2 BIOPHYSICALLY ACTIVE X-RAYS 



in a low-pressure gas. They travel in straight lines until they collide 

 with the target, from the surface of which the roentgen rays are emitted. 

 The thin platinum targets of the early gas tubes were rapidly replaced 

 by thin disks of platinum, backed by massive pieces of copper. The 

 copper support served as a good heat conductor, since it was early recog- 

 nized that most of the energy of the cathode rays after the rays collided 

 with the target appeared as heat. For a metal to be suitable as material 

 for a practical x-ray target, in medical diagnostic work, it was found 

 that it must possess the following properties: (1) a high melting point, 

 (2) a low vapor pressure, (3) a high thermal conductivity, (4) a high 

 atomic number. Tungsten fulfills all these requirements and hence 

 replaced platinum in most of the later gas tubes. 



Coolidge High-Vacuum Tube 



No radical changes took place in the construction and method of 

 generating x-rays until the introduction, in 1913, of the Coolidge x-ray 

 tube with a hot tungsten-filament cathode. 



By 1909 O. W. Richardson and his students at Princeton had shown 

 under what conditions electrons may be expected to be emitted by high- 

 temperature metal filaments in vacuo. This work terminated in 

 Richardson's thermionic law, which was subsequently slightly modified 

 in its theoretical derivation through the use of the Fermi-Dirac law of 

 distribution of energies in the electrons in the metal, giving 



/ = AT 2 e- b/T 



where I is the saturation current in amperes per square centimeter of 

 the emitting filament surface; A, a universal constant, is the same for 

 all emitting metals; T is the absolute temperature of the filament; and 

 b = e0/3OO/c, in which k is Boltzmann's constant. e$/300 is called the 

 work function of the metal emitting the thermoelectrons, and its magni- 

 tude is expressed in electron volts. The work function is not the total 

 potential barrier at the surface through which the inner electrons of the 

 metal must pass, but the difference between this energy and the inner 

 energy of the electrons. The value of A for tungsten is 60.2 amp/cm 2 / 

 degree 2 , and its work function is equal to 4.52 volts. 



The electron emission from a metal surface, heated in vacuo, increases 

 exponentially with the temperature of the metal, so that a small change 

 in temperature will result in a very large increase in electron emission. 

 If a metal electrode is placed opposite such an emitting surface and a 

 difference of potential is applied, so as to accelerate the free electrons 

 toward the plate, it is found that when the temperature of the filament 



