Introduction

Abstract

In its action on matter, including living matter, ionizing radiation is uniquely efficient because it transfers energy to atoms in a highly concentrated form. The average energy absorbed per unit mass of irradiated medium, the absorbed dose, is minute compared to the energy densities that elicit comparable effects by other physical agents. The effectiveness of ionizations is further enhanced by their association in the tracks of charged particles. Depending on its microscopic distribution the energy required for a given level of biological injury may vary as much as a hundredfold between different ionizing radiations. Thus while the absorbed dose is a useful and standard quantity in the specification of irradiation, effects depend on the pattern in which a given amount of energy is deposited in the irradiated medium. A knowledge of such energy distributions is required not only in any explanations of the relative effectiveness of different kinds of ionizing radiation but it also can be expected to provide insight into the action of ionizing radiation in general. This has been recognized since the earliest days of radiobiology and led to the fundamental contributions by such investigators as Crowther, Dessauer, Timofeff-Resowsky, Zirkle and Lea.