PRINCIPLES OF RADIOLOGICAL PHYSICS 121 



rays, for which this unit was designed and for which measuring tech- 

 niques are highly developed, but attempts to extend the application of the 

 roentgen to other ionizing radiations are likely to subside. On the other 

 hand, measurements of ionization in air may well remain a most impor- 

 tant method for the indirect measurement of energy dissipation (see 

 Fano and Taylor, 1950). 



The X-ray dose, expressed in roentgens, and the energy dissipation 

 per gram of a material are related in a simple manner if the material is 

 homogeneous and the X-ray energy is low or moderate. For example, 

 the energy dissipation in a mass of soft tissue exposed to X rays of a few 

 hundred thousand electron volts amounts to approximately 93 ergs/gram 

 per roentgen. 



Under more complicated conditions, the conversion of the value of the 

 dose from roentgens to energy units offers some difficulty. The X-ray 

 dose in roentgens at a point depends only on the flow of X rays at that 

 point and on the properties of a standard reference material (air). The 

 energy dissipation depends on the properties of the material actually 

 present at that point and in the surrounding space as well. The methods 

 for calculating the energy dissipation in a material have been outlined in 

 earlier sections. 



The techniques for measuring radiation doses are discussed in Chap. 2. 



5-lb. Methods of Expressing Macroscopic Effects. In order to establish 

 and analyze quantitative relationships between the macroscopic effects of 

 radiation and the quantity and quality of the radiation, some meaningful 

 quantitative index of the effect under consideration must be adopted. 



Among the characteristics of biological systems which are subject to 

 quantitative measurement are the sizes and masses of organisms or of 

 their various parts, the duration of certain stages of development, and 

 the chemical composition of various biological components. Many 

 kinetic studies of the influence of radiation on variables of this kind have 

 been made. 



However, much greater emphasis seems to have been laid on biological 

 effects in which the frequency of occurrence of a certain event among a 

 population of organisms is observed. Biological objects other than 

 separate organisms, for example, different cells of the same organism, may 

 be regarded as the elements of a population. In some instances, as in 

 the example of genetic mutations, the event constitutes, by its own 

 nature, a clear-cut all-or-none phenomenon. (Thus, the "white" eye or 

 the "cut" wing mutant of a fruit fly constitutes a well-defined inheritable 

 character of the organism.) In other instances the event is defined more 

 or less arbitrarily, as, for example, when the inability of a bacterial cell to 

 develop into a visible colony serves as a criterion of the "death" of the 

 cell. 



In either case, the observation of the frequency of events produced by 



