Schottky-like phase transition in the fission of atomic nuclei: U235

Abstract

Background: The physics of fission and the features of the fission fragment yields---symmetric or asymmetric shapes for different atomic nuclei and their energies of excitation or temperatures $T$-are among the ``popular puzzles'' of nuclear physics. Explanation of these features requires understanding both the nature of the interaction between the nucleons and the thermodynamics of nuclear matter transformations. The new statistical method for investigation of the ordering of the postscission ensemble of fission fragments is also essential.Purpose: The goal of this article is to demonstrate the possibility of a new type of phase transition in nuclear fission within the fixed temperature range when there is a change in the shape of fission fragment yields from asymmetric to symmetric. The temperature dependence of the thermodynamic functions indicates a Schottky-like phase transition, known from solid-state physics.Methods: We used the proposed statistical method based on the study of thermodynamic ordering for a postscission ensemble of fission fragments. This method allows us to investigate the temperature evolution of the yields (both mass and charge) and the features of the change of the thermodynamic functions of the ensemble of the fragments of fission. The isotope $^{235}\mathrm{U}$ for which the data of the fission fragment yields are well known was chosen for the numerical investigations.Results: We have found anomalies of the thermodynamic functions of the ensemble of fragments of fission of $^{235}\mathrm{U}$ in the temperature range when the shape of the yields of fragment fission changes from asymmetric to symmetric. In particular, the peaklike form of the heat capacity $C(T)$ indicates a Schottky-like phase transition. We also point out the experimental possibility of observing such a phase transition within the nuclear temperature range of 1--2 MeV.Conclusions: This article shows that a new Schottky-like phase transition can be observed under nuclear fission. It differs from known phase-type transformations under nuclear fission, which have been intensively investigated lately and may be due to fundamental factors such as loss of statistical nonequivalence or the identity of nucleons in different fission fragments.