Experimental $\gamma$-decay strength in ${}^{59,60}$Ni compared with microscopic calculations

Abstract

Nuclear level densities and $\gamma$-ray strength functions have been extracted for ${}^{59,60}\mathrm{Ni}$, using the Oslo method on data sets from the ${}^{60}$Ni(${}^3$He,${}^3$He${}^{\prime }\gamma$)${}^{60}$Ni and ${}^{60}$Ni(${}^3$He,$\alpha \gamma$)${}^{59}$Ni reactions. Above the neutron separation energy, S${}_n$, we have measured the $\gamma$-ray strength functions for ${}^{61}$Ni and ${}^{60}$Ni in photoneutron experiments. The low-energy part of the ${}^{59,60}$Ni $\gamma$-ray strength functions show an increase for decreasing $\gamma$ energies. The experimental $\gamma$-ray strength functions are compared with $M1$ $\gamma$-ray strength functions calculated within the shell model. The $E1$ $\gamma$-ray strength function of ${}^{60}$Ni has been calculated using the QTBA framework. The QTBA calculations describe the data above $E_{\gamma }\approx$ 7 MeV, while the shell-model calculations agree qualitatively with the low energy part of the $\gamma$-ray strength function. Hence, we give a plausible explanation of the observed shape of the $\gamma$-decay strength.