Advances in geochemistry and geochronology are often closely linked to development of new technologies for improved measurement of elemental and isotopic abundance. At the beginning of the past decade, thermal ionization mass spectrometric (TIMS) methods were just beginning to be applied for long-lived uranium-series nuclide measurement (Edwards et al. 1987; Goldstein et al. 1989; Bard et al. 1990), with considerable advances in measurement speed, precision, and sensitivity over decay-counting methods. This opened up a vast number of applications in uranium-series geochronology and geochemistry of young sediments, volcanic rocks, and aqueous systems. Over the past decade there have continued to be advances in thermal ionization techniques, and the advent of alternative mass spectrometric methods, particularly multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS), has continued to improve the quality of uranium-series studies.

So the past decade has been a particularly dynamic time for not only development of mass spectrometric techniques, but initiation of other methods related to long-lived uranium-series nuclide measurement. In the area of sample preparation, further development of microwave digestion methods had led to advances in speed and cost of analysis. In chemical separations, development of extraction chromatographic resins for isolating specific elements have simplified many separation problems and consequently improved analytical characteristics including sensitivity, speed of analysis, waste generation, and cost. With regard to instrumental analysis, advances in both decay-counting and mass spectrometry instrumentation have improved either measurement sensitivity or precision, speed of analysis, or analytical cost. One could argue that instrumental developments will continue to drive scientific breakthroughs in the application of uranium-series nuclides as tracers and chronometers in the earth and other sciences.

In this chapter we discuss improvements documented in the literature over the past decade in these areas and others. Chemical procedures, decay-counting spectroscopy, and mass spectrometric techniques published prior to 1992 were …

Techniques for Measuring Uranium-series Nuclides: 1992–2002

Radioecological survey at selected sites hit by depleted uranium ammunitions during the 1999 Kosovo conflict

Detection efficiencies of CR-39 and LR-115 type II solid state nuclear track detectors (SSNTDs) for α-particles emitted by the nuclei of uranium and thorium series inside different natural materials were calculated by using a new Monte Carlo computer code. Variation of these detection efficiencies as a function of initial α-particles energies was investigated. Uranium and thorium contents have been determined in the studied samples by exploiting the data obtained for detection efficiencies of the SSNTD, utilized for α-particles from uranium and thorium families and measuring the resulting track densities. Data obtained were compared with those obtained by using other methods.

A new method for evaluating uranium and thorium contents in different natural material samples by calculating the CR-39 and LR-115 type II SSNTD detection efficiencies for the emitted α-particles

Deposition and distribution of Chernobyl fallout fission products and actinides in a Russian soil profile.

A new and accurate method for the determination of uranium isotopes (238U, 234U and 235U) in environmental samples by alpha-spectrometry has been developed. Uranium is preconcentrated from filtered water samples by coprecipitation with iron(III) hydroxide at pH 9-10 using an ammonia solution and the precipitate is dissolved in HNO3 and mineralized with H2O2 and HF; uranium in biological samples is ashed at 600 °C, leached with Na2CO3 solution and mineralised with HNO3, HF and H2O2; uranium in soil samples is fused with Na2CO3 and Na2O2 at 600 °C and leached with HCl, HNO3 and HF. The mineralized or leaching solution in 2M HNO3 is passed through a Microthene-TOPO (tri-octyl-phosphine oxide) column; after washing, uranium is directly eluted into a cell with ammonium oxalate solution, electrodeposited on a stainless steel disk and measured by alpha-spectrometry. The lower limits of detection of the method is 0.37 Bq.kg-1 (soil) and 0.22 mBq.l-1 (water) for 238U and 234U and 0.038 Bq.kg-1 (soil) and 0.022 mBq.l-1 (water) for 235U if 0.5 g of soil and 1 litre of water are analyzed. Five reference materials supplied by the IAEA have been analyzed and reliable results are obtained. Sample analyses show that, the 238U, 234U and 235U concentrations are in the ranges of 0.30-103, 0.49-135 and 0.02-4.82 mBq.l-1 in waters, of 1.01-7.14, 0.85-7.69 and 0.04-0.32 Bq.kg-1 in mosses and lichens, and of 25.6-53.1, 26.4-53.8 and 1.18-2.48 Bq.kg-1 in sediments. The average uranium yields for waters, mosses, lichens and sediments are 74.5±9.0%, 80.5±8.3%, 77.8±4.9% and 89.4±9.7%, respectively.

Determination of uranium isotopes in environmental samples by alpha-spectrometry

An improved method is described utilizing extraction chromatography, anion exchange, and alpha spectroscopy for measurement of uranium isotopes in human and environmental surveillance studies. These methods provide a sensitivity of {approximately}0.7 mBq per isotope per sample and are generally accurate within the precision of the measurements. The extraction chromatography methods greatly simplify separation of uranium from iron in silicate matrices and provide increased sample throughput and data quality for water, soil, and air filter samples. For bioassay samples, the coprecipitation/anion exchange/alpha spectrometric methods provide rapid throughput and sufficient sensitivity to meet new analytical performance standards in human monitoring studies. In addition, the {sup 234}U:{sup 238}U data can be used as a fingerprint of natural vs. anthropogenic sources of uranium. For 1995 data from our laboratory, a large percentage (79-94% by matrix) of samples appear to be of natural {sup 234}U:{sup 238}U isotopic composition. For all matrices, samples with higher uranium concentration generally have more depleted isotopic composition (smaller {sup 234}U:{sup 238}U). A small percentage of soil (11%), air filter (3%), urine (3%), and water (3%) samples have depleted isotopic signatures at the 95% confidence interval, indicating anthropogenic contributions of uranium to these samples. 29 refs., 5 figs., 2 tabs.

Nancy Lujan

Papers

Measurement and Application of Uranium Isotopes for Human and Environmental Monitoring