6 



RADIATION BIOLOGY 



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sign toward two electric terminals (Fig. 1-4). Vessels designed for this 

 purpose are called "ionization chambers" and constitute basic depend- 

 able tools for radiation detection and 

 measurement (see, for example, Halliday, 

 1950, p. 170). 



Neutrons are detected indirectly, 

 through the effect of secondary radiations 

 arising from the collision of neutrons with 

 atomic nuclei (see Sect. 2-5). 



1-lb. Types of Corpuscular Radiations. 

 As stated before, any stream of atomic or 

 subatomic particles constitutes a corpus- 

 cular radiation. The main characteristics 

 of the various kinds of particles are their 

 masses and their electric charges. 



The masses are conveniently expressed 



9 9 ® 



+ + + 



{} 



+ + + 



Q- 



<Z> 



Fig. 1-4. Schematic functioning 

 of an ionization chamber. 



in the scale of atomic weights in which 



1 unit atomic weight == 



1 gram 

 Avogadro's number 



1.66 X 10--4gram (1) 



The electric charges are conveniently expressed in terms of the charge of 

 an electron, whose sign is negative and whose magnitude is usually indi- 

 cated by the symbol 



e = 4.80 X 10-'" esu = 1.60 X lO-'^ coulomb 



(2) 



The charges of atomic particles may be positive or negative but are 

 always integral multiples of the charge of an electron. 



The corpuscular radiations hsted in Table 1-1 fall into the following 

 three groups: 



(1) Light charged particles, namely: 



(a) The electron, with atomic weight Mg-is and a negative charge 

 equal to e. Electrons are a normal constituent of all kinds of 

 matter. They spring out from matter when sufficient energy 

 becomes available for them to overcome the attraction of the 

 positive charges carried by the atomic nuclei. This energy may 

 be supplied by radiation or simply by heat. Classical studies 

 have been made of electrons as they emerge from the negative 

 terminal ("cathode") of an electric circuit inserted in an evacu- 

 ated vessel and form a beam of cathode rays (see, for example, 

 Richtmyer and Kennard, 1947, pp. 80 ff.). Electrons are also 

 called "negative /S particles." 

 (5) The positron, with atomic weight ^82.3 and a positive charge 

 equal to e. Positrons are the positively charged counterpart of 



