Measurement of the G-value for 1.5 keV X-rays.

Although there are several theoretical predictions of the dependence of the G-value on X-ray energy, measurements have not been made below approximately equal to 7 keV. Using a ferrous sulfate solution modified by the addition of benzoic acid, we have measured the relative G-values for Alk characteristic X-rays (1.5 keV), 238Pu alpha-particles (3.7 MeV), 60Co (1.17 MeV) and 137Cs (0.66 MeV) gamma-rays. This modified ferrous sulfate solution gave a 4-fold increase in sensitivity relative to the conventional solution, making measurements with the Alk X-rays feasible. The relative ferrous-to-ferric conversions as a function of dose were similar for the two gamma-ray energies, yielding G-values of 1.62 and 1.59 mumol J-1 for the 60Co and 137Cs radiations, respectively. The alpha-particle G-value was 0.52 mumol J-1, or 31 per cent of that for the 60Co gamma-rays, in good agreement with previous measurements. The Alk X-rays had a G-value of 0.92 mumol 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.