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

Abstract

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