A modified lead attenuation method to determine the fast neutron sensitivity kU of a photon dosemeter

Various lead attenuation techniques to determine the fast neutron sensitivity kU of a photon dosemeter are reviewed and a modified method is proposed to determine the kU value of a commercially available Geiger--Müller detector. The dependence on the 'effective' photon energy of the gamma-component of the mixed n--gamma field is illustrated and the most probable photon energy is deduced from measurements outside the radiation field. Using the relevant photon mass attenuation coefficient, measurements carried out with a well-collimated neutron beam at the MRC Cyclotron, Hammersmith Hospital, London, show that the Dg component contributes 2.41 +/- 0.13% to the total radiation field. Using a detector with a PTFE sleeve in the place of a Perspex sleeve, highly consistent results are obtained and the enhancement of the kU value by the Perspex is demonstrated. The neutron sensitivities of two MX 163 GM detectors with Perspex and PTFE sleeves were respectively determined as 0.73 +/- 0.07% and 0.98 +/- 0.14%.