In 1979 ICRU recommended the value W = (33.85 *0.05) eV for the average energy required for electrons to produce an ion pair in dry air. A re-evaluation is presented which is based on recently published stopping-power data. The new result is W / e = (33.97i0.05) J C", where all stated uncertainties are estimated standard deviations.
Two experimental determinations of W, the average energy required to produce an ion pair in dry air, for electrons arising from 6oCo radiation, are presented, One is based on calorimetric and ionometric measurements of absorbed dose in graphite, the other on activity and exposure measurements. The values obtained by using the most recent data for the electron stopping powers are W / e = 33.96 f 0.08 and 33.8 I f 0.14 J C", respectively. The good agreement with the W values from other authors whose methods do not involve stopping powers gives support to the new stopping-power values.
An experimental determination has been made of the perturbation correction to be applied to a graphite cylindrical (thimble-type) ionisation chamber used for the measurement of absorbed dose in graphite. The results show that the magnitude of this correction increases with depth and depends on the size of the cavity. The knowlede of this correction makes possible the use of such a chamber as a standard for absorbed dose in graphite for 6 0 ~o gamma rays.
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