18 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



test object causes variation of the amount of radiation reaching the 

 detector. 



Two geometric configurations of source, sample, and detector have 

 been used: that of the abso?ytion gauge, Fig. (1-1)16, which requires 

 access to both sides of the sheet to be measured, and that of the 

 backscatter gauge, Fig. (1-1)17, which requires access to one side only. 

 In the absorption gauge, source and detector are on opposite sides 

 of the measured sheet of material, and the amount of radiation enter- 

 ing the detector varies with the attenuation in the sheet and hence 



Source Detector 



Test 

 object 





f 



Detector 



Fig. (1-1)16. Radiation thick- 

 ness gauge, principle. 



Test object 



Backing 

 Fig. (1-1)17. Backscatter 

 radiation thickness gauge, 

 principle. 



with its thickness. In the backscatter gauge, source and detector are 

 on the same side of the measured sheet. The sheet must be in contact 

 with a backing material of different atomic number; the amount of 

 backscattered radiation from the measured sheet and the backing 

 material depends upon the amount of scattering and absorption of 

 radiation in the measured sheet and the backing material, and varies 

 with the thickness of the sheet. 



If a beta-, gamma-, or X-ray beam of homogeneous energy (wave- 

 length) and of the initial intensity I penetrates an absorbing medium 

 of the thickness d, it will emerge with the intensity 



I=I e-» d (1) 



where /j, is the linear absorption coefficient. 1 For any substance, /.t 

 is proportional to the density p of the absorbing material, i.e., the 

 mass m per unit volume V. The exponent in Eq. (1) is, therefore, 



md m 



ad = const pd = const -—- = const — 

 r V a 



1 Instead of absorption coefficient, sometimes the words "extinction coeffi- 

 cient" or "attenuation coefficient" are used. These latter expressions are more 

 correct, since attenuation of X-ray radiation comes about by absorption, scat- 

 tering, and pair formation. 



