$\beta$ decay of the 21/2${\mathrm{}}^{+}$ isomer in ${}^{93}\mathrm{Mo}$ and level structure of ${}^{93}\mathrm{Nb}$

The $\gamma$ rays associated with $\beta$ decay of the 21/2${\mathrm{}}^{+}$ isomer in ${}^{93}\mathrm{Mo}$ ($E_x=2.425$ MeV, $T_{1/2}=6.85$ h) were measured with a selective sensitivity to long-lived isomer decays. A new 1262-keV transition was found in the $\gamma$-$\gamma$ coincidence measurement, and it was attributed to a transition in ${}^{93}\mathrm{Nb}$, which is the daughter nucleus of the $\beta$ decay of the ${}^{93}\mathrm{Mo}$ isomer, from the 2.753- to the 1.491-MeV levels. Accurate $\gamma$-ray intensity balances have determined the $\beta$-decay intensity from the ${}^{93}\mathrm{Mo}$ isomer to the 2.753-MeV level in ${}^{93}\mathrm{Nb}$ and placed no appreciable intensity for the previously reported $\beta$-decay branching to the 2.180-MeV level, for which a recent in-beam $\gamma$-ray experiment assigned to be $I^{\pi }$ $=$ 17/2${\mathrm{}}^{-}$. Based on the $\gamma$-ray intensities from the 2.753-MeV level, spin-parity assignment of this level was revised from 21/2${\mathrm{}}^{+}$ to 19/2${\mathrm{}}^{+}$. The observed $\beta$-decay intensity and the spin-parity assignment were explained by the $\mathit{jj}$-coupling shell model calculations.

Figure 1

Layout of the secondary beamline, the EN beamline, at RCNP [8, 9]. The dipole, quadrupole, and sextupole magnets are denoted by D, Q and SX, respectively. The reaction products from the target T are focused at the dispersive focal plane F1 and achromatically focused at F2, where a Ge array is placed.

Figure 2

Total projection spectrum of $\gamma$-$\gamma$ coincidence measurement. Energies are shown in keV units.

Figure 3

$\gamma$-ray spectra gated on (a) 950-, (b) 541-, and (c) 573-keV $\gamma$ rays in ${}^{93}\mathrm{Nb}$. Spurious peaks due to Compton cross talk between Ge detectors are marked by diamonds. The newly found 1262-keV $\gamma$ ray is seen in (a) and (b) but not in (c).

Figure 4

Decay scheme of the 21/2${\mathrm{}}^{+}$ isomer at 2.425 MeV in ${}^{93}\mathrm{Mo}$, established in the present work. Transition and level energies are in units of keV and MeV, respectively. $\gamma$-ray intensities relative to that of the 685-keV transition in ${}^{93}\mathrm{Mo}$ are shown in parentheses.

Figure 5

Levels in ${}^{93}\mathrm{Mo}$ and ${}^{93}\mathrm{Nb}$ calculated by the $\mathit{jj}$-coupling shell model. Main components of the configurations are shown.