Breakup of spectator residues in relativistic nuclear collisions

Abstract

Low energy nuclear fragments ($12\le A\le 140$) from the bombardment of Au by high energy protons, ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, and ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ projectiles are measured to yield information on the breakup of the target nucleus. We discuss the shapes of the fragment energy spectra across this broad range of fragment mass and pick out the fission component by its binary breakup. Multiplicities of coincident fast charged particles are presented as a measure of the degree of violence of the initial stage of the collision. Angular distribution of light fragments ($8\le Z\le 11$) give the average velocities imparted to the residue in central collisions, and these values are compared to the results of cascade calculations. Measurements of slow fragment multiplicities from central events reveal a many-fragment final state. In the light of this finding we develop a simple calculation of the Coulomb features of the observed spectra. The applicability of a statistical breakup model is discussed.