522 



H. 



GRAY 



Biological specimens should never be placed in close proximity to 

 the window of the tube in which the electrons are accelerated. Not 

 only is the window apt to become warm on account of the electron 

 beam energy it absorbs, but it also is a source of slow secondary elec- 

 trons. These electrons are the 8 rays discussed in the preceding sec- 

 tion, but characteristic of the passage of an electron through the ma- 

 terial of the window, and since all 8 rays commence to travel in the 

 forward direction at the moment of ejection, a cell placed in immediate 

 contact with the window would be exposed to more than half this 

 total 8-ray emission. The complementary proportion of 8 rays will, 

 of course, come from the material that supports the cell. Although 



25 







15 



;G 10 



oo 

 I- ' 



< UJ 



Ola: 



12 3 4 5 6 



WATER ABOVE IONIZATION CHAMBER, mm. 



Fig. 12. Distribution of 

 ionization with depth of 

 penetration into water for 

 cathode raj's incident with 

 different energies (50). 



5-ray ionization is only about 20% of the total it may completely ob- 

 scure any dependence of biological effect on the spacing of the clusters 

 along the track of the ionizing particles in any cases in which the high 

 ion density associated with the slow electrons has a high biological 

 efficiency (cf. the production of chromosome structural change dis- 

 cussed in Sect. B2). In certain circumstances it is therefore of inter- 

 est to reduce 5-ray ionization to a minimum, which may be done by 

 supporting the biological specimen on the thinnest possible film at a 

 distance from the tube window and from all other solid objects. 

 When it is desired to study the effect of the primary particle together 

 with the 8 rays that would accompany it as it traverses a mass of tis- 

 sue, the specimen should be mounted at a distance from the window 

 but with tissue-like material on both sides. 



Pulsed Electronic Radiation. Very small currents of 20 m.e.v. 

 electrons have been extracted from the betatron by Kerst and Skaggs 

 {52). The microwave linear accelerator ma}^ ultimately prove a 



