Level densities and $\gamma$-ray strength functions in Sn isotopes

Abstract

The nuclear level densities of ${}^{118,119}$Sn and the $\gamma$-ray strength functions of ${}^{116,118,119}$Sn below the neutron separation energy are extracted with the Oslo method using the (${}^3$He, \,$\alpha \gamma$) and (${}^3$He,${}^3$He${}^{\prime }\gamma$) reactions. The level density function of ${}^{119}$Sn displays step-like structures. The microcanonical entropies are deduced from the level densities, and the single neutron entropy of ${}^{119}$Sn is determined to be $(1.7\pm 0.2)k_B$. Results from a combinatorial model support the interpretation that some of the low-energy steps in the level density function are caused by neutron pair-breaking. An enhancement in all the $\gamma$-ray strength functions of ${}^{116-119}$Sn, compared to standard models for radiative strength, is observed for the $\gamma$-ray energy region of $\simeq (4-11)$ MeV. These small resonances all have a centroid energy of 8.0(1) MeV and an integrated strength corresponding to $1.7(9)\%$ of the classical Thomas-Reiche-Kuhn sum rule. The Sn resonances may be due to electric dipole neutron skin oscillations or to an enhancement of the giant magnetic dipole resonance.

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