$E5$ decay from the $J^{\pi }=11/2^{-}$ isomer in ${}^{137}\mathrm{Ba}$

A new $\gamma$-decay branch has been found from the well-known 661.659(3)-keV $J^{\pi }=11/2^{-}$, $T_{1/2}=2.552\left(1\right)$ min isomer in ${}^{137}\mathrm{Ba}$ which is populated in the $\beta$ decay of ${}^{137}\mathrm{Cs}$. The new 377.9(3)-keV $\gamma$ ray connects the isomer to the low-lying 283.5 keV, $J^{\pi }=1/2^{-}$ state. It is of near-pure $E5$ character. The decay has a $\gamma$ branching ratio (${\mathrm{Br}}_{\gamma }={\Gamma }_{\gamma }/{\Gamma }_{\mathrm{tot}}$) of $1.12\left(9\right)\times {10}^{-7}$. The new decay has a $B\left(E5\right)$ of 0.71(6) W.u. [$B\left(E5\right)\downarrow =6.5\left(6\right)\times {10}^5{e}^2{\mathrm{fm}}^{10}$], a value consistent with other “single-particle” $E5$ decays in the region. The new decay branch is of topical interest, as it competes with the much-sought “two-photon” second-order electromagnetic decay from this state.

Figure 1

Key features of the decay of ${}^{137}\mathrm{Cs}$ to states in ${}^{137}\mathrm{Ba}$. $\gamma$-ray transitions are labeled by their $\Sigma I(\gamma +ce)$ values.

Figure 2

Time-selected, time-random-subtracted, compressed $\gamma \gamma$ coincident matrix. All the events of interest lie on the sloping line with a total energy of 662 keV, though the overall topology of this matrix reveals much about interfering background processes.

Figure 3

A projection along the 662-keV ridge, with no angle selection. The total spectrum is dominated by Compton scattering between detectors, either across the array (large energy differences) or into near neighbors (small energy differences).

Figure 4

Energy difference vs opening angle matrix. The suppressed Compton events in the opening angle range from ${53}^{\circ }$ to ${130}^{\circ }$ creates an ideal region for investigating the presence of real two-photon decays.

Figure 5

As for Fig. 3, but with an opening angle selection of ${53}^{\circ }$ to ${130}^{\circ }$ between detectors. This subset of events has the lowest Compton scattering, so it is most sensitive for a search for the two-step cascade.

Figure 6

Distribution of currently known $B\left(E5\right)$ matrix elements (in log Weisskopf units). The current determination lies right in the middle of the single-particle transitions.