Relativistic Coulomb Excitation of Neutron-Rich 54,56,58 Cr Herbert Hübel

1. Relativistic Coulomb Excitation of Neutron-Rich 54,56,58Cr 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. Görgen G. Hammond CEA Saclay, France Keele University, UK P. Reiter, P. Doornenbal M. Kmiecik, A. Maj, W. Meczynski University of Köln, GermanyUniversity of Krakow, Poland S. Muralithar Z. 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), Dl = 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: Position of p1/2 uncertain 50 g9/2 T = 1 (2p1/2)2 monopole strongly binding in some interactions Modification of the spin-orbit splitting p1/2 f5/2 p3/2 28 Prediction subshell at N = 32,34 Differences between effective potentials Experimental data are needed to test the potentials used in calculations f7/2 M. Honma et al., Phys. Rev. C 69, 034335 (2004) E. Caurier et al., Eur. Phys. J. A 15, 145 (2002)

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

6. Experimental quantities sensitive to shell closure:Separation energies2+ energies and B(E2) values 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 54Ti32 low

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 GSIRadioactive beams produced by fragmentation and separated by FRS Primary beam: 86Kr 480 MeV/A Production target: 8Be 2.5 g/cm2 Reaction target: Au 1.0 g/cm2 54,56,58Cr ions: 100 MeV/A SCI1 and SCI2 give TOF: v/c, MW1,2: multiwire detectors MUSIC ionization chamber gives energy loss: Z HECTOR: BaF2 scintillation detectors, not used here 15 Ge-Cluster detectors, 7 encapsulated Ge crystals each CATE: Si-CsJ CAlorimeter TElescope for DE, E

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

10. Ge-Cluster detectorsSeven encapsulated Ge crystals in common vacuumEfficiency ~60 % each, hexagonal tapered

11. Ge Cluster detectors • 15 Clusters arranged in two rings at 150 and 360 • Absolute efficiency determined with 60Co source: 1.15% at 1.332 MeV, with Lorentz boost 2.31% • Energy dependence determined with 152Eu source Good timing of BaF2 detectors of HECTOR array used to identify and suppress background

12. tracking: γ θγ θs po pi MW1 MW2 Au target CATE 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 g emission Multiwire extrapolation to target 20 x 20 cm2, Resolution: 1mm ⇒ 5mm @ target

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

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

15. CATE events

16. tracking: γ θγ θs po pi MW1 MW2 Au target CATE Event-by-event Doppler correction of g-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 g-ray emission angle 30 keV Counts 16 keV 834 g-Ray Energy (keV)

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

18. Details of the three experiments • 54Cr: ~4 x 103particles/s, 22 h, 45% 54Cr • 56Cr: ~1 x 103 particles/s, 20 h, 35% 56Cr • 58Cr: ~3 x 102 particles/s, 55 h, 25% 58Cr 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 54,56,58Cr 54Cr 835 keV 56Cr 1006 keV 58Cr 880 keV

20. Comparison to theory PRELIMINARY Experimental B(E2) value lower for 56Cr32 than for 54Cr and 58Cr Experimental 2+ energy high for 56Cr32 Theory does not reproduce the 56Cr B(E2) value Similar results for 52,54,56Ti (MSU) D.-C. Dinca et al., preprint 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)

21. Summary • 54,56,58Cr ions produced by spallation of high-energy 86Kr on Be and separated by FRS • 54,56,58Cr Coulomb excited on Au target at 100 MeV/A • B(E2,0+ - 2+) determined • E(2+) higher and B(E2) smaller for 56Cr32 than for neighbors (preliminary) • Evidence for subshell closure at N = 32 • Discrepancy to large-scale shell model calculations