Fluctuations of fragment observables

Abstract

This contribution presents a review of our present theoretical as well as experimental knowledge of different fluctuation observables relevant to nuclear multifragmentation. The possible connection between the presence of a fluctuation peak and the occurrence of a phase transition or a critical phenomenon is critically analyzed. Many different phenomena can lead both to the creation and to the suppression of a fluctuation peak. In particular, the role of constraints due to conservation laws and to data sorting is shown to be essential. From the experimental point of view, a comparison of the available fragmentation data reveals that there is a good agreement between different data sets of basic fluctuation observables, if the fragmenting source is of comparable size. This compatibility suggests that the fragmentation process is largely independent of the reaction mechanism (central versus peripheral collisions, symmetric versus asymmetric systems, light ions versus heavy ion induced reactions). Configurational energy fluctuations, that may give important information on the heat capacity of the fragmenting system at the freeze out stage, are not fully compatible among different data sets and require further analysis to properly account for Coulomb effects and secondary decays. Some basic theoretical questions, concerning the interplay between the dynamics of the collision and the fragmentation process, and the cluster definition in dense and hot media, are still open and are addressed at the end of the paper. A comparison with realistic models and/or a quantitative analysis of the fluctuation properties will be needed to clarify in the next future the nature of the transition observed from compound nucleus evaporation to multi-fragment production.

Effect of system size on the traditional signatures of critical behavior in projectile multifragmentation

“…Similar distribution fitted with a power law has also been reported in nuclear fragmentation of 238 U at 0.96 AGeV, 84 Kr at 1.25 AGeV, 84 Kr at 0.95 AGeV and 131 Xe at 1.22 AGeV in nuclear emulsion [16,19,23,[28][29][30].…”

“…For Mg it is observed in the range of 5-9 with a peak at m = 6±1. Recently, Gulminelli et al [23] have pointed out that finiteness of the system under investigation smooth the fluctuation effect to such an extent that not only the transition point is loosely defined and shifted, but also the signal is qualitatively the same for a critical point, a first order phase transition or even a continuous change or cross over. Thus the heap like structures that has been observed in the figure 3 may not carry as much information as one would expect for cluster approximation of critical behavior analysis for an infinite system.…”

Onset of quenching of the giant dipole resonance at high excitation energies

“…In particular, in the last decade, the study of the nuclear caloric curve, i.e., the relation between the system temperature and its excitation energy, has yielded evidence of a trend that is reminiscent of liquid water heating to the boiling point, the plateau region being interpreted as a liquid-gas phase transition in nuclear matter [43,53]. Other possible signatures of a liquid-gas phase transition or critical behavior in nuclei were found by using different experimental probes and theoretical approaches [54][55][56][57]. It has been proposed that the presence of collective states can be a signature of the existence of a compound nucleus and that the disappearance of the collective motion at high excitation energies could therefore provide further evidence for a phase transition in nuclei [58,59].…”

Nuclear multifragmentation and phase transition for hot nuclei

“…That procedure was applied by the MULTICS and INDRA collaborations on different sources produced in peripheral and central heavy ion collisions in the incident energy range 30-80 MeV/nucleon [158,245,246,247,248,249,250]. Figure 38 summarizes the results obtained by the MULTICS collaboration: on the left side it is seen that normalized kinetic energy fluctuations overcome the estimated canonical fluctuations, C k ; the right part of the figure illustrates the microcanonical negative heat capacities observed, the distances between the poles being associated with the latent heat.…”