692 



TABLES 759-784.— X-RAYS 



X-rays, which are short wavelength (.06 — 102(L4) radiant energy, are, in 

 general, generated whenever swiftly moving electrons are suddenly stopped by 

 striking any material substance. The electrons may come from a cold cathode 

 (gas-filled tube) and the current increased by ionization of the gas in the tube, 

 or they may come from a hot cathode (Coolidge tube) in a tube of very low 

 gas pressure. Soft and hard X-rays are terms applied to X-rays produced by 

 low or high applied voltage respectively. 



Two types of X-rays are generated when the electrons hit the target — con- 

 tinuous spectrum (over a limited wavelength) and the radiation that is charac- 

 teristic of the material of which the anode is made. The continuous X-ray 

 spectrum has a very definite short-wave limit that depends upon the voltage 

 applied to the tube. Thus 



V e = hv = hc/\ 



If V is given in volts, this wavelength A will be in angstroms if the other 



units are properly chosen. 



x ,. ., 12395 

 A (in A) = 



The characteristic spectra are designated K, L, M, N, O, etc., where these 

 letters refer to the various electron shells (Table 658). 



X-rays, like any type of radiant energy, have two characteristics ; intensity 

 (i.e., the rate of energy transfer), and wavelength. These two quantities are 

 connected thus : the energy E = hv — hc/k. 



This, of course, assumes monochromatic radiation or the energy for a nar- 

 row wavelength interval, which is not always the case ; all electrons do not hit 

 the anode with the same energy nor do all materials react alike to electron 

 bombardment. Some of the characteristics of X-rays and the reaction of 

 X-rays to various materials are given in the following tables. 



TABLE 759.— X-RAY PRODUCTION 288 



Quantity of X-rays emitted by a tungsten-target tube per kilowatt of energy in 



cathode-ray beam.* ' 



Effective 



wavelength 



(unfiltered) 



angstrom units 



.56 



.40 



.28 



.14 



.056 



.028 



.014 



Roentgens (r) 



per second at 



1 meter from 



target 



(unfiltered) 



1.2 



.62 



.34 



.39 

 1.1 

 2.1 

 4.0 



238 Clark, George L., Applied X-rays, McGraw-Hill Book Company, Inc. 

 of the publishers. 



* Compiled by A. H. Compton. 



1940. Used by permission 



TABLE 760.— CRITICAL ABSORPTION WAVELENGTHS (A), K SERIES* 



12 Mg 9.5112 



13 Al 7.9470 



17 CI 4.3938 



24 Cr 2.0663 



26 Fe 1.7405 



29 Cu 1.3780 



35 Br 9182 



40 Zr 6874 



42 Mo 61842 



47 Ag 4852 



53 I 3738 



56 Ba 3308 



74 W 17807 



78 Pt 1581 



79 Au 1534 



82 Pb 1410 



92 U 1075 



* For reference, see footnote 236, above. 

 SMITHSONIAN PHYSICAL TABLES 



