Emission of Alpha Particles in the Fission ofU238by 11- to 21-MeV Protons

Abstract

The rate of emission of alpha particles in coincidence with fission has been measured for ${\mathrm{U}}^{238}$ bombarded with protons of energies 11, 12, 13, 14, 15, 17, and 21 MeV. The results of these measurements may be summarized by the formula ${Y}_{\ensuremath{\alpha}}=(1.53\ifmmode\pm\else\textpm\fi{}0.18)+(0.044\ifmmode\pm\else\textpm\fi{}0.010){E}_{p}$, where ${Y}_{\ensuremath{\alpha}}$ is the number of alpha particles per thousand fissions and ${E}_{p}$ is the incident proton energy in MeV. For spontaneous fission of ${\mathrm{Cf}}^{252}$, ${Y}_{\ensuremath{\alpha}}$ is now known to include (2\ifmmode\pm\else\textpm\fi{}1)% of ${\mathrm{He}}^{6}$ and possibly much smaller amounts of slightly heavier particles. ${Y}_{\ensuremath{\alpha}}$ increases by about 2.2% per MeV from 11 to 21 MeV, with no evidence for a minimum in this range. The angular correlations between alpha particles and fission fragments have been measured at 11-, 15-, 17-, and 21-MeV proton energy and for the spontaneous fission of ${\mathrm{Cf}}^{252}$. When corrected for the angular resolution of the detectors, the distributions are found to be approximately Gaussian with means and standard deviations 94\ifmmode\pm\else\textpm\fi{}0.3\ifmmode^\circ\else\textdegree\fi{} (center-of-mass) and 14.7\ifmmode\pm\else\textpm\fi{}0.4\ifmmode^\circ\else\textdegree\fi{} (nearly independent of incident proton energy) for the induced fission, and 93\ifmmode\pm\else\textpm\fi{}0.6\ifmmode^\circ\else\textdegree\fi{} and 12.0\ifmmode\pm\else\textpm\fi{}0.4\ifmmode^\circ\else\textdegree\fi{} for ${\mathrm{Cf}}^{252}$. Absolute values of ${Y}_{\ensuremath{\alpha}}$ were obtained relative to the absolute rate of emission of alpha particles from the spontaneous fission of ${\mathrm{Cf}}^{252}$ as given in the literature and by an independent measurement reported in this paper.