560 



BIOLOGICAL EFFECTS OF RADIATION 



in the path. This is at present impracticable with any other type of 

 radiation. Absolute values of doses of alpha particles can be determined 

 directly with ease and accuracy, and expressed in terms of the number 

 of particles striking in unit time a unit area at a definite distance from 

 the source. It is to be noted that expression of dose in this manner makes 

 possible a direct and precise calculation of the number of alpha-particle 

 paths which traverse any cell or part of a cell during administration of 

 any given dose. This also is at present impracticable with any other 

 type of radiation. Duplication of dose with the same source is extremely 



Distance from Source 



3.87 cm. air 

 32 u wafer 



Fig. 1. 



-Relation between ionization per unit path of a beam of alpha particles and distance 

 from the source (polonium). (.After Curie and Behounek, 4.) 



easy, because the rate of emission of alpha particles varies with time in 

 accordance with a simple exponential law. 



SUITABLE SOURCES OF ALPHA PARTICLES 



Two substances — radon (radium emanation) and polonium (radium 

 F) — have been used as sources in nearly all biological work with alpha 

 particles. 



Radon. — This substance is a chemically inert gas. Its chief advan- 

 tages over polonium are: (A) If necessary equipment and a suitable 

 amount of radium are available, the preparation of sources, especially 

 of very active ones, is much more rapid. (B) The gamma-ray activity 

 of its decay products makes possible a rapid and accurate indirect 

 measurement of the rate of emission of alpha particles. Its chief dis- 

 advantages are: (a) It decays to half value in less than four days. This 

 necessitates a new source every few days. (6) It has decay products 

 which emit beta and gamma rays. These rays require special investi- 

 gation, so as to make sure that the biological effects are certainly due to 

 alpha particles, (c) Radon has two decay products — radium A and 

 radium C — which also emit alpha particles. Since the particles from 

 these two substances have velocities different from that of the particles 

 from radon itself, this means that the alpha-ray beam from a radon source 

 is heterogeneous. In many types of experiments this may be of no 



