Heavy-fragment ($Z>\mathrm{}2\mathrm{}$) energy spectra from 720-MeV $\alpha$-induced nuclear reactions on Al, Ag, and Ta have been measured at 30°, 60°, 90°, and 120°. The angular distributions are not well reproduced by thermodynamic evaporation medels with either one or two emitting sources. The anisotropy of the low-$Z$ fragments is, however, better described by three sources whose evenly spaced velocities and temperatures indicate that the three sources may represent a discrete approximation to an actual continuum of sources. The observed anisotropy may also be a manifestation of large contributions to the forward-angle cross sections from pre-equilibrium fragment emission. Extension of this analysis to our hydrogen- and helium-isotope data indicates that the light fragments continue the trends exhibited by the heavy-fragment data. A comparison with data from proton-induced reactions indicates that the anisotropy observed may be a manifestation of higher energy and momentum deposition in the target nucleus by $\alpha$-particles than by protons.

NUCLEAR REACTIONS Al, Ag, Ta ($\alpha$,$X$), $E=720\mathrm{}$ MeV; measured $\sigma (E,\theta )$, $X=\mathrm{Li}\mathrm{to}\mathrm{O}$. Thermodynamic models, evaporation, firestreak, pre-equilibrium emission.

• Received 6 September 1978

DOI:https://doi.org/10.1103/PhysRevC.19.1408