PRINCIPLES OF RADIOLOGICAL PHYSICS 19 



The amount of energy hv which electromagnetic radiation of frequency 



V is capable of transferring to an electron in a single process is called a 

 "photon" or "radiation quantum." 



All pertinent experiments of atomic physics show that electromagnetic 

 radiation always transfers energy to matter in finite lumps, i.e., one 

 photon at a time. In fact, all experiments which can indicate the size of 

 energy transfers with sufficient accuracy confirm that the transfer of 

 energy "photon by photon" is a general law. 



The diseontinous absorption of radiation energy in lumps is not detected in 

 macroscopic systems such as radio antennas for the following reasons: In the 

 first place, radio circuits respond only to electromagnetic radiation whose fre- 

 quencies are many orders of magnitude lower than the frequency of light. The 

 photons absorbed by radio circuits have correspondingly lower energies. In 

 the second place, even an amount of energy whose absorption has a substantial 

 effect upon a single atom has a wholly negligil)le effect on a wire circuit. Radio- 

 frequency circuits in practice absorb or emit huge numbers of photons within 

 short time intervals, and the discontinuity of the energy transfers escapes 

 detection. 



The frequency of electromagnetic radiation is thus a clearcut index of 

 its "potency." The photon energy hv determines the ability of a radia- 

 tion to accomplish or not to accomplish any given effect on matter. 

 Low-frequency radiation cannot eject electrons from atoms. Higher 

 frequency radiation is photoelectrically active, but the amount of kinetic 

 energy which it imparts to an ejected electron is limited by the magnitude 

 of each photon. 



Electromagnetic radiations are entered in the chart of radiation 

 potencies (Fig. 1-5) by equating the potency of radiation of frequency 



V to that of a corpuscular radiation whose particles have a kinetic energy 

 equal to hv. 



The following example illustrates the interconnection of the potencies 

 of corpuscular and electromagnetic radiations. Consider an X-ray tube 

 to which is applied a potential difference of 85 kv. The electrons within 

 the tube strike the target with an energy of 85 kev. The target radiates 

 X rays consisting of a variety of monochromatic components whose fre- 

 quencies range up to a maximum value v^^^. This value is fixed by the 

 condition that an electron cannot possibly radiate more energy than it 

 possesses initially. Therefore the maximum photon energy equals the 

 kinetic energy of the electron: 



^I'max = 85 kev 

 v^^^ = 85,000/4.15 X 10-1^ = 2.05 X lO^^ cycles/sec (11) 



The X-ray wave lengths exceed a corresponding minimum value: 



Xn.in = c/v„,^, = 3 X 10172.05 X W = 1.46 X 10-« cm = 0.146 A 



