Proton states in the N = 88 nuclei 149Pm, 151Eu and 153Tb populated in the (τ, d) and (α, t) reactions

The (τ, d) and (α, t) reaction on targets of 148Nd, 150Sm and 152Gd have been studied, using beams of 24 MeV 3He and 27 MeV 4He from the McMaster University FN tandem Van de Graaff accelerator. The reaction products were analyzed with a magnetic spectrograph and detected with photographic emulsions. The (α, t) spectra were measured at two angles for each target, and the (τ, d) reactions were studied at 8 or 9 angles. The l-values for a number of low-spin states were determined from the (τ, d) angular distributions, and ratios of the (α, t) and (τ, d) cross sections were used to obtain l-values for several other states. There are some striking similarities in the observed structures of the three final nuclei, 149Pm, 151Eu and 153Tb. In each case there are low-lying strongly populated 112− states and a higher lying l = 5 level somewhat below 1 MeV of excitation energy. Several states (10 in 149Pm, 17 in 151Eu and 8 in 153Tb) appear to be populated via l = 2 transitions, and there are strongly excited 12+ levels at ≧ 1 MeV of excitation energy in each case. Of particular interest is a 72− state located ≦ 50 keV above the lowest 112− state in each nuclide. The relatively strong populations of these 72− levels in the present experiments are contrary to expectations based on the simple shell model as there are no f72 states in the 50 < Z < 82 shell.