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Los Alamos _ _ - N A T I o N A L L A B o R A T o R Y

MODELING (P, XN) REACTIONS PRODUCING

STUDIES PROTON-RICH NUCLIDES FOR RADIOACTIVE ION BEAM

M.B. Chadwick, T-2, MS-B283, Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545, USA

Laboratory, P.O. Box 1663, Los Alamos, NM 87545, USA

P.G. Young, T-2, MS-B283, Los Alamos National

Proceedings of the American Nuclear Society 1997 Transactions of Winter Albuquerque Meeting, November 16-20, 1997, Volume 77, Albuquerque, NM, USA n

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MODELING (P,XN) REACTIONS PRODUCING

PROTON-RICH NUCLIDES FOR RADIOACTIVE ION BEAM

STUDIES

M. B. Chadwick and P.G. Young

University of California, Theoretical Division,

Los Alamos National Laboratory, Los Alamos, N M 87545, USA

INTRODUCTION

Over the last few years, an intense worldwide interest has grown in the use of radioactive

ion beams (RIB) to study the properties of nuclei far from stability. The relatively recent

technological developments that have enabled the production of high-quality radioactive

beams promise to lead to a renaissance in nuclear structure and nuclear reaction physics, as

well as nuclear astrophysics. Some of the principal issues that will be addressed with RIB

research are: (1) The nature of residual interactions, particularly among valence nucleons, in

orbitals not accessible near stability; (2) New nuclear shapes and collectivities; (3) Neutron

halos for extreme N/Z rations; (4) Properties of N=Z nuclei far from stability; ( 5 ) Nuclear

reaction mechanisms sensitive to new nuclear structure properties in RIB nuclei; and (6)

Study of astrophysical nuclear reactions, for the first time, involving unstable nuclei in

nucleosynthesis.

Group T-2 at Los Alamos has been working with US Nuclear Reaction Data Network

Radioactive Ion Beam Task Force [l] to address RIB nuclear data needs. The current focus

is nuclear data needed to guide the choice of targets for the production of various radioactive

product species using the isotope-separation on line (ISOL) method; future work will include

the study of RIB nuclear reaction and structure physics. Laboratories that are undertaking

RIB studies with the ISOL method in the US are principally Oak Ridge National Laboratory

(ORNL) and Argonne National Laboratory (ANL).

’

1

MODEL CALCULATIONS

To produce proton-rich RIBS, (p,xn) reactions are used on targets that are typically the

most proton-rich stable isotope of a given element. When selecting targets candidates and

radionuclide extraction methods in a RIB facility it is important to have reliable estimates of

the production cross sections of various radionuclides produced in proton-induced reactions,

for incident energies up to 200 MeV. Since measured cross section data exist in only a few

cases, for a limited number of incident energies, nuclear model calculations can be used

to estimate these cross sections once they have been benchmarked against existing data to

validate their accuracy.

The Los Alamos GNASH code, which applies direct, preequilibrium, and compound

nucleus reaction theories, has been used for calculating (p,xn) reactions. Such calcula-

tions require accurate information on nuclear level densities, optical potentials, masses, as

well as physically-realistic reaction models (for instance, preequilibrium emission processes

significantly effect the population of various radionuclide products). Recent theoretical de-

velopments that we have made for performing such calculations include: Incorporation of

multiple preequilibrium emission within both the exciton-model version and the Feshbach-

Kerman-Koonin (FKK) version (FKK-GNASH) [2]; Study of the importance of multiple

preequilibrium processes through comparison with predictions by the Quantum Molecular

Dynamics theory [3]; and Development of a two-component FKK theory that explicitly fol-

lows neutron and proton particle-hole excitations in the preequilibrium reaction, as well

as making use of shell-model states for the single-particle excitations [4]. Such studies are

important for correctly determining the relative partitioning of reaction flux into secondary

neutron and proton ejectiles, and are therefore important for predicting (p,xn) cross sections.

To date, nuclear model calculations of (p,xn) cross sections using the GNASH code have

been completed for RIB-producing targets of 28Si, 40Ca, 58Ni, 64Zn, and 70Ge. Addition-

ally, proton-induced cross sections on 48Ca and 64Ni have been calculated for studying the

possibility of producing neutron-rich products. The evaluation methodology consist of first

obtaining existing experimental compiled cross sections from the NNDC CSISRS database

2

and from the literature, performing model calculations and comparing the results against

these data (to validate the calculations), and using the model calculations to predict un-

measured cross sections of interest to the RIB community. These results have been supplied

to ORNL [ 5 ] . Calculations of (p,xn) reactions on Au (for the release of proton-rich Hg

isotopes), Pb, Ta, and other targets are currently underway, at the request of ANL.

Future research that is required to respond to RIB nuclear reaction data needs includes:

0 Calculation and evaluation of additional (p,xn) reaction cross sections as requests are

obtained from the RIB community.

0 Improved theoretical treatments of preequilibrium reaction mechanisms for energies

up to 200 MeV, using both semiclassical and quantum mechanical theories.

0 Use of improved optical potentials in reaction calculations that use an isospin-

dependent formalism, so that particle decay from nuclides far from stability can be

more accurately determined.

0 Use of more accurate level densities determined from experimental measurements.

0 Improvement of fission models for nucleon-induced fission at energies up to a few hun-

dred MeV, particularly for the fission fragment distribution for characterizing neutron-

rich RIB production. Forthcoming experimental results using the Los Alamos GEANIE

gamma-ray array combined with the Los Alamos intense neutron source will be use-

ful for determining radionuclide production and therefore guiding such studies. The

GEANIE array will also provide useful information on the nuclear structure of such

radioactive nuclides.

3

REFERENCES

[ 11 See ht tp: //www.dne. bnl.gov/ burrows/usnrdn/ribtf. html

[2] M.B. Chadwick, P.G. Young, and D.C. George, Physical Review C 50, 996 (1994).

[3] M.B. Chadwick, S. Chiba, K. Niita, T. Maruyama, and A. Iwamoto, Physical Review C

52, 2800 (1995).

[4] A.J. Koning and M.B. Chadwick, accepted for publication by Physical Review C (1997).

[5] J. Garrett, Oak Ridge National Laboratory, Private communication to MBC (1997).

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