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 ⁴He, ¹⁶O, and ⁴⁰Ar 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≥2gₘᵢₙ, corresponding to protons of E≤250 MeV. Both range and angle measurements have been made for fragment ranges {≤}4 mm, corresponding to protons of E≤31 MeV. The data are analyzed in terms of a modified Maxwell-Boltzmann distribution from which we obtain estimates of the longitudinal velocity β_∥ and the characteristic spectral volocity β₀ 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.NUCLEAR REACTIONS AgBr(⁴He,x), $E=2.1$ GeV/A; AgBr(¹⁶O,x), $E=2.1$ GeV/A; and AgBr(⁴⁰Ar,x), $E=1.8$ GeV/A; measured fragment multiplicities; range and angular distributions for ranges {≤}4 mm (Eₚᵣₒₜₒₙ≤31 MeV); angular distribution for Eₚᵣₒₜₒₙ≤250 MeV. Nonperipheral collisions, Maxwell-Boltzmann parametrization.