Ion chamber gas-to-wall conversion factors for fast neutron dosimetry

Modern ionising photon dosimetry is essentially entirely based upon gas-filled cavity determinations. For photons, ion chamber response is largely independent of photon energy almost perfectly transforming absorbed dose in the gas to the surrounding media. Absolute uncertainties are <1-2%. For fast neutron dosimetry, this is certainly not the case. Interpretation of the response of the cavity filling material, usually a gas, to the charged particle spectrum induced in the walls and interacting with the cavity gas is fraught with uncertainties. Despite these challenges, gas filled cavities surrounded by various mixtures, compounds and elements, have proved to be essential for integral determinations of the indirectly ionising neutrons, generating dosimetric quantities, such as kerma and absorbed dose. The transformation from gas response to wall dose is material dependent and varies with neutron energy. This study discusses recent advances in cavity response interpretation using the results from complex nuclear modelling of microscopic cross sections as well as estimates of secondary particle production enabling much improved cavity gas-to-wall media conversion factors. (authors)