G-value determination for 20 MeV protons

There are appreciable differences in some G values for protons reported by different laboratories using the same measuring technique. The water equivalent Fricke dosemeter is suitable for radiobiological investigations involving X-rays, fast electrons and heavy ions of different LET. A description is given of measurements made using the emergent beam of a cyclotron. Beam monitoring was performed by a transmission type of ionization chamber, and the absorbed dose was determined with a standard ionization chamber operated with atmospheric air. The Fricke dosemeter solutions used were 1mM in ferrous ammonium sulphate, 1mM in NaCl and 0.8N in H2SO4. The ferric ion concentration was determined as usual by measurement of the optical extinction at 304nm. The results demonstrated linearity between absorbed dose and ferric yield at least up to 30 krad. A value for G of (14.5 +- 0.6)(100 eV)-1 was found for (19.3 +- 2.0) MeV protons. This value was independent of dose rate between 5 and 100 krad min-1, as expected for sparsely ionizing radiation. (U.K.)