The geophysical techniques described in previous chapters have depended on variations in the mechanical, chemical, electrical, or magnetic properties of rocks and minerals. Since about 1945 another property of certain elements has become of considerable economic importance. This property is known as radioactivity.
The original discovery was made by Becquerel in 1896, shortly after Röentgen had announced in 1895 the discovery of X-rays. Becquerel found that minerals containing uranium, as well as salts of uranium, emitted radiations that passed through material opaque to ordinary light, affected photographic emulsions in a manner similar to X-rays, and would ionize a gas.
The discovery of other radioactive elements soon followed. Mme. Curie, investigating minerals of uranium, extracted two new elements, polonium and radium, which were much more active than uranium. About the same time Schmidt discovered that thorium was radioactive and Debierne found the new radioactive element actinium.
Although at least 20 naturally occurring elements are now known to be radioactive, only uranium (U), thorium (Th), and an isotope of potassium (K) are of importance in exploration. One other, rubidium, is useful in determining ages of rocks, but the rest are either so rare or so weakly radioactive, or both, as to be of no significance in applied geophysics. A complete list, with characteristic radiations and other pertinent data, is given in Table 10.1.
The two elements, uranium and thorium, are important today as a source of fuel for the generation of heat and power in nuclear reactors.