Improved Calculation of Photoneutron Yields Released by Incident Electrons

Calculations are presented of yields of low-energy neutrons released by electrons incident on semi-infinite targets of natural C, Al, Fe, Ni, Cu, Ag, Ba, Ta, W, Au, Pb and U. Yields are based on photon differential track-length distributions derived for semi-infinite targets from Approximation B of analytical shower theory. By the introduction of additional corrections in the treatment of the photon track-length distributions, calculational accuracy in the electron energy region near the photoneutron threshold is considerably improved over a previous report. The calculations confirm the yields of the previous report for high incident electron energies but show them to be overestimates close to threshold. Systematics of yields are re-examined and a parameterization is offered which can be used to estimate yields very close to the threshold for any material. Results are relevant to the energy range used for radiation therapy.