8 RADIATION BIOLOGY 



Protons may turn into neutrons through a similar transformation, ejecting a 

 positive electron and, presumably, a neutrino. However, this transformation is 

 energetically impossible unless the proton is part of a nucleus and the rest of the 

 nucleus supplies the necessary energy. 



The transformations of protons into neutrons and of neutrons into protons 

 within a nucleus constitute the processes of "/3 disintegration." 



1-lc. Potency of Corpuscular Radiations. The ability of a corpuscular 

 radiation to affect the properties of matter clearly depends not only on 

 the nature of its constituent particles and on their total energy but also 

 on the kinetic energy of each single particle. A few particles of high 

 kinetic energy can produce effects which a far larger number of particles of 

 low kinetic energy cannot possibly achieve. Therefore an indication of 

 the speed or of the kinetic energy of individual particles serves as a 

 measure of what we may call the "potency" of the radiation. 



The basic unit employed with reference to the kinetic energy of atomic 

 particles is the "electron volt" (ev), i.e., the increase of kinetic energy 

 experienced by an electron while being accelerated across a potential 

 difference of 1 volt. This unit has a direct significance when applied to 

 a particle carrying the same quantity of electric charge as an electron, 

 but it is currently used as a unit of energy with reference to particles 

 carrying any quantity of charge or no charge at all. 



1 ev = 1.6 X 10-19 joule = 1.6 X IQ-^^ erg * (3) 



When the kinetic energy £* of a particle is expressed in electron volts and 

 its mass M in units of atomic weight, its velocity is given by 



y = 1.4 X 10^ ^E/M cm/sec (4) 



(This formula is valid only if its result is much less than 10 ^^ cm/sec; 

 otherwise it must be replaced by a more complicated formula of relativ- 

 istic mechanics.) 



The kinetic energy of individual particles may also be expressed in 

 terms of the chemical energy unit "calorie per mole." One mole of a 

 substance consists of Avogadro's number, A^ = 6.0 X 10^^, of molecules. 

 If each of these molecules has, for example, a kinetic energy of 1 ev, the 

 entire mole has a kinetic energy equal to 



6.0 X 10^^ X 1.6 X 10-12 ergs = 9.6 X lO^i ergs = 23,000 cal 



In the same way, if a chemical reaction yields 1000 cal/mole, this means 

 that each molecule participating in the reaction has yielded 3=^2 3 6V. 

 The equation 



1 ev = 23,000 cal/mole (5) 



serves to estimate the chemical "potency" of a radiation. For example, 

 if a certain gas reaction requires an activation energy of 50,000 cal/mole, 



