536 L. H. G R A Y 



be negligible. The millicurie activity of such sources after decay of 

 RaA will be perhaps a quarter that of the radon used in their prepara- 

 tion, and the activity decays with a half-life of somewhat less than 

 half an hour (^P). 



The exposure to a radiation of specimens larger than the a-ray 

 range may sometimes be accomplished by immersion in a solution 

 containing radon. This method has been used for the irradiation of 

 root tips (77), microspores contained within the intact anther (2), 

 and tadpoles (78) . Radon solution has also sometimes been used for 

 the irradiation of viruses (79) and bacteria {80, Chapter V) in sus- 

 pension, and in a variety of radiochemical studies. When a specimen 

 is uniformly perfused by radon in eciuilibrium with its decay products 

 RaA and RaC, which emit a rays, the a-ray dose rate is 810 C 

 r.e.p. per minute, where C is the concentration of radon in milli- 

 curies per gram of tissue. If required, therefore, many thousands 

 of roentgens can conveniently be delivered in this way. The speci- 

 mens will be exposed in addition to |8 radiation from RaB and RaC 

 to an extent that depends on the volume of the radon solution in 

 which the specimen is immersed. At most the jS-ray dose will be 

 5% of the a-ray dose. The 7-ray dose will generally be negligible. 



Application of this technique to the irradiation of roots was in- 

 vestigated by Gray and Read (77) in some detail. They found that 

 when the root was introduced into the solution the radon entered at 

 roughly the rate to be expected for simple diffusion of an inert gas of 

 high molecular weight, and at equilibrium the concentration was the 

 same in the root as in the solution. When the root is transferred 

 again to pure water the radon diffuses out at the same rate. In each 

 case half equilibrium concentration is reached in the root tip in about 

 1.5 minutes. The atoms of RaA and RaC formed in the solution do 

 not enter the root and those formed in the root do not leave. In these 

 circumstances, the total dose is related to the radon concentration in 

 the root in the manner given above even when the exposure is too 

 short for radioactive equilibrium to be established between radon, 

 RaA, and RaC, provided the radon concentration reaches its equilib- 

 rium value at the center of the root. The dose is, of course, spread 

 over a longer period than the duration of immersion because it is de- 

 livered in part by the RaA and RaC remaining in the root. The 

 RaA and RaC formed in the solution were found to accumulate on 

 the surface of the root thereby very greatly increasing the dose to the 

 superficial cells. This effect can give rise to entirely misleading re- 



