Explosion-evaporation model for fragment production in intermediate-energy nuclear collisions

Nuclear collisions at intermediate energies may create transient systems of hot nuclear matter that decay into many nuclear fragments. The disassembly of such a nuclear fireball is described as a two-stage process. In the primary explosion stage the system quickly fragments into nucleons and composite nuclei according to the available phase space. The explosion produces excited nuclei with half-lives longer than the time associated with the breakup. In the secondary evaporation stage, these nuclei decay, first by sequential emission of light particles (neutrons, protons, alphas), later by electromagnetic radiation. The secondary stage in general changes the relative abundancies of the various fragment species. This general feature makes it essential to take account of the composite fragments before using d/p as a measure of the entropy of the initial source. The formation of unbound nuclei at the explosion stage also has the desirable effect of enhancing the final abundancies of particularly stable nuclei, e.g., 4He. For neutron-excessive sources the presence of composite nuclei amplifies the ratio of observed neutrons and protons; this effect persists for heavier mirror systems. Predictions of the model are qualitatively compared to available experimental data. The model offers a convenient way to augment existing dynamical models, such as intra-nuclear cascade and nuclear fluid dynamics, to yield actual nuclear fragments rather than merely matter distributions