Measurement of the G-value for 1·5 keV X-rays

Summary

Although there are several theoretical predictions of the dependence of the G-value on X-ray energy, measurements have not been made below ≈ 7 keV. Using a ferrous sulfate solution modified by the addition of benzoic acid, we have measured the relative G-values for Al_k characteristic X-rays (1·5 keV), ²³⁸Pu α-particles (3·7 MeV), ⁶⁰Co (1·17 MeV) and ¹³⁷Cs (0·66 MeV) γ-rays. This modified ferrous sulfate solution gave a 4-fold increase in sensitivity relative to the conventional solution, making measurements with the Al_k X-rays feasible. The relative ferrous-to-ferric conversions as a function of dose were similar for the two γ-ray energies, yielding G-values of 1·62 and 1·59 µmol J⁻¹ for the ⁶⁰Co and ¹³⁷Cs radiations, respectively. The α-particle G-value was 0·52 µmol J⁻¹, or 31 per cent of that for the ⁶⁰Co γ-rays, in good agreement with previous measurements. The Al_k X-rays had a G-value of 0·92 µmol J⁻¹ or 57 per cent of that of the ⁶⁰Co radiation. This G-value for the 1·5 keV X-rays is within 20 per cent of the values predicted by current theories, and theoretical values are within the error range of our measurement. The consistency between the experimental value reported here and theoretical G-values for ultrasoft X-rays should be valuable for models of radiation action on biological systems.