1. Review of PHYSICAL REVIEW C 70, 024301 (2004) Stability of the N=50 shell gap in the neutron-rich Rb, Br, Se and Ge isotones Y. H. Zhang, LNL, Italy David Scraggs

2. Overview  Motivation  Background  Experimental Details  Experimental Results  Shell-Model Calculations  Summary

3. Motivation  Populate low and medium spin states in N=50 neutron rich isotones  Compare experimentally observed excited states in N=50 shell gap region with shell-model calculations  Investigation of the neutron-core breaking excitations and therefore the N=50 shell gap  Explore the energy level structure

4. Background  Precise analytical form of the effective interaction between nucleons due to their substructure is not known  This fundamental goal of nuclear structure can be achieved by probing nuclei under extreme conditions  Phys. Rev. C70 explores exotic nuclei that have been produced far from stability

5. Background  Nuclei can be represented on nuclear chart  Isotones at N=50 126 82 50 28 50 82 20 8 2 2 8 neutron number N proton number Z N=Z nuclei

6. Background  Adding neutrons in succession creates valence neutrons  Eventually, neutron is no longer bound and neutron emission occurs  This defines the neutron drip line  Radically new features may occur in these highly exotic nuclei

7. Background  Wave functions of valence neutrons extend a remarkable distance from nucleus centre  Halos and neutron skins can be formed  Energy levels shift and re-order as the limits of stability are reached and possibly the breakdown of shell gaps  Cornerstone of NS for 50 years

8. Background  Shell gaps can be understood from the shell model  Considers a potential well with a series of energy levels  Neutrons and protons accommodated according to the Pauli exclusion principle  This leads to completely filled shells (closed shells) and magic numbers!  N=50 is a magic number

9. Background  Large energy gap at N=50, hence stable nucleus N=50

10. Background  N=20 and N=28 have exhibited properties inconsistent with shell closure  N=20 shell gap disappearance also predicted by Hartree-Fock calculations  Predictions for N=50 come to differing conclusions! Hence experiment

11. Experimental Details  87 Rb, 85 Br, 84 Se and 82 Ge excited states populated using (450MeV) heavy-ion multi- nucleon transfer reactions  Expected distribution of neutron rich products  Products stopped in target  Emitted photons detected with GASP spectrometer

12. Experimental Details  GASP – 4 spectrometer consisting of 40 Compton- suppressed, large- volume Ge detectors and an inner BGO ball  Experiment ran for six days

13. Experimental Details  Minimum of 3 Ge and 2 BGO fired in coincidence  Both products detected!  rays assigned to nuclide by gating on previously known rays in conicidence  Spatial distribution of photons determine the parity of emitting level (use ADO)  Spectroscopic data summarises nuclei

14. Experimental Results  Analysis of single- and double-gated spectra identified new -rays from the isotones  The energy levels were populated and compared with the shell model  Results for isotones follow

15. Rubidium - 87  Previously – Highest excited state was I  =9/2 + at 1578keV  Only two rays (1175.3)(402.6)  Level scheme extended by coincidence relationship between these rays  Extended up to 6.8MeV

16. Rubidium - 87 Previously known

17. Rubidium - 87 Too weakly populated

18. Bromine - 85  Level scheme extended up to 4.343MeV  Seven -rays added  Ordered according to relative intensities

19. Selenium - 84  Excited states in band other than the yrast band  Notice the spin and parity of the ground state  This is an even- even nuclei  Low intensities for 704 and 1249keV

20. Germanium - 82 Low intensity and no ADO.

21. Shell-Model Calculations  Two shell-model calculations using RITSSCHIL  SM2 Allows particle-hole excitations across the N=50 neutron core  SM1 Does not  Results are similar for all isotones

22. Shell-Model Calculations – 87 Rb  Up to 17/2 + (4.1MeV) there is good agreement with SM1 and SM2  Then SM2 has good agreement indicating importance of particle-excitations across N=50 neutron core

23. Summary  For all isotones explored it is necessary to introduced particle-hole excitations across the N=50 gap  The size of the gap has been kept constant in calculations  Shell-model predictions reproduce observed spectra  Therefore, moving away from stability down to Z=32 the N=50 shell gap remains stable  Persistence of closed shells (or not?)