.338 |{.\l)l A riO.N HKtLOCJY 



so-called "target theory," accordiiifi; to which the effective hits are those 

 that occur within a .specific physical domain which may coincide with all 

 or parts of the l)iolo<i;ical ohjcci in\('.stifi;ated. Iiil'ormation concerning 

 the geometry of this domain is sought in the following way. An ideal 

 "target " or "sensitive xolunic" is delinetl by the value of the constant A; 

 in Eq. (9-1): 



If D is measured in acts of absorption per unit volume, /,• will have the 

 dimension.'-' of a \olume (sensitive volume); if D is measured in number of 

 paths of ionizing particles (such as protons) crossing a unit area, then k 

 will have the dimension of an area, the "sensitive cross section." Clearly, 

 the target or sensitive volume thus defined is not a priori identifiable with 

 a physical portion of the biological object. Two extreme po.ssibilities, 

 and several intermediate ones, are conceivable: 



Hypothesis 1. The target corresponds to a real volume, within which 

 each hit is effective, whereas all hits without are ineffective. 



Hypothesis 2. There is no such completely sensitive " real target " ; the 

 probability that a hit in a given unit volume is effective is distributed o\'er 

 a more or less large volume, which may even extend beyond the recognized 

 boundaries of the organism. 



In the case of viruses the recognition of the direct effect and its distinc- 

 tion from the indirect effects suggests that the probability that hits out- 

 side the physical borders of the virus particles are effective is not appre- 

 ciably different from zero. The effectiveness of a hit within the particle, 

 however, may be lower than unity and may vary from point to point, with 

 a distribution p{x,y,z) over the volume of the particle. If c is the \()lume 

 of the particle, Eq. (9-1) becomes 



If p{x,y,z) = I* is constant, then 



Hypothesis 1 would divide a virus into two parts, one with P = 1 and 

 the rest (if present) with P = 0. This point of view was vigorously 

 defended by Lea (194G) who made extensive measurements of A* for dif- 

 ferent biological effects of radiation and developed methods for analyzing 

 the dependence of k on ionization density for various radiations. For the 

 application of this type of analysis to viruses, the reader is referred to 

 re\iews by Lea (19-1:()) and Bonet-Maury (1948). A brief summary will 

 be sufficient here because the target theory is discussed by Fano (Vol. I of 

 this series) and because, in the opinion of this writer, the information 

 at)out \irus('s obtainable by this type of analysis is of limited \alue. 



