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

Abstract

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%.

Effective frequency conversion of radiation in sodium vapor

“…The ionization chamber and GM tube responses are calculated in terms of absorbed dose per monitor unit (Gy MU −1 ). Hough (1979) has shown that for the modified lead attenuation method for the determination of the k U value of a detector one has…”

“…Although there are considerable data on k U values measured in low-energy neutron beams and 14 MeV beams produced in D-T generators, there are relatively few data for the high-energy neutron beams now used in neutron radiotherapy. In the present paper the k U value for a commercially available miniature energy compensated GM detector (Far West Technology, Goleta, CA; type GM 1-S) is determined in the d(48.5)-Be neutron beam produced by the superconducting cyclotron based neutron therapy facility at Harper Hospital in Detroit, using the modified lead attenuation method of Hough (1979). The superconducting cyclotron facility has been described in detail elsewhere (Maughan et al 1994, Maughan andYudelev 1995).…”