Angular Distributions of Protons from the 19-Mev Alpha-Particle Bombardment of Na23, Al27, and Si28

Abstract

Using ∼19-Mev alpha particles, angular distributions have been obtained for the three highest-energy proton groups from ($\alpha , p$) reactions with ${\mathrm{Na}}^{23}$, ${\mathrm{Al}}^{27}$, and ${\mathrm{Si}}^{28}$. The differential cross sections are interpreted with the aid of an expression similar to that given by Butler for the "knock-out" process, and normalizing factors $G$, approximately proportional to the product of two reduced widths, are extracted. Reasonable looking fits are obtained for most of the angular distributions with the $l$ values allowed by the theory. Both the reduced total cross sections, $\frac{\sigma }{(2\mathrm{}{J}_f+1)}$, and the $G$ factors increase for the ground-state transitions in the series ${\mathrm{F}}^{19}$-${\mathrm{Na}}^{23}$-${\mathrm{Al}}^{27}$, but decrease for the excited-state transitions. These trends are shown to be in agreement with arguments based on the shell model and the knock-out mechanism. The cross sections for ${\mathrm{Si}}^{28}$ are unexpectedly large, indicating either a strong ${s_{\frac{1}{2}}}^2$ component for the last two protons in ${\mathrm{Si}}^{28}$ or alpha-particle stripping.