^20 



L. H. GRAY 



has been suggested (4^) that the distribution of such elements in tis- 

 sue might be investigated by an obvious adaptation of standard elec- 

 tron microscope techniciue. Whether or not this use of nuclear iso- 

 meric transformation proves of value in biological tracer research, 

 it appears extremely doubtful if sources could ever be prepared in suf- 

 ficient intensity free from isotopes that emit (3 rays of continuous en- 

 ergy distribution to be useful in radiobiological research. 



In principle, the artificial production of electron beams of con- 

 stant energ}^ requires only a source of electrons, which may conven- 

 iently' be a heated tungsten filament, a source of high potential, and 



100 



I- 



10 



o 



_l 



UJ 



C5 



< 



100 ^ 



(ft 

 (/) 



1- 

 u. 

 O 

 (/> 



q: 



UJ 



10 tz 



in 



z 

 o 

 cr 



10 



100 1000 



ELECTRON ENERGY, e.kv. 



10,000 



1 



Fig. 11. Range in tissue of electrons of energy between 2 kv. and 1 m.e.v. 

 Above 1 m.e.v., the range, R, in millimeters, is related to the energy, E, in m.e.v. 

 by R = 4.8/; - 0.8 {cf. Feather's foi-mula R = 0.52£' - 0.09, when R. is expressed 

 in grams per square centimeter). For the purpose of estimating rate of loss of 

 energy by electrons the total length of track may be taken to be about 1.4 times 

 the range. 



an evacuated tube in which the electrons may be accelerated. Appa- 

 ratus of this kind especially designed to meet certain radiobiological 

 requirements in the energy ranges 1-15 kv. and 10-100 kv. has been 

 described (46,47). After acceleration the electrons pass through a 

 defining slit onto the specimens contained in an evacuated box. 

 Irradiation in vacuo is practically unavoidable at the low range of 

 energies since the penetrating power of even a 10 kv. electron is little 

 over a millimeter of air at atmospheric pressure (cf. Fig. 11). It is 

 possible, however, to make extremely thin windows that will with- 



