Track Theory and Radiation Quality

TRACK THEORY AND RADIATION QUALITY. Radiation detection and damage data from several physical, chemical, and biological systems have been analysed by a unified track theory, in which observed effects are attributed to the interaction of secondary electrons with the medium. Gamma-ray dose-response curves are combined with calculations of the spatial dose distribution about an ion's path to yield dose-response curves (survival curves) for heavy ion bombardment, through appropriately defined parameters. Perplexing phenomena associated with high LET radiation are sorted out according to track regime (grain-count or track-width), inactivation mode (gamma-kill or ion-kill), structural complexity (elementary, cellular, multicellular), and end-point. The central problem in assigning a quality factor to radiation lies in the fact that the variables describing the bombarding particle and those describing the medium are not separable. What seems to be required is a theory of survival curves in which cells are represented by measured parameters, from which their response to a particular radiation environment may be calculated. A start has been made in this direction. In the past decade a new theory of track structure has been built, based on the concept that track effects arise principally from the interaction of secondary electrons with the surrounding medium, and that the relevent variable is the local dose deposited in sensitive elements by secondary electrons (13). Since the response of a system to gamma-rays also arises from the interaction of the medium with secondary electrons, it is possible to use the gamma-ray dose-response (survival) curve as the basis for the understanding of particle tracks. We assert that the probability P for the production of a sensitized (inactivated) element may be written as where is the dose experienced by a sensitive element, and E, and m are the extrapolated D-37 dose (for gamma-rays) and the extrapolation number (for gamma-rays). Where m = 1, the system is said to have a 1-or-more hit response to dose. E, is then the dose at which 0.63 of the elements are sensitized, or at which there is an average of 1 interacting event per sensitive element. Such a system shows exponential survival to gamma-rays. Many detecting systems have 1-hit response, including nuclear emulsion, dry enzymes and viruses, and scintillation counters. Current work implies that such a description may also be applicable to the bubble chamber, the observation of free radicals in solids by esr, and to the Fricke dosimeter. The survival of some cellular systems is given by Eq. (I), with m>l. Their survival curves are sigmoidal rather than exponential, and are usually described as multi-target single-hit * Supported by the U. S. Atomic Energy Commission and the National Science Foundation.