Nonequilibrium effects in the ${}_{}{}^{139}{}_{}{}^{}\mathrm{Ar}$ reaction at 10 MeV per nucleon observed in a study of neutron emission

Energy spectra of neutrons emitted in the damped reaction ${}_{}{}^{139}{}_{}{}^{}\mathrm{Ar}$ at $E_{\mathrm{lab}=400\mathrm{}}$ MeV were measured at eight angles in coincidence with projectile-like reaction fragments, using a time-of-flight technique. Apart from a small high-energy component, the angular and energy distributions of neutrons were found to be well described by assuming two sources moving at the velocities of the light and heavy reaction fragments emitting isotropically in their rest frames. The logarithmic slopes of the neutron energy spectra and the corresponding multiplicities of neutrons from the two sources suggest that the projectile-like fragment receives more than its equilibrium share of excitation energy for all values of total kinetic energy loss. Detailed statistical calculations indicate that the amount of excitation energy generated in each fragment is well described by the one-body nucleon exchange model. The high-energy component in the neutron energy spectra has properties that are consistent with preequilibrium neutron emission. Averaged over all energy losses greater than 20 MeV, this ‘‘preequilibrium’’ neutron multiplicity is approximately equal to 0.1. However, the intensity of this component increases as the impact parameter decreases.