In the half century that has elapsed since a French physicist discovered that gamma rays would fog a photographic plate, much progress has been made in the field of nuclear physics and instrumentation. Commercial gamma-logging service for oil wells has been offered for the past 15 years and during the past 5 years, considerable work has been done by the A.E.C. and others, in the development of slim-hole-logging equipment for use in exploration for radioactive minerals.

Gamma rays are the most penetrating form of natural radiation given off by the daughter products of uranium, thorium, and potassium. Traces of these elements occur in varying amounts in all common rocks, so that the gamma log has become a valuable geophysical tool both for stratigraphic correlation and for the determination of the presence of radioactive material.

During 1955, 5½ million feet of exploratory shallow holes were drilled in the search for uranium in the United States. The development of reliable scintillation-logging equipment has now made it possible rapidly to assay radioactive ore bodies in place, and at the same time, produce a stratigraphic log of overlying barren formations. These data can be correlated between holes to give a geological structure map. Because of improved gamma-logging instruments which are now available, and the greater experience which mining operators have in interpreting the logs, it is now possible accurately to evaluate a buried ore body by means of bore holes instead of core holes. As core holes cost 2-3 times as much as bore holes, the new, more reliable gamma-logging equipment quickly pays for i self by reducing the exploration costs.

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