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

Abstract

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 approximately 63 keV (absolute intensity = 3.7%) and the (234m)Pa 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 approximately 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 K(alpha1) (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 K(alpha1) 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 1 sigma 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.

Using atmospheric fallout to date organic horizon layers and quantify metal dynamics during decomposition

“…We determined detector efficiency for U-series radionuclides in different counting geometries using certified uranium ore (Canadian Certified Reference Materials Project BL-5 U ore) and used a mixed radionuclide solution (Isotope Products) for efficiency calibration at other energies. Corrections are made for self-attenuation of the 46 keV 210 Pb photon using standard techniques (Cutshall et al, 1983), and the 186 keV 226 Ra line was corrected for the 235 U interference by simultaneously measuring the uranium content of the sample at 63 keV (Dowdall et al, 2004;Kaste et al, 2006b). Samples were measured for weaponsderived fallout radionuclides 241 Am and 207 Bi by quantifying their gamma emissions at 59.5 and 569.7 keV, respectively, and cosmogenic 7 Be was measured via its 477.6 keV gamma emission.…”

Chemical and radiological characterization of fly and bottom ash landfill of the former sulfate pulp factory Plaški and its surroundings

“…40 [1,13] or 0.04605. [14] This gives a ratio A( 238 U)/A( 235 U) of 21.7 [15] or 21.5. [16] 235 U activity concentration was calculated using the 185.71 keV peak after subtracting the 226 Ra contribution whose peak at 186.1 keV overlaps with the 235 U peak.…”

Characterization and treatment of water used for human consumption from six sources located in the Cameron/Tuba City abandoned uranium mining area

“…238 U activity concentration was estimated indirectly through the 235 U activity concentration using the activity ratio A( 235 U)/A( 238 U) which is constant in almost all natural samples and equals to 0.04603 [11−12] or 0.04605. [13] This gives a ratio A( 238 U)/A( 235 U) of 21.7 [14] or 21.5. [15] 235 U activity concentration was calculated using the 185.71 keV peak after subtracting the 226 Ra contribution whose peak at 186.1 keV overlaps with the 235 U peak.…”