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Nuclear Structure far from Stability at LNL: From high intensity stable to radioactive nuclear beams
Nuclear Physics A 787 (2007) 74c–83c
Nuclear Structure far from Stability at LNL: From high intensity stable to radioactive nuclear beams Giacomo de Angelisa a
Istituto Nazionale di Fisisca Nucleare, Laboratori Nazionali di Legnaro, I 35020 Legnaro, Italy Future perspectives in nuclear structure rely on radioactive nuclear beams as well as on high intensity beams of stable ions. Deep-inelastic and multi-nucleon transfer reactions with stable beams of heavy elements can be used to populate yrast and non yrast states of neutron rich nuclei. Particularly powerful is here the combination of large acceptance spectrometers with highly segmented γ-detector arrays. Such devices, eventually complemented by large coverage particle detectors, can provide the necessary channel selectivity to identify very rare signals. An example is the CLARA γ-ray detector array coupled with the PRISMA spectrometer at the Legnaro National Laboratories (LNL). The physics aims achievable complement studies performed with current radioactive beam (RIB) facilities. With such set-up we have recently investigated the stability of the N=50 shell closure when moving towards more exotic systems. Here the comparison of the experimental data with shell model calculations seems to indicate a persistence of the N=50 shell gap down to Z=31. Future perspectives at LNL are based on an increase in intensity as well as on the availability of heavy ion species. Beams like 136 Xe or 208 Pb, provided by the new PIAVE injector, can be used to drive the multinucleon flux toward the most exotic regions. Moreover a new ISOL facility (SPES) dedicated to the production and acceleration of radioactive neutron rich species is now under development at LNL. Among the new instrumentation, the concept of γ-ray tracking has been recently introduced in nuclear spectroscopy. A new γ-ray detector array (AGATA) based on such technique is now under study in a wide european collaboration. The first sub-cluster of AGATA is foreseen to be installed at the PRISMA spectrometer. 1. Probing Nuclear Structure with Radioactive and Stable ion beams The most critical ingredient in determining the properties of a nucleus from a given effective interaction, is the overall number of nucleons and the ratio N/Z of neutrons to protons. It is the extremes in these quantities, which define the limits of existence for nuclear matter, that is going to be opened up for study with present and future radioactive ion beam accelerators. A presently available alternative approach, at least for systems not very far away from the valley of stability, is offered by high intensity beams of stable ions in combination with very efficient detectors. High intensity beams of heavy ions can be used to reach proton and neutron rich systems where new exciting phenomena are expected to happen. Of high interest is the investigation of the density dependence of 0375-9474/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2006.12.017
G. de Angelis / Nuclear Physics A 787 (2007) 74c–83c
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the effective interaction in nuclei with large proton/neutron ratio. In fact, changes of the nuclear density and size in nuclei with increasing N/Z ratios are expected to lead to different nuclear symmetries and excitations. A relevant aspect related to changes in size and diffusivity encountered in neutron rich nuclei is the modification of the average field experienced by a single nucleon [1,2] (see figure 1). This is a basic ingredient in the many-body theories used to describe nuclear properties. The experimental study of the single-particle levels with neutron excess is therefore very important, inducing changes in the standard magic numbers and, possibly, even the breakdown of shell gaps and magicity.
Figure 1. Spin-orbit splitting among single particle energies in the neutron-rich Ni isotopes calculated in the framework of the relativistic mean field theory. 2. Neutron Rich Nuclei and the N=50 shell closure Neutron-rich nuclei close to shell gaps have recently attracted a particular interest triggered by a possible existence of anomalies into the shell structure. Different from proton-rich systems, which are stabilized by the Coulomb barrier, nuclei close to the neutron drip-line are weakly bound and therefore valence neutrons can be very extended spatially. Here, new features like neutron skins or halos have been predicted and major effects are expected due to the pairing interaction and to the influence of the particle continuum. Neutron-rich nuclei around the shell-model magic numbers N=20 and 28 have exhibited properties inconsistent with shell closure [3,4]. Several experimental results have shown indications of a quenching of the N=20 shell in neutron-rich isotopes. The disappearance of the N=20 shell gap, predicted by Hartree-Fock calculations and by numerous shell-model studies [5], has, as a consequence an increased collectivity and eventually the stabilization of deformation in light mass, semi-magic nuclei. Similarly, it has been suggested, on the basis of self-consistent mean field calculations, that the major N=28 shell-gap disappears when approaching Z=16. Here the potential energy surfaces become very soft with close lying shallow minima corresponding to different deformations. Experimentally,
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G. de Angelis / Nuclear Physics A 787 (2007) 74c–83c
Figure 2. View of the CLOVER array coupled to the PRISMA spectrometer. intermediate-energy Coulomb excitation measurements of the B(E2; 0+ → 2+ ) values have shown evidence of collectivity for 44 S and 46 Ar [6,7]. Recently indication of shape coexistence has been found by Azaiez et al.[8]. A possible shrinking of the shell-closure feature has also been suggested from the comparison of the measured and calculated solar nuclear abundances for heavy elements. Network calculations for the solar isotopic abundances coming from the rapid neutron-capture processes involved in the explosive stellar nucleosynthesis reproduce the three peaks observed at A ≈80, 130 and 195 if, for very neutron-rich nuclei, the magic neutron numbers are less pronounced than assumed from nuclear structure studies [9]. The N=50 shell-gap has been predicted to be quenched already at Z ≈32 by calculations using mass predictions from the infinite nuclear matter model. Experimental evidence for setting up of collectivity was shown by Kratz et al. [10] in the N=49 80 Ga from the decay of the r-process waiting-point isotope 80 30 Zn50 . Here the gross β-decay properties as well as the quasi-particle structure have been interpreted as a clear indication of shape coexistence in 80 Ga suggesting a rather rapid weakening of the shell strength far from β-stability around 78 Ni. Predictions of various theoretical models for the N=50 isotones come to differing conclusions. Hartree-Fock-Bogoliubov (HFB) calculations based on Gogny’s two-body effective interaction [11] and shell-model calculations [12] predict a persistence of the shell closure for the N=50 nuclei close to 78 Ni. In contrast a more recent HFB calculation [13], in which pairing (with a density dependent particle-particle interaction) is treated on the same footing as particle-hole interactions, predicts a significant reduction of the shell gap. 3. Exotic nuclei populated by means of binary reactions In the last few years, the use of binary reactions, quasi-elastic (multinucleon transfer) or deep inelastic scattering, combined with modern γ−ray arrays (GASP, Gammasphere,
77c
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Euroball, etc.) with or without efficient ancillary detectors, has increased substantially the amount of information available on the structure of previously inaccessible nuclei far from stability. An example is the neutron-rich nucleus 68 Ni, where investigation of the structure have revealed the doubly-magic character of N=40 Z=28 subshell closure [14]. The neighboring 71 Cu nucleus [15] has also been investigated that way. The knowledge of the determined residual interactions has opened the way for shell model calculations for nuclei in the region around N=40 Z=28 [15].
fission events
Z=50
'E proton pick-up channels Z=34
proton stripping channels
E
Figure 3. Energy loss versus Total energy at the PRISMA focal plane for the 82 Se + 238 U reaction at 505 MeV of beam energy. Proton stripping reaction channels leading to more neutron rich nuclei are clearly visible. Deep-inelastic collisions have also been used to access different neutron rich nuclear regions at medium and high spin. The Sn isotopes with N=72,74 and 76 have been reached, allowing the identification of the 10+ isomeric states with (νh11/2 )n configuration [16]. In the region of doubly-magic 208 Pb, the two body, neutron-neutron residual interaction and the neutron single particle energies have been determined from the structure of the 210 Pb and 209 Pb nuclei [17], also populated in the afore-mentioned collisions. The information extracted on this nuclei is very important for the understanding of the states in nuclei with valence neutrons above the shell closure at N=126. 3.1. The CLARA-PRISMA detector array As already mentioned, deep-inelastic reactions are a powerful mechanism to access neutron-rich nuclei at medium and high spin. Such reaction mechanism populate ”hundreds” of nuclei close to the projectile and the target, following a N/Z ratio equilibration. The multinucleon flux moves from proton stripping and neutron pck-up to vice-versa when going from proton rich to proton deficient projectiles. Therefore using the proper beamtarget combination, one expects that the distribution of the projectile-like products will be shifted toward more neutron-rich nuclei. Due to the large amount of final products it is extremely important to provide the necessary selectivity to identify in mass and charge
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Figure 4. Mass distributions for the different elements populated in the reaction at 505 MeV of beam energy.
82
Se +
238
U
the produced systems. Such selectivity can be achieved combining highly segmented γ-ray detector arrays with large acceptance spectrometers. The detailed knowledge of the kinematical conditions of the reaction products, achievable in a spectrometer which tracks the individual trajectories, is also highly important for achieving the appropriate Doppler correction of the γ-ray spectra. Recently a new γ-detector array (CLARA) [18] dedicated to such binary reactions has started operation at Legnaro National Laboratories in combination with a magnetic spectrometer (PRISMA) [18]. PRISMA is a large acceptance magnetic spectrometer for heavy ions [19,20]. It has has been designed for the A=100-200, E = 5-10 MeV×A heavy-ion beams accelerated by the LNL accelerator complex. The optical design of PRISMA consists of a quadrupole singlet at 50 cm from the target, and a dipole placed 60 cm further away. The most interesting features are its large solid angle of 80 msr; momentum acceptance ±10%; mass resolution 1/300 via TOF; energy resolution up to 1/1000 and rotation around the target in a large angular range (-20o ≤ θ ≤130o ). The above performance is achieved by software reconstruction of the ion tracks using the position, time and energy signals from the entrance (start) and focal-plane detectors. The use of the PRISMA spectrometer coupled to an anti-Compton γ−ray detector array marks a step forward with respect to the previous spectroscopy studies with deep inelastic or multinucleon transfer reactions. The high resolving power of PRISMA gives, for most of the reaction products, the full identification of mass and
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Figure 5. Doppler corrected γ-ray spectra measured with the CLARA Ge-detector array for Z=33, 32, 31 and N=50 selected with the PRISMA Specrometer. Z. This makes available information from reaction products of very low cross section and thus allows measurements on nuclei further away from stability. The CLARA γ-detector array is based on the composite EUROBALL CLOVER detectors. They are composed of four Ge-HP crystals, each with a diameter of 50 mm, mounted in a single cryostat. In figure 2 it is shown the PRISMA spectrometer with the CLOVER array CLARA positioned at the PRISMA target position. The detector system, installed on a mobile platform, rotates together with the spectrometer, in such a way that reaction products detected in the spectrometer focal plane, in coincidence with the γ−rays, have a forward trajectory with respect to the array.
82Ge
84Se
S.M.
Exp.
Figure 6. Level scheme of the N=50 tions.
80Zn
S.M. 84
Se,
82
Exp.
S.M.
Ge nuclei compared to Shell model calcula-
4. Nuclear structure studies with the PRISMA spectrometer 82 81 87 85 Excited states of the 83 33 As, 32 Ge and 31 Ga N=50 isotones and 36 Kr and 34 Se N=51 isotones have been populated using heavy-ion multi-nucleon transfer reactions and studied through γ-ray spectroscopy in two measurements, a ”thin target” measurement performed
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using the CLARA γ-ray spectrometer at the PRISMA target position and a ”thick target” measurement [22] performed using the GASP Ge array. The combination of the Tandem-XTU and the superconducting LINAC ALPI accelerators at the LNL was used to accelerate beams of 82 Se ions at energies of 505 and 460 MeV for the two reactions. The targets, of 238 U and 192 Os respectively, isotopically enriched, were of a thickness of 300 μg/cm2 and 60 mg/cm2 . In the thin target experiments single γ-ray spectra of the CLARA array during 5 days of beam time have been collected in coincidence with the PRISMA spectrometer in order to achieve mass and charge identification (see figures 3, 4 and 5). Once identified the γ-rays of the final products, triple and higher fold γ-γ coincidences were acquired in the thick target experiment with the 4π spectrometer GASP consisting of 40 Compton-suppressed, large-volume germanium detectors and of an inner BGO ball acting as a multiplicity filter and total-energy spectrometer.
83As
81Ga
Exp.
S.M.
Exp.
S.M.
Figure 7. Level scheme of the N=50 83 As and 81 Ga compared to Shell model calculations. The spins and parities of the levels were deduced from the GASP experiment, where possible, from angular distribution ratios from oriented states (ADO) as well as from the decay branches. 5. The neutron-rich As, Ge, and Ga isotones end the N=50 shell gap 82 81 New experimental information has been obtained on the 83 33 As, 32 Ge and 31 Ga N=50 87 85 isotones (figure 6,7) and 36 Kr and 34 Se N=51 isotones (figure 8). The excited states observed in their decay have been compared with shell-model calculations allowing neutron particle-hole excitations across the neutron core. The comparison between measured and calculated excitation energies of the levels at different spin values is used here to investigate the microscopic configuration of such nuclear systems as well as to test the N=50 shell gap and its stability down to Z=31. Shell-model calculations have been performed using the Oxbash code. The shell-model space used includes the active proton orbitals π(0f5/2 , 1p3/2 , 1p1/2 , 0g9/2 ) relative to the 78 28 Ni50 core. The effective interaction in the proton shells was taken from Ref.[21]. As examples I report in figures 6 and 7 the level schemes obtained for the N=50 isotones compared with shell model calculations. The study of how their excited struc-
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Figure 8. Level scheme of the N=51 nuclei
85
Se and
87
81c
Kr.
tures evolve as a function of the proton number can be taken as a test of the persistence (or not) of the neutron shell-gap when moving away from the line of β-stability. The fact that the shell-model calculation seems to reproduce the observed spectra can be considered as evidence for an adequate description of the residual nucleon-nucleon interaction and of the single-particle energies (shell gap) thus indicating a continued stability of the N=50 shell gap in the vicinity of 78 28 Ni [22]. 6. Future perspectives at LNL High intensity beams of stable heavy ions (like Xe or Pb) or neutron rich radioactive nuclear beams offer the interesting possibility to further extend our knowledge of neutron rich nuclei. Since, as already mentioned, the multinucleon flux moves from protonstripping and neutron-pick-up to vice-versa when going from proton-rich to neutron-rich beams, heavy neutron-rich projectiles can be used to populate the most exotic final products. Figure 9 shows the production of neutron rich nuclei calculated using the program GRAZING [23]. One should notice that the relative intensity of the neutron rich target like channels, marked in black, strongly increases when using an heavy neutron-rich radioactive beam. The present accelerator complex of LNL is based on a TANDEM XTU and a new superconducting ion injector, both accelerating the beams into a superconductive LINAC. The new heavy ion injector, called PIAVE, consists of an ECR ion source coupled to two superconductive RFQs and to 8 low β resonators. It provides the optimum velocity matching to the ALPI superconductive LINAC. The new accelerator complex, when completed, will allow to boost all stable ion beams, up to uranium, to energies above the Coulomb barrier.
82c
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Figure 9. Coupled channel calculations performed with the program GRAZING for the Xe + 206 Pb reaction. 6.1. The mid-term Isol facility SPES Source TRIPS
Be converter
BNCT
U target
5 MeV
RFQ
ISCL Ion source 100 MeV
Isotope separator
Charge breeder
Charge state sep arator
SPES block diagram Exp.
ALPI
BRIC High resolution spectrometer
SRFQ
Figure 10. Schematic lay-out of the SPES RIB-ISOL facility. The use of beams of high intensity stable and unstable ions will definitely allow to push the nuclear structure studies to the most exotic systems. A new mid-term radioactive nuclear beam facility dedicated to the production of neutron rich beams is now under study at LNL (see figure 10 and Ref.[24]). It is based on an proton LINAC, the proton beam impinging on a U carbide target. Radioactive species are produced by induced fission and subsequently accelerated by means of the present accelerator complex (PIAVE-ALPI). Prototypes of the RFQs and of the accelerating cavities are presently under development at LNL. 6.2. Increasing the sensitivity by means of gamma ray tracking Gamma detector arrays of the present generation are built of Compton suppressed Ge detectors arranged in tightly packed spherical configurations. Despite the fact that these arrays are already composed of more than 100 detectors, their performance is limited to an
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efficiency of only about 10% due to the presence of the surrounding BGO shields. Recent advances in crystal segmentation technology and digital signal processing has opened the possibility to operate the detectors in a position sensitive mode. This enables to build a compact array solely out of Ge detectors omitting the BGO shields (the AGATA project). As it is expected from simulations it can have unprecedented features: an efficiency of up to 40% while maintaining a P/T-ratio of 60% . The commissioning of the first sub-array of AGATA, called the Demonstrator, is foreseen at LNL in year 2007. 7. Summary Future perspectives in nuclear structure are based on radioactive as well as on high intensity stable nuclear beams. Neutron rich nuclei far from stability can be investigated at medium spin using deep-inelastic and multinucleon transfer reactions. Here particularly powerful is the combination of large γ-ray detector array with large acceptance spectrometers like the CLARA-PRISMA detector at LNL. High intensity stable beams with masses up to U are going to be accelerated at LNL by the new positive ion injector PIAVE. Future perspectives for more exotic systems will be based on new generation radioactive ion beam facilities. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
G.A. Lalazissis et al., Phys. Lett. B 418, 7 (1998) J. Dobaczewski et al., Phys. Scr. T56, 15 (1995). N.A. Orr et al., Phys. Lett. B258, 29 (1991). O. Sorlin et al., Phys. Rev. C47, 2941 (1993). N. Fukunishi et al., Phys. Lett. B296, 279 (1992). T. Glasmacher et al., Phys. Lett. B395, 163 (1997). H. Scheit et al., Phys. Rev. Lett. 77, 3967 (1996). F. Azaiez et al., Nucl. Phys. A704, 37c (2002). B. Chen et al., Phys. Lett. B355, 37 (1995). K.L. Kratz et al., Phys. Rev. C38, 278 (1988). M. Girod et al., Phys. Rev. C37, 2600 (1988). J.M. Daugas et al., Phys. Lett. B476, 213 (2000). T.R. Werner et al., Z. Phys. A358, 169 (1997). R. Broda et al., Phys. Rev. Lett. 74, 868 (1995). I. Ishii et al., Phys. Rev. Lett. 81, 4100 (1998) and ref. therein. P. Bhattacharyya et al., Phy. Rev. C, 64 054312 (2001). M.Rejmund et al., Eur. Phys. J. A 1 (1998) 261. A. Gadea et al., Eur. Phys. J. A 20, 193 (2004) A. M. Stefanini et al., Nucl. Phys. A 701, 217c (2002). A. Latina et al., Nucl. Phys. A 734, E1 (2004) Lisetskiy et al., Phys. Rev. C 70, 044314 (2004). Y.H. Zhang et al., Phys. Rev. C. 70, 024301 (2004). A. Winter, Nucl. Phys. A594, 203 (1995) SPES Thechnical Design for an Advanced Exotic Ion Beam Facility at LNL, LNLINFN (REP) 181/02, June 2002
Nuclear Structure far from Stability at LNL: From high intensity stable to radioactive nuclear beams Giacomo de Angelisa a
Istituto Nazionale di Fisisca Nucleare, Laboratori Nazionali di Legnaro, I 35020 Legnaro, Italy Future perspectives in nuclear structure rely on radioactive nuclear beams as well as on high intensity beams of stable ions. Deep-inelastic and multi-nucleon transfer reactions with stable beams of heavy elements can be used to populate yrast and non yrast states of neutron rich nuclei. Particularly powerful is here the combination of large acceptance spectrometers with highly segmented γ-detector arrays. Such devices, eventually complemented by large coverage particle detectors, can provide the necessary channel selectivity to identify very rare signals. An example is the CLARA γ-ray detector array coupled with the PRISMA spectrometer at the Legnaro National Laboratories (LNL). The physics aims achievable complement studies performed with current radioactive beam (RIB) facilities. With such set-up we have recently investigated the stability of the N=50 shell closure when moving towards more exotic systems. Here the comparison of the experimental data with shell model calculations seems to indicate a persistence of the N=50 shell gap down to Z=31. Future perspectives at LNL are based on an increase in intensity as well as on the availability of heavy ion species. Beams like 136 Xe or 208 Pb, provided by the new PIAVE injector, can be used to drive the multinucleon flux toward the most exotic regions. Moreover a new ISOL facility (SPES) dedicated to the production and acceleration of radioactive neutron rich species is now under development at LNL. Among the new instrumentation, the concept of γ-ray tracking has been recently introduced in nuclear spectroscopy. A new γ-ray detector array (AGATA) based on such technique is now under study in a wide european collaboration. The first sub-cluster of AGATA is foreseen to be installed at the PRISMA spectrometer. 1. Probing Nuclear Structure with Radioactive and Stable ion beams The most critical ingredient in determining the properties of a nucleus from a given effective interaction, is the overall number of nucleons and the ratio N/Z of neutrons to protons. It is the extremes in these quantities, which define the limits of existence for nuclear matter, that is going to be opened up for study with present and future radioactive ion beam accelerators. A presently available alternative approach, at least for systems not very far away from the valley of stability, is offered by high intensity beams of stable ions in combination with very efficient detectors. High intensity beams of heavy ions can be used to reach proton and neutron rich systems where new exciting phenomena are expected to happen. Of high interest is the investigation of the density dependence of 0375-9474/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2006.12.017
G. de Angelis / Nuclear Physics A 787 (2007) 74c–83c
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the effective interaction in nuclei with large proton/neutron ratio. In fact, changes of the nuclear density and size in nuclei with increasing N/Z ratios are expected to lead to different nuclear symmetries and excitations. A relevant aspect related to changes in size and diffusivity encountered in neutron rich nuclei is the modification of the average field experienced by a single nucleon [1,2] (see figure 1). This is a basic ingredient in the many-body theories used to describe nuclear properties. The experimental study of the single-particle levels with neutron excess is therefore very important, inducing changes in the standard magic numbers and, possibly, even the breakdown of shell gaps and magicity.
Figure 1. Spin-orbit splitting among single particle energies in the neutron-rich Ni isotopes calculated in the framework of the relativistic mean field theory. 2. Neutron Rich Nuclei and the N=50 shell closure Neutron-rich nuclei close to shell gaps have recently attracted a particular interest triggered by a possible existence of anomalies into the shell structure. Different from proton-rich systems, which are stabilized by the Coulomb barrier, nuclei close to the neutron drip-line are weakly bound and therefore valence neutrons can be very extended spatially. Here, new features like neutron skins or halos have been predicted and major effects are expected due to the pairing interaction and to the influence of the particle continuum. Neutron-rich nuclei around the shell-model magic numbers N=20 and 28 have exhibited properties inconsistent with shell closure [3,4]. Several experimental results have shown indications of a quenching of the N=20 shell in neutron-rich isotopes. The disappearance of the N=20 shell gap, predicted by Hartree-Fock calculations and by numerous shell-model studies [5], has, as a consequence an increased collectivity and eventually the stabilization of deformation in light mass, semi-magic nuclei. Similarly, it has been suggested, on the basis of self-consistent mean field calculations, that the major N=28 shell-gap disappears when approaching Z=16. Here the potential energy surfaces become very soft with close lying shallow minima corresponding to different deformations. Experimentally,
76c
G. de Angelis / Nuclear Physics A 787 (2007) 74c–83c
Figure 2. View of the CLOVER array coupled to the PRISMA spectrometer. intermediate-energy Coulomb excitation measurements of the B(E2; 0+ → 2+ ) values have shown evidence of collectivity for 44 S and 46 Ar [6,7]. Recently indication of shape coexistence has been found by Azaiez et al.[8]. A possible shrinking of the shell-closure feature has also been suggested from the comparison of the measured and calculated solar nuclear abundances for heavy elements. Network calculations for the solar isotopic abundances coming from the rapid neutron-capture processes involved in the explosive stellar nucleosynthesis reproduce the three peaks observed at A ≈80, 130 and 195 if, for very neutron-rich nuclei, the magic neutron numbers are less pronounced than assumed from nuclear structure studies [9]. The N=50 shell-gap has been predicted to be quenched already at Z ≈32 by calculations using mass predictions from the infinite nuclear matter model. Experimental evidence for setting up of collectivity was shown by Kratz et al. [10] in the N=49 80 Ga from the decay of the r-process waiting-point isotope 80 30 Zn50 . Here the gross β-decay properties as well as the quasi-particle structure have been interpreted as a clear indication of shape coexistence in 80 Ga suggesting a rather rapid weakening of the shell strength far from β-stability around 78 Ni. Predictions of various theoretical models for the N=50 isotones come to differing conclusions. Hartree-Fock-Bogoliubov (HFB) calculations based on Gogny’s two-body effective interaction [11] and shell-model calculations [12] predict a persistence of the shell closure for the N=50 nuclei close to 78 Ni. In contrast a more recent HFB calculation [13], in which pairing (with a density dependent particle-particle interaction) is treated on the same footing as particle-hole interactions, predicts a significant reduction of the shell gap. 3. Exotic nuclei populated by means of binary reactions In the last few years, the use of binary reactions, quasi-elastic (multinucleon transfer) or deep inelastic scattering, combined with modern γ−ray arrays (GASP, Gammasphere,
77c
G. de Angelis / Nuclear Physics A 787 (2007) 74c–83c
Euroball, etc.) with or without efficient ancillary detectors, has increased substantially the amount of information available on the structure of previously inaccessible nuclei far from stability. An example is the neutron-rich nucleus 68 Ni, where investigation of the structure have revealed the doubly-magic character of N=40 Z=28 subshell closure [14]. The neighboring 71 Cu nucleus [15] has also been investigated that way. The knowledge of the determined residual interactions has opened the way for shell model calculations for nuclei in the region around N=40 Z=28 [15].
fission events
Z=50
'E proton pick-up channels Z=34
proton stripping channels
E
Figure 3. Energy loss versus Total energy at the PRISMA focal plane for the 82 Se + 238 U reaction at 505 MeV of beam energy. Proton stripping reaction channels leading to more neutron rich nuclei are clearly visible. Deep-inelastic collisions have also been used to access different neutron rich nuclear regions at medium and high spin. The Sn isotopes with N=72,74 and 76 have been reached, allowing the identification of the 10+ isomeric states with (νh11/2 )n configuration [16]. In the region of doubly-magic 208 Pb, the two body, neutron-neutron residual interaction and the neutron single particle energies have been determined from the structure of the 210 Pb and 209 Pb nuclei [17], also populated in the afore-mentioned collisions. The information extracted on this nuclei is very important for the understanding of the states in nuclei with valence neutrons above the shell closure at N=126. 3.1. The CLARA-PRISMA detector array As already mentioned, deep-inelastic reactions are a powerful mechanism to access neutron-rich nuclei at medium and high spin. Such reaction mechanism populate ”hundreds” of nuclei close to the projectile and the target, following a N/Z ratio equilibration. The multinucleon flux moves from proton stripping and neutron pck-up to vice-versa when going from proton rich to proton deficient projectiles. Therefore using the proper beamtarget combination, one expects that the distribution of the projectile-like products will be shifted toward more neutron-rich nuclei. Due to the large amount of final products it is extremely important to provide the necessary selectivity to identify in mass and charge
78c
G. de Angelis / Nuclear Physics A 787 (2007) 74c–83c
Figure 4. Mass distributions for the different elements populated in the reaction at 505 MeV of beam energy.
82
Se +
238
U
the produced systems. Such selectivity can be achieved combining highly segmented γ-ray detector arrays with large acceptance spectrometers. The detailed knowledge of the kinematical conditions of the reaction products, achievable in a spectrometer which tracks the individual trajectories, is also highly important for achieving the appropriate Doppler correction of the γ-ray spectra. Recently a new γ-detector array (CLARA) [18] dedicated to such binary reactions has started operation at Legnaro National Laboratories in combination with a magnetic spectrometer (PRISMA) [18]. PRISMA is a large acceptance magnetic spectrometer for heavy ions [19,20]. It has has been designed for the A=100-200, E = 5-10 MeV×A heavy-ion beams accelerated by the LNL accelerator complex. The optical design of PRISMA consists of a quadrupole singlet at 50 cm from the target, and a dipole placed 60 cm further away. The most interesting features are its large solid angle of 80 msr; momentum acceptance ±10%; mass resolution 1/300 via TOF; energy resolution up to 1/1000 and rotation around the target in a large angular range (-20o ≤ θ ≤130o ). The above performance is achieved by software reconstruction of the ion tracks using the position, time and energy signals from the entrance (start) and focal-plane detectors. The use of the PRISMA spectrometer coupled to an anti-Compton γ−ray detector array marks a step forward with respect to the previous spectroscopy studies with deep inelastic or multinucleon transfer reactions. The high resolving power of PRISMA gives, for most of the reaction products, the full identification of mass and
79c
G. de Angelis / Nuclear Physics A 787 (2007) 74c–83c
Figure 5. Doppler corrected γ-ray spectra measured with the CLARA Ge-detector array for Z=33, 32, 31 and N=50 selected with the PRISMA Specrometer. Z. This makes available information from reaction products of very low cross section and thus allows measurements on nuclei further away from stability. The CLARA γ-detector array is based on the composite EUROBALL CLOVER detectors. They are composed of four Ge-HP crystals, each with a diameter of 50 mm, mounted in a single cryostat. In figure 2 it is shown the PRISMA spectrometer with the CLOVER array CLARA positioned at the PRISMA target position. The detector system, installed on a mobile platform, rotates together with the spectrometer, in such a way that reaction products detected in the spectrometer focal plane, in coincidence with the γ−rays, have a forward trajectory with respect to the array.
82Ge
84Se
S.M.
Exp.
Figure 6. Level scheme of the N=50 tions.
80Zn
S.M. 84
Se,
82
Exp.
S.M.
Ge nuclei compared to Shell model calcula-
4. Nuclear structure studies with the PRISMA spectrometer 82 81 87 85 Excited states of the 83 33 As, 32 Ge and 31 Ga N=50 isotones and 36 Kr and 34 Se N=51 isotones have been populated using heavy-ion multi-nucleon transfer reactions and studied through γ-ray spectroscopy in two measurements, a ”thin target” measurement performed
80c
G. de Angelis / Nuclear Physics A 787 (2007) 74c–83c
using the CLARA γ-ray spectrometer at the PRISMA target position and a ”thick target” measurement [22] performed using the GASP Ge array. The combination of the Tandem-XTU and the superconducting LINAC ALPI accelerators at the LNL was used to accelerate beams of 82 Se ions at energies of 505 and 460 MeV for the two reactions. The targets, of 238 U and 192 Os respectively, isotopically enriched, were of a thickness of 300 μg/cm2 and 60 mg/cm2 . In the thin target experiments single γ-ray spectra of the CLARA array during 5 days of beam time have been collected in coincidence with the PRISMA spectrometer in order to achieve mass and charge identification (see figures 3, 4 and 5). Once identified the γ-rays of the final products, triple and higher fold γ-γ coincidences were acquired in the thick target experiment with the 4π spectrometer GASP consisting of 40 Compton-suppressed, large-volume germanium detectors and of an inner BGO ball acting as a multiplicity filter and total-energy spectrometer.
83As
81Ga
Exp.
S.M.
Exp.
S.M.
Figure 7. Level scheme of the N=50 83 As and 81 Ga compared to Shell model calculations. The spins and parities of the levels were deduced from the GASP experiment, where possible, from angular distribution ratios from oriented states (ADO) as well as from the decay branches. 5. The neutron-rich As, Ge, and Ga isotones end the N=50 shell gap 82 81 New experimental information has been obtained on the 83 33 As, 32 Ge and 31 Ga N=50 87 85 isotones (figure 6,7) and 36 Kr and 34 Se N=51 isotones (figure 8). The excited states observed in their decay have been compared with shell-model calculations allowing neutron particle-hole excitations across the neutron core. The comparison between measured and calculated excitation energies of the levels at different spin values is used here to investigate the microscopic configuration of such nuclear systems as well as to test the N=50 shell gap and its stability down to Z=31. Shell-model calculations have been performed using the Oxbash code. The shell-model space used includes the active proton orbitals π(0f5/2 , 1p3/2 , 1p1/2 , 0g9/2 ) relative to the 78 28 Ni50 core. The effective interaction in the proton shells was taken from Ref.[21]. As examples I report in figures 6 and 7 the level schemes obtained for the N=50 isotones compared with shell model calculations. The study of how their excited struc-
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Figure 8. Level scheme of the N=51 nuclei
85
Se and
87
81c
Kr.
tures evolve as a function of the proton number can be taken as a test of the persistence (or not) of the neutron shell-gap when moving away from the line of β-stability. The fact that the shell-model calculation seems to reproduce the observed spectra can be considered as evidence for an adequate description of the residual nucleon-nucleon interaction and of the single-particle energies (shell gap) thus indicating a continued stability of the N=50 shell gap in the vicinity of 78 28 Ni [22]. 6. Future perspectives at LNL High intensity beams of stable heavy ions (like Xe or Pb) or neutron rich radioactive nuclear beams offer the interesting possibility to further extend our knowledge of neutron rich nuclei. Since, as already mentioned, the multinucleon flux moves from protonstripping and neutron-pick-up to vice-versa when going from proton-rich to neutron-rich beams, heavy neutron-rich projectiles can be used to populate the most exotic final products. Figure 9 shows the production of neutron rich nuclei calculated using the program GRAZING [23]. One should notice that the relative intensity of the neutron rich target like channels, marked in black, strongly increases when using an heavy neutron-rich radioactive beam. The present accelerator complex of LNL is based on a TANDEM XTU and a new superconducting ion injector, both accelerating the beams into a superconductive LINAC. The new heavy ion injector, called PIAVE, consists of an ECR ion source coupled to two superconductive RFQs and to 8 low β resonators. It provides the optimum velocity matching to the ALPI superconductive LINAC. The new accelerator complex, when completed, will allow to boost all stable ion beams, up to uranium, to energies above the Coulomb barrier.
82c
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Figure 9. Coupled channel calculations performed with the program GRAZING for the Xe + 206 Pb reaction. 6.1. The mid-term Isol facility SPES Source TRIPS
Be converter
BNCT
U target
5 MeV
RFQ
ISCL Ion source 100 MeV
Isotope separator
Charge breeder
Charge state sep arator
SPES block diagram Exp.
ALPI
BRIC High resolution spectrometer
SRFQ
Figure 10. Schematic lay-out of the SPES RIB-ISOL facility. The use of beams of high intensity stable and unstable ions will definitely allow to push the nuclear structure studies to the most exotic systems. A new mid-term radioactive nuclear beam facility dedicated to the production of neutron rich beams is now under study at LNL (see figure 10 and Ref.[24]). It is based on an proton LINAC, the proton beam impinging on a U carbide target. Radioactive species are produced by induced fission and subsequently accelerated by means of the present accelerator complex (PIAVE-ALPI). Prototypes of the RFQs and of the accelerating cavities are presently under development at LNL. 6.2. Increasing the sensitivity by means of gamma ray tracking Gamma detector arrays of the present generation are built of Compton suppressed Ge detectors arranged in tightly packed spherical configurations. Despite the fact that these arrays are already composed of more than 100 detectors, their performance is limited to an
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efficiency of only about 10% due to the presence of the surrounding BGO shields. Recent advances in crystal segmentation technology and digital signal processing has opened the possibility to operate the detectors in a position sensitive mode. This enables to build a compact array solely out of Ge detectors omitting the BGO shields (the AGATA project). As it is expected from simulations it can have unprecedented features: an efficiency of up to 40% while maintaining a P/T-ratio of 60% . The commissioning of the first sub-array of AGATA, called the Demonstrator, is foreseen at LNL in year 2007. 7. Summary Future perspectives in nuclear structure are based on radioactive as well as on high intensity stable nuclear beams. Neutron rich nuclei far from stability can be investigated at medium spin using deep-inelastic and multinucleon transfer reactions. Here particularly powerful is the combination of large γ-ray detector array with large acceptance spectrometers like the CLARA-PRISMA detector at LNL. High intensity stable beams with masses up to U are going to be accelerated at LNL by the new positive ion injector PIAVE. Future perspectives for more exotic systems will be based on new generation radioactive ion beam facilities. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
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