Constant-temperature level densities in the quasi-continuum of Th and U isotopes

TL;DR

This study measures gamma-ray spectra of Th and U isotopes to analyze their level densities, revealing a constant temperature behavior and insights into nuclear pairing and entropy in the quasi-continuum.

Contribution

It provides the first detailed experimental evidence of constant-temperature level densities in Th and U isotopes using the Oslo method.

Findings

Level densities exhibit constant temperature behavior.

Even-odd isotopes show an entropy increase Delta S.

Microcanonical temperature and heat capacity suggest Cooper pair melting.

Abstract

Particle-gamma coincidences have been measured to obtain gamma-ray spectra as a function of excitation energy for 231-233Th and 237-239U. The level densities, which were extracted using the Oslo method, show a constant temperature behavior. The isotopes display very similar temperatures in the quasi-continuum, however, the even-odd isotopes reveal a constant entropy increase Delta S compared to their even-even neighbors. The entropy excess depends on available orbitals for the last unpaired valence neutron of the heated nuclear system. Also, experimental microcanonical temperature and heat capacity have been extracted. Several poles in the heat capacity curve support the idea that an almost continuous melting of Cooper pairs is responsible for the constant-temperature behavior.