Spectroscopic study of the N = 27 nuclei by means of the (τ, α) reaction

Abstract

The (τ, α) reaction on 48Ca, 50Ti, 52Cr and 54Fe target nuclei has been studied at 25 MeV incident energy. Angular distributions have been measured from 5° to 40° with a split-pole spectrometer in a large range of excitation energy. A local zero-range DWBA analysis has been carried out, using an isospin-dependent potential for the calculation of the neutron form factor, in order to get a coherent set of spectroscopic factors for both T> and T< levels in different nuclei. Assignments of l-values have been done for a large number of levels, most of them previously unknown, and energy centroids of hole states have been determined. Spectroscopic factors in 39Ca, 47Ti, 49, 52, 53Cr have also been obtained for strongly excited states. A sum rule analysis has been carried out for the N = 27 nuclei: the 1d32 and 2s12T< hole strengths are generally fully exhausted by the observed levels, whereas only a fraction of the 1d52 strength has been evidenced. The 1f72, 1d32 and 2s12 analog states have been observed in all nuclei; in 53Fe, the 1d32 and 2s12 analog states appear to be split in several components. In addition, a CRC analysis has been carried out for some levels with angular distributions not accounted for by a direct pick-up process. These levels are tentatively identified with states resulting from the coupling of one f72 neutron hole with excited states of the target nucleus. In particular, the 52− and 92− members of the [2+ ⊗ f72−1] configuration have been identified in each final nucleus. Unambiguous Jπ assignments are made; and the two-step (τ, α) reaction therefo e appears as a useful spectroscopic tool, especially for investigating high spin states.