Production of neutrons from water, polyethylene, tissue equivalent material and CR-39 irradiated with 2.5-30 MeV photons.

Photoneutron yields from water, polyethylene, tissue substitute and CR-39 have been calculated for the photon energy range of 2 to 30 MeV, using a previously established method and photoneutron production data on hydrogen, carbon, nitrogen and oxygen. The rarer isotopes of the constituent elements of these compounds, namely 2H, 13C, 15N, 17O and 18O, have been taken into account and neutrons are shown to be produced for photon energies above 2.2 MeV, the (gamma, n) threshold for 2H. The data are useful for estimating neutron production in materials located in the vicinity of a megavoltage radiotherapy beam. Substances such as those considered here are often used as filtration, phantom or scattering material and as components of neutron dosimetry detectors. Photoneutrons produced in such materials may need to be taken into consideration when carrying out neutron dosimetry in the presence of photons in this energy range, especially when the neutron flux is several orders of magnitude less than that of the photons.