Level densities, gamma-ray strength functions and the evolution of the pygmy resonance in Sn isotopes

Abstract

Experimental measurements of statistical properties at high temperatures in atomic nuclei have long been the main research field of the nuclear physics group at the Oslo Cyclotron Laboratory. This thesis presents measurements of the level densities and the gamma-ray strength functions for a wide range of tin isotopes. All the experiments have been performed in Oslo. In the experiments, accelerated helium nuclei were impinged on metallic foils of tin. The nuclear reactions increase the temperature of the target nuclei up to about ten billion degrees Celsius. In this way, the nucleus is excited to high-energy levels. When the nucleus cools down, light particles and gamma rays are emitted. From measuring the energy and time of these, the decay of the nucleus can be studied by means of statistical methods. Pronounced steps in the level densities, interpreted as signatures of neutron pair breaking, as well as the small enhancement of strength in the low-energy region, interpreted as a pygmy resonance, are observed in the tin isotopes in this thesis. Moreover, comparison of the pygmy resonance in the tin isotopes gives an increase in the centroid energy with increasing isotope number, while no significant difference is seen in the integrated strength. The observed evolution is in contradiction to theoretical predictions of the neutron-skin oscillation mode, which is often suggested as the origin of this resonance. More experiments are strongly motivated, and theory is challenged to explain this behaviour of the pygmy resonance in tin isotopes.