512 L. H. G R A Y 



An interesting situation arises when the volume to be ionized is 

 not roughly isometric but is, for example, a spherical shell, a thin disc, 

 a rod, or its equivalent, a large number of smaller spheres. The /3- 

 and 7-ray inactivation dose depends only on the total volume mthin 

 which an ionization is to be produced. The extent of the inefficiency 

 of the a-ray ionization depends on the average path of the particle 

 within the region to be ionized, which is obviously less for the cases 

 cited than it would be for a sphere of equal volume. Such a result 

 was found by Lea and Salaman (31) for the inactivation of vaccinia 

 virus. The 7-ray inactivation dose for this virus was found to be 

 80,000 r. If the exponential relation between virus inactivation and 

 dose, the observed independence of radiation intensity, and other 

 circumstances are interpreted as indicating that inactivation is 

 brought about whenever a certain region of the virus is ionized, the 

 volume of that region is about 1% of the whole virus and equal to 

 that of a sphere 31 ran in diameter. If the region were in fact a 

 sphere the o;-ra,y inactivation dose should be eleven times this, or 

 880,000 r. instead of 211,000 r. as determined experimentally. Lea 

 and Salaman (31) concluded that the sensitive material is nonspheri- 

 cal and probably in the form of some 100 regions each ~ 7 m/x in 

 diameter. 



An alternative way of regarding the difference between j8 and a 

 radiation is to consider that since an a particle produces nearly two 

 ion clusters per millimicron of track it is virtually certain to ionize 

 any structure of dimensions greater than 1 m/i that it actually trav- 

 erses. If, therefore, one ionization suffices to produce the effect 

 under consideration the a-ray dose measures the cross-sectional area 

 of the ''target," by contrast with /? and y radiation, which measure 

 the volume. Lea has treated this problem in very great detail in his 

 papers and in his book (1 ) , where all the necessary physical data are 

 given for the analysis of results obtained with a wide range of ionizing 

 radiations. 



It is very interesting to note that the discovery by Freer (98) — 

 that the material "kappa" (which is responsible for the killer proper- 

 ties of certain paramecia) consists of microscopically visible par- 

 ticles — was the direct result of irradiation studies. Finding that 

 kappa was exponentially inactivated, and that the inactivation dose 

 was about 3500 r., Freer concluded that the kappa particles should 

 be at least as large as the largest viruses, and was thereby encouraged 

 to undertake the cytological investigations which lead to their identi- 



