Protons from the Bombardment of Several Elements with 40-Mev Alpha Particles

Thin targets of Au, Ag, and Cu were bombarded with 40-Mev alpha particles and the energy distributions of protons emitted at 150° were measured. According to the compound-nucleus model, the level density of the residual nucleus is equal to: const $\frac{N}{E{\sigma }_c}$, where $N$ is proportional to the probability that the compound nucleus emits a proton of energy $E$, and ${\sigma }_c$ is the cross section for the inverse reaction. For each element, $log\left(\frac{N}{E{\sigma }_c}\right)$ plotted as a function of the excitation energy of the residual nucleus, $E_r$, is concave downward. This is in qualitative agreement with the Fermi gas level density formula: $\mathrm{const}\exp {\left(A{E}_r\right)}^{\frac{1}{2}}$. For Au, $\frac{N}{E{\sigma }_c}$ fits this formula with $A=5.8\mathrm{}$ ${\mathrm{Mev}}^{-1\mathrm{}}$ when $E_r>2\mathrm{}$ Mev; when $E_r<2\mathrm{}$ Mev, $\frac{N}{E{\sigma }_c}$ increases less rapidly with increasing $E_r$ than the formula. For Ag, $\frac{N}{E{\sigma }_c}$ fits with $A=4.7\mathrm{}$ ${\mathrm{Mev}}^{-1\mathrm{}}$ for all $E_r$. For Cu, $\frac{N}{E{\sigma }_c}$ fits with $A=5.6\mathrm{}$ ${\mathrm{Mev}}^{-1\mathrm{}}$ when $E_r>4.5\mathrm{}$ Mev; when $E_r<4.5\mathrm{}$ Mev, $\frac{N}{E{\sigma }_c}$ increases more rapidly than the formula. In the region of 150°, the cross section for the emission of lower-energy protons is isotropic, but the cross section for high-energy protons decreases slightly with increasing angle. Thus the energy distributions in the region of small $E_r$, are probably contaminated with protons from noncompound-nucleus processes.