No abstract

The properties of fragments and light charged particles emitted in multifragmentation of single sources formed in central 36 A.MeV Gd+U collisions are reviewed. Most of the products are isotropically distributed in the reaction c.m. Fragment kinetic energies reveal the onset of radial collective energy. A bulk effect is experimentally evidenced from the similarity of the charge distribution with that from the lighter 32 A.MeV Xe+Sn system. Spinodal decomposition of finite nuclear matter exhibits the same property in simulated central collisions for the two systems, and appears therefore as a possible mechanism at the origin of multifragmentation in this incident energy domain.

Characteristics of the primary fragments produced in central collisions of129 Xe + nat Sn from 32 to 50 AMeV have been obtained. By using the correlation technique for the relative velocity between light charged particles (LCP) and fragments, we were able to extract the multiplicities and average kinetic energy of secondary evaporated LCP. We then reconstructed the size and excitation energy of the primary fragments. For each bombarding energy a constant value of the excitation energy per nucleon over the whole range of fragment charge has been found. This value saturates at 3 AMeV for beam energies 39 AMeV and above. The corresponding secondary evaporated LCP represent less than 40% of all produced particles and decreases down to 23% for 50 AMeV. The experimental characteristics of the primary fragments are compared to the predictions of statistical multifragmentation model (SMM) calculations. Reasonable agreement between the data and the calculation has been found for any given incident energy. However SMM fails to reproduce the trend of the excitation function of the primary fragment excitation energy and the amount of secondary evaporated LCP's.

Enhanced production of events with almost equal-sized fragments is experimentally revealed by charge correlations in the multifragmentation of a finite nuclear system selected in 129 Xe central collisions on nat Sn. The evolution of their weight with the incident energy: 32, 39, 45, 50 AMeV, is measured. Dynamical stochastic mean field simulations performed at 32 AMeV, in which spinodal instabilities are responsible for multifragmentation, exhibit a similar enhancement of this kind of events. The above experimental observation evidences the spinodal decomposition of hot finite nuclear matter as the origin of multifragmentation in the Fermi energy regime.

Multifragmentation of a "fused system" was observed for central collisions between 32 MeV/nucleon 129Xe and (nat)Sn. Most of the resulting charged products were well identified due to the high performances of the INDRA 4pi array. Experimental higher-order charge correlations for fragments show a weak but nonambiguous enhancement of events with nearly equal-sized fragments. Supported by dynamical calculations in which spinodal decomposition is simulated, this observed enhancement is interpreted as a "fossil" signal of spinodal instabilities in finite nuclear systems.