Level densities and γ-ray strength functions in Sn isotopes

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The nuclear level densities of ¹¹⁸,¹¹⁹Sn and the γ-ray strength functions of ¹¹⁶,¹¹⁸,¹¹⁹Sn below the neutron separation energy are extracted with the Oslo method using the (³He, αγ) and (³He,³He′γ) reactions. The level density function of ¹¹⁹Sn displays step-like structures. The microcanonical entropies are deduced from the level densities, and the single neutron entropy of ¹¹⁹Sn is determined to be (1.7 ± 0.2)kB. 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 γ-ray strength functions of ¹¹⁶⁻¹¹⁹Sn, compared to standard models for radiative strength, is observed for the γ-ray energy region of ≃ (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.