Simultaneous investigation of the $T=1(J^{\pi }=0^{+})$ and $T=0(J^{\pi }=9^{+})$ $\beta$ decays in ${}^{70}\mathrm{Br}$

The $\beta$ decay of the odd-odd nucleus ${}^{70}\mathrm{Br}$ has been investigated with the BigRIPS and EURICA setups at the Radioactive Ion Beam Factory (RIBF) of the RIKEN Nishina Center. The $T=0\left(J^{\pi }=9^{+}\right)$ and $T=1\left(J^{\pi }=0^{+}\right)$ isomers have both been produced in in-flight fragmentation of ${}^{78}\mathrm{Kr}$ with ratios of 41.6(8)% and 58.4(8)%, respectively. A half-life of $t_{1/2}={2157}_{-49}^{+53}$ ms has been measured for the $J^{\pi }=9^{+}$ isomer from $\gamma$-ray time decay analysis. Based on this result, we provide a new value of the half-life for the $J^{\pi }=0^{+}$ ground state of ${}^{70}\mathrm{Br},t_{1/2}=78.42\pm 0.51$ ms, which is slightly more precise, and in excellent agreement, with the best measurement reported hitherto in the literature. For this decay, we provide the first estimate of the total branching fraction decaying through the $2_1^{+}$ state in the daughter nucleus ${}^{70}\mathrm{Se},R\left(2_1^{+}\right)=1.3\pm 1.1\%$. We also report four new low-intensity $\gamma$-ray transitions at 661, 1103, 1561, and 1749 keV following the $\beta$ decay of the $J^{\pi }=9^{+}$ isomer. Based on their coincidence relationships, we tentatively propose two new excited states at 3945 and 4752 keV in ${}^{70}\mathrm{Se}$ with most probable spins and parities of $J^{\pi }=\left(6^{+}\right)$ and $\left(8^{+}\right)$, respectively. The observed structure is interpreted with the help of shell-model calculations, which predict a complex interplay between oblate and prolate configurations at low excitation energies.

Figure 1

$\beta$-decay time spectra of ${}^{70}\mathrm{Br}$ including (a) ion-$\beta \left(\gamma \right)$ correlations. The time distributions of the 945-, 964-, 1034-, and 1094-keV $\gamma$ transitions, attributed to the decay of the $T=0(J^{\pi }=9^{+})$ isomer, are summed (b) ion-$\beta$ correlations with no $\gamma$-ray conditions. The activities of the ground state and $9^{+}$ isomer are indicated as dashed and dot lines, respectively, while the background is shown as a dot-dashed line. The fitting functions are indicated as continuous lines. The residual plots are presented at the bottom of each panel. The half-life and reduced ${\chi }^2$ obtained from each spectrum is shown together with the uncertainty from the fit.

Figure 2

Singles $\gamma$-ray energy spectrum in coincidence with $\beta$ particles detected within 20 s after the implantation of ${}^{70}\mathrm{Br}$ ions. Random background has been subtracted using the backward-time correlations described in the text. The $\gamma$ rays attributed to ${}^{70}\mathrm{Se}$ are labeled with their energies. In the central panel, the part of the spectrum on the right of the vertical dashed line (shown in red online) is scaled by a factor 6 to facilitate the observation of the $\gamma$ rays.

Figure 3

$\beta$-delayed $\gamma \text{−}\gamma$ coincidence spectra gated on the 958-, 1609-, 1103-, 1749-, 661-, and 1561-keV $\gamma$ rays attributed to the $\beta$ decay of the $T=0(J^{\pi }=9^{+})$ isomer of ${}^{70}\mathrm{Br}$. The parts of the spectra on the right of the vertical dashed lines (shown in red online) are scaled by the factors indicated in each line to enhance the low-intensity $\gamma$ rays. The transition marked by an asterisk does not show a mutual coincidence relationship.

Figure 4

Partial level scheme of ${}^{70}\mathrm{Se}$ attributed to the $\beta$ decay of the $T=0(J^{\pi }=9^{+})$ isomer of ${}^{70}\mathrm{Br}$. Spins and parities $J^{\pi }$ of the observed states are indicated on the left side of the levels. Widths of the arrows are proportional to absolute intensities of $\gamma$ rays. New information is indicated in gray. See text for details.

Figure 5

Measured half-lives (empty triangles) for the $T=1(J^{\pi }=0^{+})$ ground state as a function of (a) the fitting range, (b) the $\beta$ threshold, (c) the X strips, and (d) the Y strips. The half-lives shown in panels (a), (c), and (d) are obtained without imposing any specific condition on the $\beta$ threshold. Error bars indicate fit uncertainties for each data point. The overall deduced half-life is shown by a thick continuous line, while the total error is indicated as a dot-dashed line. Panels (a) and (b) include in dots the isomeric ratio of the $T=0(J^{\pi }=9^{+})$ state obtained from each lifetime fit. The corresponding scale is shown on the right axis, and the mean value as a thick dashed line. The dots in panel (b) are slightly shifted to the left to facilitate their view.

Figure 6

$\beta$-delayed $\gamma$ energy spectrum following the implantation of ${}^{70}\mathrm{Br}$ ions. Though the time interval 2–400 ms is selected to enhance the decay of the $T=1(J^{\pi }=0^{+})$ ground state, the only $\gamma$ rays observed are attributed to the decay of the $9^{+}$ isomer.

Figure 7

Experimental (exp) and theoretical (cal) low-lying level schemes of ${}^{70}\mathrm{Se}$. Shell-model calculations using the PMMU [66] (left) and JUN45 [67] (right) interactions are shown. Spectroscopic quadrupole moments are indicated in units of $\mathrm{e}{\mathrm{fm}}^2$ on the left side of the calculated states, in boldface. Arrows in the experimental level scheme indicate observed electromagnetic transitions, while in the theoretical ones they stand for states connected by large $B(E2\downarrow )$ values. Experimental [24] and theoretical $B(E2\downarrow )$ values are shown next to the corresponding transitions.