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

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 $Y_{\alpha }=(1.53\mathrm{}\pm 0.18)+(0.044\pm 0.010)E_p$, where $Y_{\alpha }$ is the number of alpha particles per thousand fissions and $E_p$ is the incident proton energy in MeV. For spontaneous fission of ${\mathrm{Cf}}^{252}$, $Y_{\alpha }$ is now known to include (2±1)% of ${\mathrm{He}}^6$ and possibly much smaller amounts of slightly heavier particles. $Y_{\alpha }$ 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±0.3° (center-of-mass) and 14.7±0.4° (nearly independent of incident proton energy) for the induced fission, and 93±0.6° and 12.0±0.4° for ${\mathrm{Cf}}^{252}$. Absolute values of $Y_{\alpha }$ 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.