ALPHA PARTICLES 561 



importance, but in experiments such as those involving accurate control 

 of penetration it may be very disadvantageous. 



Radon is used in two ways: (i4) dissolved in the culture medium; (B) 

 confined in very thin-walled glass bulbs (alpha-ray bulbs). The first 

 method is very convenient for certain types of qualitative work, but its 

 use in quantitative work is open to several criticisms : (a) The distribution 

 of radon between organism and culture medium is unpredictable. The 

 radon may not be absorbed at all by the cells, it may be concentrated in 

 the cells, or its concentration may be the same in the cells and in the 

 medium. As with any other dissolved substance, the only way to make 

 sure of the degree of absorption of radon is to make direct determinations 

 upon the specific organism being used. The same is true for each of 

 the decay products — radium A and radium C — which also emit alpha 

 particles. (6) The radium A and radium C further compHcate matters. 

 If pure radon is introduced into the closed culture vessel, it distributes 

 itself between the gas and liquid phases in a known way. The formation 

 of radium A and radium C becomes significant almost immediately. 

 (At the end of 3 min., half as many, and at the end of 9 min., seven- 

 eighths as many alpha particles are being emitted by radium A as by 

 radon itself.) The rates of formation are accurately known, and these 

 substances should present no insuperable difficulties if all the radon decays 

 in the liquid phase. However, it would be very difficult to predict what 

 fractions of the radium A and radium C formed in the gas phase find 

 their way into the Uquid phase where they become effective. This 

 uncertainty is significant whenever the volume of the gas phase is of as 

 large an order as that of the hquid phase, for at 20°C. the distribution 

 coefficient of radon between water and gas phases is about 1:4.^ The 

 uncertainty becomes worse if the radon is introduced along with the 

 equilibrium amounts of radium A and radium C, for these substances 

 are solids deposited in a very thin layer on the walls of the original radon 

 container and may or may not dissolve readily in the culture solution. 



The use of radon in bulbs is much more satisfactory. The chief dis- 

 advantages are: (a) The glass walls seriously reduce the penetrating 

 ability of the emergent alpha particles, (b) Particles ejected at various 

 points in the bulb traverse different thicknesses of glass before emerging. 

 This introduces further heterogeneity into a beam of particles which is 

 already heterogeneous because of different initial velocities (see above). 



Polonium. — This substance has the following advantages: (a) It 

 decays to half value in 136 days. A source of satisfactory original 

 activity may be used upon most biological material for at least 2 or 3 

 months without the necessary increase in length of exposure becoming a 

 serious factor. (6) It is the only known substance which emits alpha 

 particles and has no radioactive decay products, (c) It can be readily 



^ Derived from International Critical Tables 3 : p. 257. 



