992 EXPLORATION GEOPHYSICS 



together with half -lives and decay constants is shown graphically in Figure 

 605. 



The two uranium families always occur together in nature with the 

 parent elements intimately mixed in a fixed proportion which is constant 

 throughout the earth (^7235^^^238 — I/139.2), The thorium family is some- 

 times found with uranium, but more frequently occurs in minerals by 

 itself. Most of the commercial ores of uranium contain no thorium what- 

 ever. 



There are other naturally radioactive elements : the alkalis potassium 

 and rubidium, the rare earths samarium and lutecium, and the heavy metal 

 rhenium. 



The radioactive isotope of potassium (igi^^^) comprises 0.012 per cent 

 of all natural potassium and has a half-life of 4.5 • 10^ years.f It emits 

 y8-rays, orbital electron capture x-rays and y-rays, and occasionally may be 

 the principal contributor to the radioactivity of a rock.| Natural potas- 

 sium compounds can be quantitatively analyzed by geophysical means with 

 great accuracy. § The activity of common K amounts to roughly 30 betas 

 and 3.3 gammas per second per gram. The activity of the uranium series 

 (in equilibrium) is approximately 40,000 betas* per second per gram oi U. 

 Consequently, a very small uranium contamination may cause a large 

 error in a potassium analysis, but the error caused by potassium in uranium 

 and thorium analyses is usually negligible. 



Rubidium receives its radioactivity from sjRb^"^ which has an isotopic 

 abundance of 27.2 per cent in natural rubidium and a half-life of 5.8 • 10^® 

 years, or about 20 times the accepted age of the earth. Common Rh^O has 

 a specific activity approximately 10 times greater than the activity of com- 

 mon K2O. The rubidium j3, however, is 10 times weaker than that of 

 potassium.** As a chemical element, rubidium is comparatively rare 

 in nature. Its richest "ore" is lepidolite, which usually contains about 

 1.5 per cent Rh20, and lesser amounts are found in amazonite, pollucite, 

 and some varieties of microHne and muscovite. 



The radioactive samarium isotope is probably q2Sw}^^, which makes up 

 14.2 per cent of natural samarium. It is the only known a emitter outside 

 of the heavy elements, but the half-life of the activity is extremely long 

 (1.4 • 10^^ years). The element itself is found only in minute quantities 

 in samarskite, cerite, monazite, and a few other rare minerals. The lutecium 

 activity also has a long life (7 • 10^° years) and the concentration of the 



t L. H. Ahrens and R. D. Evans, "The Radioactive Decay Constants of K*° as Determined 

 from the Accumulation of Ca*° in Ancient Minerals," Phys. Rev. 74, 279-86 (1948). 



t F. E. Senftle, "The Effect of Potassium in Prospecting for Radioactive Ores," Can. Mining 

 Jour., Nov., 1948. 



§ H. C. Spicer, "Gamma Ray Studies of Potassium Salts and Associated Geological Forma- 

 tions," U. S. Geol. Survey Bull. 950, 143-61 (1942-45). 



R. B. Barnes and D. J. Salley, "Analysis for Potassium by its Natural Radioactivity," Ind. 

 and Eng. Chem., Anal. Ed., 15, 4-7 (1943). 



A. M. Gaudin and J. H. Pannell, "Radioactive Determination of Potassium in Solids," Anal. 

 Chem. 20, 1154-6 (1948). 



* Neglecting the weak ^'s of r/i==" and Pb'^\ 

 ** Maximum /3 energies : Rb — 0.13 Mev, K — 1.3 Mev. Rb also emits weak y rays. 



