Proton Inelastic Scattering onNi56in Inverse Kinematics

G. Kraus, P. Egelhof, C. Fischer, H. Geissel, A. Himmler, F. Nickel, G. Münzenberg, W. Schwab, A. Weiss, J. Friese, A. Gillitzer, H. J. Körner (+8 others)
1994 Physical Review Letters  
Inelastic proton scattering to the first excited 2+ state of the doubly magic ' Ni nucleus was investigated in inverse kinematics, using a secondary beam of radioactive Ni nuclei. At an incident energy of 101 MeV/nucleon, a value B(E2, 0+ 2+) = 600~120 e~fm4 was measured. This result completes the set of experimental data for the first excited 2+ states in the 1f 2p shell w-ith a closed shell of neutrons or protons. These data are compared to recent shell-model calculations. PACS numbers:
more » ... PACS numbers: 25.40.Ep, 23.20.Js, 25.60.+v, 27.40.+z The availability of radioactive beams of good quality opens a new regime for nuclear structure studies through the use of direct reactions in inverse kinematics. In particular, nuclei become accessible to experiments in the region of shell closures away from stability (for example 6Ni, '3 Sn), nuclei that are often important in astrophysical scenarios for element synthesis. Ni is a doubly closed-shell nucleus, well known for its major role in stellar nucleosynthesis: For instance the decay in the light curve of supernova 1987A was dominated over the first year by the mass 56 decay curve, and the solar systems abundance of 56Fe is a result of the stability of Ni. Nucleon stripping and pickup reactions and inelastic scattering can provide valuable information on the single-particle states that are the foundation of our understanding of nuclear structure, on the matrix elements of the effective nucleon-nucleon interaction, on collective excitations, and on neutron-capture cross sections of astrophysical interest in regions that so far have been inaccessible. As a first such experiment with a medium-heavy nucleus, elastic and inelastic proton scattering on Ni have been investigated in inverse kinematics at E],b = 101 Me V/nucleon. The experiment, performed at the SIS accelerator at GSI, Darmstadt, was intended to provide a more accurate value of the poorly known matrix element [1]for the transition connecting the 0+ ground state to the first excited 2+ state at 2.7 MeV in Ni. A successful measurement would complete the data set of first excited 2+ excitations for N and/or Z = 28 nuclei in the 1 f 2pshell and provide a sensitive test of recent shell-model calculations. The measurement focused on the inelastic cross section in the vicinity of the first diffraction maximum in the angular distribution. It occurs in the favorable region of the kinematics of the inverse reaction where the largest center-of-mass angular range is covered in a limited range of laboratory angles, an interval of about 2 around H~, b --79. 1' [ Fig. 1(a) ]. Because of the kinematics and the large level spacing in Ni, the measurement can provide unambiguous excitation spectra, even with a spread in beam energy of several percent. This favorable situation allowed us to perform this experiment with a relatively low-quality secondary beam (b, E),b = 2%, a = 20vr mmmrad), limited by the fragmentation process. The 56Ni beam was obtained via fragmentation of a primary beam of Ni, 2 X 10 /s at 350 MeV/nucleon, incident on a 4 g/cmz beryllium target. The isotopes among the fragmentation products were separated in fIight by the GSI Fragment Separator (FRS) [2] . The FRS is a large (= 70 m length) zero-degree magnetic spectrometer operated in the achromatic mode. It combines magnetic rigidity and energy-loss separation; for the latter a profiled aluminum degrader was used with a total thickness of about half the range of the fragments. It provides full
doi:10.1103/physrevlett.73.1773 pmid:10056883 fatcat:swjnsefi3fh7fdbhprgi3gl7t4