Experimentally Determined Electron Energy Distribution produced by Cobalt 60 Gamma Rays

X Rays interact with matter to set in motion electrons which, on losing energy, give rise to the chemical and biological effects of the radiation. Because the rate at which these electrons give up energy depends on their energy, their energy spectrum is a necessary link in our understanding of the complicated chemical and biological effects of radiation. When the spectrum of the X rays is known, the distribution of the electrons set in motion by them may be calculated. This method has been used for a number of types of radiation by Cormack and Johns (1952). Unfortunately, difficulties arise in trying to take into account all the effects of scattered radiation. The change in photon spectrum in a scattering medium has been calculated theoretically in but a few geometries, none of which corresponds to that used in conventional therapy (Spencer and Fano, 1951). In general these calculations are very complicated. For one of these geometries, a point isotropic 60Co source in water, the electron spectrum has been measured with an organic scintillation spectrometer and was in good agreement with theory (Hayward, 1952). The use of a similar spectrometer to measure the distribution of electron energies in a water phantom at points down the central axis of a collimated 60Co γ-ray beam will be reported here. The apparatus used is illustrated in Fig. 1. The radiation beam was directed from the left of the figure.