Spectra of (p, α) and (p, p′) Reactions and the Evaporation Model

Abstract

The energy spectra of $\alpha$ particles from V, Fe, Co, Ni, and Rb bombarded with 17.5-Mev protons were obtained at a number of angles. Beyond 30°, the spectra are essentially isotropic and have been analyzed in terms of the evaporation model. The level density deduced is of the form $\exp \left(\frac{E_x}{T}\right)$ for $E_x=3\mathrm{} \mathrm{to} 12$ Mev. The values of $T$ are in the range 1.2-1.5 Mev if the cross sections of the continuum model with $R=(1.4\mathrm{}{A}^{\frac{1}{3}}+2.2)$ fermis are used, and in the range 1.3-1.6 Mev if optical model cross sections are used. This "constant temperature" level density is also consistent with alpha spectra of Ni and Co bombarded 15- and 19-Mev protons. Analysis of published inelastic proton scattering data for elements in the Ti-Zn region, with bombarding energies of 11.3 to 23 Mev, indicates that the ($p, p^{\prime }$) spectra (in the above excitation interval) are only partially attributable to evaporation. At 11.3 Mev the upper limit of the contribution of evaporation to the proton spectra is estimated to be ≤96%, while at 23 Mev it is probably much smaller than 22%. The value of $T$ for ${\mathrm{Fe}}^{56}$ deduced from ($p, \alpha$) and ($\alpha , {\alpha }^{\prime }$) reactions is ∼1.5 Mev, while inelastic neutron scattering data yield a value of 0.95 Mev. The resolution of this disagreement will require modification of the usual calculations of inverse cross sections.