Angular-Momentum Effects in Compound-Nucleus Reactions Induced by 27-MeVN14Ions

The angular distribution of particles emitted in the statistical decay of a compound nucleus will show forward-backward peaking if the angular momentum available in the final system is limited. Furthermore, the decay will not be independent of the mode of formation. These effects were demonstrated by measurements of the energy and angular distributions of alpha particles emitted from reactions of ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ with ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$, and ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ at about 27 MeV. The angular distributions were analyzed by the semiclassical theory of Ericson and Strutinski to determine the spin cutoff parameter $\sigma$, or equivalently the moment of inertia $\mathcal{I}$. The values of $\sigma$ ranged from 2.6 for the residual nucleus ${}_{}{}^{26}{}_{}{}^{}\mathrm{Al}$ to 3.5 for ${}_{}{}^{37}{}_{}{}^{}\mathrm{Ar}$. The moment of inertia was approximately 0.9 ${\mathcal{I}}_{\mathrm{rigid}}$. The energy dependence of the level density was determined from the energy spectra. With the simple form $\exp \left[2\mathrm{}{\left(a_{1}E\right)}^{\frac{1}{2}}\right]$, good fits were obtained with $a_1\approx \frac{A}{10}$ for the two heavier residual nuclei. The data did not distinguish between this expression and the form $E^{-2\mathrm{}}\exp \left[2\mathrm{}{\left(a_{2}E\right)}^{\frac{1}{2}}\right]$; equally good fits were obtained with $a_2\approx \frac{1}{6}A$.