Cross sections are reported for ($p, \mathrm{pn}$) and ($p, 2p$) reactions of ${\mathrm{Ce}}^{142}$ with protons of 0.4, 1.0, 2.2, and 3.0 Bev. To evaluate the contribution of proton and neutron evaporation from excited ${\mathrm{Ce}}^{142}$ nuclei to the observed cross sections, this evaporation competition was studied by means of cross-section measurements on the reactions ${\mathrm{Ba}}^{138}(\alpha , n)$ and ${\mathrm{Ba}}^{138}(\alpha , p)$ with $\alpha$ particles of 19- to 40-Mev kinetic energy. The ratio $\frac{{\sigma }_{\alpha ,n}}{{\sigma }_{\alpha ,p}}$ is between 2.4 and 4.4 in this region as compared with a ratio of 1.58±0.14 for $\frac{{\sigma }_{p,pn}}{{\sigma }_{p,2p}}$ at 0.4 Bev. It is concluded that at this energy the ($p, \mathrm{pn}$) and ($p, 2p$) reactions proceed at least partially by a pure knock-on mechanism. This conclusion is supported by comparison of the data with recent Monte Carlo calculations of intranuclear cascades. With increasing proton energy both ${\sigma }_{p,2p}$ and ${\sigma }_{p,pn}$ are found to decrease, the former more rapidly than the latter. This energy dependence is discussed in terms of the elementary nucleon-nucleon cross sections. The Monte Carlo calculations mentioned agree within their rather poor statistical accuracy with the measured cross-section ratios and with the energy dependence of the cross sections, but predict cross-section values about a factor of three too small.

• Received 12 February 1958

DOI:https://doi.org/10.1103/PhysRev.110.1169