$\beta$-decay properties of the very neutron-rich isotopes ${}^{86}\mathrm{Ge}$ and ${}^{86}\mathrm{As}$

The $\beta$-decay properties of very neutron rich nuclei ${}^{86}\mathrm{Ge}$ and ${}^{86}\mathrm{As}$ were measured at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. Spectroscopic information on new excited states in ${}^{86}\mathrm{As}$ and in ${}^{86}\mathrm{Se}$ was obtained and is interpreted within an advanced shell model approach. These calculations, previously explaining well the structure of ${}^{84}\mathrm{Ge}$ and ${}^{85}\mathrm{Ge}$, are not able to reproduce all the experimentally determined features of the measured level schemes of ${}^{86}\mathrm{As}$ and ${}^{86}\mathrm{Se}$. The Gamow-Teller decay of ${}^{86}\mathrm{Ge}$ and ${}^{86}\mathrm{As}$ is also investigated in a shell-model framework. The fission yield for ${}^{86}\mathrm{Ge}$ is discussed.

Figure 1

Schematic decay path of ${}^{86}\mathrm{Ge}$. Energies are to scale and the presented discrete levels (solid lines) are from this work. $Q_{\beta }$ values and neutron-separation energies $(S_n$, dashed lines) are taken from [4]. Neutron-unbound states are represented by the shaded regions.

Figure 2

Parts of the $\beta$-gated $\gamma$-ray spectrum of mass 86 (background subtracted). $\gamma$ lines belonging to the $\beta \gamma$ $(•)$ and $\beta n\gamma$ $(◯)$ decay branches of ${}^{86}\mathrm{Ge}$ are marked, as well as those belonging to the $\beta \gamma$ decay of ${}^{86}\mathrm{As}$ $\left(▵\right)$. Unmarked lines belong to room- and beam-induced background. See text for details.

Figure 3

Parts of the $\beta$-gated $\gamma$-ray spectrum (background subtracted) in coincidence with the 112 keV transition. See text for details.

Figure 4

Low-energy part of the $\beta$-gated $\gamma$-ray spectrum (background subtracted) in coincidence with the 125 keV transition. See text for details.

Figure 5

Schematic representation of ${}^{86}\mathrm{Ge}$ $\beta$ decay. Energies are given in keV, apparent $\beta$ feedings $\left(I_{\beta }\right)$ in %. $Q$ values and neutron-separation energy $\left(S_n\right)$ stem from [4], half-life from [2], and $P_n$ from [11]. Neutron-unbound states are represented by the shaded regions. Drawing not to scale. See text for details.

Figure 6

Schematic representation of ${}^{86}\mathrm{As}$ $\beta$ decay. The level at 1115 keV in ${}^{85}\mathrm{Se}$ and the transition de-exciting it to the g.s. are depicted as dotted line and arrow, respectively. This is due to the fact that from this data it is not possible to disentangle the contributions to the 1115 keV transition from $\beta n$ decay of ${}^{86}\mathrm{As}$ and from $\beta$ decay of ${}^{85}\mathrm{As}$ (see Fig. 1). Energies are given in keV, $I_{\beta }$ in %; $Q$ values and $S_n$ stem from [4], half-life from [2], and $P_n$ from [18]. Upper limits for $I_{\beta }$ were determined by assuming no decay to the g.s. of ${}^{86}\mathrm{Se}$ and normalizing the relative $\gamma$ intensities to the 704 keV transition. Neutron-unbound states are represented by the shaded regions. Drawing not to scale. See text for details.

Figure 7

Portions of the $\beta$-gated $\gamma$-ray spectrum (background subtracted) in coincidence with the 704 keV transition. See text for details.

Figure 8

Lowest-lying excited states in ${}^{86}\mathrm{As}$ predicted in shell-model (SM) calculations in comparison with the results of this work (EXP). The vertical (energy) axis is plotted to scale. See text and Fig. 5 for details.

Figure 9

Excited states in ${}^{86}\mathrm{Se}$ predicted in shell-model calculations in comparison with the results of this work (EXP). The vertical (energy) axis is plotted to scale. See text and Fig. 6 for details.

Figure 10

$B$(GT) strength distribution for ${}^{86}\mathrm{Ge}$. The black dots represent the apparent $B$(GT) calculated from the present data and the histogram shows the results of the calculation. The part plotted in blue is multiplied by 20. All calculations use nominal s.p.e. and use (a) interactions constructed from jj44pn; (b) interaction constructed on the basis of the jj45pn; and (c) as in (a) but with modified $T=0$ parts interaction. The horizontal-axis represents $\beta$ energies from 0 to $Q_{\beta }=9.2\left(3\right)$ MeV. The black vertical line highlights the $S_n$ value. See text for details.

Figure 11

$B$(GT) strength distribution for ${}^{86}\mathrm{As}$. The histogram shows the results of the calculation, the part plotted in blue is multiplied by 20. All calculations use nominal s.p.e. and use (a) interactions constructed from jj44pn; (b) interaction constructed on the basis of the jj45pn; and (c) as in (a) but with modified $T=0$ parts interaction. The horizontal-axis represents $\beta$ energies from 0 to $Q_{\beta }=11.541\left(4\right)$ MeV. The black vertical line highlights the $S_n$ value. See text for details.