Interpreting microdosimetric spectra

The field of microdosimetry is the study of those physical characteristics of energy transfers other than dose that produce variations in biological consequences of exposures to radiation. Historically, this area developed primarily from a need to understand and quantify the differences in human responses to different types of radiation, in particular the variations in late complications for the same level of acute effects such as edema and vomiting or tumor control in radiation therapy, and further applications in radiation health. In space applications, this field has been used to interpret variations in responses of personnel (and sometimes electronics) to the different radiation fields present in space environments, especially in comparison with exposures on the Earth’s surface from which the preponderance of our radiobiological database originates. These variations result in differing risks which must be evaluated for the health and welfare of astronauts and for establishing compliance with regulatory limits. Research on the physical aspects of the radiations includes the investigation of probability density functions for energy transfers, molecular physics, track structure, thermodynamics, and even radiation chemistry as evidenced by the variety of articles published in the proceedings of the microdosimetry symposiums [Proceedings of the 15 International Symposium on Microdosimetry, 2011, and published proceedings of earlier symposiums] as well as other scientific literature [e.g., M. Zaider and J. F. Dicello, 2004 and references therein]. Again, scientists not in the field usually will encounter this area in terms of efforts to evaluate acute and late consequences of exposures or to determine levels of exposure for regulatory purposes [NCRP Report No. 137, (2001)]. The purpose of this article is to briefly present the fundamentals of the most common methods of presenting such data in the literature in order to briefly provide the casual reader sufficient background to better understand the data in context. To fulfill this objective, the article will focus on the most common method of presenting such data, i.e., probability distributions for energy deposited as a function of energy deposited, probability density functions, leaving the reader seeking further details to peruse more extensive publications such as NCRP 137 [2001].