Coulomb excitation of $^{124}$Te: Emerging collectivity and persisting seniority structure in the $6_1^+$ level

Abstract

The low-lying energy spectra of even-even tellurium isotopes near midshell have long been interpreted as `textbook' examples of vibrational collective motion. However, in many cases electric-quadrupole observables, which are a particularly sensitive probe of collectivity, remain undetermined. Coulomb-excitation measurements were performed to measure transition strengths connecting the ground and low-excitation states in $^{124}$Te. This isotope lies at a transitional point between collective structure near the neutron midshell and seniority structures near the $N=82$ shell. A transition strength, $B(E2; 6_1^+ \to 4_1^+)$, of 27(9)~W.u. was measured for the $6^+_1\rightarrow4^+_1$ transition for the first time in this nucleus; this value is significantly below that expected for a spherical vibrator, as well as other collective models. We examine the transition strengths in $^{124}$Te and its neighbors by comparison with large-basis shell-model calculations and by comparison with General Collective Model (GCM) fits. A GCM description of $^{120}$Te agrees with experimental $E2$ transition strengths, but no comparable description of $^{124}$Te is possible with the GCM. In contrast, there is remarkably good agreement between the $B(E2; 6_1^+ \to 4_1^+)$ values and shell-model calculations for $^{124-134}$Te. It appears that, despite approaching midshell, $^{124}$Te retains a seniority structure for the $6^+_1$ level, i.e. a significant $\pi 0g_{7/2}^2$ contribution. This persistence of the shell structure at the $6^+_1$ state is in contrast to the $B(E2)$ values of the lower-excitation $2^+_1$ and $4^+_1$ states in $^{124}$Te, and neighboring $^{120}$Te and $^{122}$Te, for which the collectivity becomes enhanced as more neutrons are removed from $N=82$.