Biological effectiveness of low energy protons

The biological effectiveness of monoenergetic protons was investigated with the track-segment method. Protons were accelerated by a Tandem Van de Graaff accelerator and their final energies were 3.0 and 7.4 MeV. The biological system used was Chinese hamster V-79 cells and their survival ability following proton irradiation was investigated. Cobalt-60 γ-rays were used as reference radiation to assess proton relative biological effectiveness (RBE). Survival curves were obtained for the γ-ray and proton irradiations, and the relation S=exp(-αD-βD2) was fitted to the data and the parameters α and β were determined. The RBE values, calculated on the basis of the mean inactivation dose D-bar and other pertinent parameters, were found to be 1.7+-0.1 and 2.8+-0.2 for 7.4 and 3.0 MeV protons, respectively. Comparisons were made with the results published by other investigators and it was concluded that in this low energy range the biological effectiveness increases substantially with decreasing proton energy. (author)