Comparison of Gamma-Ray Multiplicity for Compound-Nucleus and Direct-Reaction Mechanisms

This paper reports an experimental comparison of $\gamma$-ray multiplicity (the average number of $\gamma$ rays emitted in the decay of residual nuclei left by nuclear reactions) for compound-nucleus and direct-reaction mechanisms. The $\gamma$-ray multiplicity was determined for ($p, p^{\prime }$) reactions with targets ${\mathrm{Sn}}^{122}$, ${\mathrm{Cd}}^{116}$, ${\mathrm{Ni}}^{64}$ [for which ($p, p^{\prime }$) reactions are direct] and ${\mathrm{Sn}}^{112}$, ${\mathrm{Cd}}^{106}$, ${\mathrm{Ni}}^{60}$ [for which ($p, p^{\prime }$) reactions are to a considerable extent compound nucleus]. Two bombarding energies, 17 and 12 MeV, were used. The $\gamma$-ray multiplicity was found to be approximately the same for compound-nucleus and direct reactions for similar targets, although there is some indication that it is slightly larger for the former in Sn and Cd at 12-MeV bombarding energy, but not at 17-MeV bombarding energy and not for Ni at either bombarding energy. The observed compound-nucleus-direct-reaction differences do not explain why the $\gamma$-ray multiplicity is larger for $\mathrm{Ni}(p, p^{\prime })$ than for $\mathrm{Sn}(p, p^{\prime })$ reactions.