An investigation has been made of the energy and angular distributions of $\alpha$ particles emitted from silver and bromine nuclei in Ilford $D-1\mathrm{}$ (200 μ) nuclear emulsions, during bombardments in the Brookhaven Cosmotron with 1.0-, 2.0-, and 3.0-Bev protons. The $\alpha$ energies studied were in the interval from 0-50 Mev with particular emphasis placed on the low-energy region. An attempt has been made to correct the observed spectra for the center-of-mass motion of the emitting nucleus and then to compare these spectra with those calculated from nuclear evaporation theory. Two sets of center-of-mass transformations were made. In one case the beam direction was considered to be that of the moving system, and in the second case the direction of the observed recoil was considered to be that of the moving system. Good agreement was obtained with the theoretical spectra throughout the energy region studied. An apparent excess of low-energy $\alpha$ particles in the uncorrected spectra was removed by assuming that the emitting nucleus moves in the observed direction of the recoil at $0.015c$ at 1.0 Bev and $0.02c$ at 2.0 Bev. These velocities were consistent with the measured lengths of the observed recoil nuclei. Both the angular distributions of the recoil fragments and of the $\alpha$ particles were consistent with random emission from this moving system. It seems likely, therefore, that one can, at the same time, explain the observed angular distributions and the low-energy $\alpha$ particles by isotropic evaporation from a moving system.

• Received 23 September 1959

DOI:https://doi.org/10.1103/PhysRev.117.1352