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A matter of growing concern amongst users of electron accelerators for medical application is the problem of photoneutron production in target and collimating assemblies and the consequent whole body neutron dose to a patient undergoing radiotherapeutic treatment. Measurements on a 35 MeV linear accelerator constructed by the M.E.L. Equipment Company Limited indicated that the neutron dose-equivalent rate in the x-ray beam was of the order of 0.5% of the central axis x-ray absorbed dose rate. Measurements reported by various workers on betatrons lead to values between 1.0 and 3.0% (Frost and Michel 1964). Various manufacturers in the United Kingdom and other countries are now supplying accelerators capable of operating in the 15-45 MeV range which will give rise to neutron production. There is an additional complication in that photoneutrons produced by high energy photons incident on the patient contribute to patient dose.Rough calculations based on measurements of photoneutron production by Barber and George (1 959) lead to the results shown in table 1.

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This is a report of an international comparison of determinations of neutron-source emission rates. Contributions received from fourteen laboratories describing emission-rate measurements of three sources by four different methods are analysed by a least-squares minimization procedure to determine the best values for the three emission rates and for the residual differences in the measured values. The residual differences are considerably reduced when the input values are normalized to a common set of standard cross-sections and to a common basis for the calculation of corrections.