186 



Cellular Events Produced by Ionizing Radiations / 1 : I 



systems manufacturing blood cells; complete destruction of certain 

 tissues: and induction of malignant growths. The entire subject of 

 somatic responses to ionizing radiations is very complex; empirical 

 knowledge extends beyond that which can be explained in terms of the 

 basic cellular events. No attempt is made in this text to describe the 

 details of the responses of complex organisms to ionizing radiation. 

 Rather., in this chapter, the cellular events are emphasized. These in 

 turn can be described in terms of molecular phenomena, the presentation 

 of which comprises Chapter 16. Genetic effects, in contrast to the 

 somatic ones, occur originally in only one cell, even in higher plants 

 and animals. These genetic effects are also discussed in this chapter. 

 Ionizing radiations are destructive to living cells. In most cases, this 



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c 



Q 





Distance from Source 



Figure I. The attenuation of a proton beam passing through 

 tissue. 



destruction is undesirable to humans. However, in controlled labora- 

 tory experiments, the effect of ionizing radiations can be used to study 

 the organization of the biological cell. In particular, the effects of 

 ionizing radiation are useful for studies of cellular division and of 

 genetics. The use of ionizing radiation as a tool to study biological 

 systems is emphasized in this text. 



The various types of ionizing rrdiations and related subatomic 

 particles are summarized in Appendix D, for the benefit of those un- 

 familiar with atomic physics. It is sufficient here to note that all these 

 types produce ionization along their path. The heavier ones follow a 

 straight path of definite length; the uniformity' of this path length is 

 illustrated by the graph in Figure 1. The lighter ionizing radiations 

 cannot be described in terms of a definite path length, because the path 



