V51A-3026
Modern Measurements of Uranium Decay Rates

Friday, 18 December 2015
Poster Hall (Moscone South)
Tashi Parsons-Moss1, Sherry A Faye1, Ross W Williams1, Tzu-Fang Wang1, Paul R. Renne2, Roland Mundil2, Mark Harrison3, Bryan B Bandong1, Kenton Moody1 and Kimberly B Knight1, (1)Lawrence Livermore National Laboratory, Livermore, CA, United States, (2)Berkeley Geochronology Center, Berkeley, CA, United States, (3)University of California Los Angeles, Los Angeles, CA, United States
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 λ235U and λ238U. [1] Alpha counting experiments in 1971[2] measured λ235U and λ238U with ~0.1% precision, but have never been independently validated. We are embarking on new direct measurements of λ235U, λ238U, λ234Th, and λ234U using independent approaches for each nuclide. For the measurement of λ235U, 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 λ238U 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 λ238U with less than 0.1% uncertainty, if combined with improved λ234Th 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 λ234U at the same time. The high-precision λ234U 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.


[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