Medium-spin states of the neutron-rich nucleus ${}^{87}\mathrm{Br}$

Medium-spin excited states of the neutron-rich nucleus ${}^{87}\mathrm{Br}$ were observed and studied for the first time. They were populated in fission of ${}^{235}\mathrm{U}$ induced by the cold-neutron beam of the PF1B facility of the Institut Laue-Langevin, Grenoble. The measurement of $\gamma$ radiation following fission was performed using the EXILL array of Ge detectors. The observed level scheme was compared with results of large valence space shell model calculations. The medium-spin level scheme consists of three bandlike structures, which can be understood as bands built on the $\pi f_{5/2}$, $\pi (p_{3/2}+f_{5/2})$, and $\pi g_{9/2}$ configurations. The behavior of the observed $\pi g_{9/2}$ band at high spins shows a considerable deviation from the shell model predictions. This deviation in this band is probably the result of an increased collectivity, which can be understood assuming that the $\pi g_{9/2}$ high-$\mathit{j}$ proton polarizes the core.

Figure 1

Double-gated $\gamma \gamma \gamma$-coincidence spectra with one gate set on the strong 167 keV ${}^{147}\mathrm{La}$ transition and the other gate set on one of the ${}^{87}\mathrm{Br}$ candidate transitions.

Figure 2

Partial level scheme of ${}^{87}\mathrm{Br}$, seen in prompt coincidence with ${}^{146}\mathrm{La}$ and ${}^{147}\mathrm{La}$. The energies are given in keV, the widths of the transitions are proportional to their relative intensities.

Figure 3

Example double-gated $\gamma \gamma \gamma$-coincidence spectra confirming the placements of the newly observed ${}^{87}\mathrm{Br}$ transitions in the level scheme.

Figure 4

Angular-correlation analysis for the 589–875 keV cascade in ${}^{87}\mathrm{Br}$. See text for more details.

Figure 5

Comparison of the observed (exp.) and calculated (SM) levels of ${}^{87}\mathrm{Br}$.

Figure 6

Comparison of Band 3 in ${}^{87}\mathrm{Br}$ with the ground-state bands of the neighboring $N=52$ even-even nuclei, ${}^{86}\mathrm{Se}$ and ${}^{88}\mathrm{Kr}$.