1. Relativistic Coulomb Excitation of Neutron-Rich 54,56,58 Cr Herbert Hübel Helmholtz-Institut für Strahlen- und Kernphysik Universität Bonn Germany

2. Participants A. Bürger, H. Hübel, A. Al-Khatib, P. Bringel, A. Neußer, A.K. Singh, D. Mehta, T.S. Reddy University of Bonn, Germany T. Saito, A. Banu, T. Beck, F. Becker, P. Bednarczyk, H. Geissel, J. Gerl, M. Gorska, H. Grawe, J. Grebosz, M. Hellström, M. Kavatsyuk, O. Kavatsyuk, Kojouharov, N. Kurz, R. Lozeva, S. Mandal, N. Saito, H. Schaffner, H. Weick, M. Winkler, H.J. Wollersheim GSI Darmstadt, Germany G. Benzoni, A. Bracco, F. Camera, B. Million, O. Wieland University of Milano, Italy E. Clement, A. GörgenG. Hammond CEA Saclay, FranceKeele University, UK P. Reiter, P. DoornenbalM. Kmiecik, A. Maj, W. Meczynski University of Köln, GermanyUniversity of Krakow, Poland S. MuralitharZ. Podolyak NSC New Delhi, IndiaUniversity of Surrey, UK C. Wheldon HMI Berlin, Germany

3. Physics Motivation Shell structure of nuclei far off stability may differ from that of nuclei near the valley of stability Shell structure is also important for astrophysics applications, e.g. for nuclear synthesis r-process abundance calculations Shell structure is related to the monopole part of the NN interaction e.g. S = 0 (spin flip),  l = 0 (spin-orbit partners), T = 0 (proton-neutron interaction): strongly binding in the two-body interaction Causes large monopole shifts at large neutron or proton excess due to missing interaction partners Effect on spin-orbit splitting T. Otsuka et al., Eur. Phys. J. A 13, 69 (2002) E. Caurier et al., Eur. Phys. J. A 15, 145 (2002) M. Honma et al., Phys. Rev. C 69, 034335 (2004) H. Grawe, Springer Lecture Notes Phys. 651, 33 (2004)

4. Neutron-rich nuclei with N = 28 to 40: p 1/2 f 5/2 p 3/2 f 7/2 T = 1 (2p 1/2 ) 2 monopole strongly binding in some interactions Modification of the spin-orbit splitting M. Honma et al., Phys. Rev. C 69, 034335 (2004) E. Caurier et al., Eur. Phys. J. A 15, 145 (2002) g 9/2 50 28 Position of p 1/2 uncertain Prediction subshell at N = 32,34 Differences between effective potentials Experimental data are needed to test the potentials used in calculations

5. Neutron-rich region around Z = 24, N = 32

6. In the Ca isotopes E(2 + ) increases at N = 32, but not in the Ni isotopes Ti and Cr isotopes also show the increase in E(2 + ), B(E2) for 54 Ti 32 low Experimental quantities sensitive to shell closure: Separation energies 2 + energies and B(E2) values

7. Experiments with FRS-RISING setup at GSI FRS = FRagment Separator RISING = Rare ISotope INvestigation at GSI GSI = Gesellschaft für SchwerIonenforschung Darmstadt, Germany

8. Layout of the FRS-RISING setup at GSI Radioactive beams produced by fragmentation and separated by FRS Primary beam: 86 Kr 480 MeV/A Production target: 8 Be 2.5 g/cm 2 Reaction target: Au 1.0 g/cm 2 54,56,58 Cr ions: 100 MeV/A SCI1 and SCI2 give TOF: v/c, MW1,2: multiwire detectors MUSIC ionization chamber gives energy loss: Z HECTOR: BaF 2 scintillation detectors, not used here 15 Ge-Cluster detectors, 7 encapsulated Ge crystals each CATE: Si-CsJ CAlorimeter TElescope for  E, E

9. RISING  -ray detectors around the Au reaction target

10. Ge-Cluster detectors Seven encapsulated Ge crystals in common vacuum Efficiency ~60 % each, hexagonal tapered

11. Ge Cluster detectors 15 Clusters arranged in two rings at 15 0 and 36 0 Absolute efficiency determined with 60 Co source: 1.15% at 1.332 MeV, with Lorentz boost 2.31% Energy dependence determined with 152 Eu source Good timing of BaF 2 detectors of HECTOR array used to identify and suppress background

12. Multiwire detectors MW1 and MW2 used for incoming beam tracking: Extrapolation to interaction point on the target Together with CATE ➔ determine scattering angle and angle of  emission 20 x 20 cm 2, Resolution: 1mm ⇒ 5mm @ target tracking: popo MW1 MW2 CATEAu target γ pipi θsθs θγθγ Multiwire extrapolation to target

13. Fragment Identification Fragment identification before Au target Z: 0.8% 56 Cr Z A/Q A/Q:1.1% (with Z gate)

14. CAlorimeter TElescope CATE ∆E 0.3 mm thick Si detectors Z identification Position sensitive E CsI detectors Mass identification 56 Cr (Coulomb excitation) 56 Cr + 197 Au ∆E∆E E Ion identification after the target

15. CATE events

16. Event-by-event Doppler correction of  -ray energies Determine v/c from TOF Tracking of incoming and outgoing Cr ions and angle of Ge crystal with respect to ion gives actual  -ray emission angle tracking: popo MW1 MW2 CATEAu target γ pipi θsθs θγθγ  -Ray Energy (keV) 30 keV 16 keV 834 Counts

17. Scattering angle of Cr ions Selection of Coulomb-excitation events Scattering angle (deg) 200 C o u n t s 0 Limit in scattering angles 0.6 o to 2.8 o corresponds to impact parameters of 40 to 10 fm, respectively

18. Details of the three experiments 54 Cr: ~4 x 10 3 particles/s, 22 h, 45% 54 Cr 56 Cr: ~1 x 10 3 particles/s, 20 h, 35% 56 Cr 58 Cr: ~3 x 10 2 particles/s, 55 h, 25% 58 Cr Trigger condition: SCI2 and one CATE CsI Time gate on prompt peak, Doppler-shift correction, gate on scattering angle, gate on incoming and outgoing Cr ions

19. Gamma-ray spectra of 54,56,58 Cr 1006 keV 58 Cr 880 keV 54 Cr 835 keV 56 Cr

20. Comparison to theory Calculations: T. Otsuka et al., Phys. Rev. Lett. 87, 082502 (2001) T. Otsuka et al., Eur. Phys. J. A 13,69 (2002) M. Honma et al., Phys. Rev. C 69, 034335 (2004) E. Caurier et al., Eur. Phys. J. A 15, 145 (2002) Experimental B(E2) value lower for 56 Cr 32 than for 54 Cr and 58 Cr Experimental 2 + energy high for 56 Cr 32 Theory does not reproduce the 56 Cr B(E2) value Similar results for 52,54,56 Ti (MSU) D.-C. Dinca et al., preprint PRELIMINARY

21. Summary 54,56,58 Cr ions produced by spallation of high-energy 86 Kr on Be and separated by FRS 54,56,58 Cr Coulomb excited on Au target at 100 MeV/A B(E2,0 + - 2 + ) determined E(2 + ) higher and B(E2) smaller for 56 Cr 32 than for neighbors (preliminary) Evidence for subshell closure at N = 32 Discrepancy to large-scale shell model calculations