Radioactivity of Rocks: an Improvement in the Photographic Technique

Abstract

EXISTING methods^1–3 for the study of the radioactivity of thin sections of rock using ‘Nuclear Research’ photographic plates suffer from difficulties in technique. The use of a photographic plate to study the distribution of radioactivity in a thin section of rock entails the removal of the rock section from the emulsion during processing, followed by accurate replacement so that emulsion and section can be examined together. Accurate replacement of the section by mechanical means in its original position relative to the emulsion is obviously difficult, and in addition, since emulsion and thin section must be viewed through the thickness of the glass plate, normal immersion objectives of high resolving power cannot be used. The accurate correlation of alpha-tracks in the emulsion with specific mineral grains or boundaries is also complicated by the presence of an air gap between section and emulsion during exposure, especially in the case of alpha-tracks which may enter the emulsion at small angles of incidence. The error in correlation due to the presence of this air gap may be aggravated if distortion of the emulsion should occur during processing, or if the surface of the emulsion should need to be cleaned before examination.