CHAPTER X 



RADIOACTIVITY METHODS* 



When Henri Becquerel discovered that a lump of pitchblende fogged 

 his photographic plate,f he had stumbled upon a phenomenon of far-reach- 

 ing implications. The subsequent researches of the Curies in France and 

 Rutherford in England drew attention to the fact that here at last was a 

 way in which man could learn about the subatomic particles and the forces 

 that hold them together. The field of nuclear physics was born and grew 

 at an ever-increasing rate. 



Important geophysical implications of the new science became apparent 

 shortly after the turn of the century. First, since the ever-present radio- 

 active substances liberate energy in their decay, they must contribute 

 materially to the internal heat of the earth. ij: Second, since the rate of decay 

 can be measured with good accuracy, we can deduce the absolute age of an 

 igneous rock by analyzing for the amounts of the decaying element and 

 the product of the decay. The age determinations that have been particularly 

 successful are based on the decay of uranium and thorium into lead,§ the 

 accumulation of heliumff (dead alpha particles), and the decay of rubidium 

 into strontium.$$ 



Throughout the first third of this century, sporadic attempts were made 

 to use radioactivity for geophysical exploration. The work of Lind§§ in the 

 carnotite fields of western Colorado and of Ambronnf f f in Germany is prob- 

 ably outstanding. Almost all measurements of this era were made by gold- 

 leaf electroscope combined with various types of ionization chambers. 



The first large-scale application of radioactivity to exploration came in 

 the late thirties, when it was discovered that logs of gamma-ray activity$|$ 

 in oil wells are very useful in stratigraphic correlation. The success of the 



* Contribution from the Laboratory for Nuclear Science and Engineering and the 

 Department of Geology, Massachusetts Institute of Technology. 



t H. Becquerel, "Sitr les radiations emises par phosphorescence," Compt. rend. 122, 420-1 

 (1896). See also ibid.. 1086-8. 



% L. B. Slichter, "Cooling of the Earth," Bull. Geol. Soc. Am. 52, 561-600 (1941), 

 W. D. Urry, "Significance of Radioactivity in Geophysics — Thermal History of the Earth," 

 Trans. Am. Geoph. Union 30, 171-80 (1949). 



§ A. Holmes, "The Age of the Earth." Th. Nelson and Sons. London, 1937. 

 tt C. Goodman and R. D. Evans, "Age Measurements by Radioactivity," Bull Geol. Soc. Am. 

 52, 491-544 (1941). See also Phys. Rev. 65. 216-27 (1944). 



P. M. Hurley and C. Goodman, "Helium Age Measurement", Bull. Geol. Soc. Am. 54, 305-24 

 (1943). 



tt L. H. Ahrens, "Measuring Geologic Time by the Strontium Method," Bull. Geol. Soc. Am. 

 60, 217-266 (1949). 



§§ S. C. Lind and C. F. Whittemore, "The Radium-Uranium Ratio in Carnotites," U. S. Bur. 

 Mines Tech. Paper 88. 29 pp. (191.5). 



ttt R. Ambronn, "Elements of Geophysics," McGraw-Hill, New York, 1928. 



ttt W. Green and R. E. Fearon, "Well Logging by Radioactivity," Geophysics V, 272 (1940). 



