Coulomb excitation of Te 124 : Emerging collectivity and persisting seniority structure in the 6 1 + level

Le résumé fourni par la source

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 $^{124}\mathrm{Te}$. 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, $B(E2;{6}_{1}^{+}\ensuremath{\rightarrow}{4}_{1}^{+})$, of 27(9) W.u. was measured for the ${6}_{1}^{+}\ensuremath{\rightarrow}{4}_{1}^{+}$ 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 $^{124}\mathrm{Te}$ and its neighbors by comparison with large-basis shell-model calculations and by comparison with general collective model (GCM) fits. A GCM description of $^{120}\mathrm{Te}$ agrees with experimental $E2$ transition strengths, but no comparable description of $^{124}\mathrm{Te}$ is possible with the GCM. In contrast, there is remarkably good agreement between the $B(E2;{6}_{1}^{+}\ensuremath{\rightarrow}{4}_{1}^{+})$ values and shell-model calculations for $^{124\text{--}134}\mathrm{Te}$. It appears that, despite approaching midshell, $^{124}\mathrm{Te}$ retains a seniority structure for the ${6}_{1}^{+}$ level, i.e., a significant $\ensuremath{\pi}0{g}_{7/2}^{2}$ 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 $^{124}\mathrm{Te}$, and neighboring $^{120}\mathrm{Te}$ and $^{122}\mathrm{Te}$, for which the collectivity becomes enhanced as more neutrons are removed from $N=82$.

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