A detailed re-evaluation of Clambda and Ce with application to ferrous sulphate G-values

The theoretical basis of Clambda and Ce is re-examined and consistent expressions derived involving the quantity Fwa which is a combination of a stopping-power ratio and an energy-absorption coefficient ratio, thereby taking into account the extent to which the ionisation chamber acts as an 'electron detector' and as a 'photon detector' for a particular radiation quality. An extension of the Spencer-Attix cavity ionisation theory to include energy dissipation due to photon interactions with the cavity wall and gas given in a recent paper by Nahum has been used to calculate values for Fwa appropriate to the Baldwin-Farmer ionisation chamber assuming an air-equivalent inner wall, and new values of Clambda and Ce are presented for this and similarly constructed instruments. Clambda also depends on whether the radiation is produced by a linac or a betatron of a given maximum photon energy. The new set of Clambda and Ce has been used to recalculate the experimental ferrous sulphate G-values of Law and Naylor with the result that the difference between G for high energy photons and electrons disappears.