1014 



EXPLORATION GEOPHYSICS 



radioactively inert minerals of various densities. We can then plot specific activity 

 against apparent density of the various mixtures (Figure 630). Knowing the apparent 

 densities of our unknown samples, we can now adjust their measured beta activities to 

 any arbitrarily established "mean density" by using the curve. 



Samples containing minerals with high 

 emanating power require additional treatment 

 to bring back the radioactive equilibrium dis- 

 turbed by loss of radon. It is necessary to 

 render the sample impermeable in some way 

 to stop the outward diffusion of the gas and 

 then store the fixed sample for about two to 

 four weeks to reestablish equilibrium. Prob- 

 ably the simplest way of "sealing" a sample 

 is by sintering, i.e., heating to a temperature 

 where the emanating mineral begins to fuse. 

 Figure 631 shows the beta activity of two 

 samples of carnotite that have been sintered 

 at about 1000° C. for one hour. Heating 

 drives off most of the radon, but the sintering 

 lowers the emanating power to a very small 

 value, and equilibrium reestablishes itself 

 with time. It is interesting to note that apart 

 from a slight discoloration, sintering produces 

 no change visible to the naked eye in most 

 powdered carnotite ore samples. Only rarely 

 is it necessary to re-grind the sample after 

 sintering. The overall loss of water which 

 results from the sintering process must be considered in some samples, although the 

 correction is usually very small. 



Gamma Counting 



Measurement of y activity in the laboratory is a method no more diffi- 

 cult than ^ counting, but considerably less sensitive. If a large sample is 

 available, it is usually placed coaxially around the counter, in a layer up to 

 several inches thick. Special multiple anode G-M tubes or scintillation 

 counters can be used on small samples. It is generally difficult to make good 

 y determinations on samples smaller than about 20 g. 



The y-rays from the uranium series and from potassium are fairly weW 

 known, so that theoretical calibration is possible if we know the y counting 

 efficiency of our detector. The empirical method of calibration is much 

 simpler, however, in spite of the fact that the geometrical relation of the 

 sample to the counter must be taken into account. The problem has been 

 analyzed by Davisf and by Hushley and Dixon^ who present a compre- 

 hensive theoretical treatment. 



Fig. 631. — Increase in the ^ activity of 

 carnotite samples after sintering. The orig- 

 inal activity (before heating) is indicated by 

 solid circles and arrows. 



Economic Applications 



Probably the most important economic application of radioactivity in 

 geophysics to date is the gamma- ray logging of wells drilled for petroleum. 



t F. J. Davis, "An Improved Geiger Counter Arrangement for Determination of Radium 

 Content," Jour. Res. Nat. Bur. Sids. 38. 513-518 (1947), (Res. Paper RP 1792). 



t Hushley and Dixon, "The Gamma-Ray Measurement of Radium Ore Concentrates, Can. 

 Jour. Res. 25A, 210-22 (1947). 



