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

Nuclear level densities and $gamma$-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$^{prime}gamma$)$^{60}$Ni and $^{60}$Ni($^{3}$He,$alphagamma$)$^{59}$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.