PRINCIPLES OF RADIOLOGICAL PHYSICS 



83 



Fig. 1-49. Cloud-chamber pictures of a-particle tracks in a rarefied gas. {Gentner 

 et al., 1940.) 



chamber pictures reflect the actual distribution of activations. Part of 

 the observed blending of clusters certainly arises from the migration of 

 electrons prior to the formation of negative ions (see Sect. 3-6b), and thus 

 appears as a somewhat spurious effect. Notice also that the pictures 

 taken in increasingly rarefied gases show an increasing amount of detailed 

 5-ray structure and give less and less the impression of an even columnar 

 distribution of ions. 



3-7. SUMMARY 



The main lasting effects of radiation of a material consist of changes in 

 the chemical state of aggregation of the material. Changes in the aggre- 

 gation of particles within atomic nuclei are overshadowed in numbers by 

 chemical changes, by many orders of magnitude. 



Chemical changes result, in the main, from the disturbance, or "activa- 

 tion," of the external electrons of atoms and molecules. Ripping of 

 chemical bonds through bodily bumping of whole atoms by radiation 

 particles is infrequent under ordinary conditions. 



Two phases are distinguishable in the chain of processes that follow the 

 introduction of radiation energy into a material: (1) the first phase 

 develops through radiation phenomena and ends when most of the energy 

 absorbed is distributed in separate packets having the form of chemical 

 activations ; (2) the second phase of the process develops through chemical 

 (molecular) processes and is accompanied by the eventual complete 

 degradation of the radiation energy into heat. We have much better 

 information on the first phase of the process than on the second. The 

 present treatment deals only with the first phase. The first phase is 

 missing in the case of low-potency radiations (see Sect. 3-5), whose initial 



