64 APPLIED RADIOACTIVITY 



Alpha Rays 



Alpha particles are ejected as if they were high-velocity projectiles 

 originating in the nucleus. They are doubly charged positive frag- 

 ments of matter and have a structure like that of the helium nucleus. 

 On the average they have a velocity equal to about one-twentieth the 

 velocity of light. They are able to penetrate about 6.97 cm of air at 

 standard atmospheric pressure before dissipating their energy. A single 

 particle during its passage through air produces as many as 150,000 ions 

 by collision. 



TABLE II-2 



Ranges in Solid Elements as Compared with Water and Mica 



Alpha particles of RaC. Units of range are 10~ 4 cm. 



Velocity of light is 3 X 10 10 cm/sec. 



The number of alpha particles emitted from 1 gram of pure radium 

 per second is 3.7 X 10 10 . When radium is in radioactive equilibrium 

 with its products, the same number of particles is also emitted per second 

 by each of its important alpha rayers, viz., Rn, RaA, RaC', and RaF. 

 Consequently, the number of alpha particles from 1 gram of radium 

 in radioactive equilibrium is 5 X 3.7 X 10 10 , or 18.5 X 10 10 . 



We are now in a position to appreciate the terrific bombardment that 

 the walls of a glass ampule are subjected to when they enclose as little 

 as a milligram of radium salt. 



The ranges in some metals used as alpha-ray filters are listed in 

 Table II-2. These data show what thickness of metallic capsules can 

 be used for radium containers which will completely absorb the alpha 

 particles. The range in mica is about 0.036 mm. Since mica and glass 

 may be considered comparable it can be appreciated why a glass radium 

 needle with 0.04-mm wall thickness is sufficient to absorb all the alpha 

 particles. An aluminum filter 0.04 mm thick is sufficient protection 

 against the physiological effects of alpha particles when open radium 

 applicators are used in dermatology. 



