Diversity of fragment sizes in multifragmentation of gold nuclei induced by relativistic3Heions

Abstract

The charge-moment technique has been used to study the fragment charge distribution for the 3 He͑4.8 GeV͒ϩ 197 Au reaction. A large variety of fragment charges characterized by a relative variance ϳ2.3, is observed for excitation energies around 5.5 MeV/nucleon. Similar signals related to a phase transition are predicted by the percolation model and the statistical multifragmentation model. Effects of detector acceptance and contribution from fission are discussed. ͓S0556-2813͑98͒50309-3͔PACS number͑s͒: 25.70. Pq, 25.55.Ϫe A nucleus under extreme conditions of temperature can break into many pieces of different sizes. This phenomenon, referred to as multifragmentation, is often related to the possible existence of phase transition in nuclear matter ͓1-5͔. The occurrence of a second-order phase transition ͑critical behavior͒ was suggested over ten years ago by the observation of the fragment mass yield exhibiting a power law dependence ͓6͔. Since that time, various statistical tools have been developed in order to extract a signal of critical behavior from the fragment size distributions ͓7-14͔. In particular, Campi in his pioneering work ͓7͔ has proposed to study moments of the fragment size distributions. Within the framework of infinite percolation models, the moments of order of 2 or greater diverge at the critical point ͓7,15͔. For finite systems, the divergence is replaced by a maximum near the critical point. This indicates a strong connection with the continuous limit, which makes those quantities of great interest in searching for signals of a critical behavior.In this contribution, we present the charge moment analysis applied to the 3 He͑4.8 GeV͒ϩ 197Au reaction. The transport and thermal properties of this system have been studied in Refs. ͓16-19͔. The interest in this system lies in the fact that the heating process is achieved over a small time scale without leading to sizable compressional or rotational effects. In fact, unlike most heavy-ion induced reactions, the present data show a unique multifragment source ͓16,18͔. In contrast with the present system, extracting signals from heavy-ion reactions can be controversial due to the problem of source separation.Our first attempt in searching for a critical regime is the examination of the relative variance of the fragment charge distributions. This quantity is expected to reach the largest values in the vicinity of the critical point ͓7͔. As will be shown below, mean values of the relative variance are not distorted by our detector acceptance in case of multifragment events. Experimental evidence of such a signature is scarce ͓7,20,21͔. Two results from emulsion experiments seem to lead to different conclusions ͓7,20͔: one is in accord with a three dimensional percolation model whereas no similar agreement exists in the other. Advantages of the present technique over emulsion experiments are that the preequilibrium contribution can be partially removed from the analysis and that the transferred energies can be evaluated.The experiment was perform...

Light-ion-induced multifragmentation: The ISiS project

“…Calculations with both percolation and statistical multifragmentation models predict that the relative variance γ 2 of the charge distributions will exhibit maxima of γ 2 ≈ 2.1-2.3 near the critical point. Brzychczyk [66] analyzed the relative moments of the 4.8 GeV 3 He + 197 Au reaction and found a variance of γ 2 = 2.3 ± 0.1 near E*/A ≈ 5.5 MeV. Thus, the ISiS data are in good agreement with phase transition models and provide further consistency with arguments for a liquid-gas phase transition and possible critical behavior in hot nuclei.…”

“…The tendency to form lighter clusters at high excitation energy is most likely due to the dissolution of the larger clusters in the heat bath and/or the formation of highly-excited clusters that undergo secondary decay. The average variances of the Z distributions have also been measured [66] and the relation of both τ and the average variance is discussed in Sec.6 with regard to phase transition arguments.…”

Distributions of the largest fragment size in multifragmentation: Traces of a phase transition

“…Besides the characteristic evolution of its mean-value, event-to-event fluctuations reflected in the probability distribution of the largest fragment size are of considerable interest. Experimental examinations have focused on the appearance of particularly large fluctuations [6,9,[11][12][13][14][15], on bimodal characteristics representative of a two-phase coexistence [6,8,10,[16][17][18], on so-called ∆-scaling features [1, 5,6,8], and on the connection with dynamical observables as, e.g., radial flow [19].…”