Rapidity dependence of entropy production in proton- and nucleus-induced reactions on heavy nuclei

The entropy of hot nuclear systems is deduced from the mass distribution of fragments emitted from high energy proton- and nucleus-induced reactions via a quantum statistical model. It is found that the entropy per baryon, $\frac{S}{A}$, of intermediate rapidity ("participant") fragments is higher than the entropy of target rapidity ("spectator") fragments. The spectator fragments exhibit $\frac{S}{A}$ values of ≅ 1.8 independent of the projectile energy from 30 MeV/nucleon up to 350 GeV. This value of the entropy coincides with the entropy at which nuclear matter becomes unbound.