Track structure theory in radiobiology and in radiation detection

The response of biological cells, and many physical radiation and track detectors to ionizing radiations and to energetic heavily ionizing particles, results from the secondary and higher generation electrons ejected from the atoms and molecules of the detector by the incident primary radiation. The theory uses a calculation of the radial distribution of local dose deposited by secondary electrons (delta-rays) from an energetic heavy ion as a transfer function, relating the dose-response relation measured (or postulated) for a particular detector in a uniform radiation field (gamma-rays) to obtain the radial distribution in response about the ion's path, and thus the structure of the track of a particle. Subsequent calculations yield the response of the detector to radiation fields of arbitrary quality. The models which have been used for detector response arise from target theory, and are of the form of statistical models called multi-hit or multi-target detectors, in which it is assumed that there are sensitive elements (emulsion grains, or biological cell nuclei) which may require many hits (emulsion grains) or single hits in different targets (say, cellular chromosomes) in order to produce the observed end-point. Physically, a hit is interpreted as a ‘registered event’ caused by an electron passing through the sensitive site, with an efficiency which depends on the electron's speed. Some knowledge of size of the sensitive volume and of the sensitive target is required to make the transition from gamma-ray response to heavy ion response. Critical differences in the pattern of response of biological systems and physical detectors to radiations of different quality arise from the number of electrons which must pass through the sensitive volume to produce the recorded end-point. For biological cells this is typically 2 or more. This characteristic multi-hittedness results in survival curves with shoulders, or supralinear dose-response relations for gamma-irradiation, and for an ‘RBE’ which can exceed 1 at appropriate values of the ‘LET’. One-hit detectors cannot mimic the response of biological cells to radiations of different quality. From the beginning it has been clear that SSNTD's (etchable plastics) are not 1-hit detectors. But even now, we do not know their characteristic response to gamma-rays. We are not able to produce a satisfactory theory of track structure in these detectors. There is only a hint, that etching rate is nominally proportional to the quantity $\text{z}{}^4\text{β}{}^4$ of the incident ion, suggesting the possibility of a ‘2-or-more’ hit detector.

Recent work has demonstrated that many-hit physical detectors do exist. From both emulsion sensitometry and from the structure of tracks of heavy ions, we are able to show that emulsion-developer combinations exist which yield many-hit response. There is also some evidence that the supralinearity in thermoluminescent dosimeters arises from a mixture of 1-hit and 2-hit response, perhaps of different trap structures within the same TLD crystal. These detectors can be expected to mimic the response of biological cells to radiations of different quality. Their patterns of response may help us to understand better the structure of particle tracks in SSNTD's.