CALCULATION OF ENERGY AND CHARGE DEPOSITION AND OF THE ELECTRON FLUX IN A WATER MEDIUM BOMBARDED WITH 20‐MEV ELECTRONS *

During the past few years, a small-scale but steady effort has been underway in the Radiation Theory Section of the National Bureau of Standards to calculate the transport of fast electrons through extended media by an approach which combines random sampling with analytical procedures. This work has been done becauseof an interest in transport theory and with a view toward applications in space science and technology. The invitation to speak at this symposium provided a stimulus to undertake applications in medical physics. We believe that our calculational approach may be of help to people concerned with dosimetry for electron-irradiation therapy, and we should therefore like to indicate here the kind of results that can be obtained. This paper presents information on the penetration of high-energy electrons (mainly 20-mev beams) into a water phantom, with particular reference to the following topics: (1) energy and charge deposition; (2) electron flux (differential in energy), including primary and secondary electrons down to a spectral energy of 400 ev; (3) distribution of delta-ray track ends. All of these quantities have been obtained as a function of the depth for a broad incident beam. Some results for energy dissipation have also been obtained for a narrow beam, as functions of depth as well as radial distance from the beam axis. The results of this paper are for the most part recent and are considered preliminary. Their implications and significance have not yet been studied sufficiently, and there has not been time to make comparisons with all other pertinent calculations and measurements. Our coverage of topics is similar to that in important work by Kessaris'" and but we avoid some of the approximations made by these authors. Kessaris did his calculations by the moment method, on the basis of the continuous-slowing-down approximation. Harder used a mixed approach combining experimental and calculated information. The results of both authors were helpful to us in setting up our investigation. We also acknowledge our indebtedness to the pioneering work of Spencer and Fano' and of Spencer"'