20 RADIATION BIOLOGY 



The X-ray photons can in turn eject electrons by photoelectric effect from 

 any material around the X-ray tube. The kinetic energies which these 

 "photoelectrons" receive from the radiation range up to 85 kev. Thus 

 the energy of the corpuscular radiation in the tube is transferred by 

 intermediary X rays to a new corpuscular radiation generated outside the 

 tube. The decrease of potency involved in the transfer may be ciuite 

 small. 



l-3c. Classification of Electromagnetic Radiations. There are two fre- 

 quency ranges at which electromagnetic radiation is very strongly 

 absorbed by practically any kind of matter. One is the range between 

 approximately 10" and 10^^ cycles/sec (corresponding to photon energies 

 of 0.0004 to 0.04 ev, or wave lengths of 0.3-0.003 cm). Electromagnetic 

 radiation of these frequencies induces resonating currents associated with 

 the motion of whole atoms with respect to one another. 



The other range of strong absorption runs approximately from 10^^ to 

 10^^ cycles/sec (corresponding to photon energies from 4 to 400 ev or 

 wavelengths of 30-3000 A). Oscillating currents associated with the 

 motion of electrons within atoms are responsible for this absorption. 

 Radiation in these two ranges of strong absorption is unwieldy. There- 

 fore the frecjuency spectrum which finds most practical application con- 

 sists of three separate portions lying below, between, and above the 

 ranges of strong absorption (see Fig. 1-5). 



Radiation of the lower frequency range, up to 10" cycles/sec, propa- 

 gates freely through insulators (but drives oscillating currents through 

 conductors, and is absorbed in doing so). It is usually called "radio 

 waves" or "hertzian waves"; radiation whose frequency lies in the upper 

 part of this range (above 10" cycles/sec) is usually called "microwaves." 



Radiation of the frequency band 3.85 to 7.7 X 10^* cycles/sec affects 

 the human eye and is called "light." The term "ultraviolet hght" 

 applies to radiations of higher frequency, perhaps up to 10^^ cycles/sec. 

 The term "infrared light" applies to radiation of frequency lower 

 than visible light and higher than radio waves. Infrared, visible, and 

 ultraviolet light together cover the middle or "optical" portion of the 

 spectrum. Radiations of this range penetrate easily through homogene- 

 ous pure substances in which the external electrons of each atom or 

 molecule are held comparatively tightly. 



Radiations of such high frecjuency as to be poorly absorbed by most 

 atomic electrons (see Sect. 2-3) are called "X rays"^ and cover the third 

 main frequency range, above 10^^ to 10^^ cycles/sec. X rays emitted by 



^ The term " X rays " is tentatively applied in this chapter to cover all high-frequency 

 radiations, no matter how produced, because the need is felt for a single expression to 

 be used in this connection. No such term is at present in general use. The term 

 "X ray" is often understood to refer to radiation produced by the impact of electrons 

 on a target, or otherwise by atomic electrons, and thus not to include y rays. 



