Direct measurements of neutron emission in complete and incomplete 12C-induced fusion reactions.

Abstract

The multiplicity of neutron emission in $^{12}\mathrm{induced}$ nuclear reactions with $^{197}\mathrm{Au}$, $^{209}\mathrm{Bi}$, $^{232}\mathrm{Th}$, and $^{238}\mathrm{U}$ was measured directly on-line using a Gd-loaded scintillation neutron multiplicity counter with 1 m diameter recording single neutrons with a registration efficiency of about 80%. Using a well-focused carbon beam one could measure up to about 30 neutrons per nuclear interaction directly, when the neutron multiplicity counter was triggered with a prompt fission fragment. For $^{197}\mathrm{Au}$ and $^{209}\mathrm{Bi}$ targets, fission was observed to follow compound-nucleus formation, whereas in the cases of $^{232}\mathrm{Th}$ and $^{238}\mathrm{U}$, fission following incomplete fusion was seen to contribute also. An average multiplicity of \ensuremath{\nu}\ifmmode\bar\else\textasciimacron\fi{}=(4\ifmmode\pm\else\textpm\fi{}1) neutrons was measured for incomplete fusion, while for complete fusion, \ensuremath{\nu}\ifmmode\bar\else\textasciimacron\fi{} varied between 8 and 12, depending on target and bombarding energy. The results complement measurements of Viola et al. on fragment-angular correlations and agree with them.