The Determination Of Uranium And Thorium Isotopes In Water Rock And Soil Samples By Alpha Spectrometry

This publication details the analytical methods used at the Freshwater Institute for the radiochemical analysis of aquatic and terrestrial samples. Sample collection methods are described with emphasis on water sampling. A detailed "Calculations" section contains the mathematical formulae used to determine the absolute activity of each isotope analyzed.

Inductively coupled plasma-mass spectrometry (ICP-MS), using standard sample introduction by peristaltic pumping, is presented as a method to determine total and isotopic uranium (234U, 235U, 236U, and 238U) and thorium (232Th) in soil samples. Initial sample preparation consists of oven drying to determine moisture content, and grinding and mixing the soil to make it homogenous. This is followed by a nitric/hydrofluoric acid digestion to bring the uranium into solution. Bismuth (209Bi) is added prior to digestion to monitor for losses due to sample preparation and analysis. An additional digestion, using nitric/perchloric acid is performed if the total thorium concentration is required on the sample. The uranium and thorium content of this solution and the 235U/238U ratio are measured on an initial pass through the ICP-MS. The total uranium measurement is based on the 238U isotope measurement with correction for the presence of the U isotopes. To determine the concentration of the less abundant 234U and 236U isotopes, the digestate is further concentrated by using a solid phase extraction column (TRUSpec by EiChrom Industries, Inc.) before a second pass through the ICP-MS.

The U.S. Geological Survey (USGS) Yucca Mountain Project Branch laboratory in Denver, Colorado, conducts routine high-precision isotope analyses of uranium (U) and thorium (Th) using thermal ionization mass-spectrometry (TIMS). The measurements are conducted by a solid-source mass-spectrometer equipped with a Faraday multi-collector system and an energy filter in front of an active-film-type secondary electron multiplier (SEM). The abundance sensitivity of the instrument (signal at mass 237 over {sup 238}U in natural U) with the energy filter is {approx} 15 x 10{sup -9} and peak tails are reduced by a factor of {approx}100 relative to the Faraday cup measurements. Since instrument installation in April 2004, more than 500 rock and water samples have been analyzed in support of isotope-geochemical studies for the U.S. Department of Energy's Yucca Mountain Project. Isotope ratios of sub-nanogram to microgram U and Th samples are measured on graphite-coated single-filament and double-filament assemblies using zone-refined rhenium filaments. Ion beams less than 5 millivolt (mV) are measured with the SEM, which is corrected for non-linearity on the basis of measurements of National Institute of Standards and Technology (NIST) U-500 and 4321 B standards with ion beams ranging from 0.01 to 8 mV. Inter-calibration between the SEM and the Faraday multi-collector is performed for every mass cycle using a {approx}5 mV beam switched between Faraday cup and SEM ('bridging' technique), because SEM-Faraday inter-calibrations prior to the measurement failed to produce acceptable results. Either natural ({sup 235}U) or artificial ({sup 236}U, {sup 229}Th) isotopes were used for the bridging. Separate runs are conducted for minor isotopes using SEM only. These techniques result in high within-run precisions of

Thorium is a naturally occurring element for whom all isotopes are radioactive . Many of these isotopes are alpha emitting radionuclides, some of which have limits for inhalation lower than plutonium in current regulations . Neutron activation analysis can provide for the low-level determination of 232Th but can not determine other isotopes of dosimetric importance . Biological and environmental samples often have large quantities of materials which activate strongly, limiting the capabilities of instrumental neutron activiation analysis . This paper will discuss the application of a combined technique using alpha spectrometry and radiochemical neutron activiation analysis for the determination of isotopic thorium.