Identification of excited states and collectivity in ${}^{88}\mathrm{Se}$

The $\gamma$ decays of excited states in the very neutron-rich nucleus ${}^{88}\mathrm{Se}$ have been observed following the neutron-induced fission of ${}^{235}\mathrm{U}$. The measurement was performed using the EXILL array of Ge detectors at the PF1B cold-neutron beam facility of the Institut Laue-Langevin, Grenoble. The level scheme of ${}^{88}\mathrm{Se}$ was established using $\gamma \text{−}\gamma \text{−}\gamma$ coincidences. A low $\left(2_1^{+}\right)$ energy hints at the onset of quadrupole deformation, and the identification of possible members of a $\left(2_2^{+}\right)$ band provide evidence for $\gamma$ vibrations. Shell-model calculations using a ${}^{78}\mathrm{Ni}$ core reproduce the decay scheme well. Equivalent deformation and $B\left(E2\right)$ values have previously been predicted for ${}^{88}\mathrm{Se}$ using pseudo-SU(3), shell-model, and beyond mean-field frameworks implying that interactions between particles in the $\pi f_{5/2}p$ and $\nu sd$ orbits are mostly responsible for the collectivity present.

Figure 1

Average mass of Ce partner nuclei measured with $\gamma$-ray gates set on different Se fission fragments. The gates used are labeled in blue.

Figure 2

Two $\gamma$-ray coincidence spectra obtained by double gating on the 258.2- and 502.8-keV $\gamma$-ray transitions in ${}^{146}\mathrm{Ce}$: showing a range (a) between 50 and 600 keV and (b) between 600 and 1250 keV. Contaminant lines are labeled with c.

Figure 3

Coincidence $\gamma$-ray spectrum obtained by double gating on the 589.4- and 961.9-keV lines. Contaminant lines are labeled with c.

Figure 4

Coincidence spectrum created by double gating on the new 1242.5- and 556.1-keV transitions.

Figure 5

Level scheme of ${}^{88}\mathrm{Se}$ as obtained in the present Rapid Communication. The relative $\gamma$-ray intensities are given in square brackets.

Figure 6

(a) Experimental energies of the $\left(2_1^{+}\right)$ states, (b) ratios of the level energies $E\left(4_1^{+}\right)/E\left(2_1^{+}\right)$, and (c) $E\left(2_2^{+}\right)/E\left(2_1^{+}\right)$ of $50\le N\le 58$ Ge, Se, Kr, and Sr isotopes. Shell-model calculated values for ${}^{88}\mathrm{Se}$ are highlighted by a + symbol.

Figure 7

Calculated and experimental energies of excited states in ${}^{88}\mathrm{Se}$. Linewidths represent transition intensities, including any $M1$ and $E2$ components. $B\left(E2\right)$ values in W.u. are labeled on the arrows. See the text for more details.