PRINCIPLES OF RADIOLOGICAL PHYSICS 21 



oscillating currents within the atomic nuclei are usually called "7 rays." 

 The oscillating currents within nuclei cause the absorption of X rays, but 

 only to a small extent. Cosmic rays include electromagnetic radiation 

 of the highest frequency known; this radiation may be called the "7-ray 

 component of cosmic rays." 



l-3d. Detection of Electromagnetic Radiation. The detection relies 

 directly or indirectly on the effects of the electric currents which electro- 

 magnetic radiation induces within matter. 



In the range of radio or microwave frequencies, i.e., of wave lengths 

 from miles down to millimeters, the currents induced in a receiving 

 antenna or in an equivalent circuit are studied conveniently by electrical 

 engineering techniques. 



In the range between the radio and the optical frequency, i.e., of wave 

 lengths between 1 mm and a few microns, the detection is less sensitive 

 and specific. It is usually based on the observation of small temperature 

 rises caused by the dissipation of radiation energy in any material. 



Beginning with the optical frequency range, radiation becomes capable 

 of exerting lasting effects on individual atoms or molecules. Some of 

 these effects result in chemical transformations. In particular the photo- 

 graphic effect is convenient and fairly sensitive. The electric effects, due 

 to the separation of atomic charges or even to the generation of secondary 

 charged corpuscular radiations, are often still more convenient and sensi- 

 tive. For example, photoelectrons ejected from atoms can be scored one 

 by one (see Sect. 1-la). It is generally also possible to measure the 

 energy of these electrons. In general, high-frequency radiations are 

 studied, through the secondary electrons which they release, by the 

 methods indicated in Sect. 1-la. 



1-4. PRODUCTION OF ELECTROMAGNETIC RADIATION 



The general method of producing electromagnetic radiation of given 

 frequency is, naturally, to pj-oduce electric currents in systems apt to 

 oscillate with that frequency. This is done in the radiofrequency range 

 by electrical techniques which are highly developed but discussion of 

 which is beyond our present concern. Suitable techniques of the same 

 nature are not available to generate electric currents of frequency much 

 above 10^^ cycles/sec (100,000 megacycles/sec). Hence resort is made to 

 methods which utilize the properties of atoms. 



l-4a. Production of Optical Radiation. Infrared, visible, and ultra- 

 violet light of all frequencies is emitted in substantial amounts by the 

 atoms of any condensed material at a sufficiently high temperature. 

 The average frequency of the radiation emitted increases in direct ratio 

 to the absolute temperature of the emitting body as indicated by the 

 correspondence of temperature and frequency in Fig. 1-5. Figure 1-13 

 shows an example of the "spectral distribution" of intensity among 



