Direct measurement of prompt neutrons emitted in fission of Ra226 by 12 MeV protons

Prompt neutron energy distributions were measured at 0\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{} with respect to the fission axis, in correlation with the mass-energy distribution of the fragments, for the fission of $^{226}\mathrm{Ra}$ induced by 12 MeV protons. The average number and average kinetic energy of prefission and postfission neutrons were obtained as a function of fragment mass and total kinetic energy. The average number of prefission neutrons emitted is 0.33 \ifmmode\pm\else\textpm\fi{} 0.15 $\frac{n}{\mathrm{fission}}$ (this includes also contributions from scission neutrons). By comparing this result with calculations of the average number of "true" prefission neutrons, it was found that the number of true prefission neutrons is zero. The average number of postfission neutrons emitted from both fragments is 3.20 \ifmmode\pm\else\textpm\fi{} 0.20 with an average kinetic energy of 1.33 \ifmmode\pm\else\textpm\fi{} 0.07 MeV. The dependence of the number of postfission neutrons on the fragment mass can be interpreted as a combination of a sawtooth structure for the asymmetric mass division and a linearly increasing function for the symmetric mass division.NUCLEAR REACTIONS, FISSION $^{226}\mathrm{Ra}(p,xnf)$, $E=12$ MeV; measured coin. fragment energy, neutron velocity at $\ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{}, 90\ifmmode^\circ\else\textdegree\fi{}$ with respect to fragment; deduced number and kinetic energy of prefission, postfission neutrons.