Radiological physics and dosimetry research with a small positive ion accelerator

The distribution of energy along and around the path of charged particles is the fundamental determinant of the chemical and biological effects of ionizing radiation. This energy is deposited initially in a relatively small number of excited molecules lying generally close to the path of the secondary charged particles. This discreet initial structure of the track is of special importance to radiological research. The task confronting the radiation physicist is to develop a comprehensive description, on a microscopic scale, of the primary energy depositions prevailing when a radiation field interacts on biological matter. In view of the inhomogeneity and complexity of this system, this is a monumental task. This paper reviews the current status for measuring directly the energy deposition in simulated biological matter. Inasmuch as current radiobiological information indicates that the sensitive volume is very small, diameter of order 1 μm or smaller, deposition in volumes of this dimension are of primary interest. Limitations of experimental techniques make the direct measurement of energy deposition in such small volumes difficult and hence very uncertain. A Monte Carlo method for calculating this discreet initial track structure is discussed. The input data for calculations of microscopic energy depositions are double differential cross sections for electron ejection by energetic ions. The current status of the experimental measurement of these cross sections is reviewed