Coulomb excitation of ¹²⁴Te: Emerging collectivity and persisting seniority structure in the 6₁ ⁺ level

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 ¹²⁴Te. 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 → 4₁ ⁺ ), of 27(9) W.u. was measured for the 6⁺ ₁ → 4₁ ⁺ 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 ¹²⁴Te and its neighbors by comparison with large-basis shell-model calculations and by comparison with general collective model (GCM) fits. A GCM description of ¹²⁰Te agrees with experimental E2 transition strengths, but no comparable description of ¹²⁴Te is possible with the GCM. In contrast, there is remarkably good agreement between the B(E2; 6⁺ ₁ → 4₁ ⁺ ) values and shell-model calculations for ^124–134Te. It appears that, despite approaching midshell, ¹²⁴Te retains a seniority structure for the 6⁺ ₁ level, i.e., a significant π0g²₇/₂ contribution. This persistence of the shell structure at the 6⁺ ₁ state is in contrast to the B(E2) values of the lower-excitation 2₁ ⁺ and 4₁ ⁺ states in ¹²⁴Te, and neighboring ¹²⁰Te and ¹²²Te, for which the collectivity becomes enhanced as more neutrons are removed from N = 82.