Low-Energy Neutron Energy Spectra from (p,n) Reactions

Coincidence experiments indicate that ($p,np$) reactions on ${\mathrm{Zr}}^{90}$ and ${\mathrm{Sn}}^{112}$ leave the residual nuclei in their ground states, whence neutron spectra can be deduced from measurements of proton spectra. Results are very different for the two nuclei. Analysis indicates that the variation of ${\sigma }_i$-the cross section for the inverse process-is larger than expected and in opposite directions for the two nuclei.