Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 873 



gamma rays depends on the density of the substance traversed and is, 

 therefore, usually expressed in terms of a "mass-absorption-coefficient." 



It is seen that all radioactive radiations are analogous to tube discharges 

 in high electrical fields. For instance, the energy of an alpha particle of 

 Ra C is 8 million electron-volts. To produce electrons having the same 

 velocity as the beta rays from Ra C, 3 million volts would be required. 

 To excite X rays of the frequency of gamma rays (10^^ to 10^° cycles), 

 a tube with 2-3 million volts would be necessary. Incidentally, the volt- 

 ages employed in atom-smashing machines are of this magnitude. 



It was mentioned before that radium breaks up into one atom of (ionized) 

 helium and another of radon (radium emanation). This gas is of special 

 importance in the technique of radioactivity measurements, since its alpha 

 radiation and the quantity of radium in radioactive equilibrium with it 

 can be readily determined. At normal pressure and temperature, 0.6 mm 

 of radon weighing 6 • 10~ grams is in equilibrium with 1 gram of Ra and 

 is called a curie. For radioactivity tests of liquids, the Mache unit 

 (1 M.U. = 4-10"^'' curie units) is often used. This represents the amount 

 of radon in one liter producing a "saturation" current of 1-10~ e.s.u., 

 that is, 1 curie produces a saturation current of 2.5-10 e.s.u. 's. Com- 

 pared with radon, the corresponding gases in the thorium and actinium 

 series are unimportant because of their rapid decay. The half-value 

 period of thoron is 54 seconds, that of actinon 3.9 seconds, and that of 

 radon 3.82 days. 



B. Radioactivity of Rocks 



The radioactive elements and decay products, respectively, of uranium, 

 thorium, actinium, rubidium, and potassium are, geologically, of very 

 unequal distribution and importance. Rubidium and actinium are so rare 

 that they may be disregarded completely. Although potassium occurs 

 abundantly and often rather uniformly throughout geologic formations, 

 its radiation is of low intensity and noticeable only where rocks contain 

 potassium compounds in chemically recoverable quantities.^ Of the two 

 remaining radioactive elements, uranium is geologically more important. 

 Its decomposition series contains a greater number of products of long 

 life and strong radiation than does the thorium series. In the uranium 

 series, radium and associated products are most readily detected in quan- 

 tities much beyond the reach of the analytical chemist. It is customary to 

 express the radioactivity of rocks in units of 10~^^ grams per gram (or 



^ See p. 880. 



BQ. Kirsch, Handb. Exper. Phys., 26(2), 32 (1931). 



