22 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



with increasing atomic number. The mass absorption coefficient 

 depends, of course, upon the energy of the beta radiation and can be 

 found, for aluminum, by the empirical equation 



H 17 



P ^raax 



where E max is the maximum energy in the beta-radiation spectrum, 

 in million electron volts. 1 



Absorption-type beta gauges can be used for thickness measure- 

 ments up to about 1,200 mg/cm 2 , that is, for aluminum, a thickness 

 of 4.5 mm. The maximum range of the backscatter gauge is about 

 one-half that of an absorption gauge. 



The radiation flux must be high enough to produce the required 

 sensitivity, but not so high as to make shielding too unwieldy. Source 

 strengths used in industrial applications range from 5 millicuries to 

 1 curie. 2 



alpha gauges. The general form of an absorption characteristic 

 for alpha rays is shown in Fig. (1-1)20, curve a. The intensity of the 



radiation passing through an 



"N. \ b \^a absorbing medium, such as a 



\ , _\ gas, remains unchanged until 



\ i jl at a distance R from the source 



\ I Jl (range) it decreases rapidly. 



V^ \ jy^ The range R depends upon the 



Thickness R energy level of the radiation 



and the absorption in the me- 



Fig. (1-1)20. Attenuation of alpha rays dium; for polonium 210 in dry 



(a) in air; (b) after penetrating a homo- ^ ^ ^oq &nd ?6() mm jj it ig 



geneous foil; (c) after penetrating a 



(thicker) nonhomogeneous foil. aDOUt O.O Cm. 



The test object must be thin 

 enough so that the range R of the radiation, after passing the object, 

 exceeds the thickness of the test object. The introduction of such 

 an object in the beam decreases the range R by an amount Ai? 

 (curve b). The range and the range variation AR can be measured 

 either by varying the distance between the sample and the detector 3 

 or by varying the pressure of the gas between the sample and the 

 detector. 4 The stopping power of the object (in milligrams per 



1 M. Curie et al., Revs. Mod. Phys., 3, 427 (1931). 



2 G. B. Foster, Industrial Thickness Gages, Nucleonics, 14, 66 (May, 1956). 



3 W. C. Barber, Rev. Set. Instr., 24, 469 (1953). 



4 W. H. T. Davison, J. Sci. Instr., 34, 418 (1957). 



