Properties of low-lying states in ${}^{65}\mathrm{Co}$ from lifetime measurements

The low-energy structure of ${}^{65}\mathrm{Co}$ was studied by means of $\gamma$- and fast-timing spectroscopy at the ISOLDE/CERN facility. The known level scheme of ${}^{65}\mathrm{Co}$ populated following the ${\beta }^{-}$ decay of ${}^{65}\mathrm{Fe}$ was expanded. The experimental results were compared with large-scale shell-model calculations. The measured long lifetime of the $(1/2_1^{-})$ level confirms its nature as a highly collective state with proton excitations across the $Z=28$ gap and neutrons across the $N=40$ subshell.

Figure 1

HPGe energy spectrum with the ${}^{65}\mathrm{Fe}$ activity enhanced and the ${}^{65}\mathrm{Co}$ transitions identified; ${}^{65}\mathrm{Mn}$ activity and long-lived contaminants have been subtracted. ${}^{*}$, transitions observed in the ${}^{65m}\mathrm{Fe}$ ($J^{\pi }=9/2^{+}$) decay. $▾$, transitions observed in ${}^{65}\mathrm{Ni}$.

Figure 2

Half-life of the $(1/2^{-}){}^{65}\mathrm{Fe}$ ground state. The exponential fit was done after all the ${}^{65}\mathrm{Mn}$ had decayed away (between the second and third proton impacts of the cycle). See text for details.

Figure 3

HPGe-HPGe coincidence energy spectrum with a gate on the 882.8-keV transition in ${}^{65}\mathrm{Co}$.

Figure 4

${}^{65}\mathrm{Co}$ level scheme populated in the $\beta$ decay of ${}^{65}\mathrm{Fe}$ from this work. Levels and transitions populated in the ${}^{65m}\mathrm{Fe}$ decay have not been included, even when they were observed in this work.

Figure 5

HPGe (red) and ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ (black, renormalized) energy spectra with the ${}^{65}\mathrm{Fe}$ activity enhanced and the most intense ${}^{65}\mathrm{Co}$ transitions labeled in keV. It is shown with the same conditions as the fast-timing analysis was performed. ${}^{+}$, ${}^{65}\mathrm{Mn}$ decay transitions. ${}^{*}$, ${}^{66}\mathrm{Ga}$ [$T_{1/2}=9.49\left(3\right)\mathrm{h}$] decay transitions.

Figure 6

Mean lifetime of the 882.8-keV excited state. Black circle: $\beta$-340.2(HPGe)-882.8 [${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$] coincidence. Red triangle: $\beta$-882.8(HPGe)-340.2 [${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$] coincidence. The $\mathrm{\Delta }\tau =75\mathrm{ps}$ observed must then be corrected by time walk and the Compton contribution; see text for details.

Figure 7

Half-life of the 1095.6-keV state measured in $\beta \gamma \left(\mathrm{t}\right)$ coincidences using the convolution method with a gate on the 212.7-keV transition. The fit was done to a Gaussian prompt plus a double exponential decay.

Figure 8

Occupation numbers from the shell-model LNPS calculations for neutrons (left) and protons (right). The blue bars show the occupation of the $7/2^{-}$ ground state and the red bars the $1/2^{-}$ 1095.6-keV state.

Figure 9

Comparison of experimental (left) and calculated (right) level schemes. The levels in blue correspond to the spherical $\pi f_{7/2}^{-1}{\otimes }^{66}\text{Ni}\left(2_1^{+}\right)$ quintuplet, while the levels in red are the deformed $K=1/2$ band. The experimental levels not observed in this work are taken from Ref. [18].