Modern Measurements of Uranium Decay Rates

Abstract:

It has been widely recognized that accurate and precise decay constants (λ) are critical to geochronology as highlighted by the EARTHTIME initiative, particularly the calibration benchmarks λ_235U and λ_238U. [1] Alpha counting experiments in 1971[2] measured λ_235U and λ_238U with ~0.1% precision, but have never been independently validated. We are embarking on new direct measurements of λ_235U, λ_238U, λ_234Th, and λ_234U using independent approaches for each nuclide. For the measurement of λ_235U, highly enriched ²³⁵U samples will be chemically purified and analyzed for U concentration and isotopic composition by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Thin films will be electrodeposited from these solutions and the α activity will be measured in an α-γ coincidence counting apparatus, which allows reduced uncertainty in counting efficiency while achieving adequate counting statistics. For λ_238U measurement we will measure ingrowth of ²³⁴Th in chemically purified, isotopically enriched ²³⁸U solutions, by quantitatively separating the Th and allowing complete decay to ²³⁴U. All of the measurements will be done using MC-ICP-MS aiming at 0.05% precision. This approach is expected to result in values of λ_238U with less than 0.1% uncertainty, if combined with improved λ_234Th measements. These will be achieved using direct decay measurements with an E-∆E charged particle telescope in coincidence with a gamma detector. This system allows measurement of ²³⁴Th β-decay and simultaneous detection and identification of α particles emitted by the ²³⁴U daughter, thus observing λ_234U at the same time. The high-precision λ_234U obtained by the direct activity measurements can independently verify the commonly used values obtained by indirect methods.[3] An overarching goal of the project is to ensure the quality of results including metrological traceability in order to facilitate implementation across diverse disciplines.

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[1] T.M. Harrison et al., (2015) It’s About Time:Opportunities and Challenges for U.S. Geological Survey. Institute of Geophysics and Planetary Physics Publication 6539, University of California, Los Angeles

[2] A. H. Jaffey et al., Physical Review C, 4, 5, (1971), 1889-1906

[3] H. Cheng et al., Chemical Geology, 169, (2000), 17-33

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