Cross Sections for Photoneutron Emission Induced by the Lithium Gamma Rays

Cross sections for emission of photoneutrons produced by ${\mathrm{Li}}^7(p, \gamma )$ gamma rays in thirteen middle-weight and heavy nuclei have been measured. Neutrons were detected by B${\mathrm{F}}_3$ counters embedded in paraffin, and the gamma-ray intensity was measured with a NaI scintillation detector. Measurements made with two groups of neutron counters whose efficiencies depended differently on neutron energy indicated that the energy distributions of the emitted neutrons are evaporation-like in shape. The present cross section values are about 20% higher than previous measurements of McDaniel, Walker, and Stearns, but the difference is within the combined errors. The measurements provide an independent check on the accuracy of the cross-section curves obtained with bremsstrahlung beams; agreement is obtained with University of Pennsylvania betatron data but not with some other betatron results.