Microdosimetry

Abstract

"Microdosimetry" is the latest title to be issued by the International Commission on Radiation Units and Measurements in their well known and well respected Report series. The subject is concerned with the study of the spatial distribution of energy deposition along the tracks of ionizing radiations. On a microscopic scale such energy deposition is far from uniform and, while a full description of the interaction of ionizing radiations with matter together with the consequences of such interactions is probably unmanageable in interpreting the microscopic effects of radiation, nevertheless a study of energy deposition in volumes comparable to the elemental structures being affected is an obvious starting point. The Report is a comprehensive review of microdosimetric concepts and techniques, both experimental and theoretical and forms a very useful state-of-the-art review.The first major chapter describes the quantities needed for microdosimetry and their interrelation. The next chapter very briefly reviews the primary interactions between ionizing radiations and matter, whilst in the following one the physical processes involved in the interaction of charged particles (mainly electrons and fast ions) are described in more detail together with the processes by which such particles lose energy, and the spatial distribution of energy deposits. Then come two chapters on, respectively, the experimental determination and theoretical calculation of various microdosimetric quantities. The principal detector for microdosimetric measurements is, of course, the gas proportional counter, usually operated at low pressure. The various techniques used to simulate small volume cavities are discussed, with their limitations, while an appendix describes in detail some of the designs of proportional counters that have been used. A fairly reliable indication that any new frontier of science has emerged from the "gee whizz" stage to more mature respectability is given when its proponents start to discuss uncertainties seriously; in this respect it is significant that there is a section in the experimental chapter on just this topic. In the theoretical calculations of microdosimetric quantities two methods have been used, viz., the analytical and the Monte Carlo methods, although so far mainly for fast neutrons only. Most interest centres on the differences between the theoretical and experimental data rather than on the relative merits of the two theoretical approaches, the limitations of the experimental method generally being identified as the culprit for most discrepancies.The final chapter is perhaps the most interesting, especially to the reader with a more general interest. It deals with the various applications of microdosimetry to the real world of radiation, including the vexed question of the specification of radiation quality as well as the microdosimetric implications in radiation chemistry, radiobiology and radiation protection. Interesting though these applications are, it has to be said that the overall impact of microdosimetry on ionizing radiation activities generally has been less than one might have been led to expect. Perhaps the pay-off has yet to come when the techniques become even more refined.The Report has several useful appendices and an extensive list of references. It is certainly a "must" for anyone entering the field and it is a very convenient reference datum for practitioners.