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.
The average number of neutrons ( V ) emitted per fission event in the spontaneous fission of 252Cf has been determined by separate absolute measurements of the fission rate and the neutron emission rate per mg of a solution of californium chloride. The fission rate was determined by a hitherto untried method based on fission-fission coincidences, and the neutron emission rate was determined by the well-known manganese sulphate bath technique. The measurements were carried out during the period 1968 to 1972, and several provisional values of V have been released from time to time. The results have now been subjected to a rigorous statistical treatment in which correlations were treated by covariance matrices.The value for V (total) for 252Cf is (3.7509 k 0.0107) neutrons per fission. This value supersedes all previously released provisional values.
The neutron emission rates of the UK national standard Ra-Be photoneutron source and a 252Cf spontaneous fission source have been remeasured by the manganese sulphate bath technique. The measurements were performed as part of a dilution experiment in which the hydrogen-to-manganese capture-cross-section ratio was determined following replacement of the original manganese sulphate by purer material. The data have been subjected to rigorous analysis in which the correlations were treated by covariance matrices. These measurements confirm the re-evaluation of previously determined source emission rates published in 1982. A value for the hydrogen-to-manganese capture-cross-section ratio of 0.02477 ± 0.00005 (one standard deviation) is recommended for use with other manganese baths and other neutron source spectra.
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