IONIZATION AND DETECTION 



105 



absorption 

 by the nucleus 



decay of 

 unstable nucleus 



photoelectric 

 absorption 



Depth in Tissue 



Figure 5-1. Schematic Representation of Tracks of a Neutron (n), and of 

 Alpha (a), Beta (/?) and Gamma (7) Rays in Tissue. Note that the density of 

 ionization increases as energy is lost from the impinging ray. The alpha trail of 

 ionization is dense, the beta trail is spotty, and the gamma and neutron trails 

 are composed of spurs. 



From these considerations and the properties given in Table 5-1, one can 

 understand that the differences among alphas, protons, and deuterons art- 

 more those of degree than of kind. All are positive, heavy particles with 

 high LET. 



Electrons 



The beta is a very small particle — a very fast electron. Its charge is either 

 negative (as is the beta from P 32 ) or positive (as is the beta from P 30 ), al- 

 though the negative is the more common among biologically interesting iso- 

 topes. Because it is of light weight, with a mass only somewhat greater (rela- 

 tively) than the mass of the electrons in the atom, a collision can result in 

 energy transfer and a change in direction, similar to billiard balls in play. **] 

 As a result the path traversed by the beta will be governed more or less by 

 chance collision. It will have many changes of direction. Along the straight 

 portions of the path, when the beta flies through the electron cloud of the 



