Conversion factors for calculating mass stopping power ratios.

Abstract

In order to use an air-filled ionisation chamber for dosimetry of a photon or electron beam in a material, it is necessary to know the restricted (Spencer-Attix) mass stopping power ratio of the material, relative to air. A method is described in which a conversion factor, k, is derived by comparing unrestricted stopping power ratios of various materials (relative to air) with those for water (relative to air). Provided that the material does not differ too greatly in effective atomic number from that of water, this ratio is independent of energy and is a constant for any material both for unrestricted and restricted stopping powers. Values of k have been calculated for 31 materials and application of any one such factor to the restricted mass stopping power ratio of water relative to air allows the appropriate restricted mass stopping power of the material, relative to air, to be derived.