Measurement of Stopping-Power Ratio for Cavity Ionization Chambers

Abstract

THE energy absorbed in a medium per unit mass, E_m, exposed to X- or γ-radiation can be related to the ionization produced per unit mass of gas, J_m, situated in a cavity in the medium by the Bragg–Gray equation: where W is the average energy associated with the production of one ion pair in the gas, F_m is the mass stopping-power ratio of the gas to the solid for electrons. F_m has been evaluated theoretically by assuming electrons lose energy continuously^1,2 and also by allowing for discrete losses of energy^3,4. However, experimental investigation has shown that these theoretical treatments are only useful for a limited range of size of cavity⁵, as indeed the initial assumptions in their derivations imply. In many situations the greatest uncertainty in measuring exposure or absorbed dose arises from uncertainty in this factor. Thus a direct measurement of F_m is desirable.