Phase transition dynamics in hot nuclei and N/Z influence

Bernard Borderie,Nicolas Le Neindre,Pierre Désesquelles

doi:10.1051/epjconf/201922301006

Abstract

An abnormal production of events with almost equal-sized fragments was theoretically proposed as a signature of spinodal instabilities responsible for nuclear multifragmentation in the Fermi energy domain. On the other hand finite size effects are predicted to strongly reduce this extra production. High statistics quasifusion hot nuclei produced in central collisions between Xe and Sn isotopes at 32 and 45 MeV per nucleon incident energies have been used to definitively establish, through the experimental measurement of charge correlations, the presence of spinodal instabilities. N/Z influence was also studied. The nature of the phase transition dynamics i.e. the fragment formation was the last missing piece of the puzzle concerning the liquidgas transition in nuclei.

Highlights

An important challenge of heavy-ion collisions at intermediate energies was the identification and characterization of the nuclear liquid-gas phase transition in hot nuclei, which has been theoretically predicted for nuclear matter
Statistical mechanics for finite systems appeared as a key issue to progress, revealing specific first-order phase transition signatures related to the consequences of the local convexity of the entropy
Note that the set of reaction trajectories in the density - temperature plane, close to the border of the spinodal region at 45 MeV per nucleon can be slightly different for the two reactions. To summarize on these experimental results one can say that, using charge correlations, the fossil signature of spinodal instabilities i.e. the abnormal presence of nearly equal-sized fragments, even if very low as expected, was definitively established at a confidence level of around 67 σ. Associated to this weak signal, it is important to underline again the dominating role of chaotic/stochastic dynamics driven by spinodal instabilities for fragment formation, especially for finite systems

Summary

Introduction

An important challenge of heavy-ion collisions at intermediate energies was the identification and characterization of the nuclear liquid-gas phase transition in hot nuclei, which has been theoretically predicted for nuclear matter. By considering the microcanonical ensemble with energy as extensive variable, the convex intruder implies a backbending in the temperature (first derivative of entropy) at constant pressure [3] and correlatively a negative branch for the heat capacity (second derivative of entropy) These two converging signatures have been observed in hot nuclei from different analyses [4,5,6,7] of homogeneous event samples. It is important to recall here that signals of phase transition for finite systems are only meaningful at the level of statistical ensembles constructed from the outcome of carefully selected collisions Another consequence of the entropy curvature anomaly manifests itself when systems are treated in the canonical ensemble. Stochastic mean field approaches predict the transition dynamics to follow the

Theoretical predictions

Experimental results

Conclusions