RADIOACTIVITY METHODS 



1011 



Neither can we ignore the natural radioactivity of the formation. Some hydro- 

 genous rocks, notably shale, would appear as lows on the neutron curves, were it not 

 for their high radioactivity. The 'V detector does not differentiate between quanta from 

 neutron absorption, from Compton collisions or from naturally radioactive nuclei. It 

 records them all quite indiscriminately, and the reading from Compton quanta alone is 

 frequently greater than the neutron effect. 



The slowing down of neutrons in barite drilling mud is about as good as in pure 

 water. Therefore the presence of mud in the immediate vicinity of the neutron source 

 will seriously affect the measurement, and the probe should be large enough to dis- 

 place most of the mud. It is important to consider carefully the probability of mud- 

 filled caves behind the casing, lest we interpret them as porous zones filled with water 

 or oil. Bore-hole calipering is an important adjunct. (See Chapter XI.) 



LABORATORY TECHNIQUES 

 Direct a Counting^ 



The range of the natural a particles in average rock is only about 15 

 to 50 microns. It follows that in any measurement of a activity we deal 

 primarily with the surface of the sample, and grain size becomes a critical 

 parameter. The sample may be prepared as either a thin source or an 

 "infinitely thick" source, depending on the type of measurement. 



Very thin deposits of metallic uranium can be made by electrolytic deposition on a 

 flat metal plate (usually nickel), but the preparation of thin layers from rock samples 

 is considerably more difficult, for the maximum permissible particle size is on the order 

 of only a few microns. One of the methods is to 

 deposit the sources from a thin slurry made with 

 a volatile organic solvent. Sources that are suffi- 

 ciently thin to cause only slight absorption of the 

 weakest a will give a lower counting rate than 

 thick sources, but the conversion of the counting 

 rate to actual specific a activity is easier for thin 

 layers and does not necessitate accurate knowl- 

 edge of the a stopping power /x in that particular 

 material. We may define counting efficiency as 

 the observed a counting rate divided by the actual 

 rate of a disintegration within the sample. For a 

 sample of zero thickness, the efficiency would be 

 50 per cent. Half the alphas would spend their 

 energy in the gas of the chamber and be counted, 

 while the others would imbed themselves in the 

 sample tray and would not produce detectable 

 ionization. As we increase the thickness of the 

 sample, more and more of the as are absorbed 

 in the sample layer and the counting efficiency 

 decreases, as shown in Figure 629. The decrease 

 is almost rectilinear up to a sample thickness of about 5 mg/cm'. 



Thick sources have been used more widely in geological laboratories, for they do 

 not present the problem of preparation, which is inherent to the thin source technique. 



t R. D. Evans and C. Goodman, "Radioactivity of Rocks," Bull. Geol. Soc. Am. 52, 459-90 

 (1941). 



N. B. Keevil and W. E. Grasham, "Theory of a-ray Counting from Solid Sources," Canadian 

 Jour. Res. 21A,2\-ZQ, {I'SiZ). 



H. H. Nogami and P. M. Hurley, "The Absorption Factor in Counting Alpha Rays from 

 Thick Mineral Sources," Trans. Am. Geoph. Union 29, 335-40 (1946). 



THICKNESS, 



12 14 



Fig. 629. — Alpha counting efficiency as 

 a function of sample thickness for instru- 

 ments of fair, good, and excellent sensi- 

 tivity; r is the minimum detectable resid- 

 ual a range in air-cm. (After Keevil and 

 Grasham, loc. cit.) 



