The nuclear level densities of 118,119 Sn and the γ-ray strength functions of 116,118,119 Sn below the neutron separation energy are extracted with the Oslo method using the ( 3 He, αγ) and ( 3 He, 3 He γ) 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±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 γ-ray strength functions of 116−119 Sn, 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.

The level densities and γ-ray strength functions of 205−208 Pb have been measured with the Oslo method, utilizing the ( 3 He, 3 He ′ γ) and ( 3 He, αγ) reactions on the target nuclei 206 Pb and 208 Pb. The extracted level densities are consistent with known discrete levels at low excitation energies. The entropies and temperatures in the micro-canonical ensemble have been deduced from the experimental level density. An average entropy difference of ∆S ∼ 0.9k B has been observed between 205 Pb and 206 Pb. The γ-ray strength functions in 205−208 Pb have been extracted and compared with two models; however, none of them describe the data adequately. Intermediate structures have been observed in the γ-ray strength functions for γ-ray energies below neutron threshold in all the analyzed Pb nuclei. These structures become less pronounced while moving from the doubly-magic nucleus 208 Pb to 205 Pb.

The level densities and radiative strength functions (RSFs) of 50,51 V have been extracted using the ( 3 He,αγ) and ( 3 He, 3 He ′ γ) reactions, respectively. From the level densities, microcanonical entropies are deduced. The high γ-energy part of the measured RSF fits well with the tail of the giant electric dipole resonance. A significant enhancement over the predicted strength in the region of Eγ < ∼ 3 MeV is seen, which at present has no theoretical explanation.

No abstract

The scandium isotopes 44,45 Sc have been studied with the 45 Sc( 3 He,αγ) 44 Sc and 45 Sc( 3 He, 3 He ′ γ) 45 Sc reactions, respectively. The nuclear level densities and γ-ray strength functions have been extracted using the Oslo method. The experimental level densities are compared to calculated level densities obtained from a microscopic model based on BCS quasiparticles within the Nilsson level scheme. This model also gives information about the parity distribution and the number of broken Cooper pairs as a function of excitation energy. The experimental γ-ray strength functions are compared to theoretical models of the E1, M1, and E2 strength, and to data from (γ,n) and (γ,p) experiments. The strength functions show an enhancement at low γ energies that cannot be explained by the present, standard models.