CHEMICAL SPECIES INDUCED BY X-RAYS IN CELLS 



93 



(Class 11); and an oxygen -dependent poilion that consists of long-lived 

 free radicals (Class 111). 



THE BACTERIAL SPORE SYSTEM 

 We use spores of a strain (ATTC #8245) of Bacillus megaterium that 

 has demonstrated good spornlating capacity. The spores are mounted 

 in known numbers on precipitated cellulose discs (called the Millipore 

 Filter) O-l nun thick (Powers et al. 1957 ; Kaleta and Powers, 1958). The 

 discs carrying the spores are dried in a chamber at less than 1 mm Hg. 

 pressure over a drying agent for several hours, and are kejjt in this 

 chamber until ready for use. Colony formation is induced by putting 

 the paper disc on an absorbent pad saturated with liquid nutrient 

 medium. The spores germinate and give rise to colonies on the surface 

 of the disc. Control, unirradiated discs demonstrate 100 per cent re- 

 covery of mounted spores. 



Experimental methods 

 When exposing the spores to X-rays, we utilize the chaml)er shown 

 in Fig. 1. (Webb et al, 1958). The discs are put on the bottom of a stain- 

 less steel cylinder. The gas surrounding them can be controlled by 



X-ray tube 



Vacuum seal 



— Liquid heliufTi 



Rubber sleeve 



Potentiometer 



Membrane 

 filters 



2in exposure 

 cylinder 



Thermocouples 

 Liquid helium 



Liquid nitrogen 



Fig. 1. 



.—Diagram of the basic exposure chamber, for controlhng atmosphere and tem- 

 perature before, dm'ing, and after irradiation. (From "Webb et al., 1958). 



means of pumps and valves connected to the manifold that is between 

 the cylinder and the X-ray tube. The temperature within the cylinder 



