988 



EXPLORATION GEOPHYSICS 



gamma-ray log led to reexamination of the radioactivity of sedimentsf 

 and was the principal impetus behind the unpublished radioactivity research 

 of most major oil companies. 



With the discovery of fission and the subsequent development of 

 nuclear power (1939-42) came a renewed interest in uranium ores, and 

 considerable attention was directed toward the methods of searching for 

 radioactive minerals 4 Most of the procedures described in this chapter 

 are products of this development. 



As geophysical methods go, however, radioactivity exploration is still 

 in its infancy. Most of the theoretical analysis and the practical refinement 

 of field technique is yet to come. 



Particles and Quanta 



Let us first consider the missiles by which radioactive atoms make 

 known their decay. The projectiles may be either charged particles of 

 matter with a definite rest mass (alphas or betas), or merely bundles of 

 energy with zero rest mass, called quanta (gamma-rsiys) . 



The energy of these radiations is usually expressed in electron-volts 

 (ev), or the same unit multiplied by 10*^ (Mev). One ev is the energy 

 acquired by an electron in falling through a potential difiference of one 



volt, and it is equal to 1.6 

 decay which interest us here : 



10^^^ ergs. There are three types of nuclear 



Ra' 



(1) Alpha Emission. — A radioactive nucleus of atomic number 

 Z and mass number A may emit a stripped helium atom (a-particle) and 

 decay into a daughter nucleus with atomic number (Z-2) and mass number 



(A-4). No further event will take place in this 

 particular decay if the a removes sufficient 

 energy to leave the nucleus in the ground state. 

 However, a weaker a may be emitted, leaving 

 the daughter nucleus in a quantized excited state. 

 The excited nucleus then falls to ground state 

 by emitting its excess energy in the form of a 

 gamma-ray. The decay of radium^^^ into 

 radon^^- is an example of such a process (Figure 

 602). The energies of a's and y's emitted in a 

 decay are definite and discrete, and may be 

 thought of as a line spectrum. 



(2) Beta Emission. — In another type of disintegration, a fast elec- 

 tron (/? particle) is emitted from the nucleus, Z increases by one unit, and 



t K. G. Bell, C. Goodman, and W. L. Whitehead, "Radioactivity of Sedimentary Rocks and 

 Associated Petroleum," Bull. Am. Assn. Pet. Geol. 24. 1529-47 (1940). 



B. Pontecorvo, "Radioactivity Analysis of Oil Well Samples," Geophysics VII, 90-94 (1942). 

 W. L. Russell, "The Total Gamma Ray Activity of Sedimentary Rocks as Indicated by Geiger 

 Counter Determinations," Geophysics IX, 180-216 (1944). 



t H. Faul, "Radioactivity Exploration with Geiger Counters," A.I.M.E. Tech. Pub. 2460, Min- 

 ing Technology, Nov., 1948. 



Rh222 



Fig. 602. — Decay scheme of 

 radium228. Energies a^ 4.79 Me"; 

 a, 4.61 Mev; 7, 0.18 Mev. 



