Charge Correlations and Dynamical Instabilities in the Multifragment Emission Process.

A new, sensitive method allows one to search for the enhancement of events with nearly equal-sized fragments as predicted by theoretical calculations based on volume or surface instabilities. Simulations have been performed to investigate the sensitivity of the procedure. Experimentally, charge correlations of intermediate mass fragments emitted from heavy ion reactions at intermediate energies have been studied. No evidence for a preferred breakup into equal-sized fragments has been found. 1 In recent years, multifragmentation of nuclear systems has been extensively studied, and many efforts have been made to clarify the underlying physics [1]. It has been suggested that fragment production can be related to the occurrence of instabilities in the intermediate system produced by heavy ion collisions [2–13]. In particular, two kinds of instabilities are extensively discussed in the literature: volume instabilities of a spinodal type (see e.g. Ref. [12]) and surface instabilities [3]. Spinodal instabilities are associated with the transit of a homogeneous fluid across a domain of negative pressure, where the homogeneous fluid becomes unstable and breaks up into droplets of denser liquid. Surface instabilities can be subdivided into Rayleigh or cylinder instabilities which are responsible for the decay of shapes like long necks or toroids [2], and sheet instabilities which cause the decay of bubbles or disklike structures [3]. A variety of models have predicted the formation of these exotic geometries which may develop after the initial compression of nuclei in the early stage of the collision for both symmetric and asymmetric systems [3,4,6,7,10–13]. Although the scenarios and the models vary, breakup into several nearly equal-sized fragments has been predicted for both kinds of instabilities. Thus it would be interesting to search model-independently for this signal. In this paper, we examine the signatures of a breakup configuration which would decay into a number of nearly equal-sized fragments by investigating charge correlations from both experimental data and simulations. We have experimentally studied the reactions Xe+Cu at 50 MeV/nucleon. The measurements were performed at the National Superconducting Cyclotron Laboratory of Michigan State University using the Miniball [14] and a Si-Si(Li)-plastic forward array [15]. Detailed information on the experiment can be found in Ref. [16]. For comparison, and in order to determine the sensitivity of our analysis, Monte Carlo calculations have been performed. The created events obey two conditions: the sum charge of all fragments is conserved within an adjustable accuracy, and a fragment is produced according to the probability …