Emission of light charged particles in photon induced fission.

Abstract

The emission of light charged particles in the photofission of $^{230,232}\mathrm{Th}$, $^{233,234,235,238}\mathrm{U}$, $^{237}\mathrm{Np}$, and $^{242}\mathrm{Pu}$ has been studied with bremsstrahlung with end-point energy of 12, 15, and 20 MeV. The light charged particles are measured using a setup consisting of eight \ensuremath{\Delta}E-E particle identification detector telescopes. The kinetic energy distributions and the emission probabilities of tritons and long-range alpha particles were determined. The presence of photoprotons obscures the identification of ternary protons while $^{6}\mathrm{He}$ particles were observed clearly, however, with insufficient statistics to allow meaningful conclusions concerning their energy distribution or emission probability. The average kinetic energy of the ternary tritons and \ensuremath{\alpha} particles and the width of the triton kinetic energy distributions seems to be practically independent of the compound nucleus excitation energy or ${\mathit{Z}}^{2}$/A. Only the width of the long-range alpha particle kinetic energy distribution shows a slight increase with increasing ${\mathit{Z}}^{2}$/A of the compound nucleus. Our results point toward a slight increase of the ternary alpha particle yield with increasing excitation energy of the compound nucleus. As already observed by Wild et al. for spontaneously fissioning nuclei, this yield shows a clear correlation with 〈Q〉-〈${\mathit{E}}_{\mathrm{tot}}^{\mathrm{*}}$〉, where ${\mathit{E}}_{\mathrm{tot}}^{\mathrm{*}}$ is the total kinetic energy, suggesting that the light charged particle emission probability is determined by the deformation of the system at scission, and that, for the compound nucleus excitation energy studied in our experiments, the excitation energy transferred to the compound nucleus remains in the system as internal heat, and is not transformed into deformation energy.The triton emission to binary fission ratio, t/B, for $^{237}\mathrm{Np}$ is not substantially higher than for the neighboring nuclei, indicating that the unpaired proton in $^{237}\mathrm{Np}$ does not influence in an important way the triton emission probability.