1012 EXPLORATION GEOPHYSICS 



Fine crushing is still essential, however, lest we emphasize the effect of any radioactive 

 material which may merely coat the rock grains. "Infinitely thick" layers (about 1 mm 

 or about ISO mg/cm^) are frequently used, for they are easily spread with a straight- 

 edged tool. The counting rate from a thick layer is givenf by 



C = iip [89.4 U + 26.6 Th] • 10" counts/sec/cm* 



where mp is the stopping power for alphas in the sample, U and Th are the contents of 

 uranium and thorium in radioactive equilibrium with their daughter products in grams 

 per gram, and r, the minimum detectable residual air range is assumed to be 0.5 air-cm 

 for the a counter used. The value of MP for any material follows approximately from 

 the Bragg-Kleeman relation 



/t = ^ -2>2 • lO-'V^F 



where Ro and R are the ranges of the a particle in the sample material and in air, and 

 W is the mean atomic weight given by 



y/W= fi VWi + /2 VWl + .... 



where the f's are weight proportions and the Ws atomic weights of all the elements 

 present in the sample. 



Radon Method of Radium Detection. — The amount of radium in a 

 rock can be determined indirectly but very accurately by the radon method, 

 independent of any other radioactive elements that might be present. The 

 rock is fused with sodium carbonate or any other inert flux. Fusion 

 releases all gases from the rock (including radon). The gas passes into a 

 large pulse chamber where the a activity is measured. The inethod is labor- 

 ious but can be extremely sensitive. $ A good radon chamber will detect 

 the radon in equilibrium with as little as 10^^^ g. of radium. 



Beta Counting 



The determination of relative content of radioactive material in rock 

 samples by measurement of their /^-activity has become a widespread tech- 

 nique in geology as well as in mineral exploration for uranium. The exten- 

 sive use of the method can be largely ascribed to its relative simplicity and 

 reliability. Beta-ray assaying with chemical control is frequently adopted 

 when large numbers of samples are to be tested, § and when speed is an 

 important factor. 



The usual procedure is to place a fixed volume of the finely crushed rock sample 

 in the immediate vicinity of the sensitive region of a thin-wall or mica-window beta 

 counter. The thickness of the sample is usually chosen to be somewhat greater than the 

 maximum range of the most energetic /3-ray emitted from the sample to be measured. 

 In this way the recorded activity will consist almost exclusively of /3's. The "V-radiation 

 of the sample will have little effect due to the low efficiency (on the order of 1 per 

 cent) of Geiger-Miiller counters for T-rays and the small amount of sample. Even 

 though some mica-window counters are sufficiently delicate to admit and count the 



t J. Beharrell, "Absorption of Alpha-Rays in Thick Sources," Trans. Am. Geoph. Union 30, 

 333-6 (1949). 



t L. F. Curtiss and F. J. Davis, "A Counting Method for the Determination of Small Amounts 

 of Radium and of Radon," Jour. Res. Nat. Bur. Stds. 31. 181-96 (1943), (Res. Paper RP 1557). _ 



§ W. C. Peacock and W. M. Good, "An Automatic Sample Changer for Measuring Radioactive 

 Samples," Rev. Sci. Inst. 17, 255-61 (1946). 



