For the ferrous-ferric dosimeter, numerous G values (that is, the number of molecules oxidized per 100 ev) have been reported in the literature. For cobalt-60 γ-radiation the generally accepted value for this radiation is 15.6 molecules/100 ev. Recent measurements with 16-MEV and 6-MEV electrons (1) yielded the same value to within the limits of experimental error. Some other measurements (2) indicate slightly lower values for 200-kv x-rays, and certainly, as the energy decreases further, the G value appears to drop; for example, with tritium beta particles at 5.5 kev average, the G value is 12.9 (3). In the course of radiobiological experiments comparing the three radiations listed above, it became necessary to measure the dose by chemical means. A comparison was therefore made between the values of G for the Fricke dosimeter for each radiation. These values depend on our current estimates of absorbed dose from exposure dose measurements, which are based in turn on Victoreen chambers calibrated at the National Bureau of Standards for 200-kv x-rays and cobalt-60 γ-radiation and by intercompari-son with Dr. J. S. Laughlin for 22-Mevp x-rays (4). The chemical comparison WE-S made by using an arrangement in which six Teflon cups containing 2 ml. of the dosimeter (1 mM FeS04' 1 mM NaCl in 0.8 N H2SO4) could be irradiated uniformly and simultaneously with doses of 1,000 to 9,000 rads. For each of the three radiations the exposures took place at a depth in Mix D tissue-equivalent wax through two parallel opposing fields, in order to obtain a uniform dose distribution. Measurements, were made with a 250-r chamber placed at the center of the exposure volume and calibrated against a 25-r substandard with a constant amount of stem exposure. A correction was made for the softening of the x-ray beam from its primary value (200 kv, 0.5 mm. Cu + 1 mm. AI, h.v.1. 1.4 mm. Cu, equivalent wave length 96.5 Kev) to an effective h.v.1. of 0.78 mm. Cu (equivalent wave length 75.2 Kev) for a depth of 5 em. in Mix D wax. Further corrections were made for the difference between the absorption properties of Mix D wax and water and for the absorption of x-radiation in the dosimeter components as compared with water for a given exposure dose. The concentration of ferric ion was determined by direct absorption at 3040 A°; the molar extinction coefficient was measured with known concentrations of ferric ion to be 2,157 at 25°C. The results obtained for the three experiments are shown in Table I. There is very good agreement between each of the three series, and the standard deviation due to the scatter of points is shown. The main errors are in the calibration of exposure dose and the estimate of absorbed dose. In view of the inaccuracies involved, the conclusion may be drawn from these measurements that the G values for cobalt 60 and for betatron 22-Mevp x-rays agree to within the limits of experimental error, but that the G value for 200-kv x-rays (0.78 mm. Cu) is about 8 per cent lower.
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Sinclair et al. (1958) studied this question.