Microdosimetric Characterisation of Radiation Fields with a High Pressure Ionisation Chamber: A Comparison with the Low Pressure Proportional Counter

Abstract

Abstract The application is described of a tissue-equivalent (TE) high pressure ionisation chamber that permits evaluation of the mean quality factor in mixed neutron-gamma radiation fields and compares the data obtained with those acquired with a TE proportional counter (Rossi counter). Irradiations are produced by a research fission reactor. The radiation spectrum composition is modified by moderating the beam by various heavy water (D2O) or lead thicknesses and by inserting boral shutters in the neutron beam. Thus, radiation fields with most of the radiation dose originating from thermal neutrons or fast neutrons are obtained, with associated gamma ray absorbed doses ranging from 35% to 90%. The mean quality factors derived are in agreement (±10%) for situations where most of the radiation dose is due to fast neutrons. When most of the delivered dose is due to thermal or intermediate neutrons, the Rossi counter systematically predicts smaller (30 to 40%) quality factors than the high pressure TE chamber, this observation being confirmed on theoretical grounds, as most of the neutrons interacting with the Rossi counter do not cross the counter at these energies (i.e. they are 'stoppers'). The practical advantages and disadvantages of using either instrument in a particular situation are discussed.