(p, n) Threshold-Curve Shapes and Measurements of Threshold Enelrgy withH1+Beams

Yield curves have been calculated for ($p, n$) threshold reactions and the results have been applied to data previously obtained with the two-meter electrostatic analyzer at the Naval Research Laboratory. Taken into account in the calculation are the finite beam-energy spread from the analyzer, Doppler broadening due to thermal motion of the target nuclei, and the effect due to the statistical nature of the energy loss of protons in the target. A comparison of the yield-curve shape when the cross section is proportional to the emitted neutron velocity and the yield-curve shape when there is a nearby resonance is made. There is a small but calculable difference between the extrapolated yield curve intercept and true threshold energy. The threshold energies for several reactions are determined from data previously reported: ${\mathrm{T}}^3(p, n){\mathrm{He}}^3$, 1019.76±0.51 keV; ${\mathrm{Li}}^7(p, n){\mathrm{Be}}^7$, 1881.27±0.94 keV; ${\mathrm{C}}^{13}(p, n){\mathrm{N}}^{13}$, 3237.1±1.6 keV.