Prescission and postscission neutrons from the reactions p+235,236,238U with Ep

Abstract

Prescission (${\ensuremath{\nu}}_{\mathrm{iso}}$) and postscission (${\ensuremath{\nu}}_{\mathrm{post}}$) neutron multiplicities have been measured for the reactions p${+}^{235,236,238}$U at several projectile energies between 12.7 and 25.6 MeV in coincidence with binary fragments. Separation of ${\ensuremath{\nu}}_{\mathrm{iso}}$ from ${\ensuremath{\nu}}_{\mathrm{post}}$ was achieved under the assumption of isotropic emission in the respective source frames. Both multiplicities increase with initial excitation energy ${\mathit{E}}_{\mathrm{CN}}^{\mathrm{*}}$ with comparable rates \ensuremath{\Delta}${\mathit{E}}_{\mathrm{CN}}^{\mathrm{*}}$/\ensuremath{\Delta}\ensuremath{\nu}\ensuremath{\approxeq}17\ifmmode\pm\else\textpm\fi{}3 MeV. ${\ensuremath{\nu}}_{\mathrm{iso}}$(${\mathit{E}}_{\mathrm{CN}}^{\mathrm{*}}$) extends existing data of heavy-ion induced fusion-fission with ${\mathit{E}}_{\mathrm{CN}}^{\mathrm{*}}$\ensuremath{\ge}50 MeV; for ${\mathit{E}}_{\mathrm{CN}}^{\mathrm{*}}$\ensuremath{\ge}20 MeV it is incompatible with the transition-state model and cannot be reproduced without a delay time for the fission competition in the order of 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$\char21{}${10}^{\mathrm{\ensuremath{-}}19}$ s. The fragment mass dependence ${\ensuremath{\nu}}_{\mathrm{post}}$(m) shows a clear sawtooth structure for all three targets at ${\mathit{E}}_{\mathit{p}}$=12.6 MeV that is gradually washed out with increasing ${\mathit{E}}_{\mathit{p}}$ or decreasing total kinetic energy TKE, because the heavier fragment receives most of the additional excitation energy. The nuclear temperatures of the heavy fragments exceed those of the light ones, but seem to approach each other with increasing fragment excitation.