Central collisions produced by relativistic heavy ions in nuclear emulsion

We have performed an experimental study of the angular and momentum distributions of fragments emitted from central collisions between emulsion nuclei (AgBr) and heavy-ion projectiles ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ at beam rigidity 5.72 GV. Central collisions are here defined as interactions that exhibit an absence of projectile fragmentation, i.e., no beam-velocity fragments are produced within 5° of the incident beam direction. Production angles have been measured for all fragments having a restricted gain density $g\ge 2g_{min}$, corresponding to protons of $E\le 250$ MeV. Both range and angle measurements have been made for fragment ranges ≤4 mm, corresponding to protons of $E\le 31$ MeV. The data are analyzed in terms of a modified Maxwell-Boltzmann distribution from which we obtain estimates of the longitudinal velocity ${\beta }_{\parallel }$ and the characteristic spectral volocity ${\beta }_0$ of the particle-emitting systems. We find that no unique Maxwellian distribution can account for the observed fragment distributions. The angular distributions do not display statistically significant structure attributable to collective phenomena.