Matthew Douglas

Verifying the presence of low levels of neptunium in a uranium matrix with electron energy-loss spectroscopy

This paper examines the problems associated with analysis of low levels of neptunium in a uranium matrix with electron energy-loss spectroscopy (EELS) on the transmission electron microscope (TEM). The detection of neptunium in a matrix of uranium can be impeded by the occurrence of a plural scattering event from uranium (U-M(5)+U-O(4,5)) that results in severe overlap on the Np-M(5) edge at 3665 eV. Low levels of Np (1600-6300 ppm) can be detected in a uranium solid, uranophane [Ca(UO(2))(2)(SiO(3)OH)(2)(H(2)O)(5)], by confirming that the energy gap between the Np-M(5) and Np-M(4) edges is at 184 eV and showing that the M(4)/M(5) ratio for the neptunium is smaller than that for uranium. The Richardson-Lucy deconvolution method was applied to energy-loss spectral images and was shown to increase the signal to noise ratio.

Liquid scintillation counting of environmental radionuclides: a review of the impact of background reduction

Liquid scintillation counting (LSC) supports a range of environmental science measurements. At Pacific Northwest National Laboratory, we are constructing an LSC system with an expected background reduction of 10–100 relative to values reported in the literature. In this paper, a number of current measurement applications of LSC have been considered with an emphasis on determining which aspects of such measurements would gain the greatest benefit: improved minimum detectable activity (MDA), reduction in sample size, and reduction in total analysis time.

Integrated separation scheme for measuring a suite of fission and activation products from a fresh mixed fission and activation product sample

Mixed fission and activation materials resulting from various nuclear processes and events contain a wide range of isotopes for analysis spanning almost the entire periodic table. This work describes the production of a complex synthetic sample containing fission products, activation products, and irradiated soil, and determines the percent chemical recovery of select isotopes through the integrated chemical separation scheme. Based on the results of this experiment, a complex synthetic sample can be prepared with low atom/fission ratios and isotopes of interest accurately and precisely measured following an integrated chemical separation method.

Optimization and evaluation of mixed-bed chemisorbents for extracting fission and activation products from marine and fresh waters

Chemically selective chemisorbents are needed to monitor natural and engineered waters for anthropogenic releases of stable and radioactive contaminants. Here, a number of individual and mixtures of chemisorbents were investigated for their ability to extract select fission and activation product elements from marine and coastal waters, including Co, Zr, Ru, Ag, Te, Sb, Ba, Cs, Ce, Eu, Pa, Np, and Th. Conventional manganese oxide and cyanoferrate sorbents, including commercially available Anfezh and potassium hexacyanocobalt(II) ferrate(II) (KCFC), were tested along with novel nano-structured surfaces (known as Self Assembled Monolayers on Mesoporous Supports or SAMMS) functionalized with a variety of moieties including thiol, diphosphonic acid (DiPhos-), methyl-3,4 hydroxypyridinone (HOPO-), and cyanoferrate. Extraction efficiencies were measured as a function of salinity, organic content, temperature, flow rate and sample size for both synthetic and natural fresh and saline waters under a range of environmentally relevant conditions. The effect of flow rate on extraction efficiency, from 1 to 70 mL min(-1), provided some insight on rate limitations of mechanisms affecting sorption processes. Optimized mixtures of sorbent-ligand chemistries afforded excellent retention of all target elements, except, Ba and Sb. Mixtures of tested chemisorbents, including MnO(2)/Anfezh and MnO(2)/KCFC/Thiol (1-3 mm)-SAMMS, extracted 8 of the 11 target elements studied to better than 80% efficiency, while a mixture of MnO(2)/Anfezh/Thiol (75-150 μm)-SAMMS mixture was able to extract 7 of the 11 target elements to better than 90%. Results generated here indicate that flow rate should be less of a consideration for experimental design if sampling from fresh water containing variable amounts of DOM, rather than collecting samples from salt water environments. Relative to the capability of any single type of chemisorbent tested, optimized mixtures of several sorbents are able to increase the number of elements that can be efficiently and simultaneously extracted from natural waters.

Development of a low background liquid scintillation counter for a shallow underground laboratory

Pacific Northwest National Laboratory has recently opened a shallow underground laboratory intended for measurement of low-concentration levels of radioactive isotopes in samples collected from the environment. The development of a low-background liquid scintillation counter is currently underway to further augment the measurement capabilities within this underground laboratory. Liquid scintillation counting is especially useful for measuring charged particle (e.g., β and α) emitting isotopes with no (or very weak) gamma-ray yields. The combination of high-efficiency detection of charged particle emission in a liquid scintillation cocktail coupled with the low-background environment of an appropriately designed shield located in a clean underground laboratory provides the opportunity for increased-sensitivity measurements of a range of isotopes. To take advantage of the 35m-water-equivalent overburden of the underground laboratory, a series of simulations have evaluated the scintillation counters shield design requirements to assess the possible background rate achievable. This report presents the design and background evaluation for a shallow underground, low background liquid scintillation counter design for sample measurements.

Optical design considerations for efficient light collection from liquid scintillation counters.

Liquid scintillation counters measure charged particle-emitting radioactive isotopes and are used for environmental studies, nuclear chemistry, and life science. Alpha and beta emissions arising from the material under study interact with the scintillation cocktail to produce light. The prototypical liquid scintillation counter employs low-level photon-counting detectors to measure the arrival of the scintillation. For reliable operation, the counting instrument must convey the scintillation light to the detectors efficiently and predictably. Current best practices employ the use of two or more detectors for coincidence processing to discriminate true scintillation events from background events due to instrumental effects such as photomultiplier tube dark rates, tube flashing, or other light emission not generated in the scintillation cocktail vial. In low-background liquid scintillation counters, additional attention is paid to shielding the scintillation cocktail from naturally occurring radioactive material present in the laboratory and within the instruments construction materials. Low-background design is generally at odds with optimal light collection. This study presents the evolution of a light collection design for liquid scintillation counting (LSC) in a low-background shield. The basic approach to achieve both good light collection and a low-background measurement is described. The baseline signals arising from the scintillation vial are modeled and methods to efficiently collect scintillation light are presented as part of the development of a customized low-background, high-sensitivity LSC system.

Ultra-Low Level Plutonium Isotopes in the NIST SRM 4355A (Peruvian Soil-1)

For more than 20 years, countries and their agencies which monitor radionuclide discharge sites and storage facilities have relied on the National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 4355 Peruvian Soil. Its low fallout contamination makes it an ideal soil blank for measurements associated with terrestrial-pathway-to-man studies. Presently, SRM 4355 is out of stock, and a new batch of the Peruvian soil is currently under development as future NIST SRM 4355A. Both environmental radioanalytical laboratories and mass spectrometry communities will benefit from the use of this SRM. The former must assess their laboratory procedural contamination and measurement detection limits by measurement of blank sample material. The Peruvian Soil is so low in anthropogenic radionuclide content that it is a suitable virtual blank. On the other hand, mass spectrometric laboratories have high sensitivity instruments that are capable of quantitative isotopic measurements at low plutonium levels in the SRM 4355 (first Peruvian Soil SRM) that provided the mass spectrometric community with the calibration, quality control, and testing material needed for methods development and legal defensibility. The quantification of the ultra-low plutonium content in the SRM 4355A was a considerable challenge for the mass spectrometric laboratories. Careful blank control and correction, isobaric interferences, instrument stability, peak assessment, and detection assessment were necessary. Furthermore, a systematic statistical evaluation of the measurement results and considerable discussions with the mass spectroscopy metrologists were needed to derive the certified values and uncertainties. The one sided upper limit of the 95% tolerance with 95% confidence for the massic (239)Pu content in SRM 4355A is estimated to be 54,000 atoms/g.

Background characterization of an ultra-low background liquid scintillation counter

The Ultra-Low Background Liquid Scintillation Counter developed by Pacific Northwest National Laboratory will expand the application of liquid scintillation counting by enabling lower detection limits and smaller sample volumes. By reducing the overall count rate of the background environment approximately 2 orders of magnitude below that of commercially available systems, backgrounds on the order of tens of counts per day over an energy range of ~3-3600keV can be realized. Initial test results of the ULB LSC show promising results for ultra-low background detection with liquid scintillation counting.