Study of the 58Ni( τ, α) 57Ni reaction at 25 MeV

Abstract

The 58Ni( τ, α) 57Ni reaction has been studied at 25 MeV incident energy. Angular distributions have been measured from 5° to 50° with a split-pole spectrometer up to 13.5 MeV excitation energy. A local zero-range DWBA analysis has been carried out, allowing l-assignments for about eighty levels, most of them previously unknown. An isospin-dependent potential has been used in the calculation of the neutron form factor for both T <and T > states, and the C 2 S values deduced using this procedure are compared to those obtained with the usual separation energy method. Analog states of eleven 57Co levels have been identified and the eventuality for isospin mixing in 57Ni has been discussed. A sum rule analysis has been carried out and energy centroids of hole states have been determined. About 60% of the 1 d 5 2 and 2 s 1 2 T < strengths and the full 1 d 3 2 and 1 f 7 2 hole strengths are observed. It is shown that the two excess neutrons in the 58Ni ground state mainly populate the 2 p 3 2 , 1 f 5 2 and 2 p 1 2 orbitals, whereas the occupancy number of the 1 g 9 2 subshell is found to be smaller than 0.1%. Some non-pickup angular distributions have also been observed and a CRC analysis assuming two-step processes in the (τ, α) reaction and weak-coupling wave functions for final states has been attempted. Assignments of J π values are proposed for four 57Ni levels, based on this analysis.