Lifetime measurements in neutron-rich ${}^{63,65}$Co isotopes using the AGATA demonstrator

Lifetimes of the low-lying $(11/2^{-})$ states in ${}^{63,65}$Co have been measured employing the recoil distance doppler shift method (RDDS) with the AGATA $\gamma$-ray array and the PRISMA mass spectrometer. These nuclei were populated via a multinucleon transfer reaction by bombarding a ${}^{238}$U target with a beam of ${}^{64}$Ni. The experimental $B\left(E2\right)$ reduced transition probabilities for ${}^{63,65}$Co are well reproduced by large-scale shell-model calculations that predict a constant trend of the $B\left(E2\right)$ values up to the $N=40$ ${}^{67}$Co isotope.

Figure 1

Doppler-corrected $\gamma$-ray spectra for ${}^{63}$Co showing both before (blue) and after-degrader (red), background subtracted, components of the $\gamma$-ray transition ${11/2}^{-}\to {7/2}^{-}$ at 1674 keV for different target to degrader distances. Peaks after degrader are indicated by a dashed line.

Figure 2

As Fig. 1, Doppler-corrected $\gamma$-ray spectra for ${}^{65}$Co showing both before (blue) and after-degrader (red), background subtracted, components of the $\gamma$-ray transition $\left({11/2}^{-}\right)\to {7/2}^{-}$ at 1480 keV and $\left({13/2}^{-}\right)\to \left({11/2}^{-}\right)$ at 1190 keV for different target to degrader distances. Peaks after degrader for both transitions are indicated by dashed lines.

Figure 3

Experimental $R\left(t\right)$ values as a function of time and fitted decay curves for the $(11/2^{-})\to {7/2}^{-}$ transitions (in black circles) in both (a) ${}^{63}$Co at 1674 keV and (b) ${}^{65}$Co at 1480 keV. The decay curve for the 1480-keV transition is fitted taking into account the feeding from the excited state $(13/2^{-})$ at 2670 keV, decaying with a $\gamma$-transition energy of 1190 keV (red curve).

Figure 4

Experimental (dashed black lines) and theoretical (solid blue lines) $B\left(E2\right)$ values for the $11/2^{-}\to {7/2}^{-}$ transitions in Co isotopes (triangles), $2^{+}\to 0^{+}$ transitions in Fe (diamonds), and $2^{+}\to 0^{+}$ transitions in Ni isotopes (circles). Results from this work are shown with red triangles. The effective charges used for the shell-model calculations with the LNPS interaction are $1.31e,0.46e$ for protons and neutrons, respectively. The experimental values for the Ni and Fe isotopes are taken from Refs. [25, 26, 27] and [9, 28, 29], respectively.

Figure 5

Neutron occupation numbers for the (a) $g_{9/2}$ and (b) $d_{5/2}$ orbitals for the ${11/2}^{-}$ states in Co isotopes, and the $2^{+}$ states in Ni and Fe isotopes.