Multifragmentation at Intermediate Energy: Dynamics or Statistics?

Abstract

Since the observation of a power-law behaviour in the charge distributions, characteristic of critical phenomena^{1, 2}, in proton induced reactions at relativistic energies, the production of multiple intermediate mass fragments (IMF)^{6, 7}, typically 3 ≤ Z ≤ 20, has been touted as a signature of the nuclear liquid-gas phase transition^{3, 4, 5}. While this may be the case in peripheral reactions e. g. projectile or spectator breakup^{8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18}, the situation becomes less clear when one looks at more central reactions. In particular, it has been shown that the dissi-pative binary mechanism^{19, 20, 21, 22, 23} contributes 95% or more of the reaction cross section^{22, 23}. Yet, as long as the sources are thermalized, it has been shown that a characteristic signature for phase coexistence can be extracted from the charge distributions^{24, 25}. The situation is further complicated by the experimental observation of a significant contribution to the fragment yields from a third source formed between the projectile and target^{26, 27, 28, 29, 30, 31}. Most of these observations were made using velocity plots (see for example ref. 27) which are useful in assigning a given particle to its primary source. This evidence points out the importance of dynamics in the entrance channel. Unfortunately, it tells very little about the intrinsic properties of the sources themselves. In particular, it does not disclosed the nature of the fragmentation process producing the detected “cold” IMF, i. e. at t → ∞.