PRINCIPLES OF RADIOLOGICAL PHYSICS 81 



The number of activations or the number of ions within a cluster 

 depends primarily on the energy of the secondary electron which gen- 

 erates the cluster. The number of ion pairs is roughly equal, on the 

 average, to the total energy dissipated within the cluster divided by 

 30 ev (see Sect. 3-lb). Therefore the frequency of clusters containing 

 different numbers of ion pairs shown in Table 1-7 parallels the frequency 

 of secondary electrons of different energies shown in Fig. 1-38. Notice 

 that this distribution is completely skew; a large proportion of the 

 clusters consist of a single pair of ions, corresponding to energy-poor 

 secondary electrons. The average number of ion pairs per cluster is 

 approximately three for most gases. 



Table 1-7. Frequency of Ion Clusters Containing Various Numbers of Ion 



Pairs 

 (Experimental data from Wilson, 1923; 



The larger the initial energy of a secondary electron, the longer and the 

 more nearly straight is its path. The transition from cluster formation to 

 an arrangement of activations along a clear track takes place gradually, 

 of course, as the energy increases from about 100 to about 500 to 1000 ev. 

 Secondary electrons whose energy amounts to at least several hundred 

 electron volts are called "5 rays." 



The track length of 8 rays of various initial energies is not known satis- 

 factorily. The theoretical methods for calculating the "range" of fast 

 charged particles (see Sect. 4-2a) are less reliable when applied to com- 

 paratively slow electrons. Experimental evidence is scarce. Lea (1946) 

 has given tables of the numbers and ranges of 8 rays produced by various 

 kinds of particles in living tissue, but these data should be used with 

 caution. The preparation of more comprehensive and reliable data 

 requires further study. 



3-6c. Over-all Distribution of Activations around the Tracks of Fast 

 Particles. It follows from the preceding discussion that the activations 

 produced by a fast charged particle are grouped in clusters of different 

 sizes spaced at irregular intervals along the track of- the particle. Many 

 "clusters" actually consist of a single activation. The activations pro- 

 duced by the few secondary electrons whose energy is particularly high 

 constitute a 5-ray track which branches out from the main track. 



The speed and charge of a particle determine the average spacing of 

 clusters along its track, but the relative frequency of clusters of different 

 sizes is nearly independent of the characteristics of the incident particle. 



Cloud-chamber pictures of charged-particle tracks give a visual impres- 



