XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 511 



with the Poisson distribution. The chance that a particle of virus 

 protein 25 m^u in diameter will be ionized by 10^ r. is thus: 



1 _ e-««« = 0.49 



The chance that the virus particle is ionized is less than the aver- 

 age number of clusters per particle with the large dose chosen because 

 more than one jS particle will ionize the same virus in a proportion of 

 cases. 



The chance of ionizing the virus would be almost the same with 

 7 radiation or high voltage X radiation as with /3 radiation. It 

 would be very different if the virus were exposed to the same dose of 

 (X radiation. Table I shows that about 4000 ions per micron are 

 formed along an a-particle track in w^ater. The mean number of 

 clusters per micron is therefore about 1300, so that each a particle 

 will leave some twenty-two clusters within a sphere 25 mju in diameter. 

 Those virus particles that are ionized at all will be heavily ionized, 

 and correspondingly, for exposure to the same total dose, many more 

 virus particles will be unaffected than w^as the case with jS radiation. 

 a. radiation is thus clearly inefhcient with respect to any effect that 

 requires only the production of a single ionization mthin a small 

 spherical volume of virus dimensions. This has been demonstrated 

 by Lea and collaborators {!), as the figures in Table III show. The 



TABLE III 



-■ Inactivation of Phage S 13 {1) 



X rays or rays 



Radiation 7 rays (1.5 A.) (4 m.e.v.) 



Inactivation dose" 0.58 0.99 3.5 



Target diameter 15.5 15.9 16.3mM 



Unhydrated virus diameter = 16 m/j (hydrated, 18 m^) 



" Unit = 10" r. 



a-ray inactivation dose is seen to be six times the 7-ray inactivation 

 dose. The figures in the last line show the calculated size of the 

 sphere which is such that there is a chance of 0.63 {i.e., 1 — 1/e) that 

 such a sphere contain at least one ion cluster. The diameters esti- 

 mated by the use of each of the three types of radiation agree re- 

 markably well with the estimated size of the phage particle, which 

 makes it possible to assert with some confidence that inactivation 

 results when almost any part of the bacteriophage particle is ionized. 



