PRINCIPLES OF RADIOLOGICAL PHYSICS 137 



action of the different radiations are equal when expressed as a prob- 

 ability of an "event" per unit energy dissipation. The rates of action 

 are also equal when the dose is expressed by the number of any kind of 

 physical processes which correspond, on the average, to equal dissipation. 

 Thus the "effective volume" for the production of a biological event 

 under consideration by an average activation (see Sect. 5-2c) has eciual 

 values for the different radiations. Similarly, the effective volumes for 

 production of that effect by an ionization or by a collision of a fast ion- 

 izing particle have equal values for different radiations. 



If a certain physical process is to be regarded as the independent 

 primitive "cause" of a certain biological effect, the corresponding effective 

 volume should have a fixed value no matter what radiation is employed. 

 The application of this principle has been the object of much attention in 

 the development of the target theory. The aim was to select a single 

 basic process from the various possibilities, such as the absorption of a 

 photon or the production of an ionization in a target region or the passage 

 of an electron through that region. Experiments were then made with 

 different radiations, mostly with X rays of different energies, to determine 

 the effective volume or area for the production of a biological effect by 

 each of these processes. Since the effective volume pertaining to ioniza- 

 tions was believed to have an equal value for different radiations, it was 

 argued that the production of an ionization within a target volume con- 

 stitutes the "hit" which causes the observed biological effect. The 

 supposedly eciual effectiveness of X rays of different energies and wave 

 lengths was often referred to as the "lack of a wave length effect." 



Actually, as already mentioned, a fixed effective volume is found for 

 any physical process which occurs in numbers proportional to the energy 



newer evidence was not rapidly and generally accepted as a general rule in biological 

 effectiveness. More recently, experiments with microorganisms and on the produc- 

 tion of cytogenetic effects (Kirby-Smith et al., 1952) have provided important con- 

 tributory evidence. 



One reason for the earlier failure to detect this effect may be traced to a remark by 

 Lea (1946). Most of the earlier experiments dealt with the comparative effectiveness 

 of X rays with photon energies ranging from about 20 to about 200 kev. Lea pointed 

 out that the average energy of the secondary electrons ejected by such photons varies 

 comparatively little, despite the tenfold range of photon energies, because the mecha- 

 nism of ejection changes from predominance of photoelectric effect to predominance of 

 C'ompton effect. Therefore, evidence from X-ray work in this energy range was not 

 critical. 



In the light of the new evidence, the discussion in Sect. 5-5a becomes rather 

 academic, since it deals with "phenomena ... in which different ionizing radiations 

 produce equal effects for equal energy dissipation." Such phenomena may not exist. 

 On the contrary, nearly all the phenomena appear now to fall in the class considered 

 in Sect. 5-5b. It is not just "densely ionizing radiations" which have higher effective- 

 ness. Rather, the effectiveness appears to decrease steadily as the density of the 

 ionizations, or density of collisions, decreases to its lowest attainable value. 



