A high-pressure ionisation chamber to measure the mean neutron energy and the gamma ray dose fraction of an unspecified neutron-gamma radiation field

Abstract

A prototype small high-pressure parallel-plate tissue-equivalent ionisation chamber with a sensitive volume of 0.63 cm3 is described. The chamber was designed to measure the mean incident neutron energy or the gamma ray absorbed dose fraction of unspecified neutron-gamma radiation fields on the basis of initial recombination theory in the neutron energy range 0.1-10 MeV. Calibration was carried out separately in gamma ray and neutron fields. The angular response of the ionisation chamber was investigated with gamma rays and angular independence demonstrated. The chamber was tested at the National Physical Laboratory at neutron energies of 2.4, 1.6 and 1.05 MeV and determined the mean incident neutron energy to within 5%. Experiments were also performed in Leeds in the mixed neutron-gamma radiation field of an Am-Be source; the gamma ray absorbed dose fraction was provided independently by the microdosimetric method and the mean incident neutron energy in this case was predicted to within 8%. The present method can also be used to provide the gamma ray absorbed dose fraction in the case where the mean incident neutron energy is independently known and this is illustrated for the case of the Am-Be neutron-gamma source where the gamma ray dose fraction was determined to within 12%.