Half-life of the $15/2^{+}$ state of ${}^{135}\mathrm{I}$: A test of $E2$ seniority relations

The half-life of the $15/2_1^{+}$ state of the 3-valence-proton nucleus ${}^{135}\mathrm{I}$ has been measured to be 1.74(8) ns using the EXILL-FATIMA mixed array of Ge and ${\mathrm{LaBr}}_3$ detectors. The nuclei were produced following the cold neutron-induced fission of a ${}^{235}\mathrm{U}$ target at the PF1B beam line of the Institut Laue-Langevin. The extracted $B(E2;15/2^{+}\to 11/2^{+})$ value enabled a test of seniority relations for the first time between $E2$ transition rates. Large-scale shell-model calculations were performed for ${}^{134}\mathrm{Te}$ and ${}^{135}\mathrm{I}$, and reinterpreted in a single-orbit approach. The results show that the two-body component of the $E2$ operator can be large whereas energy shifts due to the three-body component of the effective interaction are small.

Figure 1

(a) A Ge-$\gamma \left(\mathrm{Ge}\right)\text{−}\gamma \left({\mathrm{LaBr}}_3\right)$ coincidence spectrum gated on the 572.4- and 1133.4-keV transitions of ${}^{135}\mathrm{I}$. (b) A ${\mathrm{LaBr}}_3\text{−}\gamma \left(\mathrm{Ge}\right)\text{−}\gamma \left({\mathrm{LaBr}}_3\right)$ coincidence spectrum gated on the 1133.4- and 572.4-keV $\gamma$ decays. Transitions from the most strongly produced fission-fragment partner, ${}^{99}\mathrm{Y}$, are indicated in red.

Figure 2

Background-subtracted time spectra obtained from the ${\mathrm{LaBr}}_3$ detectors when gated on the 1133.4- (Ge), 288.1- $\left({\mathrm{LaBr}}_3\right)$, and 572.4-keV $\left({\mathrm{LaBr}}_3\right)$ transitions.

Figure 3

Partial experimental and calculated decay schemes of ${}^{134}\mathrm{Te}$ and ${}^{135}\mathrm{I}$. The theoretical values for ${}^{135}\mathrm{I}$ are calculated from ${}^{134}\mathrm{Te}$, as explained in the text. $B\left(E2\right)$ values (in W.u.) are labeled on the arrows, where known or calculated.