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 NEσc, where N is proportional to the probability that the compound nucleus emits a proton of energy E, and σc is the cross section for the inverse reaction. For each element, log(NEσc) plotted as a function of the excitation energy of the residual nucleus, Eᵣ, is concave downward. This is in qualitative agreement with the Fermi gas level density formula: constexp(AEᵣ)1/2. For Au, NEσc fits this formula with $A=5.8$ Mev^-1 when Eᵣ>2 Mev; when Eᵣ<2 Mev, NEσc increases less rapidly with increasing Eᵣ than the formula. For Ag, NEσc fits with $A=4.7$ Mev^-1 for all Eᵣ. For Cu, NEσc fits with $A=5.6$ Mev^-1 when Eᵣ>4.5 Mev; when Eᵣ<4.5 Mev, NEσc 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 Eᵣ, are probably contaminated with protons from noncompound-nucleus processes.