Cross sections for ($p, \mathrm{pn}$) reactions on ${\mathrm{N}}^{14}$, ${\mathrm{F}}^{19}$, ${\mathrm{Fe}}^{54}$, ${\mathrm{Ni}}^{58}$, ${\mathrm{Cu}}^{63}$, ${\mathrm{Cu}}^{65}$, ${\mathrm{Zn}}^{64}$, ${\mathrm{Mo}}^{100}$, and ${\mathrm{Ta}}^{181}$ have been measured at proton energies of 0.4 and 3.0 Bev. For ${\mathrm{F}}^{19}$, ${\mathrm{Cu}}^{65}$, ${\mathrm{Mo}}^{100}$, and ${\mathrm{Ta}}^{181}$ cross-section measurements at several other energies between 0.3 and 3.0 Bev are reported also. Within 30%, all these ($p, \mathrm{pn}$) cross sections appear to be energy-independent in this range. At a given energy the cross sections show greater variations from nucleus to nucleus than can be explained on a purely statistical picture of knock-on processes. Among the lightest nuclei (${\mathrm{C}}^{12}$, ${\mathrm{N}}^{14}$, ${\mathrm{O}}^{16}$, ${\mathrm{Fe}}^{19}$), these variations can be correlated with the energy of the lowest lying level of the product nucleus which is stable with respect to particle emission. Among heavier nuclei this correlation disappears, and it is suggested that shell structure effects may be responsible for the fact that the ($p, \mathrm{pn}$) cross sections of ${\mathrm{Cu}}^{63}$, ${\mathrm{Cu}}^{65}$, and ${\mathrm{Zn}}^{64}$ are about 45% higher than those of ${\mathrm{Fe}}^{54}$ and ${\mathrm{Ni}}^{58}$. Apart from these individual variations which a statistical theory could not be expected to reproduce, it is found that the recent Monte Carlo calculations of intranuclear cascades by Metropolis et al. do not even predict the right magnitude and energy dependence for the cross sections. The calculated cross sections are too small by factors of 2 to 3 at 0.4 Bev and show a decrease with increasing energy. Possible reasons for these discrepancies are sought in details of the nuclear model used in the calculations. Various mechanisms which may contribute to ($p, \mathrm{pn}$) reactions are discussed. It is concluded that deuteron emission cannot contribute significantly at the energies considered. Processes involving evaporation of one of the nucleons are likely to decrease in importance with increasing energy, whereas the contribution of meson reactions, such as ($p, \mathrm{pn}{\pi }^0$), ($p, 2p{\pi }^{-}$), etc., probably increases with energy. The observed energy independence of the cross sections may result from accidental cancellation of such opposing trends.

• Received 21 July 1958

DOI:https://doi.org/10.1103/PhysRev.112.1295