Photoneutrons from Lead

The photoneutrons ejected from a small lead target by 23-Mev bremsstrahlung x-rays from a betatron have been observed by means of recoil protons in nuclear emulsions. The angular distribution of the more energetic photoneutrons was nearly isotropic. The yield, in units of 10⁷ neutrons per mole-roentgen, was 1.4±{}0.2. The observed energy distribution had its maximum near 1.75 Mev. A yield curve for photoneutrons was obtained using neutron counters in paraffin. A cross-section curve was derived from the yield curve with a normalization based on the nuclear-emulsion data. The calculated energy distribution which gave a satisfactory fit to the observed photoneutrons consisted of 90% "evaporation" from a compound nucleus and 10% emission by the "resonance" direct effect. In calculating the contribution of "evaporated" neutrons a nuclear "temperature" for the residual nucleus of 1.35 Mev was used, and the factors for transmission of $l=0$ neutrons through a centrifugal barrier were applied.