Bias in the measurement of radon gas using ionization chambers: Application to SIR.
Two main non-destructive techniques can be used to measure standard 222Rn gas ampoules: well-type ionization chambers and gamma-ray spectrometry, the former being used in the Système International de Référence (SIR) for international comparison purposes. The reliability of these techniques requires that the variability of the flame-sealed gas glass ampoules used have a negligible influence on the detector response. This variability is studied in this work by considering three parameters: the volume of the ampoule, the position of the sealing point and the thickness of the glass. Results showed that variability of the gas ampoules induced measurement bias larger than the uncertainty of the standard sources.
Primary and secondary measurements of 222Rn.
In the past few years, a primary measurement system has been established at the PTB allowing absolute measurements of 222Rn. The activity can be determined by counting the emitted alpha particles in a defined solid angle after condensing the radon at a cold point under vacuum conditions. This primary measurement system is connected to a solid 226Ra source, where 222Rn can be produced up to an activity of about 2MBq. After the measurement, radon can be completely transferred into a glass bulb or a stainless steel cylinder for use as a transfer standard. The results of the measurements, the uncertainties and the realization of the primary measurement system are described. For quality assurance and fast determination of radon activity, a secondary measurement system with an NaI(Tl) detector is also used. Together, both systems provide a reliable tool for the production of gaseous 222Rn activity standards with relative uncertainties of less than 1%.
Chemosymbiotic bivalves and stable carbon isotopes indicate hydrocarbon seepage at four unusual Cenozoic fossil localities
Four unusual Cenozoic fossil localities are identified here as ancient hydrocarbon seep sites using palaeontological, petrological, and stable carbon isotope data. Late Eocene carbonate-cemented sandstone bodies in the Wagonwheel Mountains in California were previously suspected to represent ancient hydrocarbon seep sites, but the relatively high δ13Ccarbonate values (−12.2 to −5.0‰) suggest oil rather than methane seepage. The Oligocene fauna of the Elmira asphalt mine, Cuba, was previously interpreted as a mix of freshwater and marine taxa, but all species are here identified as belonging to marine groups, including the bivalve families Lucinidae and Vesicomyidae, whose extant members live largely in symbiosis with chemoautotrophic endosymbionts. A carbonate concretion from this site showed δ13Ccarbonate values as low as −32.2‰, which most likely indicates methane seepage. A previously unpublished Oligocene fossil locality in Atlantico, northern Colombia, is dominated by large solemyid, mytilid, lucinid, and vesicomyid bivalves, which most likely lived with chemotrophic endosymbionts. Seepage of biogenic methane without a significant contribution of thermogenic methane is indicated by δ13Ccarbonate values as low as −51.3‰. We confirm that the Pleurophopsis-dominated sites of the Heath Shale and Lomitos Chert in northern Peru are ancient seep sites, although the previous identification of Pleurophopsis peruviana as a vesicomyid is doubtful.