Ferrous sulphate G-values for megavoltage photons and electrons derived from ionisation dosimetry using Csub(lambda) and Csub(E) values

The authors have recalculated their previous ferrous sulphate G-values from their earliest measurements, but using HPA revised sets of Csub(lambda) and Csub(E) values. The new results still show a continuing upward trend of G-value with photon energy. The trend is closely in line with the variation from lower energy x-rays up to 60Co radiation, and they conclude that there is no reason to suppose that the slow increase in G-value should stop there. Thus their results provide no support for the idea of a constant G-value of 15.5 from the 60Co energy upwards, and suggest that the error in any such assumption could be about 2% at around 30 MV. For megavoltage electrons, the results are consistent with a constant value close to 15.75. (U.K.)