Recently measured B(E2;$0_1^{+}$→$2_1^{+}$) values for the light xenon (Z=54) isotopes show a marked increase in deformation as the neutron numbers approach the midshell value of N=66. At first sight, this behavior is anomalous because the $2_1^{+}$ level energies are nearly the same for these isotopes. Moreover, this increase is not readily explained by several nuclear models that assign single shells to valence protons and neutrons. In particular, the single-shell asymptotic Nilsson model with current parameters seriously underpredicts the B(E2;$0_1^{+}$→$2_1^{+}$) values for ${}_{}{}^{118}{}_{}{}^{}\mathrm{Xe}_{64}^{}{}_{}{}^{}$, ${}_{}{}^{120}{}_{}{}^{}\mathrm{Xe}_{66}^{}{}_{}{}^{}$, and ${}_{}{}^{122}{}_{}{}^{}\mathrm{Xe}_{68}^{}{}_{}{}^{}$. On the other hand, several modern multishell models correctly predict these values. We examine the latter results more closely to find ways in which the single-shell asymptotic Nilsson model can be revised to correctly reproduce the measurements. We also show that the B(E2;$0_1^{+}$→$2_1^{+}$) values for lighter (N<66) barium isotopes, when they are measured, will test the predictive power of existing systematics and modeling of quadrupole deformations in nuclei.

• Received 6 April 1995

DOI:https://doi.org/10.1103/PhysRevC.52.1380