Spectroscopy and lifetime measurements in ${}^{66}\mathrm{Ge},{}^{69}\mathrm{Se}$, and ${}^{65}\mathrm{Ga}$ using fragmentation reactions

Lifetimes of low-lying excited states have been measured in ${}^{66}\mathrm{Ge},{}^{69}\mathrm{Se}$, and ${}^{65}\mathrm{Ga}$ using a $\gamma$-ray lineshape method. The results confirm the previously reported $7_1^{-}$ state lifetime in ${}^{66}\mathrm{Ge}$. The lifetime of the yrast $5/2^{-}$ state in ${}^{65}\mathrm{Ga}$ is measured for the first time. Lifetime measurements of two excited $3/2^{-}$ states in ${}^{69}\mathrm{Se}$ are also reported. Two previously unobserved $\gamma$ rays have been identified in ${}^{69}\mathrm{Se}$. $\gamma -\gamma$ coincidence measurements have been used to place one of these in the level scheme. ${}^{69}\mathrm{Se}$ excited state populations are compared to shell-model calculations using the GXPF1A interaction in the $\mathit{fp}$ model space. Theoretical spectroscopic factors to excited states in ${}^{69}\mathrm{Se}$ have identified three candidate levels for the origin of one of the new transitions.

Figure 1

(a) $\gamma$ spectrum from gating data on ${}^{69}\mathrm{Se}$ recoils for both SeGA angles. The peak labeled with a * is a radiative electron capture x ray. The peak marked with a # is a contaminant peak from excitations in ${}^{27}\mathrm{Al}$ present in the target housing. (Inset) Target-only $\gamma$ spectrum gated on the 161-keV transition. The 500-keV $\gamma$ ray is observed to come in coincidence with the 129-keV and 161-keV transitions in ${}^{69}\mathrm{Se}$. (b) $\gamma$ spectrum from gating data on ${}^{65}\mathrm{Ga}$ recoils created via 3p2n removal from the ${}^{70}\mathrm{Se}$ secondary beam (red) and via 2p2n removal from the ${}^{69}\mathrm{As}$ secondary beam (blue). The observed transitions in both nuclei are labeled.

Figure 2

Partial level scheme for ${}^{69}\mathrm{Se}$ identifying the levels populated in this work. The arrow thicknesses correspond to the relative intensities of the transitions (the 129-keV transition is set to 100), which are quantified by the values in brackets. The newly assigned state is shown at 790 keV.

Figure 3

Partial level scheme for ${}^{65}\mathrm{Ga}$ identifying the levels populated in this work. The arrow thicknesses correspond to the relative intensities of the transitions (the 191-keV transition is set to 100), which are quantified by the values in brackets (taken from weighted average contributions of ${}^{65}\mathrm{Ga}$ excited via two reaction mechanisms; see text). Levels with tentatively assigned spin parities in the literature are shown in parentheses and with a dashed level.

Figure 4

Doppler-shift corrected $\gamma$-ray spectra obtained for ${}^{69}\mathrm{Se}$ (top two panels) and ${}^{65}\mathrm{Ga}$ (bottom two panels). The simulated fits to the $\gamma$ spectra are shown (red line), along with background fits (dashed black line). Arrows denote the region of lifetime sensitivity used in the extraction of the $3/2^{-}$ state at 129 keV in ${}^{69}\mathrm{Se}$ and the $5/2^{-}$ state at 191 keV in ${}^{65}\mathrm{Ga}$. The dotted blue lines show simulations of forward-angle data for the 129-keV state in ${}^{69}\mathrm{Se}$ and the 191-keV state in ${}^{65}\mathrm{Ga}$ assuming a lifetime of 0 ps. The insets to the spectra show the ${\chi }^2$-minimization plots used to determine the lifetimes of the 129-keV state in ${}^{69}\mathrm{Se}$ and the 191-keV state in ${}^{65}\mathrm{Ga}$.