466 JOHN W. G O W E N 



eigy. The average length of these recoil electron paths is short com- 

 pared to that of a photoelectron from a quantum of the same original 

 energy; a recoil electron, for instance, from a 50 e.kv. quantum has 

 an average energy of onl,y 11 e.kv. Recoil electrons are visualized 

 as distributing the energy obtained from high voltage quanta to many 

 discrete spots in the absorbing tissue, in a manner similar to that of 

 photoelectrons with the same amount of energy. Quanta with 100 

 e.kv. energy have about 72% of their total energy dissipated as recoil 

 electrons, the mean electron energy being about 17 e.kv. With 1000 

 e.kv. quanta, onlj^ recoil electrons are formed; their average energy 



UNMODIFIED 



SCATTERED COMPTON 



X RAYS SCATTERED 



PHOTON 



CHARACTERISTIC 

 X RAYS 



INCIDENT PHOTON '//^ UNABSORBED 



PRIMARY 



X RAYS 



HEAT 



PHOTOELECTROMS RECOIL ELECTRON 



Fig. G. Sketch to indicate multiple mechanisms of en- 

 ergy dissipation from an X-i'ay beam traversing tissue. 



is about 435 e.kv. The recoil electrons, except for directional charac- 

 teristics, behave essentially as photoelectrons in that their energy is 

 dissipated by creating secondary electrons of still lower energy and 

 by creating ion pairs along their path. The degradation scheme of 

 X-ray energy is graphically represented in Figure 6. 



The conversion of the energy of large quanta into recoil electrons 

 rather than photoelectrons obviously could result in a very great dif- 

 ference in the observed effects of the X rays. For example, a theoret- 

 ical basis for wavelength effects in biological investigations will tend 

 to disappear. Work of Packard (43) on Drosophila eggs gave experi- 

 mental support to this viewpoint since he was unable to detect any 

 differential quantitative effect from the various wavelengths of X 

 rays used. 



Wyckoff (52) observed that colon bacteria were killed by X-ray 



