Experimental study of the $\beta$ decay of the very neutron-rich nucleus ${}^{85}\mathrm{Ge}$

The $\beta$-decay properties of the very neutron-rich nucleus ${}^{85}\mathrm{Ge}$, produced in the proton-induced fission of ${}^{238}\mathrm{U}$, were studied at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. The level scheme of ${}_{33}{}^{85}\mathrm{As}_{52}$ populated in ${}^{85}\mathrm{Ge}\beta \gamma$ decay was reconstructed and compared to shell-model calculations. The investigation of the systematics of low-energy levels in $N=52$ isotones together with shell-model analysis allowed us to provide an estimate of the low-energy structure of the more exotic $N=52$ isotone ${}^{81}\mathrm{Cu}$.

Figure 1

Portions of the $\beta$-gated $\gamma$-ray spectrum obtained at mass $A=85$ collected during both beam-on and beam-off periods: 0–400 keV (a), 400–800 keV (b), 800–1600 keV (c). The strongest transitions belong to the ${}^{85}\mathrm{Ge}\to {}^{85}\mathrm{As}\beta$ decay and ${}^{85}\mathrm{Ge}\to {}^{84}\mathrm{As}\beta n$ decay. The weaker transitions correspond to decays in the daughters, ${}^{85}\mathrm{As}, {}^{84}\mathrm{As}$, and ${}^{85}\mathrm{Se}$ [20]. Six lines marked with a star are unassigned.

Figure 2

The $\beta$-gated $\gamma$-ray spectra in coincidence with the 102-keV transition (a),(b), 116-keV (c), and 268-keV (d), which belong to the ${}^{85}\mathrm{Ge}\to {}^{85}\mathrm{As}\beta$ decay.

Figure 3

Partial decay scheme of ${}^{85}\mathrm{Ge}$ as obtained in this work. Dashed transitions were tentatively assigned on the basis of weak $\beta -\gamma$ coincidence. The $Q_{\beta }$ and neutron-separation $S_n$ energy were adopted from Refs. [20, 21], the half-life is taken from Ref. [8]. Listed $\beta$-feeding intensities were obtained as “apparent,” i.e., assuming no direct feeding to the ${}^{85}\mathrm{As}^{\text{g.s.}}$; compare text.

Figure 4

Experimental (a) [6, 28, 29] and shell-model (SM) predictions (b) of low-energy levels in $N=52$ isotones. The lowest-energy $5/2^{-}, 3/2^{-}$, and $1/2^{-}$ states occur within the first 200–300 keV in experimental data and calculations. In panel (a) the level scheme of ${}^{81}\mathrm{Cu}$ is predicted from systematic of heavier $N=52$ isotones. Energies are expressed in keV.