Measurement and Statistical Theory Analysis ofFe56(He3, p)andCu63(He3, p)Energy and Angular Distributions-Nuclear Shell Effects

Abstract

Thin ${\mathrm{Fe}}^{56}$ and ${\mathrm{Cu}}^{63}$ targets (approximately 1.5 mg/${\mathrm{cm}}^2$) were bombarded with 10-MeV ${\mathrm{He}}^3$ particles. The proton energy and angular distributions produced by the ${\mathrm{Fe}}^{56}({\mathrm{He}}^3, p)$ and ${\mathrm{Cu}}^{63}({\mathrm{He}}^3, p)$ reactions were measured by the $E-\Delta E$ particle identification technique. In the statistical theory interpretation of the experimental cross sections both the shape and magnitude of the angular and energy distributions were calculated. Contributions from the (${\mathrm{He}}^3$, $\mathrm{np}$) and (${\mathrm{He}}^3$, $2p$) reactions were also calculated. Large sections of the measured proton energy and angular distributions (about 90% of the total cross sections) are consistent with the predictions of the statistical theory of compound-nucleus reactions. A conventional statistical theory calculation of the ${\mathrm{Cu}}^{63}({\mathrm{He}}^3, p)$ cross sections is generally consistent with the experimental cross sections; however, a similar conventional calculation of the ${\mathrm{Fe}}^{56}({\mathrm{He}}^3, p)$ cross sections yields values 50% smaller than the experimental results. Rosenzweig has derived an expression for nuclear level densities which indicates that level densities of nuclei in the immediate vicinity of the doubly closed shell ${\mathrm{Ni}}^{56}$ nucleus can be influenced by nuclear shell structure at excitation energies as high as 15 MeV. A second statistical theory calculation of the magnitude and shape of the ${\mathrm{Fe}}^{56}({\mathrm{He}}^3, p)$ cross sections, based on the Rosenzweig level density expression, yields calculated cross sections generally consistent with the measured ${\mathrm{Fe}}^{56}({\mathrm{He}}^3, p)$ cross sections.