41Ca: Measurement by accelerator mass spectrometry and applications

Abstract

We describe in detail methods and techniques we have developed to measure 41Ca40Ca ratios to a sensitivity of 6 × 10−16 us our FN tandem accelerator. The major steps involved were (1) adoption of 41CaH3− negative ions to reduce 41K (41K/40Ca ~ 5 10−13); (2) production of typically 5 μA of 40CaH3−ions from calcium hydride in a high-intensity source; (3) refinement of a technique to efficiently convert CaO samples, as small as 15 mg, into CaH2; (4) implementation of a high resolution velocity selector to remove interferring 40Ca and 42Ca ions and (5) use of a multi-anode gas ionization detector to provide excellent discrimination of 41Ca from the remaining background ions. The system performs admirably, enabling measurements with ± 5% reproducibility on extraterrestrial samples, such as meteorites and lunar rocks. In addition, we have also carried out the first measurements, without pre-enrichment, of 41Ca in terrestrial rocks and bones. From these measurements, we conclude that radiocalcium dating of bones will most likely not develop into a viable dating technique. The 41Ca concentrations in modern bones are not only disappointingly low and quite close to our sensitivity limit, but show a marked variability. On the other hand, the study of 41Ca in extraterrestrial samples offers much promise as a means of determining terrestrial ages and pre-atmospheric sizes of meteorites, and of investigating the average solar cosmic ray flux during the past 300 000 years.