Sec. 1-1] 



.] I FA 'HAXICAL IX ITT Tli. 1 AN / > I '( ' E RS 



19 



i.e., the absorption of a test specimen depends upon the mass per unit 

 area a, measured in grams per square centimeter. 



The linear absorption coefficient // of a substance changes with the 

 density, therefore, with temperature, pressure, or the state of aggre- 

 gation of the absorber. The quantity f-t/p, the mass absorption coeffi- 

 cient (dimensions, square centimeters per gram), is independent of 

 these variations and is customarily tabulated, therefore. 



The mass absorption coefficient is different for each type of radia- 

 tion (beta, gamma rays) and changes with the energy of the radiation. 

 Gamma rays are highly penetrating; the gamma- or X-ray gauge is, 

 therefore, applicable for heavy metals and thick specimens. Beta 

 particles are much less penetrating and are used, therefore, for thick- 

 ness measurements on thin metal sheets or foils, on light metals, and 

 on paper, rubber, plastics, and similar substances. The alpha gauge 

 has been used only on foils in the range of several microns. Most 

 sources emit radiation of different kinds or of different energy levels 

 rather than monochromatic radiation. The laws of attenuation are 

 then more complicated than that described by Eq. (1) above, and 

 empirical calibration is usually required. 



The less penetrating the radiation is, the greater will be the change 

 in transmitted radiation intensity for a given increment of thickness 

 of the test object, i.e., the greater will be the sensitivity. However, 

 the less penetrating the radiation, the lower will be the level of the 

 intensity of radiation entering the detector and, therefore, the signal- 

 to-noise level, i.e., the accuracy of the system. 



gamma- and x-ray gauges. Sources of radiation are usually 

 radioactive isotopes mounted in shielded containers. The formerly 

 employed X-ray machines are rarely used nowadays. Several com- 

 mercially available gamma sources, their gamma-ray energy levels, 

 and their half-life times are given in Table 2. The mass absorption 



