The (3He, α) reaction on Yb isotopes with possible observations of indirect excitations

Angular distributions of (3He, α) reactions have been measured with 28 MeV 3He on targets of 172Yb, 174Yb and 176Yb. The alpha spectra were analysed with a magnetic spectrograph and levels up to excitation energies of 2 MeV were studied with a resolution (FWHM) of 25 keV. Distorted wave Born approximation (DWBA) calculations were carried out using optical-model parameters which had been found suitable in the lead region. The angular distributions obtained from these calculations were consistent with those measured for strongly populated states and the DWBA results have been used to extract relative spectroscopic factors for a number of levels. The transitions with l ≈ 6 yielded the largest cross sections so a comparison of the present results with earlier (d, t) data resulted in the identification of a number of new high spin states. Tentative assignments of the 52− [523] and 52+ [642] Nilsson orbitals have been made in each of the three final nuclei, although in most cases there is evidence for strong Coriolis mixing of the j = 132 states. There may also be fragmentation of the singleparticle states, as predicted by Soloviev and his coworkers. In each of the final nuclei studied, a state was observed at the expected position of the j = 172 member of the 72+ [633] band. The data indicate that this state is not populated by a direct l = 8 neutron transfer. Further deviations from the simple direct reaction process are found in a comparison of (d, t) and (3He, α) cross sections for the 52 and 72 members of the 52− [512] band in 173Yb.