The abrupt shape transition as is observed around $N\simeq 88-90$ in even nuclei has been examined for the odd-$A$ Eu isotopes ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}_{88}^{}{}_{}{}^{}$ and ${}_{}{}^{149}{}_{}{}^{}\mathrm{Eu}_{86}^{}{}_{}{}^{}$ by the ($p, t$) reaction at 18.5 and 19.0 MeV with an average resolution of 10 keV. Angular distributions, taken from 10 to 70°, enabled unambiguous indentification of many $L=0\mathrm{}$ (${J_f}^{\pi }=\frac{5}{2^{+}}$) transitions in both residual nuclei; seven for ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ and seven or eight for ${}_{}{}^{149}{}_{}{}^{}\mathrm{Eu}$. The summed $L=0\mathrm{}$ cross sections are close to those of the ($p, t$) reactions for the neighboring even-$A$ Sm isotopes connecting the same neutron numbers. A number of new levels have been found in both isotopes. We have obtained markedly different level structures for the two neighboring isoptopes. In ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$, whose ground state and low-lying states below 200 keV can be described in terms of spherical shell-model configurations, two deformed bands have been tentatively identified. One of these is a $\frac{5}{2^{+}}\mathrm{}\left[413\right]\mathrm{}$ band with members at 261 keV, $\frac{5}{2^{+}}$; 414 keV, ($\frac{7}{2^{+}}$); and possibly a 597 keV, ($\frac{9}{2^{+}}$) member. In addition a $\frac{5}{2^{+}}\mathrm{}\left[413\right]\mathrm{}$ $\beta$ band is postulated with members at 654 keV, $\frac{5}{2^{+}}$; 801 keV, ($\frac{7}{2^{+}}$). These deformed states, coexisting with the spherical states, have been strongly excited while the spherical low-lying states have shown vanishingly small ($p, t$) cross sections. The ($p, t$) cross sections to the ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ states were found to be in strong anticorrelation with $B\left(E2\mathrm{}\right)\mathrm{}$ values from Coulomb excitation and ($d, d^{\prime }$) cross sections leading to the same final states. On the other hand, ${}_{}{}^{149}{}_{}{}^{}\mathrm{Eu}$ has shown the usual ($p, t$) pattern of a strong ground state transition and weak excited state transitions, showing similar coupling schemes are involved in the ground states of ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ and ${}_{}{}^{149}{}_{}{}^{}\mathrm{Eu}$. The detailed experimental results up to 1.6 MeV excitation in both nuclei and their interpretation for some of the levels are presented.

NUCLEAR REACTIONS ${}_{}{}^{153,151}{}_{}{}^{}\mathrm{Eu}(p, t)$, $E=18.5\mathrm{} \mathrm{and} 19.0$ MeV; measured $\sigma (E_t, \theta )$, $Q$. ${}_{}{}^{151,149}{}_{}{}^{}\mathrm{Eu}$ deduced levels, $J$, $\pi$. Enriched targets.

• Received 17 December 1974

DOI:https://doi.org/10.1103/PhysRevC.12.108