Natural Radioactivity in the Eastern San Bernardino Mountains

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radioactivity, San Bernardino Mountains, uranium, natural resources, nuclear energy


Earth Sciences | Environmental Sciences | Natural Resources Management and Policy | Oil, Gas, and Energy | Physical Sciences and Mathematics


This research consists of a reconnaisance investigation of the radioactivity of the various rock types in the eastern San Bernardino Mountains involving certain steps: (1) a study of natural radioactivity, (2) the derivation of a valid procedure for determining the true radioactivity, (3) a geologic and radiometric survey, and (4) a discussion and conclusion.

The field area is located in San Bernardino County, California, east of Big Bear. The traverse, which is in effect a road profile, was aligned along the course of state Highway 38 and Highway 18 which assume a northerly bearing across the mountains. The road profile begins at South Fork Camp, near Barton Flats, continuing to Baldwin Lake, from which it moves on down into Lucerne Valley. Field locations were facilitated by the route survey stations along the highway. Pacing and a steel tape were used to supplement these positions. The major instrument used was a portable Scintillator which gave direct, quantitative measurements of the natural radioactivity. To reduce the effect of natural background, such as fallout and cosmic rays, a lead shielding was placed over the working end of the instrument. This proved very effective, allowing readings as low as one-tenth of the background.

Many natural occurring elements such as uranium and thorium are unstable and are reduced by radioactive disintegration to an isotope of lead which is stable. The disintegration is effected by the emission of alpha and beta particles, as well as gamma rays. The natural occurrence of thorium and uranium is largely controlled by geochemical and mineralogical to the generation of a magma, however these two elements are not generally existent in great quantities until the fractionation of the magma reaches the later, more acidic stages. In the very late stages, when oxidation prevails, the path of uranium begins to deviate from that of thorium. This geochemical difference is a significant factor in the occurrende of these two elements.

The geology of the Transverse Range Province, within which the field area lies, may be characterized by the existence of a profound unconformable relationship which separates the Tertiary and Quaternary sediments from the older igneous and metamorphic rocks. These older rocks are of Mesozoic, Paleozoic, and possible pre-Cambrian (?) age.

The area under investigation was chosen primarily for its great exposure of these Paleozoic sediments, as well as portion of the San Bernardino Mountains may be attributed to the intrusion of great batholithic masses of a granitic nature during the Mesozoic era. Thus, a strong correlation exists between the rock types and radioactivity. The elements uranium and thorium are located within the numerous acessory minerals which are evident throughout many phases of the intrusions. These two elements would then account for the natural radioactivity.

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