INACTIVATION OF VIRUSES 365 



where NjNo is the fraction of virus particles surviving at time t, and k is a 

 rate constant which usually depends on temperature, ionic environment, 

 hydration, etc. In irradiation experiments the dose D, delivered at time t, is 

 often substituted for t itself, and the above equation may then be written 

 NINo = e exp (— DIDo), where D^, or D~, is the so-called "inactivating 

 dose" which gives N/Nq = e \ or 37 % survivors (Lea, 1947). 



The striking discovery that inactivation by various types of radiation 

 usually is first order led to development of the so-called "target theory." ^ In 

 its general form the theory postulates that a biological unit (e.g., a virus 

 particle) has within it a sensitive volume, the target, and that a single hit 

 within that volume (e.g., an ionization or the absorption of a quantum oj 

 ultraviolet light) can, but need not, inactivate the unit. 



In experiments with ionizing radiation it is generally assumed that the 

 first ionization - to occur within the target volume causes inactivation. This 

 simple and attractive model is, to a large extent, based on early experiments 

 with certain viruses, which suggested that a single ionization, anywhere in a 

 particle, sufficed to inactivate that particle (Lea, 1947). In other words, these 

 viruses seemed to be aU target, and primary target-ionizations seemed to be 

 100 % effective. Target volumes for some large viruses and cells have been 

 calculated, assuming the same high ionization efficiency and, as a rule, it has 

 been found that the sensitive volumes are small compared to the particles 

 or cells of which they are part (Lea and Salaman, 1942; Lea, 1947; Pollard, 

 1953). 



Considering that inactivation by ionizing radiation is frequently studied 

 for the sake of estimating the size and shape of the target, it should be em- 

 phasized that the assumption that primary target-ionizations are 100 % 

 effective (i.e., that the "ionization yield" is unity) may not be generally 

 valid. It is true that this crucial assumption has not been disproved, but 

 recent findings somewhat reduce its significance. Most important are prob- 

 ably the experiments which show that the result of applying a fixed X-ray 

 dose to a dried virus depends on the method of drying, as well as on the 

 temperature maintained during irradiation (Bachofer et at., 1953; Bachofer, 

 1953; see Section B2). 



It has always been thought that the genetic material constituted the 

 radiation-sensitive target in a virus particle (Jordan, 1940; Lea, 1947); and, 

 today, this idea is as plausible as ever. Present knowledge, which tends to 

 identify the genetic material with the viral nucleic acids (Hershey and Chase, 



^ The theory was formulated by Timofeeff-RessovskyeiaZ. (1935). It has been described 

 in detail by Jordan (1940) and by Lea (1947). A generalization of the theory, applicable 

 to most biological systems, was presented by Atwood and Norman (1949). 



^ We refer to primary ionizations which have associated with them one or a few 

 secondary and closely clustered ionizations. 



