XIV. X KAYS AND X I H H A 1) 1 \ T 1 O N MM 



wavelengths lying between 4 and 0.5 A. Since semilogarithmic sur- 

 vival curves were obtained, he interpreted this to mean that the ab- 

 sorption of one energy unit in the vital volume of a bacterium was suf- 

 ficient to kill it. On this assumption, only 1 ciuantum in 4 with the 

 0.5 A. X rays was necessary for death, whereas it took 60 quanta for 

 one to be deadly with the 4 A. X rays. Instead of being more effective 

 as a killing agent the longei- wavelength X rays turned out to be some- 

 what less effective. On the basis of these and other experiments, 

 the quantum does not appear to be the primary agent in the causation 

 of the biological effects of radiation. 



Another model for explaining the effects of radiation was fur- 

 nished by Glocker (20). This model attempts to take into account 

 the spatial distribution of the energy within the tissues as well as the 

 quantum absorption. The photoelectron path through the tissues 

 is an irregular one in which the ions per unit length of path are few in 

 its initial stages, becoming greater as the energy of the electron drops, 

 until at the end of the path the ionizations are close together. With 

 this model a hit is considered to occur when the ionizing particle 

 passes through the target (see also 3, p. 88; and 40). 



More recently consideration has been given by Lea (3) to the 

 fact that ion pairs are apt to form in clusters along the radiation 

 track. Either one or several of the ions within a cluster may be con- 

 sidered sufficient to produce a given effect of irradiation. Different 

 energy particles have differing clustering effects along their ionization 

 paths. Clustering tends toward more than one ionization in a sensi- 

 tive region if that region is large. It follows that, if the volume of 

 the sensitive region is calculated on the basis of the total ion pairs 

 formed, the size of the region is underestimated. If, on the other 

 hand, the sensitive region is small, a single ion from a cluster may be 

 sufficient to produce the observed results and at the same time lie 

 outside the effective range of the other ion pairs in the cluster. This 

 will lead to the correct size of the sensitive region. For small targets 

 the method of Crowther (10), in which the ionizations are considered 

 to be distributed at random, each ionization being capable of produc- 

 ing the observed effect, would lead to approximately correct results 

 as far as size is concerned. 



Where the biological material irradiated consists of several dis- 

 crete parts. Lea has sho\\'n that it is possible to estimate the number 

 of these parts as well as the volume of each from the X-ra}^ results. 

 This is accomplished by using irradiation with widelj- spaced ion 



