High-spin $\mu$s isomeric states in ${}^{96}$Ag

The isomeric and $\beta$ decays of the $N=Z+2$ nucleus ${}^{96}$Ag were investigated at the National Superconducting Cyclotron Laboratory. A cascade of $\gamma$-ray transitions originating from the deexcitation of a microsecond isomer was observed for the first time and was found in coincidence with two previously known transitions with energies of 470 and 667 keV. The isomeric half-life was determined as 1.45(7) $\mu$s, more precise than previously reported. The existence of a second, longer-lived microsecond isomer, associated with a 743-keV $\gamma$ transition, is also proposed here. Shell model results within the $\left(p_{3/2}{p}_{1/2}{f}_{5/2}{g}_{9/2}\right)$ model space, using the jj44b interaction, reproduced level energies and isomeric decay half-lives reasonably well.

Figure 1

$\gamma$-ray spectrum collected between 1.2 and 12 $\mu$s after an ${}^{96}$Ag implantation. Transitions belonging to the isomeric decay of ${}^{96}$Ag are labeled by their energies in keV. Lines marked with an asterisk are known background from neighboring ${}^{94}$Pd. Summing peaks are marked by the letter S.

Figure 2

(a) Decay curve obtained by gating on the 257-, 630-, 667-, and 1506-keV $\gamma$-ray transitions. The curve was fitted with a single exponential and constant background. (b) Decay curve obtained by gating on the 743-keV $\gamma$-ray transition, fitted with a single exponential decay and a time-dependent background.

Figure 3

Background-corrected fragment-$\gamma \gamma$ coincidence spectra obtained by applying gates on the (a) 257-keV, (b) 470-keV, (c) 630-keV, (d) 667-keV, (e) 743-keV, (f) 1249-keV, (g) 1506-keV, and (h) 1719-keV $\gamma$ rays. These coincidences were used to deduce the ${}^{96}$Ag level scheme.

Figure 4

(a) Proposed level scheme of ${}^{96}$Ag, compared to the results of shell-model calculations made with the SLGT interaction (b) and the jj44b interaction (c). Tentative spins and parities were adopted from the results of the jj44b calculation. Deduced experimental level energies (indicated in keV) are relative to the 8${}^{+}$ state. The excitation energy of the 8${}^{+}$ state relative to the ground state of ${}^{96}$Ag is not known experimentally. The newly identified 15${}^{+}$ isomeric level is not drawn in the experimental level scheme. See text for details.

Figure 5

$\beta$-decay curve of the ${}^{96}$Ag implants that were correlated with the 470-keV line. The resulting half-life of 4.74(95) s agrees with the known half-life of the $\beta$-decaying state in ${}^{96}$Ag with tentative spin and parity 8${}^{+}$.