."^12 UADiA ri(>.\ iii()L()(;v 



tion cfticieiicy on ionization density conid result from any goometric (lex i- 

 atioii from the "single spherical target" model/' 



A study of the inactixat ion of a l)a('terioi)hage hy deutcron beams of 

 diiferent energy (Pollard and Forro, 1!)4'J) sliowed a dependence of effect- 

 iveness on the energy of the beam. This was interpreted at first as being 

 due to a thlTerence in the effective diameter of the ionization column 

 because of ultraviolet emission by excited atoms near the target but out- 

 side it. This was clearly incompatible with the known low (juantum yield 

 for ultraviolet inactivation of phage, and the deuteron data have been 

 reinterpreted (Pollard, 1951) by a method substantially eciuivalent to the 

 associated volume method of Lea (1946) and open to similar criticisms. 



In conclusion, it may be said that radiation studies on viruses based on 

 the target theory have not yet provided any basic information on the 

 nature and structure of viruses, especially because not enough attention 

 has been paid to the interaction of an irradiated virus with its host. A 

 virus particle is defined as inactive when it has become unable to produce 

 active replicas of itself. This failure of reproduction may be caused either 

 by inability of the virus to attach itself to a host or to a susceptible cell, by 

 inability to penetrate and invade the host, or by inability to carry out any 

 one of the probably numerous steps intervening between infection and 

 production of new active virus in an infected cell.^ Different portions of a 

 virus particle may be functioning in each of these processes. Chemical 

 changes may be produced in a virus particle by radiation or other means 

 without the particle registering as inactive, either because the damaged 

 portion is not essential for reproduction or because that portion, although 

 used in reproduction, may be replaceable by other portions. For example, 

 suppose that a virus particle has a discrete number of different surface 

 areas. A, B, C, . . . , any one of which can act to the exclusion of others 

 as the "receptor" involved both in adsorption of the particle on a suscep- 

 tible cell and in the following penetration, and suppose that, if receptor .1 

 is utilized for absorption, the same receptor .4 will also control penetra- 

 tion. If receptor A is damaged by a radiation hit in such a way that it 



" A similar criticism can be made of an attempt (Bonet-Maury, 1948) to interpret 

 the supposedly lower sensitivity of vaccinia virus to X rays than to a particles (an 

 unconfirmed observation; see Lea and Salaman, 1942) by assuming that the virus 

 particle consists of an agglomeration of individual units, each of \vhi(4i nmst be inacti- 

 vated for suppression of infectivity. The a rays supposedly would inactivate all the 

 units in the agglomerate by energy spread. This hypothesi.s would require that each 

 particle break apart into single units before infection ; otherwise, the X-ray inactivation 

 curve would be of the multiple-hit type. There is no evidence for .such a structure of 

 the vaccinia virus particle. 



* In the case of viruses acting on multicellular organisms, if activity is detected by 

 appearance of multicellular foci of infection or of general reactions, a virus may be 

 considered inactive if it is unable to reproduce sufficiently to overcome the host 

 defen.ses. 



