“Quantum States in the Gravitational Field of theEarth”) yields predictions for 90% confidence level ex-clusion bounds on α and λ. These limits from neutronmirror experiments are the best known in the range from1 µm < λ < 3 µm and exclude for the first time gravity-like short-ranged forces at 1 µm with strength α > 1012

and at 10 µm with strength α > 1011 (Figure 3a) [2]. Pre-vious constraints on both α and λ, adapted from [9], areshown in Figure 3b.

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Figure 2. Data and classical expectation vs. quantum expec-tation.

Figure 3. Limits for non-Newtonian gravity: Strength |α | vs.Yukawa length scale λ. (a) Experiments with neutrons placelimits for |α | in the range 1 µm < λ < 10 µm. (b) Constraintsfrom previous experiments are adapted from [9].


The Standard Model, Quark Mixing, and the CKM Matrix

This subsection is about the interplay between theStandard Model of elementary particle physics and neu-tron β-decay experiments. The energy range of the Stan-dard Model extends up to about 1TeV. It is a field-the-ory of strong and electroweak interactions at theseenergies. In view of the Standard Model, matter is builtfrom two types of fundamental fermion, called quarksand leptons. Quarks occur in several varieties or flavourslabelled up (u), down (d), charm (c), strange (s), top (t),and bottom (b). The strong interaction glues the quarkstogether and these quarks are considered to be quantummechanical mass eigenstates. Neutrons are built fromtwo d-quarks and one u-quark, whereas protons arebuilt from two u-quarks and one d-quark.

A free neutron is unstable. The decay process is gov-erned by the weak interaction converting a d-quark intoan u-quark. Thus, the lifetime τ is 885 s and a neutrondecays into a proton, an electron and an electron anti-neutrino with an energy release of 782 keV. The weakcoupling strength responsible for nuclear or neutrondecay is not identical with the coupling for µ-decay. Thedifference is about 2% and this feature of the weak in-teraction is well understood under the assumption ofquark mixing: A weak decaying quark is a mixture ofdifferent flavours of mass eigenstates. The weak eigen-states (primed) are related to the mass eigenstates (un-primed) as

with unitary CKM matrix V. All mixing is expressed interms of V operating on d, s, and b quarks. As a conse-quence, our d�-quark that is responsible for neutron β-decay, is a linear superposition of d-, s-, and b-massstates: d� = Vud

• d + Vus• s + Vub

• b. The basic idea ofV is that what is perceived to be several independentcouplings are actually components of a single force. Now,

if every quark gives as much as it takes in this mixing,then the quark-mixing CKM matrix V has to be unitary.

The values of the individual matrix elements are de-termined from weak decays of the relevant quarks. Thereexist several parametrizations of the CKM matrix. A“standard parametrization” uses three angles and aphase. The phase breaks CP violation invariance. Therange of matrix elements shown in Table 3 correspondsto 90% C.L. limits on the angles and the phase assum-ing unitarity of the CKM matrix. This unitarity con-straint has pushed Vud about one to two standard devi-ations higher than given by the experiments. TheUnitarity condition applied to the first and thirdcolumns of the CKM matrix yields VudV*ub = +VcdV*cb +VtdV*tb = 0. The so-called unitarity triangle with anglesα, β and γ is a geometrical presentation of this equation.The angles β and γ are phases of the CKM elements Vtd

and Vub. All processes can be understood by γ = 59° ±13°. The experimental results from BaBar and Belle,when averaged, yield β = 26° ± 4° [10].

So far precision tests of unitarity are only possiblefor the first row of V [11–13]. Unitarity requires that thesum of the squares of the matrix elements for each rowand column is unity, namely

Vud 2 + Vus 2 + Vub 2 = 1 – ∆. (3)

The Standard Model requires ∆ = 0. A violation ofunitarity in the first row of the CKM matrix is a chal-lenge to the three-generation Standard Model. A devia-tion ∆ has been related to concepts beyond the StandardModel like supersymmetry, couplings to exotic fermions,to the existence of an additional Z boson or the existenceof right-handed currents in the weak interaction.

Due to its large size, a determination of Vud (seeTable 3) is most important. It has been derived from a se-ries of experiments on superallowed nuclear β-decaythrough determination of phase space and measurementsof partial lifetimes. With the inclusion of nuclear struc-ture effect corrections a value of Vud = 0.9740(5)emerges in good agreement of different, independent

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Vud = 0.9741 to 0.9756 Vus = 0.219 to 0.226 Vub = 0.0025 to 0.0048

Vcd = 0.219 to 0.226 Vcs = 0.9732 to 0.9748 Vcb = 0.038 to 0.044

Vtd = 0.004 to 0.014 Vts = 0.037 to 0.044 Vtb = 0.9990 to 0.9993

Table 3. CKM quark mixing matrix Uquark with 90% C.L. The unitarity constraint has pushed|Vud | about one to two standard deviations higher than given by the experiments.


measurements in nine nuclei. Combined with Vus =0.2196(23) from kaon-decays and Vub = 0.0036(9)from B-decays, this lead to ∆ = 0.0032(14), signaling adeviation from the unitarity condition by 2.3 σ standarddeviation [14], The quoted uncertainty in Vud , how-ever, is dominated by theory due to amount, size andcomplexity of theoretical uncertainties. Although the ra-diative corrections include effects of order Zα2, part ofthe nuclear corrections are difficult to calculate. Further,the change in charge-symmetry-violation for quarks in-side nuclei results in an additional change in the pre-dicted decay rate which might lead to a systematic un-derestimate of Vud . A limit has been reached where newconcepts are needed to progress. Such are offered bystudies with neutron β-decay and with limitations withpion β-decay. The pion β-decay has been measured re-cently at the PSI. The pion has a different hadron struc-ture compared with neutron or nucleons and it offers another possibility in determining Vud . The preliminaryresult is Vud = 0.9971(51) [15]. The somewhat largeerror is due to the small branching ratio of 10-8.

The combination of neutron β-decay experiments atthe Institut Laue-Langevin now challenge the StandardModel of elementary particle physics: A measurement ofthe β-asymmetry A and the world average of the neutronlifetime τ determine the first element Vud of the quark-mixing CKM matrix. With this value and the particledata group values for Vus and Vub , the unitarity con-dition for the first row of the CKM matrix deviate fromunity by ∆ = 0.0083(28), which is 3.0 times the statederror and conflicts the prediction of the Standard Modelof particle physics.

Vud, Neutron β-Decay, and the Experiment PERKEO

In the Standard Model only two additional parame-ters describe neutron β-decay, since the Fermi decay con-stant is known from muon decay. One parameter is thefirst entry Vud of the CKM matrix. The other one is λ,the ratio of the vector coupling constant and the axialvector constant. The observables for determining Vud arethe neutron lifetime τ and a measurement of one of theangular correlation coefficients e.g. the β-asymmetry co-efficient A. The β-asymmetry A is linked to the proba-bility that an electron is emitted with angle ϑ with re-spect to the neutron spin polarization P = ⟨σz ⟩:

W(ϑ) = 1 + v/c P A cos(ϑ), (4)

where v/c is the electron velocity expressed in fractionsof the speed of light. Neglecting order 1% corrections, Ais a simple function of λ.

For a measurement of the β-asymmetry A, the in-strument PERKEO II was installed at the PF1 cold neutronbeam position at the High Flux Reactor at the InstitutLaue-Langevin, Grenoble [16]. Figure 4 shows the setup.The neutrons are polarized by a 3 x 4.5 cm2 supermirrorpolarizer. The degree of neutron polarization was meas-ured to be P = 98.9(3)% over the full cross section of thebeam.

The main component of the PERKEO II spectrometer isa superconducting 1.1 T magnet in a split pair configu-ration, with a coil diameter of about one meter. Neu-trons pass through the spectrometer, whereas decay elec-trons are guided by the magnetic field to either one of

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Figure 4. Experimental setup of the experiment PERKEO at the Institut Laue-Langevin.


two scintillation detectors. The detector solid angle ofacceptance is 2 x 2π. The measured electron spectraNi

↑(Ee) and Ni↓(Ee) in the two detectors (i = 1,2) for neu-

tron spin up and down, respectively, define the experi-mental asymmetry Aiexp(Ee) as a function of electron ki-netic energy Ee and are shown in Figure 5. Aiexp(Ee) isdirectly related to the asymmetry parameter A. After a2% correction for small experimental systematic effectsone obtains

A = –0.1189(7) and λ = –1.2739(19) [3]. (5)

Earlier experiments with large corrections [17–19]gave significant lower values for λ. With the new value,and the world average for τ = 885.7(7) s, one finds thatVud = 0.9713(13). With Vus = 0.2196(23) and thenegligibly small Vub = 0.0036(9), one obtains

Vud 2 + Vus 2 + Vub 2 = 1 – ∆ = 0.9917(28). (6)

This value differs from the Standard Model predictionby ∆ = 0.0083(28), or 3 times the stated error.

An independent test of CKM unitarity comes fromW physics at LEP where W decay hadronic branching ra-tios can be used. Since decay into the top quark channelis forbidden by energy conservation one would expectΣVij 2 to be 2 with a three generation unitary CKM ma-trix. The experimental result is 2.032(32), consistentwith (6) but with considerably lower accuracy.

The FutureAll earlier experiments on A0 made large corrections

in the 15% to 30% range. The main corrections in theexperiment PERKEO are due to neutron beam polarization(1.1%), background (0.5%) and flipper efficiency(0.3%). The total correction is 2.04%. With such smallcorrections to the data, we start to see a deviation fromthe Standard Model already in the uncorrected raw data.For the future, the plan is further to reduce all correc-tions. Since this measurement, major improvements bothin neutron flux and degree of neutron polarization hasbeen made: First, the new ballistic supermirror guide[40] at the ILL from the University of Heidelberg gives anincrease of a factor of 10 in the cold neutron flux. Sec-ond, a new arrangement of two supermirror polarizersallows to achieve an unprecedented degree of neutronpolarization P of between 99.5% and 100% over the fullcross section of the beam [20]. Third, systematic limita-tions of polarization measurements have been investi-gated: The beam polarization can now be measured witha completely new method using an opaque 3He spin fil-ter with an uncertainty of 0.1% [21, 22]. As a conse-quence, we are now in the lucky situation to improve onthe main uncertainties in reducing the main correction of1.1% to less than 0.5% with an error of 0.1%. Thus, apossible deviation from the Standard Model, if confirmed,will be seen very pronounced in the uncorrected data.

Future trends have been presented in the workshop“Quark-Mixing, CKM Unitarity” in Heidelberg, Sep-tember 19–20, 2002. Regarding the Unitarity problem,about half a dozen new instruments are planed or areunder construction to allow for beta-neutrino correla-

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Figure 5. Fit to the experimental asymmetry Aexp for detector1 and detector 2. The solid line shows the fit interval, whereasthe dotted line shows an extrapolation to higher and lowerenergies.


tion a and beta-correlation A measurements at the sub-10-3 level. With next-generation experiments measure-ments with a decay rate of 1 MHz become feasible [23].

SummaryGravitational bound quantum states have been seen

for the first time. We conclude that the measurement isin agreement with a population of quantum mechanicalmodes. Further, the spectrometer operates on an energyscale of pico-eVs and can usefully be employed in meas-urements of fundamental constants or in a search for non-Newtonian gravity. The present data constrain Yukawa-like effects in the range between 1 µm and 10 µm.

Vud , the first element of the CKM matrix, has beenderived from neutron decay experiments in such a waythat a unitarity test of the CKM matrix can be per-formed based solely on particle physics data. With thisvalue, we find a 3σ standard deviation from unitarity,which conflicts with the prediction of the StandardModel of particle physics.

AcknowledgmentsThis work has been funded in part by the German

Federal Ministry (BMBF) under contract number 06 HD854 I and by INTAS under contract number 99-705.

References1. V. Nesvizhevsky et al., Nature 415 297 (2002).2. H. Abele, S. Baeßler, and A. Westphal, Quantum states of

neutrons in the gravitational field and limits for non-New-tonian interaction in the range between 1 µm and 10 µm.In: Aspects of Quantum Gravity, ed. by C. Laemmerzahl(Springer, Berlin, Heidelberg, 2003, (Lecture Notes inPhysics)), in press.

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9. C. D. Hoyle et al., Phys. Rev. Lett. 86 1418 (2001).10. Particle Data Group, K. Hagiwara et al., Phys. Rev. D 66

010001 (2002).11. J. Deutsch, Acta Physica Hungarica, 68 129 (1990).12. D. Dubbers, Progress in Particle and Nuclear Physics, 26

173 (1991).13. H. Abele, Nucl. Instrum. Methods Phys. Res. A440 499

(2000).14. J. Hardy et al., nucl-th/9812036.15. D. Pocanic, PIBETA: A precise measurement of the pion β-

decay rate to determine Vud. In: Workshop Proceedings ofQuark-Mixing, CKM-Unitarity, ed. by H. Abele, Heidel-berg, September 19–20, 2002, in press.

16. J. Reich et al., Nucl. Instrum. Methods Phys. Res. A 440535 (2000).

17. P. Bopp et al., Phys. Rev. Lett. 56.18. B. G. Yerozolimsky et al., Phys. Lett. B 412 240 (1997).19. K. Schreckenbach et al., Phys. Lett. B 349 427 (1995).20. T. Soldner, Recent progress in neutron polarization and its

analysis. In: Workshop Proceedings of Quark-Mixing,CKM-Unitarity, ed. by H. Abele, Heidelberg, September19–20, 2002, in press.

21. W. Heil et al., Physica B 241 (1998) 56.22. O. Zimmer et al., Nucl. Instrum. Methods Phys. Res. A

440 764 (2000).23. D. Dubbers, Correlation measurements in pulsed beams.

In: Workshop proceedings of Quark-Mixing, CKM-Uni-tarity, ed. by H. Abele, Heidelberg, September 19–20,2002, in press.

24. A. Wesphal, diploma thesis, University of Heidelberg,(2001), arXiv: gr-qc/0208062.

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IntroductionModern numerical methods have made it possible to

calculate, with remarkably satisfactory results, the spec-tra of nuclei up to A = 10 on the basis of realistic phe-nomenological models for the nucleon–nucleon interac-tion, which have been fitted to nucleon–nucleonscattering data and with inclusion of a weak three-nu-cleon interaction [1]. Employment of the correspondingwavefunctions to calculate the electromagnetic and weakobservables and transition strengths of these nuclei callsfor correspondingly realistic models for the electromag-netic and axial current density operators, which are con-sistent with the interaction Hamiltonian.

Corresponding to the form of the nuclear Hamilton-ian as a sum of single nucleon kinematic energy (Ti) andtwo-nucleon (Vij) and many-nucleon interactions,

H = �i

Ti + �i<j

Vij + . . . , (1)

the nuclear current density operator may be separatedinto a sum of single nucleon currents (j�i

(1)) and two-nu-cleon (j�ij

(2)) and many-nucleon currents:

j� = �i

j�i(1) + �

i<jj�ij

(2) + . . . . (2)

If j� represents the electromagnetic current density,the continuity equation,

� . j� + i[H, ρ], (3)

where ρ is the charge density operator, demands thepresence of two-nucleon currents, when the interactionoperator V does not commute with the charge densityoperator, which in the single nucleon approximation is

ρ = e_2 �

j(1 + τ j

3)ρj(r� – r�j), (4)

where ρj is the charge density of the jth nucleon.Because of the state dependence of the nucleon–

nucleon interaction, [V, ρ] ≠ 0, and the single approxi-mation for the nuclear current operator breaks the con-tinuity equation. Electronuclear calculations therefore

have to include a two-nucleon—and if three-nucleon in-teractions are included also, a three-nucleon—“exchange”current operator in order to satisfy the continuity equa-tion (3) [2].

Pion Exchange and Electromagnetic ObservablesThe numerically most significant exchange current

operator for electromagnetic observables is the longrange pion exchange current operator, which may be de-rived by methods similar to those used to derive the pionexchange potential. The main component of this opera-tor, which is required by the continuity equation, was de-rived even before the discovery of the pion [3]. The es-tablishment of its numerical significance had, however,to await the development of realistic wavefunction mod-els for the few-body systems, by means of which it wasfound that the pion exchange current could explain boththe missing ~10% of the calculated thermal neutroncross section for np → dγ [4] and the difference betweenthe magnetic moments of 3H and 3He [5]. Subsequentlyit has been shown by means of effective field theorymethods that more complicated multipion exchangecontributes but an insignificant additional contributionto the cross section for np → dγ [6].

That reliable estimates of the exchange current con-tribution obtain already in the pion exchange approxi-mation follows from the inherently long-range nature ofthe exchange current operator, which is associated withthe isospin-dependent interaction, ν(r�1 – r�2)τ�1 . τ�2. Thecontinuity equation for this current operator is

� . j�(2)(r�;r�1,r�2) = 2ν(r�1 – r�2)[ρ1(r� – r�1) – ρ2(r� – r�2)](τ�1 × τ�2)3. (5)

As the r.h.s. of this equation vanishes as r�1 → r�2, in-dependently of the strength of the interaction, it followsthat this isovector operator j�(2) is dominated by its long-range components.

While the pion exchange current contributes a mod-est but phenomenologically required 10% enhancementof the calculated cross section for radiative capture ofthermal neutrons on protons, it is responsible for about

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Nuclear Exchange CurrentsDAN-OLOF RISKA

Helsinki Institute of Physics and Department of Physical Sciences, University of Helsinki, Helsinki, Finland

one-half of the much smaller empirical cross section forradiative capture on deuterium [7] and the bulk of theeven smaller cross section for radiative capture on 3He[8]. In the latter two reactions the matrix element of thesingle nucleon currents are suppressed by the orthogo-nality between the main components of the initial andfinal states. A particularly impressive illustration of thesignificant role of the exchange current contribution inreactions of this kind is provided by the p-d radiativecapture reaction where the exchange current contribu-tion is crucial for a description of the data on the angu-lar distributions as shown in Figure 1 [9,10].

The Nuclear Interaction and the Exchange Currents

For such nucleon–nucleon interaction models, whichare expressed as sums of single meson exchange interac-tions, the construction of the associated exchange cur-rent operators is straightforward. Modern realistic phe-nomenological interactions are not that simple, andtherefore some dynamical assumptions are required forthe construction of the corresponding exchange currentoperators that satisfy the continuity equation (3), (5).One solution to this problem is to express the interactionin a form that has the same operator structure as thesimple boson exchange models and to then replace theboson exchange Yukawa functions with the correspon-ding coefficient functions of the realistic interaction[10–13].

This method has led to very satisfactory descriptionof such electronuclear observables, that are sensitive toexchange currents. Best known of these are the cross sec-tion for backward electrodisintegration of the deuteron[14] and the electromagnetic form factors of the few-nu-cleon systems [15]. In the case of these observables therelatively large exchange current contribution is a directconsequence of a destructive interference between thematrix elements of the single nucleon currents, whichgenerates an empirically contraindicated minimum. Inthe case of the charge form factors the relatively large ex-change current contribution is a consequence of the dif-fraction minimum in the form factor, which makes thesmall exchange current contributions anomalously visi-ble. For larger nuclei the exchange current contributionsto charge form factors are less visible, because of thedominant shell structure effects [16, 17]. Three-nucleonexchange currents give but minor numerical contribu-tions [18–20].

The Axial Exchange CurrentThe exchange current contributions to the axial cur-

rent of a nucleus, which are associated with the nucleon–nucleon interaction, are smaller by a factor (v/c)2 thanthe corresponding electromagnetic exchange current op-erators, and therefore numerically of less significance[21]. The main term of the axial exchange current oper-ator is due to excitation of virtual intermediate ∆(1232)resonances.

Despite its small nuclear matrix elements the axialexchange current is required for a satisfactory descrip-tion of the Gamow-Teller transition in β-decay of 3H[22]. This empirically known matrix element may in factbe used to restrict the parameter uncertainty in the ex-

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Figure 1. Differential cross section, proton vector ana-lyzing power, and deuteron tensor analyzing power ob-servables for radiative pd capture at Ecm = 3.33 MeV.The dashed curve represents the impulse approximation,and the solid curves, the results that include the ex-change current contributions as calculated explicitly(thin lines) and with the Siegert theorem (thick lines) [9].The data points are from [10].

pression for the axial current operator, which then maybe employed to calculate astrophysically significant β-transition rates in light nuclei with some confidence [23,24].

The phenomenologically observed “quenching” of~20% of the Gamow-Teller transition strength in nuclei[25] has a simple explanation in terms of two- (andmany-) body axial pion exchange currents, with virtualintermediate ∆ (1232) resonance states [26, 27]. If re-duced to the form of an effective single nucleon axialcurrent by integration of the coordinates of all but onenucleon over the nucleus, these exchange currents com-bine into a screening factor for the axial current of thesingle nucleon, which may be expressed as

gAAa = – _________ σστa. (6)1 + g�U(k)

Here g� is a parameter, which determines the strength ofthe ∆-hole interaction, and U(k) is the pion optical po-tential. Most, if not all [28], of the phenomenologicalquenching can be explained by this screening mecha-nism.

The Axial Charge OperatorIn contrast to the case of the axial current operator,

two-nucleon mechanisms give very significant contribu-tions to the axial charge operators of nuclei, which areas large or larger than the contribution of the sum of thematrix elements of single nucleon axial charge operators.The role of the axial exchange charge operator is visiblethroughout the periodic table in the values of first for-bidden β-transitions, which are enhanced by up to~100% over the corresponding single nucleon values[29–31].

The axial charge operator has a main pion exchangecomponent, the operator structure of which is fixed bychiral dynamics and therefore is model independent [32].This by itself can explain about half of the empiricallyfound enhancement that is seen in first forbidden β-tran-sition rates. The remaining half of the enhancement isdue to the axial exchange charge operator, which is as-sociated with the short-range part of the nucleon–nucleon interaction [33, 34].

The main short-range components of the nucleon–nucleon interaction may be described as scalar and vec-tor exchange mechanisms. The former represents the at-tractive scalar field that is felt by a nuclear nucleon. Theeffect of this may be taken into account as a shift of thenucleon mass: m → m + <v>, where <v> is the average

scalar field. As the scalar field is attractive <v> < 0, thenucleon mass in a nucleus is reduced from its free value.The enhancement of the axial charge due to this scalarfield may be taken into account by implementation ofthis mass shift in the denominator of axial charge oper-ator of a single nucleon: –gAσ� . p�τa/m.

The spontaneously broken approximate chiral sym-metry of the strong interaction implies that pions coupleto hadrons and nuclei through their axial currents. Nu-clear pion absorption and production reactions aretherefore governed by the matrix elements of the axialcurrent density operator and require a realistic descrip-tion of the latter, with inclusion of the exchange currentterms.

The cross section for the reaction pp → ppπ0 nearthreshold [35, 36] provides an intriguing example ofthis. In this reaction the pion exchange contribution issuppressed because of the symmetric isospin state of thetwo protons. As a consequence, the short-range ex-change current contribution to the axial charge operator,which is due to the short-range component of the nu-cleon–nucleon interaction, plays an exceptionally signif-icant role in the description of the empirical cross section[37], which is much larger than what the single nucleonaxial charge operator would suggest [38]. Part of this en-hancement of the single nucleon term may be describedas the nucleon mass shift by the effective scalar fieldmentioned above.

Nuclear QCDAmong the goals of theoretical nuclear physics is an-

choring the phenomenologically fairly satisfactory nu-clear force–based description of nuclear structure and re-

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Proceedings: WE-Heraeus

3” wide x 2” high

NEW

actions in quantum chromodynamics (QCD). To the sev-eral different approaches to this issue belongs the largecolor limit of QCD, which allows a series expansion in1/Nc with only odd powers of 1/Nc (Nc is the number of“colors”) of hadronic observables. The first few terms inthis series have been shown to describe the key phenom-enological features of the structure of the baryons [39].The main central and tensor force components of thenucleon–nucleon interaction are of order Nc, while thesmaller spin-orbit interaction components are of order1/Nc.

The leading terms of such a 1/Nc expansion of thecomponents of the nucleon–nucleon interaction corre-spond to the strongly coupled meson exchange terms inphenomenological boson exchange models for the inter-action [40]. If applied to the exchange current operatorsthat are associated with the interaction, the correspon-ding series expansion in 1/Nc also reveals that the lead-ing terms correspond well to those exchange current op-erators, which have been found phenomenologically tobe significant [41].

The large Nc dependence of the nuclear interactionoperators is most readily derived by referring to a quarkmodel description, with Nc quark colors. The attempt to

create a quark-based description of nuclear interactionsand currents has, in fact, a good pedigree [42], althoughthe results have hitherto mainly been of qualitativevalue. In quantitative calculations of the electromagneticand weak transition rates of mesons and baryons, thereis no avoiding the concept of exchange currents, once re-alistic hyperfine interaction models for quarks are em-ployed [43–45].

Exchange Currents in MesonsMesons with one or more heavy flavor quark and

antiquark constituents form bound states with manyanalogies to the bound few-nucleon systems. To theseanalogies belong that both their M1 transition rates andpionic transition rates cannot be satisfactorily describedby the matrix elements of single quark currents alone.

A prime example of this is the M1 decay rate ofcharmonium, J/ψ → ηcγ, which is overestimated by fac-tors 3–4 in the quark model. By taking into account theexchange current that is associated with the scalar con-finement interaction in addition to relativistic correc-tions, this overprediction may be avoided [46–48]. Theexchange current term may in this case also be viewed asa consequence of an effective shift of the charm quark

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mass mc, but in this case upwards to mc + cr, where c isthe string tension of the confining interaction (c ~ 1GeV/fm).

The pion decay of the D1(2420) charm meson, whichdecays to D*π, provides a good illustration of the im-portance of the two-quark contribution to the axialcharge operator of mesons. The empirical width for thisis overestimated by factors 3–4, without inclusion of theaxial exchange charge operator, which is associated withthe scalar confining interaction [49]. This operator mayeither be derived as a pair current term that is associatedwith the negative energy pole in the nucleon propagator[50], or more directly by again making the shift mc → mc

+ mcr in the charm quark mass.

References1. S. C. Pieper, K. Varga, and R. B. Wiringa, Phys. Rev. C66,

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4153 (1995).7. E. Hadjimichael, Phys. Rev. Lett. 31, 183 (1973).8. F. Khanna and I. S. Towner, Nucl. Phys. A356, 441 (1981).9. L. E. Marcucci et al., nucl-th/0212009.

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(1999).

DAN-OLOF RISKA

feature article


IntroductionNuclear astrophysics was born

several decades ago, “to understandenergy generation in the sun andother stars at all stages of stellar evo-lution, and the nuclear processeswhich produced the relative abun-dances of the elements and their iso-topes” [1]. Experimental nuclear as-trophysics aims at the measurementof such nuclear processes, using nu-clear techniques. Nuclear reactionsin stars are organized in chains (e.g.,the p-p chain in the sun) or in cycles(e.g., the CNO cycle in stars heavierthan the sun). Since its origin, in the1930s, much work has been devotedto the measurement of reactions oc-curring in stable and quiet environ-ments, i.e., stars on the main se-quence, like the sun, people trying toreach to the energy region effectivein stars, the so-called Gamow en-ergy.* In the 1980s, the interest ofastrophysicists turned to explosivestellar environments [2], like novae,supernovae, and X-ray bursts, inwhich reaction cycles are very dif-ferent from what they are in quietenvironments; in the former, indeed,reactions involving radioactive nu-clides take an active place, the so-called hydrogen or helium explosiveburning. This posed a great challengeto experimental nuclear astrophysics:these light radioactive nuclides hadlifetimes too short to become targets,and as a consequence they had to be

used as beams, reactions on H or Hebeing performed in inverse kinemat-ics, and detectors being potentiallywrapped in the beam-induced back-ground. To be fair, one should addthat cross sections are larger by sev-eral orders of magnitude comparedto the quiet burning situation.

The Cyclotron Research Centerand Nuclear Physics group in Lou-vain-la-Neuve were involved fromthe beginning in developments lead-ing eventually to the obtention of afirst intense and pure beam of 13Nions in 1989. The presence of two-accelerators, i.e., two cyclotrons, inneighboring vaults was an incentiveto use the ISOL-method: a first cy-clotron (CYCLONE30) accelerateda high-intensity (up to 500 µA) low-energy (30 MeV) proton beam thatwas stopped in a 13C block. 13Natoms from the 13C(p,n)13N reactionwere extracted from the 13C by a ni-trogen gas flushing and transferredto an ECR ion source where theywere ionized to the 1+ state. A sec-ond cyclotron (CYCLONE110) ac-celerated the 13N1+ ions to the re-quested energy, i.e., 0.6 MeV/amu. Adetailed description of the set-up canbe found in [3]. Three Belgian uni-versities, Brussels, Leuven, and Lou-vain-la-Neuve, had joined efforts inthis undertaking. Subsequently,other beams were developed [4] andseveral reactions of astrophysical in-terest were studied in the 1990s inLouvain-la-Neuve, by groups fromEurope and the U.S. Let us mention13N(p,γ)14O [5], 18F(p,α)15O [6];19Ne(p,γ)20Na [7], 19Ne(α,p)21Na[8]. Sophisticated detection set-upshad to be installed accordingly.

Some limitations of the presentinstallation were noticed early: (i)the 30 MeV proton cyclotron of-fered little flexibility as a productionmachine; other particle–target pairswould often lead to larger produc-tion and/or easier conditions (for ex-ample, 14O could be better producedby the 12C(3He,n) reaction inducedby a high-energy 3He beam fromCYCLONE110 than by the 14N(p,n)reaction induced by the 30 MeVproton beam from CYCLONE30);(ii) in particular for the measurementof radiative capture reactions, the di-rect detection of the final heavy ionproducts appeared more attractive interms of efficiency [9] ; (iii) the per-formance of CYCLONE110 as apost-accelerator could be improvedby a more modern and dedicated ac-celerator. The CYCLONE44–ARESpair described in the following is acombined answer to the above-quoted shortcomings.

CYCLONE44CYCLONE44 is a compact iso-

chronous cyclotron, designed andbuilt by the Cyclotron Research Cen-ter team. Its main characteristics aresummarized in Table 1. WhereasCYCLONE110, built in the early1970s, was originally a low-massion accelerator (p,d,α)-modified sub-sequently to accelerate heavy ions(from Li to Xe) for nuclear physicsand nuclear astrophysics, CY-CLONE44 is dedicated to accelerat-ing low-mass heavy ions in the en-ergy range relevant to nuclearastrophysics. The energy range ac-cessible in CYCLONE44 (0.2–0.8MeV/amu) is typical of astrophysical

facilities and methods


CYCLONE44 and ARES: New tools for nuclear astrophysics

*Only recently was a measurement ofsuch a nuclear reaction performed in theGamow region, in the LUNA facility: thereaction was 3He(3He,2p)4He.

processes like the hot p-p chain orthe hot CNO-cycles and the escapeof the latter to the rp-process [10].

In the ISOL or “two-accelera-tor” scheme, the radioactive ionbeam has to be separated from itsstable isobar and accelerated. Usinga cyclotron as the post-acceleratorallows us to combine the two func-tions together. The expected majorimprovements with respect to CY-CLONE110 are a higher accelera-tion efficiency (typically 3 % in CY-CLONE110) and an improved massseparation ∆M/M. Both parametersare in fact very difficult to optimizesimultaneously, as they appear an-tagonist. To obtain a high rejectionof very near isobaric contaminants,the acceleration in CYCLONE44 isperformed on high harmonic modes(5, 6, and 8), and low accelerationvoltages are used. This implies asmall energy gain per turn and alarge number of turns. Typically, fora ∆M/M of 2.10-4 (e.g., 19F—19Ne) arejection factor of 105 was achieved.

Figure 1 represents the entire set-up including the CYCLONE30 pro-duction machine, the ECR source,the 30-m-long transport line, and fi-nally the CYCLONE44 post-acceler-ator [11].

The first radioactive beam accel-erated in CYCLONE44 was a 19Ne3+

beam of 9.6 MeV. After extraction,the beam was focused by a quadru-

pole doublet, and an intensity of 5 x109 pps was measured in a Faradaycup located two meters beyond thedoublet. The acceleration efficienciesof CYCLONE44 are of the order of10%. The presence of the ARES sep-arator has allowed us to measure thebeam purity precisely on-line: the19Ne3+ beam was in fact transmittedthrough ARES, until the ∆E-E detec-tor at the end. A particle identifica-tion was performed, yielding a 19F-over-19Ne ratio of 2.5 10-3. Thebeam stability was found to be verysatisfactory, over a one-day period.In addition to the ARES line, an-other beam line from CYCLONE44

will be installed. A large reactionchamber was constructed and will beplaced in the new line; large-sizecharged-particle detectors of theLEDA type [12] will allow classicalmeasurements of the (p,p), (p,α),(α,α), and (α,p) type, with increasedprecision.

ARESIn a (p,γ) or (α,γ) reaction in in-

verse kinematics, beam ions and ionsproduced in the reaction (hereafterthe product ions) are contained in anarrow cone beyond the target. Dueto the low momentum carried out bythe α-ray, beam and product ionshave the same momentum in firstapproximation, and in addition,both species are found in differentcharge states, the most abundantrepresenting at least 30% of thetotal. The above considerations ex-plain the ARES set-up (Figure 2): ina first step, a dipole magnet selectsthe momentum-over-charge ratio ofthe product ions corresponding tothe most abundant charge state, partof the beam ions being transmitted



Energy constant K (MeV) 44Energy range (MeV/amu) 0.2–0.8M/Q range 4–14Max average field (T) 1.54Extraction radius (m) 0.63Frequency range (MHz) 13.3–18.5Injection axial, spiral inflectorExtraction electrostatic deflectorRejection factor at ∆M/M = 2.10-4 105

Table 1. Main characteristics of CYCLONE44.

Figure 1. General layout of the facility.

as well. In a second step, a 1-m-longWien filter (or velocity filter) selectsproduct ions and deflects beam ions.In a third step, a ∆E-E counter per-forms a particle identification to re-ject the remaining beam ions.

The 19F(p,γ)20Ne reaction waschosen for the first test of ARES.This reaction has a strong resonanceof strength ωγ= 1.6 eV for an energyER = 635 keV in the cm. The fractionof the 19F beam ions surviving thesecond step of ARES was typically 3x 10-7 of the initial intensity, and atthe same time the fraction of the20Ne product ions reaching the ∆E-Ecounter was 4%. It is important topoint out that not only this percent-age but also the energy distributionof the 20Ne ions were reproduced bya simulation code. More details con-cerning these and other measure-ments performed during these tests(e.g., distribution of charge states,beam energy, ∆E gas detector per-formance, . . .) can be found in a re-cent publication [13].

Some words should be added re-garding the type of targets used for(p,γ) or (α,γ) measurements. For(p,γ) reactions, solid polyethylenefoils have been produced in our lab

in the last decade, from 20 µgr/cm2

to 500 µg/cm2. For (α,γ) reactions,helium ions from an ECR sourcewere implanted in thin (50 µg/cm2)Al foils. The helium bulk densitywas deduced from RBS measure-ments at our van de Graaff accelera-tor, while the He uniformity versusthickness was measured by theERDA method using 19F beams fromCYCLONE44. Typical areal densi-ties of 2 x 1017 at/cm2 were ob-tained. This work is described in arecent publication [14].

ConclusionCYCLONE44 is now ready for

operation in nuclear physics and nu-clear astrophysics. With respect toCYCLONE 110, increased intensi-ties of radioactive beams will be ob-tained on target, and an improvedmass separation will result in a stillbetter beam purity. The ARES sepa-rator was coupled to CYCLONE44.Extensive measurements were per-formed on a test reaction with stablebeam before reactions induced by ra-dioactive ions will be studied.

I wish to thank all collaboratorsfrom the Centre de Recherches duCyclotron and the Institut de Phy-

sique Nucléaire in Louvain-la-Neuve,who have contributed to all stages ofthe project reported here. This workhas been partially supported by theBelgian Programmes P4/18 andP5/07 on Interuniversity AttractionPoles of the Belgian State, FederalServices for Scientific, Technical andCultural Affairs. I also thank theFonds National de la Recherche Sci-entifique, Belgium, of which I am aResearch Director.

References1. W. A. Fowler, Rev. Mod. Phys. 56

(1984) 149.2. J. Truran, Ann. Rev. Nuc. Part. 34

(1984) 53.3. J. Vervier, Progr. Part. Nucl. Phys.

37 (1996) 435.4. See the present list of the available

beams in http://www.cyc.ucl.ac.be5. P. Decrock et al., Phys. Rev. Lett. 67

(1991) 808.6. R. Coszach et al., Phys. Lett. B353

(1995) 184.7. R. Page et al., Phys. Rev. Lett. 73

(1994) 3066.8. D. Groombridge et al., Phys. Rev.

C66 (2002) 055802.9. P. Leleux, in Nuclear Astrophysics,

Int. Workshop on Gross Propertiesof Nuclei and Nuclear Excitations,M. Buballa et al., editors (1998)356.

10. M. Wiescher et al., J. Phys. G: Nucl.Part. Phys. 25 (1998) R133.

11. G. Ryckewaert et al., Nucl. Phys.A701 (2002) 323c.

12. T. Davinson et al., Nucl. Instr. Meth.Phys. Res. A454 (2000) 350.

13. M. Couder et al., submitted to Nucl.Instr. Meth. Phys. Res.

14. F. Vanderbist et al., Nucl. Instr.Meth. B197 (2002) 165.

PIERRE LELEUX

Université Catholique de LouvainLouvain-la-Neuve, Belgium



Figure 2. CYCLONE44 and the ARES Spectrometer.

The NuPECC town meeting(January 30 to February 1) was wellattended with about 300 partici-pants. The meeting was organized inseveral sections, namely “Facilities,”“Nuclear Structure,” “Phases of Nu-clear Matter,” “QCD,” “Nuclei inthe Universe,” “Fundamental Inter-actions,” and “Applications,” wherethe corresponding NuPECC reportdrafts were presented and discussed.At the end of the meeting a two-hourdiscussion session was held, aimedat setting NuPECC priorities.

FacilitiesIn the “Facilities” section, Hen-

ning (GSI) gave an overview of theGSI plans for the coming 10 years.Both the physics motivation and the

proposed facility were laid out. Heexplained that the facility can runwith a 300% duty cycle by operatingthe different accelerating structuresin parallel. The EURISOL project (anext generation ISOL facility) wasdiscussed by Vervier. This project isnow in its design stage. The pro-jected 1 GeV high-intensity protondriver would also be of interest forother applications such as transmu-tation of nuclear waste and for stud-ies under the chapter of “Funda-mental Interactions.” The SPIRALfacility was presented by Mittig(GANIL) including the plans for SPI-RAL II, which should be in opera-tion by 2008. The REX facility atIsolde (CERN) was discussed byButler (CERN). In the near future

(<~5y) the experimental hall will beextended and the beam energy in-creased to 4.3 MeV/u. For the longerterm future, a further increase in en-ergy is foreseen and possibly the useof anti-protons. Pisent (Legnaro) dis-cussed the developments at Legnaro,in particular the construction of su-perconducting LINAC modules for aproton driver and the research on aconverter target for intense neutronbeams. The upgrade (to 1.5 GeV) forMAMI-C was discussed by Beck(Mainz). The physics goals rangefrom polarizability of the nucleonto open strangeness production.Habs (Munich) suggested a com-pletely new design for a radioactivebeam facility based on laser acceler-ation.

news from NuPECC


NuPECC Town Meeting

European nuclear physicists united to discuss their future. (Photo: G. Otto, GSI Darmstadt)

In the discussion session Hen-ning emphasized that the new GSIfacility will be a European facility,open to participation from othercountries.

Nuclear StructureThe theory part of the draft re-

port on nuclear structure was pre-sented by Dobaczewski (Warsaw).Due to the enormous advance incomputer capabilities, much progresshas been made in recent years in spiteof a lack of manpower. The results ofmean-field and shell-model calcula-tions are now approaching eachother. Van Duppen (Leuven) dis-cussed the experimental priorities ofthe nuclear structure community.ISOL and in-flight (IF) facilities offera nice complementarity. The recom-mendation of this working groupwas therefore to give full support toGSI (as an IF facility) and to EU-RISOL.

The plans for the AGATA detec-tor were discussed by Krücken (Mu-nich), where the high angular resolu-tion allows for superior correctionsfor Doppler shifts, thus allowing forhigh-resolution spectroscopy. In theother short contributions, Azaiez(Strassbourg) expressed the contin-ued need for stable beam facilities,Schmidt (GSI) showed that interest-ing nuclear structure information iscontained in level densities, andPochodzalla (Heidelberg) indicatedthe interesting aspects of the struc-ture of hyper nuclei. Mavrommatis(Athens) showed the use of neuralnetworks in applications of multi-parameter models.

In the lively discussion sessionseveral additional aspects of nuclearstructure were brought to the atten-tion of the writing committee, inparticular ab initio approaches.

Schutz (Nantes/CERN), repre-senting the writing committee on

“Phases of Nuclear Matter,” showedthe progress made in investigatingthe liquid–gas phase transition at in-termediate energies as well as thechiral and the hadron–quark–matterphase transition at ultra-relativisticenergies. In subsequent presenta-tions some of the topics were high-lighted. Senger (GSI) presented theimportance of vector mesons asprobes of the early high-densityphase. Trautman (GSI) emphasizedthat to improve our understandingof the liquid–gas phase transition,quantitative comparisons should bemade with molecular-dynamicsmodels. Bougault (Caen), Rivet(Orsay), and Alba (Catania) showeddifferent ways in which level densi-ties as accessed in fragmentation re-actions yield information on theshell structure at lower energies andthe liquid–gas phase transition.

The community expressed as toppriorities:

• exploiting existing facilities,• timely completion and opera-

tion of ALICE,• construction of CBM.

QCDWeise (Trento/Munich) pre-

sented the recommendations andphysics of “QCD” in a very excitingpresentation. The important openquestions in the field concern: (i) the“characteristic” mass gap, 1 GeV,separating the pion and other excita-tions; (ii) color confinement; (iii)spontaneous symmetry breaking.

Theoretically three approachesto QCD can be distinguished: (i) per-turbative QCD which applies at highmomenta where probes sense physicsat a length scale of a tenth fermi; (ii)effective field theory approacheswhich apply at extremely low ener-gies where the physics is dominatedby Goldstone bosons and chiral

symmetry and quantitative predic-tions can be made; (iii) lattice QCD,which is a rapidly developing field.

Experimentally, the importantobservables are: (i) Generalized par-ton distribution (GPD) functions,which measure quark–quark corre-lations. These GPDs are fundamen-tal to the understanding of the quarkstructure of the nucleon. For exam-ple, one may extract from these theorbital and spin contributions to thetotal angular momentum of the nu-cleon. (ii) Glue-ball and hybridstates, which are closely related tophysics of confinement. Probablypure glue-ball states do not existsince they will be mixed with qq

_-

states. Hybrid states, i.e., states withexotic JCP, may form a more cleanprobe. (iii) Charmonium states, inparticular pseudo-scalar eta-likestates, are also related to the physicsof confinement. (iv) Chiral dynamicsand spontaneous symmetry breakingare fundamental to the field. Quanti-tative predictions should be tested.(v) Propagation of mesons in nuclearmatter and transparency which is re-lated to questions of how hadronsare formed in QCD.

It will be important to use theexperience in nuclear many-bodytheory. The spin crisis, for example,stating that only 30% of spin is car-ried by the quarks, comes as no sur-prise to strong-interaction many-body physics.

Recommendations are:

• Maintain and expand, wherenecessary, adequate supportfor theoretical work, includinglattice QCD calculations.

• Fully exploit present facilities.• Strongly support HESR at GSI.• Develop a high luminosity fa-

cility (ELFE).• Engage in worldwide collabo-

rations.

news from NuPECC


Metag (Giessen) presented thephysics accessed by the PANDAproject at HESR. Explicitly, the rela-tion was shown with several of thetopics (glue balls, hybrids, charmo-nium spectroscopy, GPD) mentionedby Weise. At DEAR (DaΦne ExoticAtom Research), exotic atoms such askaonic deuterium will be measured.This yields direct information onstrangeness content of the nucleon asdiscussed by Guaraldo (Frascati).Marton (Vienna) presented the pio-nic-atom programme at PSI. Thehigh precision data on pion scatter-ing lengths form a stringent test ofchiral symmetry. Similar physics canalso be accessed through deeplybound pionic and kaonic states asdiscussed by Kienle (Munich).

In the discussion on the physicsissues, questions were asked rangingfrom the model (in)dependence ofGPDs to the structure of the confin-ing potential. In the discussion onrecommendations, it was empha-sized that the priorities have notbeen ordered.

Nuclei in the UniverseLanganke (Aarhus) presented

the case for the astrophysics aspectof nuclear physics and named a fewcharacteristic examples. (i) The re-cent results from the Sudbury neu-trino observatory (SNO) confirmsolar model predictions but are,however, sensitive to nuclear reac-tion rates. (ii) The life time of theisomeric state in 180Ta depends onthe temperature of the ambient stel-lar medium and can thus be used tolearn about stellar environment. (iii)Neutrino capture of nuclei plays arole beside the capture of protons inthe dynamics of supernova explo-sions. (iv) In supernova explosionsthe equation-of-state of the core isimportant. (v) The r-process may

occur in different stellar environ-ments resulting in different relativeabundances. To determine the stellarhistory from the abundances, oneneeds masses, half-lives, and reac-tion rates. (vi) In type II supernovae,electron capture after the explosiveburning occurs, depleting the elec-tron density and thus the electronpressure which affects flame propa-gation. (vii) Far away from the val-ley of stability, models show increas-ing discrepancies with data onmasses.

Recommendations for facilitiesare:

• need GSI,• need small facilities,• need 5 ~ MeV underground

machine, and• need meeting place (ECT*) for

theorists and experimentalists.

The astrophysics perspective waspresented by Diehl (Garching). It is,for example, assumed that we knowsupernova explosions and can usethem as standard candles for deter-mining distances. For this, more de-tails of flame evolution are neces-sary. Present models do not accountfor the evolution of known super-novas. New probes are also investi-gated, such as emitted positrons.Several space missions are plannedand one needs the nuclear physicsbacking for proper interpretation ofthe data.

In the following short contribu-tions, Rolfs (Bochum) presented thecase for the LUNAR undergroundlaboratory. Angulo (Louvain laNeuve) presented the need for low-energy radioactive nuclear beams.Kratz (Mainz) stressed that ISOL fa-cilities are essential to obtain massesand β-decay probabilities, whileSümmerer (GSI) showed that storagerings also are suited for this purpose.

At the n-TOF facility at CERN, neu-tron capture rates, important for thes-process, are measured as discussedby Mengoni (CERN). Aliotta (Edin-burgh) stressed that electron-screen-ing effects are important in extrapo-lating astrophysical cross sections tovery low energies.

Fundamental InteractionsA spirited presentation of the re-

port on “Fundamental symmetries”was made by Jungmann (KVI). Sev-eral different projects fall under thistopic. (i) Neutrino disappearancehas been observed which can be in-terpreted as neutrino oscillations.This has put the question regardingthe structure of neutrinos (Majoranavs. Dirac) center stage. Another as-pect concerns the unitarity of theCKM mass matrix, which seems tobe violated at the level of two stan-dard deviations. This calls for fur-ther measurements of neutrino-lessdouble-beta decay and the investiga-tion of rare decay branches ofmesons and baryons. (ii) CP-violat-ing static moments (electric dipolemoments) are searched for. Parity-violating moments (anapole mo-ments) have been determined. CP- orT-violation can also be accessed incertain triple-correlation functionsin beta decay. These measurementscan best be done in traps. CPT-viola-tion has been addressed with a newinteraction-based approach. Thesesymmetry violations can be tested inseveral high-precision experimentsincluding experiments with anti-hy-drogen. (iii) It has been suggestedthat the fine-structure constant istime dependent. This can be accessedthrough the Re/Os ratio in mete-orites.

Recommendations for facilitiesare:

news from NuPECC


• intense neutrino beam,• (ultra) cold neutrons (both

could be driven by an intense 1GeV proton beam),

• radioactive beams and im-proved trapping facilities,

• underground facility.

Volpe (Orsay) showed that nu-clear structure is crucial for the in-terpretation of the experiments. Heil(Mainz) showed the need for an in-tense source of cold neutrons formeasuring permanent dipole mo-ments. Maas (Mainz) has drawn at-tention to different interesting as-pects of parity-violating electronscattering. Pachucki (Warsaw) dis-cussed precise measurements inQED.

In the discussion session it wasargued that QED effects should betreated non-perturbatively because

of the high accuracy needed for theinterpretation of parity violation ex-periments on nuclei.

ApplicationsBeautiful examples of applica-

tions of nuclear physics techniquessuch as PIXE and RBS were shownby Mandò (Firenze). Calligaro(Paris) even took us on an on-linevisit to the facility at the Louvre dur-ing his presentation. Other examplessuch as applications of radiotherapyand AMS were discussed in the con-tributions of Cantone (Milano).Leray (Saclay) gave an overview ofthe status and future plans of theHINDAS project which aims atforming a comprehensive databaseof nuclear cross sections which areimportant for applications such asnuclear-waste transmutation. Patelli

(Legnaro) discussed the mutual ben-efits of interactions between nuclearphysicists and material scientists.Cinausero reported on the status ofhuman de-mining based on nucleartechniques.

GeneralJames Symons gave an eloquent

overview of the NSAC long-rangeplan, both its historical backgroundand the present recommendations.

The meeting was concluded by alively and constructive discussion re-garding the priorities of the differentprojects. The assigned two hourswere not enough to complete the dis-cussion, but consensus was met oncertain priorities.

OLAF SCHOLTEN

KVI Groningen

news from NuPECC


Under broad participation of thecommunity, two proposals of Inte-grated Infrastructure Initiatives arebeing prepared for the Sixth Frame-work Programme of the EuropeanCommission. The two proposals,EURONS (European Nuclear Struc-ture Integrated Infrastructure Initia-tive) and I3HP (Integrated Infrastruc-ture Initative in Hadron Physics),cover the main fields of researchusing Major Research Infrastruc-tures in nuclear physics.

General information can befound on the NuPECC website,

• http://www.nupecc.org/iii

More detailed information onEURONS can be found on

• http://www-new.gsi.de/informationen/EURONS/index_e.html

And more information on I3HPcan be found on

• http://www.infn.it/eu/i3hp/

news from NuPECC


Integrated Infrastructure Initiatives

The coordinators for the various activities within EURONS at their meeting at GSI Darmstadt on March 31, 2003.(Photo: A. Zschau, GSI Darmstadt)

calendar


2003

June 9–21Seville, Spain, Exotic Nuclear

Physics (VIII Hispalensis InternationalSummer School). Contact: J. M. Ariasand M. Lozano, University of Sevilla,Spain. E-mail: [email protected]

Web: www.cica.es/aliens/dfamnus/oromana/

June 10–13Paris France. International Con-

ference on Collective Motion in Nucleiunder Extreme Conditions (COMEX1). Contact: Valerie Frois, Secretary ofCOMEX 1, Institut de Physique Nu-cleaire, 91406 Orsay Cedex, France.E-mail: [email protected]. Fax:1-6915-4475. Tel.: 1-6915-7749.

Web: http://ipnweb.in2p3.fr/comex1/comex1.html

June 17–21Moscow, Russia. VIII Interna-

tional Conference on Nucleus-NucleusCollisions. Contact: Yu. Ts. Oganes-sian or R. Kalpakchieva, Flerov Labo-ratory of Nuclear Reactions, JINR,141980 Dubna, Moscow region, Rus-sia. E-mail: [email protected]. Tel: 7-09621-62151. Fax: 7-09621-65083.

Web: http://www.nn2003.ru/

June 25–28McGill University, Montreal,

Canada. Topics in Heavy Ion Colli-sions: An international conference onthe physics of hot and dense stronglyinteracting matter. Contact: [email protected]

Web: http://www.physics.mcgill.ca/HIC03/

July 13–19Crete, Greece. International

Conference on The Labyrinth in Nu-clear Structure. Contact: [email protected]

Web: http://www.INP.demokritos.gr/~kalfas/CLNS/bulletin.html

August 25–29Vrnjacka Banja, Yugoslavia. Fifth

General Conference of the BalkanPhysical Union. Contact: E-mail:[email protected]. Fax: +381 113162190. Tel. +381 11 31620099, +38111 3160598.

Web: http://www.phy.bg.ac.yu

August 31–September 7Krzyze, Poland, XXVIII Mazurian

Lakes Conference on Physics. AtomicNucleus as a Laboratory for Funda-mental Processes. Contact: KatarzynaDelegacz, The Andrzej Soltan Institutefor Nuclear Studies, Hoza 69, 00-681Warsaw, Poland. Fax: (48-22) 7793481.Tel.: (48-22) 7180583.

Web: http://zfjavs.fuw.edu.pl/mazurian/mazurian.html

September 2–6Dubna, Russia. Nuclear Structure

and Related Topics. Contact: R. Jolos,V. Voronov, Bogoliubov Laboratory ofTheoretical Physics, JINR, 141980Dubna, Moscow region, Russia. E-mail: [email protected]. Fax: (7-09621) 65084.

Web: http://thsun1.jinr.ru/~nsrt03

September 22–26Riviera, Zlatny Piasatsi (Golden

Sand), Bulgaria. Perspectives of LifeSciences Research at Nuclear Centers.Contact: Dr. M. V. Frontasyeva, E-mail: [email protected]

September 24–27Pavia, Italy. Sixth Workshop on

“Electromagnetically Induced Two-Hadron Emission.” Contact: [email protected]

Web: http://isnwww.pv.infn.it/~2hconf/

October 7–12Santorini, Greece. Electromagnetic

Interactions with Nucleons and Nuclei.EuroConference on Hadron StructureViewed with Electromagnetic Probes.

Web: http://www.esf.org/euresco/03/pc03117

October 14–17Grenoble, France. International

Workshop on Probing Nuclei via the(e,e’p) Reaction. Contact: [email protected]

Web: http://isnwww.in2p3.fr/EEP03

November 16–20Napa Valley, California, USA. 2nd

International Conference on theChemistry and Physics of the Transac-tinide Elements (TAN 03). Contact:Dianna Jacobs, e-mail: [email protected]. Fax: +1-510-486-7444. Tel.:+1-510-486-7535.

Web: http://tan03.lbl.gov

November 19–23Kurokawa Village, Niigata, Japan.

“A New Era of Nuclear StructurePhysics” (NENS03). Contact: Yasu-yuki Suzuki or Susumu Ohya, [email protected]

Web: http://ntweb.sc.niigata-u.ac.jp/nens03/

November 24–29Nara, Japan. The 8th International

Conference on Clustering Aspects ofNuclear Structure and Dynamics. E-mail: [email protected]

Web: http://ribfwww.riken.go.jp/cluster8/

nuclear physics volume 13/no. 2 newstwo-neutron halo nuclei such as 6he have been dubbed...

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