Signatures of phase transitions in nuclei at finite excitation energies

TL;DR

This paper investigates how signatures of pairing and shape phase transitions in nuclei persist despite fluctuations, using advanced shell model Monte Carlo methods to analyze heavy nuclei and their collective properties at various excitation energies.

Contribution

The study extends the shell model Monte Carlo approach to heavy nuclei, revealing correlations between collective enhancements and phase transitions in finite nuclei.

Findings

Decay of vibrational enhancement correlates with pairing transition.

Decay of rotational enhancement correlates with shape transition.

Signatures of phase transitions survive in finite nuclei despite fluctuations.

Abstract

The mean-field approximation predicts pairing and shape phase transitions in nuclei as a function of temperature or excitation energy. However, in the finite nucleus the singularities of these phase transitions are smoothed out by quantal and thermal fluctuations. An interesting question is whether signatures of these transitions survive despite the large fluctuations. The shell model Monte Carlo (SMMC) approach enables us to calculate the statistical properties of nuclei beyond the mean-field approximation in model spaces that are many orders of magnitude larger than spaces that can be treated by conventional diagonalization methods. We have extended the SMMC method to heavy nuclei and used it to study the transition from vibrational (spherical) to rotational (deformed) nuclei in families of rare-earth isotopes. We have calculated collective enhancement factors of level densities as a function of excitation energy and found that the decay of the vibrational and rotational enhancements is well correlated with the pairing and shape phase transitions, respectively.