Cavity Ion Chamber Theory

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Abstract
A new analysis is given of the cavity ion chamber principle with allowance for discontinuous energy losses by electrons. The relative energy dissipation in the cavity by electrons in the various kinetic energy ranges, is calculated by a simple method which allows for delta-track formation and radiation losses. From a consideration of the concept of electron track, it is shown that the mass stopping power ratio hitherto adopted should be replaced by a cavity mass energy-dissipation ratio. Rigorous derivation of the latter presents severe and unsolved difficulties, although in certain cases, experimental data can be used to make approximate interpolations. It is shown that there is no significant discrepancy between cavity chamber determinations, and Taylor and Singer''s free air chamber measurement of the dose rate from radium gamma-rays. Values for WAir are derived from published reports of the calibrations of gamma-emitting isotopes by ionization, calorimetric, and absolute counting methods. These values are from 4.5 to 7.5% higher than the generally accepted figure of 32.5 ev. In an appendix, specimen calculations are set out in tabular form showing the derivation of the factor needed to convert the ionization per unit mass of air observed with a spherical graphite wall chamber to the value which would be obtained with an exactly air-equivalent wall.

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