Determining 234Th and 238U in rocks, soils, and sediments via the doublet gamma at 92.5 keV

Efficient and accurate measurements of uranium (U) and U-series radionuclides in earth's materials are needed to assess its environmental impact, reconstruct geochemical histories, and quantify heat production in the crust. To date, measurements of 234Th and 238U by gamma spectrometry have relied on the quantification of 234Th gamma emissions at ∼63 keV (absolute intensity = 3.7%) and the 234mPa gamma at 1001 keV (absolute intensity = 0.84%). However, use of the 63 keV emissions can be hampered by 232Th interferences and self absorption, and the 1001 keV photon has a very low yield. Here we describe the effective use of the 234Th doublet gamma emission at ∼92.5 keV (total absolute intensity = 4.8%) for 234Th and 238U measurements. This doublet has been largely ignored because of its proximity to the Th Kα1 (93.3 keV) and thus its vulnerability to a Th self-fluorescence interference. We demonstrate that additions of U and 40K to a Th ore sample do not increase the count rate at 92–93 keV above that which would be expected from the associated additions of U and 234Th. We also show that the Th self-fluorescence interference appears to be an anomaly associated only with the analysis of relatively thick (>1 mm) Th minerals, and suggest that fluorescence will not complicate the 92–93 keV region in most environmental samples. A review of decay data reveals that Th Kα1 X-rays associated with the decay of 235U and 228Ac can significantly interfere with quantification of the 92.5 keV 234Th doublet. We show that simple experimentally-derived correction factors for these X-rays can be used to accurately determine 234Th using its strongest gammas, resulting in higher count rates and smaller self-absorption corrections relative to the traditional analytical lines. Total 1σ analytical error associated with U measurements at 92.5 keV ranged from 1 to 9% and is dominated by the relative size of the 228Ac interference. Detection limits for U in environmental samples using this technique are on the order of 50 ppb.