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Recent measurements in a p(62)Be(36) neutron therapy beam have shown that the quality of the in-phantom beam changes with depth. This variation can be ascribed to the presence of a relatively large low-energy neutron component emanating from the neutron source. As part of the pre-clinical calibration programme at a newly commissioned neutron therapy facility, radiobiological and microdosimetric observations were made to determine the magnitude of this effect on a p(66)Be(40) beam and to evaluate the hardening effect of a hydrogenous filter. The reported data identify a correlation between the two assays and quantify a linear relationship between y* and filter thicknesses less than or equal to 6 cm. Using the data obtained in the study, a filter thickness was selected to comply with clinical requirements. By employing lineal energy spectra, it is demonstrated that subtle changes in beam quality may be quantified in a reproducible manner without resorting to time-consuming radiobiological studies.

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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%.

The kerma for monoenergetic neutrons of nominal energies 27, 42 and 63 MeV was measured with commercial tissue-equivalent and carbon proportional counters. By operating the detectors in helix mode and exposing them to pulsed beams of quasi-monoenergetic neutrons, time-of-flight (TOF) techniques enabled the unambiguous identification of pulse height events associated with the neutron energies of interest. The microdosimetric spectra so obtained were used to derive gas-to-wall dose conversion factors for the TE gas filled carbon proportional counter. Subsequently the kerma ratios for carbon to A-150 plastic at the three nominal neutron energies were determined with the monoenergetic data. The results concur with other published data using different techniques.