A direct-AMS assessment of Re-Os in a Cigar Lake uranium ore specimen

Abstract

The decay constant of 187Re is known to change when its inner shell electrons are ionized; the possibility of a small change occurring in a terrestrial radioactive environment has not been assessed. A recently completed multi-isotope survey of uranium ore concentrates (UOC) has prompted the question. As an extension of that work, using direct accelerator mass spectrometry (direct-AMS), this paper reports a closer investigation of such possibility in an unaltered natural rock specimen of the Cigar Lake uranium ore, containing wt.30–60 % uranium oxide. Stimulated by this research goal, the direct-AMS quantification methods were first investigated, and later tested in connection with the Re-Os assessment. Complete sets of measurement of the anion beams of 185ReOn−, (187Re+187Os)On− and 189OsOn− (n = 0–5) were carried out to find suitable sputter target matrices that would yield self-consistent solutions of (187Os/189Os)Today and (187Re/185Re)Today. These two quantities are best determined by these multi-beam measurements to be 4400 ± 500 (2σ) and 1.635 ± 0.060 (2σ), respectively. The sample is then reasoned to contain [Re/Os]Initial ∼42,000. These results do not show apparent 187Re decay constant changes, but cannot rule out the possibility, either. As a quick follow-up inspection, a simplified approach was used to surveying 187Re/185Re in a series of uranium-concentrated materials, in which this ratio was all measured somewhat smaller than the natural known value. This work was also intended to be a pilot feasibility study of using the direct-AMS method as a form of sensitive secondary ionization mass spectrometry (SIMS) for rapid Re-Os assessment in mg-sized raw minerals and environmental samples.