Slide 1

The Atomic Nucleus The subject of Nuclear Structure is to understand how the Z protons and N neutrons arrange themselves in a bound system and how they react to external fields. Since Z and N vary over a broad range, different features are expected depending on the mass number and on the N/Z ratio. What is the effective NN interaction? What are the limits of existence for bound nuclei (driplines)? Slide 2 The Shell Model There is a surprising analogy between the description of bound electrons in atoms and bound nucleons in atomic nuclei. Nuclear Shell Model (M.Mayer et al, 1948-49) This model is a highly successful one, but the prediction capabilities are somewhat limited when going to nuclei far from stability Need to tune the model parameters collecting information on such nuclei! Slide 3 Shell structure Cooper pairing Collective modes Nuclear excitations Slide 4 neutrons protons rp process Crust processes s-processs-process r process stellar burning p process What is the origin of elements heavier than Iron? How do stars burn and explode? What is the structure of a neutron star? Nova Time (s) X-ray burst 331 330 329 328 327 Frequency (Hz) 101520 4U1728-34 T Pyxidis Neutron star Nuclear Physics and Stellar Nucleosynthesis Slide 5 Stable nuclei 10 0 heavy-ion collisions neutron drip-line protondrip-line Nuclei and Stars supernovae Slide 6 Symmetry Energy: Nuclei and Stars P sym (,) = 2 (E sym / ) 2 = 0 E sym ( o ) u +1 2 u= / o E sym () = E sym ( o ) u The symmetry energy determines the dynamics of supernovae explosions and the pressure in neutron stars: Observables (input to the models): Dynamics of heavy-ion collisions at different N/Z ratios Structure of nuclei far from stability: single-particle and collective properties Slide 7 How can we perform nuclear structure studies? In order to study the structure of nuclei, we need to find them in an excited state and to observe the radiation (light particles, electrons, -rays) which they do emit during the de-excitation. Excited nuclei can be populated naturally by decay of long-lived radioactive nuclei, or by reactions induced by cosmic rays Excited nuclei can be populated artificially by inducing nuclear reactions in our laboratories, using accelerators Slide 8 V T ~ 14,5 MV From H to 197 Au, E = 30 1.5 MeV/A CW or pulsed XTU-Tandem ECR on 350 kV platform SC-RFQs and QWRs V eq ~ 8 MV PI Injector PIAVE SC Booster ALPI 68 SC Quarter Wave Resonators (Nb, Nb/Cu) V eq ~ 48 MV, species from 28 Si to 197 Au Injected by Tandem or PIAVE TANDEM PIAVE ALPI COMPLEX Fully operational with noble Gases; ECRIS replacement in Spring 2008 Slide 9 Producing nuclei far from stability Using stable beams at energies close to the Coulomb barrier, with a careful choice of the reaction mechanism it is possible to populate nuclei far from stability in different regions of the chart of nuclides Slide 10 Observing nuclei far from stability Following the production of excited, exotic nuclei, there are two basic alternatives, implying quite different experimental problems: Transport the excited nuclei elsewhere and observe the (delayed) decays (or chains of decays) off-line Observe the prompt decays on-line, as they are produced Slide 11 Gamma Spectroscopy The observation of the electromagnetic radiation (photons or -rays) emitted by the excited nuclei provides one of the most sensitive tools to investigate the nuclear structure The emission of -rays is a well understood process from the theoretical point of view, hence the smallest effects can provide valuable information on the nuclear structure Contrary to the detection of particles, high resolution is possible Slide 12 Detectors A detector is an object translating the arrival of a radiation quantum into a (measurable) electric signal which can be: generated outside of the active volume of the detector (eg scintillators coupled to photomultiplier tubes) generated inside of the active volume of the detector (eg semiconductor detectors) Slide 13 Current mode I Detector A possible mode of operation consists in measuring the average current produced within the detector. This mode of operation is not much used in spectroscopy since there is no information on the energy of the individual radiation quanta, only on their rate. Slide 14 Pulse mode Here the information on energy and time of arrival of the individual quanta is preserved. Typically, the charge flowing at the detector electrodes is collected by a preamplifier, which we can approximate by a RC circuit. RC small compared to the charge collection time: the output signal is basically the same as the current signal: V(t)=R i(t) RC large compared to the charge collection time: the output signal rises as the current signal and decays exponentially with time constant RC. Its amplitude V=Q/C is proportional to the energy if C is constant. Slide 15 Spectra The usual way to visualize the response of a detector is a differential spectrum, namely a histogram in which each channel content is the number of events with amplitude within the bin. The integral version is not as widely used. Differential Integral Slide 16 When monochromatic radiation hits a detector, one expects that the response of the detector is always the same, in other words, that a peak appears in the differential spectrum. The resolution R is defined as the ratio between the peak full width at half maximum ( FWHM ) and its position, R=FWHM/H 0, but conventionally the values of FWHM and energy can be provided. For gaussian peaks, the statistical limit is: Resolution where N is the peak area. In practice the resolution can be better than such a limit and the Fano factor F Pair production Photons with energy larger than 2m e c 2 can materialize (in the Coulomb field of a nucleus) in an electron-positron pair with total kinetic energy E -2m e c 2. Electron and positron subsequently release their energy in the medium; the positron eventually annihilates releasing two 511 keV photons. 1.The threshold for this process is 1.022 MeV 2.This process dominates at high photon energies ( E > 10 MeV ) 3.This process depends approximately on the square of the atomic number of the medium. Slide 25 Cross Sections in NaI(Tl) NaI(Tl) Energy (MeV) Cross Section Atomic shell K Pair prod. Compton scatt. Photoelectric abs. Slide 26 Cross Sections in Germanium Compton scatt. (10 keV) ~ 55 m (100 keV) ~ 0.3 cm (200 keV) ~ 1.1 cm (500 keV) ~ 2.3 cm (1 MeV) ~ 3.3 cm Mean free path (E) = M A /(N AV. ). 1/ E Photoelectric Pair prod. Slide 27 Response function The response function is the differential spectrum obtained with a detector when hit by monochromatic radiation We can consider some schematic cases: Large detectors Small detectors Intermediate size detectors Slide 28 Large detectors In the ideal case of a very large detector (with respect to the mean free path of the radiation), the incoming radiation is fully absorbed. Since it is not possible to discriminate the time at which the individual interaction points are produced, the detector is only sensitive to the total energy deposition. Slide 29 Large detectors The response function of a large detector is very simple and it includes only a full- energy peak (which in most practical cases can be treated as a photopeak). Ideal Ge sphere, 1 m diametre. 3 MeV photons Some devices (Total Absorption Spectrometers) are a good implementation of this limit! LucreciaTAS at GSI Full-energy peak Slide 30 Small detectors In this case there is a high probability that the incoming photons are only partially absorbed. In the response function, besides the full-energy peak, one can identify a continuum generated by photons which underwent Compton scattering and, if the photon energy is larger than 1.022 MeV, peaks due to the missed detection of one or both the annihilation photons (single and double escape peaks). Compton continuum full-energy peak Double escape peak E=1.33MeV E=3MeV Slide 31 Intermediate size detectors Compton continuum Full-energy peak Double-escape peak Single-escape peak The situation is qualitatively similar to the case of small detectors, although in practice also other components can be identified, due for instance to the environmental background (eg 40 K), to the bremsstrahlung radiation, to the X-rays characteristic of the material surrounding the detector and a peak of approximately 250 keV due to the back scattering of the incoming photons. HPGe detector used with the GASP array Slide 32 In-beam -ray spectroscopy Our goal is to extract new valuable information on the nuclear structure through the -rays emitted following nuclear reactions Problems: complex spectra! Many lines lie close in energy and the interesting channels are typically the weak ones... 152 Dy What we want What we get Slide 33 The nucleus is always full of surprises Instrumentation advances New Science Slide 34 New tools for new phenomena From Sodium Iodide to Germanium detectors 104 Ag and 106 Ag With high-resolution devices the nuclear world seems quite different! Continuous effort to consistently improve our detectors and their response function Slide 35 Spectroscopic history of 156 Dy The spectroscopic history of 156 Dy is a notable example of how the progress with the acceleration and detection techniques leads to better insight on the nuclear structure. 156 Dy Slide 36 Neutron rich heavy nuclei (N/Z 2) Large neutron skins (r -r 1fm) New coherent excitation modes Shell quenching 132+x Sn Nuclei at the neutron drip line (Z 25) Very large proton-neutron asymmetries Resonant excitation modes Neutron Decay Nuclear shapes Exotic shapes and isomers Coexistence and transitions Shell structure in nuclei Structure of doubly magic nuclei Changes in the (effective) interactions 48 Ni 100 Sn 78 Ni Proton drip line and N=Z nuclei Spectroscopy beyond the drip line Proton-neutron pairing Isospin symmetry Transfermium nuclei Shape coexistence Challenges in Nuclear Structure Slide 37 Neutron-rich beams Physics with radioactive beams Slide 38 Nuclei under extreme conditions E exc J N- Z N+Z Neutron-proton ratio Angular momentum (Deformation) Coupling with continuum Binding energy Angular momentum Isospin Excitation energy Slide 39 Nuclei under extreme conditions With state-of-the-art experiments we try to study nuclei under extreme conditions in order to collect valuable information to refine the theoretical description and our knowledge of the nuclear interaction Isospin: N/Z ratios much larger (or smaller) than the stability line Spin: highly rotating nuclei Temperature: hot nuclei Isospin: N/Z ratios much larger (or smaller) than the stability line Spin: highly rotating nuclei Temperature: hot nuclei The quest for the spin frontier was a major driving force in the development of modern instrumentation Slide 40 The world of High-Spin Slide 41 Producing fast rotating nuclei Following the development of accelerators for heavy ions, it became possible to populate nuclei at high spin The stability of a nucleus under rotation can be estimated using the liquid-drop model. At low frequencies the shape of a rotating drop is oblate, at higher frequency it becomes prolate (Jacobi transition) Slide 42 Quantum rotors Only a non-spherical quantum object can rotate (around an axis different than the symmetry axis) Here J is the moment of inertia. In case of axially symmetric rotors I can only assume even values because of the symmetry for reflection. Regularly spaced transitions! Slide 43 Moments of inertia Defining the rotation frequency as: There are two possible expressions for the moment of inertia: Kinematical: requires knowledge of spin Dynamical: requires knowledge of spin differences Slide 44 Moments of inertia The two definitions of moment of inertia coincide only for rigid rotors (energy strictly proportional to I 2 ) Can we really consider the nucleus a good rigid rotor? Slide 45 Alignments Rather than rotating the nucleus as a whole, another possibility to generate angular momentum is to align the orbital angular momenta of individual nucleons in high-l orbitals (rotation around a symmetry axis) This process can continue up to the full alignment of all nucleons, producing a band terminating state Slide 46 Backbending When the rotational frequency is high enough, a pair of nucleons can break (align) with a sudden change in the moment of intertia. The regular sequence of transitions is interrupted. This is known as backbending from the characteristic behaviour of the moment of inertia vs rotational frequency plot. Slide 47 Deformation How deformed can a nucleus be? Increasing the deformation costs energy and eventually the Coulomb repulsion will cause the nucleus to fission. In particular cases, due to shell effects, a second minimum at large deformation develops, in which (weak) rotational structures can be built How can we enhance these structures experimentally? Slide 48 Ridges Using at least two detectors in coincidence, weak rotational structures can be enhanced by looking sideways at the coincidence matrix, which should result in a ridge structure. Slide 49 Superdeformed bands With the appropriate tools, it was possible to observe discrete transitions from long (20 transitions or more) and weak (populated with a cross section lower than 10 -4 of the total cross section) rotational structures. These bands were called superdeformed because the deformation corresponded to extremely elongated shapes (2:1 ratio in the ellipsoid axes) Slide 50 Linking transitions In most cases, the SD bandhead is floating and only J (2) can be determined. Only in a few cases the linking transition(s) between normal deformed and superdeformed states could be observed. These cases are obviously very important since a direct comparison between theory and experiment is possible. Slide 51 Observing discrete SD structures In our quest to observe discrete superdeformed (or hyperdeformed) structures, the experimental problem is the following: put into evidence long sequences of -rays, produced at a level 10 -4 of the total cross section, in a limited amount of time Slide 52 Channel selection In order to select a specific reaction channel, multiple coincidences can be used Each nucleus has characteristic sequences of -rays More nuclei can have transitions with similar energies but The probability of having (long) sequences of gammas with similar energies in different nuclei is much lower! Detecting more gammas in coincidence and putting conditions on the detected energies, only the channel of interest will survive (ideally, of course!) Slide 53 Doppler broadening In most cases of interest, the nuclei we are studying emit the radiation in flight at relativistic velocities. The detected energies will therefore be different than the intrinsic energies because of the Doppler effect. The Doppler shift of the photon energies cannot be fully recovered because of the finite solid angle covered by the detector(s), translating into an indetermination on the direction of the photons The final effect is a broadening of the Doppler-corrected line, thus affecting the performance of the detector(s) It should be noticed that in some cases the effect is relatively small (for instance, fusion-evaporation between medium-mass nuclei with emission of neutrons), while in other cases, such as multinucleon transfer, due to the characteristics of the reaction mechanism, the effect is not negligible. Additional detectors to recover the intrinsic energy resolution can be very useful Slide 54 Efficiency vs. Resolution With a source at rest, the intrinsic resolution of the detector can be reached; efficiency decreases with the increasing detector-source distance. With a moving source, due to the Doppler effect, also the effective energy resolution depends on the detector-source distance Small d Large d Large Small High Low Poor FWHM Good FWHM Slide 55 Efficiency vs. Granularity Photopeak efficiency is essential to collect enough statistics (in a limited amount of time) Granularity is also essential to avoid summing effects and to obtain the required selectivity through high-fold coincidences Basic idea: the regular pattern survives the coincidence, the (uncorrelated) background is smeared everywhere Slide 56 Compton suppression The cross section for Compton scattering in germanium implies quite a large continuous background in the resulting spectra Concept of anti-Compton shield to reduce such background and increase the P/T ratio P/T~30% P/T~50% Slide 57 Resolving power We need objective criteria to compare the performance of different arrangements of detectors! Concept of Resolving Power Slide 58 Resolving power The performance of our array will depend: On the effective energy resolution (narrower peaks will stand out on the background) On the P/T ratio (only peaks are interesting) On the photopeak efficiency (we dont have infinite time at our disposal to perform our measurements) Slide 59 Resolving power Defining the resolving power as: Where SE is the separation of lines in the rotational structure and E is the effective energy resolution, the peak-to-background ratio, observing at fold F, will be: Premium in improving the overall response function (large P/T, good energy resolution) Slide 60 Efficiency Efficiency does not influence the way peaks stand out of the background. On the other hand, fixing the collection time, the area of peaks will depend on the efficiency: Premium in improving the overall photopeak efficiency Slide 61 Operating point The fraction of the total cross section that can be effectively detected in a fixed time depends both on the resolving power and on the detection efficiency: The intersection of the two curves fixes the optimal operating point for the array Slide 62 Array Sensitivity and Resolving power Resolving power R: Sensitivity : Capability to resolve a -ray Minimum fraction of TOTAL observable With ancillary devices: Slide 63 Ancillary devices A very effective way of improving the N P /N B ratio without increasing the observational fold is to reduce the value of N B through auxiliary (ancillary) devices. These detector could also improve the resolving power by improving the effective energy resolution.