THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 295 



of an electron are, of course, positive ions. Tlie free electron will, in the 

 course of time, be slowed in its motion to the extent that it can join a neutral 

 atom to form a negative ion. But inasmuch as the energy change involved in 

 such attachment is very low (of the order of the excitation energy of the 

 atom) it is believed that the formation of negative ions has far less signifi- 

 cance in disrupting chemical arrangements in molecules than does the 

 formation of positive ions (Lea, 1947). 



The density of ionization along the tracks of high-speed particles is 

 strongly dependent upon the type of particle, and this fact is of importance 

 in assessing inactivation of small biological objects such as viruses. Electrons, 

 protons, and a-particles, in that order, have an increasingly greater density 

 of ion production along their tracks, and their range of path is in the inverse 

 order. For example, 1 Mev electrons have a range of over 4 millimeters in 

 tissue and produce less than 2 primary ionizations per /x, while a-particles 

 of the same electron-volt energy have a range of only about 5 jx, but produce 

 over 5000 ionizations per [x. It would be a rare occurrence for an electron to 

 produce more than one ionization within a virus particle, while an a-particle 

 would be sure to do so. 



Particles such as viruses may be irradiated either in suspension or in the 

 dried state. While the former type of preparation is easier to assay for radia- 

 tion effects than is the latter (which must be re-wet), there are fundamental 

 ambiguities inherent in interpreting the results. Dense ionization in aqueous 

 media results in the production of free radicals whose biological effect is 

 incompletely understood, and if a dilute suspension of virus particles is 

 irradiated with high-energy photons or particles, almost all of the biological 

 effects will be the indirect ones caused by chemical modifications in the 

 solvent. For this reason it is customary to irradiate viruses in the dried, or 

 frozen-dried state, or in a suspension containing a great excess of some 

 protective material, such as gelatin. In these preparations the assumption 

 is probably valid that the biological effects of radiation are direct ones, 

 i.e., caused directly by ionization of atoms within the biologically active 

 molecules. 



6. The Target Theory. When viruses are irradiated in a way such as to 

 produce direct effects of ionization it is customary to assess the results in 

 terms of loss of infectivity. By making assumptions as to the nature of 

 the relation between the number of ionizations ("hits") within a sensitive 

 structure ("target") and its loss of activity, one can calculate the target size 

 and predict the variation of ionic efficiencies of different radiations. The 

 general outline of this method of calculation is called the "target theory." 

 As applied to viruses the target theory presumes that inactivation of a virus 

 occurs when a single ionization takes place within a certain sensitive volume 

 contained within it. This postulate is probably valid if it is observed that: 



