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 $\text{11}\text{2}$^? 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 $\text{1}\text{2}$⁺ levels at $\text{≧ 1}\text{MeV}$ of excitation energy in each case. Of particular interest is a $\text{7}\text{2}$^? state located ≦ 50 keV above the lowest $\text{11}\text{2}$^? state in each nuclide. The relatively strong populations of these $\text{7}\text{2}$^? levels in the present experiments are contrary to expectations based on the simple shell model as there are no f_{$\text{7}\text{2}$} states in the 50 < Z < 82 shell.