IONIZING RADIATION AND VIRUSES 77 



widespread. This shows as a part which is temperature sensitive 

 and also possibly depends on the previous treatment of the virus. 

 As understanding of this dual action proceeds, radiation studies 

 will become more powerful. 



Experimental Methods and Some Results 



It has been pointed out that the space relationships of ioniza- 

 tion must be retained in the bombardment technique. To be 

 reasonably sure of this, the virus must be bombarded dry and in 

 vacuum. It will be seen later that this is not absolutely necessary, 

 but this conclusion can not be drawn until dry bombardment 

 has been carried out. Dry bombardment of TMV by X-rays was 

 first employed by Gowen (1940) and by Lea, Smith, Holmes, 

 and Markham (1944), who were clearly aiming at exploiting the 

 physical action of radiation just described. X-radiation of suffi- 

 cient intensity requires somewhat specialized equipment. Much 

 shorter exposure times can be used if electrons, deuterons, or 

 alpha particles are used. Lea made some use of alpha-particle 

 bombardment, but, until recently, radioactive sources of suf- 

 ficient purity, homogeneity, and intensity have been hard to 

 come by. 



The modification of a cyclotron for deuteron bombardment is 

 shown in Fig. 3.3 in schematic fashion. The deuteron beam is 

 defined by an insulated, positively charged diaphragm, and en- 

 ters a bombardment chamber without striking any metal except 

 a brass shutter or the sample holder. The beam entering the 

 chamber is slowly diverging and, because the bombardment 

 chamber is 8 ft from the cyclotron vacuum chamber, is uniform 

 in cross section over a diameter of about ^g in. The beam is 

 measured by a calibrated galvanometer which reads current col- 

 lected by the whole bombardment chamber, which is insulated. 

 The samples are placed on glass coverslips attached by a spot of 

 grease to a brass disk. These are shown in the lower part of the 

 figure. These are successively rotated into place with the cyclo- 

 tron off, the beam is turned up from the control room, and the 

 shutter is electrically operated from there for the appropriate 

 time. 



