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Abstract  

The Ionizing Radiation Division of the National Institute of Standards and Technology (NIST) has implemented several quality assurance programs to provide a consistent basis for environmental-level national and international ionizing radiation measurement credibility and comparability. These programs cut across a variety of sectors that include: (1) personnel protection; (2) survey-instrument calibration; (3) environmental radiochemistry and (4) radiobioassay. The four basic elements of the MQA programs are: (1) conformance to promulgated consensus criteria; (2) documented inhouse quality assurance and control practice; (3) periodic performance evaluations using appropriate testing materials and instruments; and (4) periodic on-site assessments by technical experts. The periodic performance evaluations are important for the demonstration of measurement traceability to the national and international physical standards. Traceability testing, however, must be augmented by the other elements to provide the strongest rationale for measurement assurance. This paper will describe the NIST programs and future directions for new programs.

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Abstract  

The National Bureau of Standards has, for the past five years, been developing natural-matrix, environmental-level radioactivity Standard Reference Materials in large quantities to be available to users over a ten year time period. These materials have been found to be useful for the evaluation of radiochemical methods and analysis, as interlaboratory comparison materials, and as quality assurance materials. To date, six Standard Reference Materials have been issued: River Sediment, Human Lung, Human Liver, Rocky Flats Soil-1, Freshwater Lake Sediment, and Peruvian Soil. The concentrations of twenty radionuclides have been certified in these materials.

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Abstract  

The National Institute of Standards and Technology (NIST) Rocky Flats Soil-II Standard reference material (SRM) is being certified through a statistical analysis of results from an interlaboratory comparison of fourteen laboratories from four countries. Mean values were calculated as the most robust and reliable certified values for each of the radionuclides. Twenty two radionuclides and four isotopes ratios were evaluated based on reported data. This article describes the approach for establishing the certified values and uncertainties for the radionuclides in the Rocky Flats Soil II SRM.

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Abstract  

The National Institute of Standards and Technology (NIST) seaweed Standard Reference Material (SRM4359) was developed through an intercomparison of 24 experienced laboratories. Several statistical techniques were evaluated for establishing reference values using the laboratories’ reported values including mean, midmean, median, Type B On Bias (BOB), Graybill-Deal, Mandel-Paule, Vangel-Rukhin, Cox and Maximum Likelihood. After reviewing the results from all these techniques, the median was chosen as a suitably robust way to calculate reference values. The uncertainties for the median values were estimated using bootstrap analysis.

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Abstract  

Radioanalytical performance testing programs provide radioassay laboratories, regulators and the public performance-based evidence that measurement capabilities are in control. Performance acceptance criteria that combine aspects of measurements difference from a known value and the associated combined uncertainty establishes a quantitative statement of statistical confidence. However, there is need for a reasonable upper limit of the reported measurement uncertainty. Evaluation of thousands of historical U.S. DOE Mixed-Analyte Performance Evaluation Program measurement results for 17 radionuclides in soil and water samples provides predictive expectations for future measurement results and a statistical basis for establishing reasonable upper limits for reported measurement uncertainties for performance evaluation programs.

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Abstract  

In the event of a radioactive disaster, one of the biggest tasks is to estimate the radiation dosage received by people to determine the actions of emergency response teams. The first and the most rapid screening method of internally contaminated people in case of an emergency response is to perform in-vivo measurements for gamma-emitters. Development of virtual gamma-ray calibration techniques will be critical for emergency invivo measurements because there are inadequate numbers of phantom types to approximate all body shapes and sizes. The purpose of this project was to find a reliable way to estimate the efficiency of gamma-systems using Monte Carlo computations, and to validate that efficiency by making measurements of a standard geometry. Two geometries, a 5-ml ampoule and a Bottle Manikin Absorption (BOMAB) phantom head, spiked with 67Ga were used as standard geometries. The radioactive objects are measured at a number of distances from a high purity germanium (HPGe) detector, and the experimental efficiency for our gamma-spectrometry system is determined. The same set of experiments was then modeled using the Monte Carlo N-Particle Transport Code (MCNP). The conclusion of this project is that computationally derived detector efficiency calibrations can be comparable to those derived experimentally from physical standards.

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Abstract  

Interlaboratory comparisons are an essential component of method validation, laboratory proficiency and equivalency evaluation, and standard reference material certification. The reliability of the results from interlaboratory comparison largely depends upon analyzing the data with appropriate statistical methods. Intercomparison data are often found to be non-normally distributed as a result of sample heterogeneity, uneven laboratory variance, and methodology bias. The statistical methods that require data normality are inappropriate for analyzing such skewed data. Instead of using data transformation or non-parametric methods, an alternative method which sets aside the assumption of data normality is utilized in the certification of new National Institute of Standards and Technology (NIST) Ocean Sediment Standard Reference Material (SRM-4357). It first determines the best-fit distribution for the data, then calculates appropriate distributional mean and tolerance limits, and finally estimates the uncertainties for the derived statistics by bootstrap analysis. The method was applied to evaluate the data from the SRM intercomparison, and revealed homogeneity for natural radionuclides and inhomogeneity for anthropogenic radionuclides in the SRM. It was found that the data of natural radionuclides in the SRM are normally distributed. In contrast, the data of anthropogenic radionuclides, such as 90Sr, 137Cs, 238Pu, and (239+240)Pu, are best fitted by Weibull distributions. The difference on data distributions for the two types of radionuclides is believed to be related to "hot" particles in the SRM. By determining the best-fit data distribution and applying bootstrap analysis, more reliable and realistic massic activity (Bq·kg-1) for anthropogenic radionuclides were certified in comparison to those obtained using data transformation and nonparametric methods. Although the developed method is computationally intensive, it provides a straightforward, rigorous procedure for computing the statistics that would otherwise be difficult to obtain when the data distribution form is complicated. With the help of advanced personal computers and use of the DataPlot software from NIST, this method becomes very practical and has enhanced interlaboratory comparison data studies and certification of the NIST standard reference materials.

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Abstract  

The preparation of performance testing (PT) materials for environmental and radiobioassay applications involves the use of natural matrix materials containing the analyte of interest, the addition (spiking) of the analyte to a desired matrix (followed by blending for certain matrices) or a combination of the two. The distribution of the sample analyte concentration in a batch of PT samples will reflect the degree of heterogeneity of the analyte in the PT material and/or the reproducibility of the sample preparation process. Commercial and government implemented radioanalytical performance evaluation programs have a variety of acceptable performance criteria. The performance criteria should take into consideration many parameters related to the preparation of the PT materials including the within and between sample analyte heterogeneity, the accuracy of the quantification of an analyte in the PT material and to what "known" value will a laboratory's result be compared. This manuscript discusses how sample preparation parameters affect the successful participation in performance evaluation (PE) programs having an acceptance criteria established as a percent difference from a "known" value or in PE programs using other acceptance criteria, such as the guidance provided in ANSI N42.22 and N13.30.

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Abstract  

This paper describes a statistical-experimental design approach for identifying sources of Pu contamination in the radioactivity material analysis of dried shellfish at the National Institute of Standards and Technology (NIST). Identification and elimination of sources of contamination were required for certifications of the radioactivity concentration of actinides in the proposed reference material. Sources of contamination included the fume hood, glassware, and reagents. The experimental design employed reduced number of required experiments by a factor of 100. The result is an orthogonal design of experiment (ODEX) and subsequent data was analyzed using Exploratory Date Analysis (EDA), which narrowed the 200+ experiments to a manageable 16. This design made it possible to identify the sources and interactions of sources of contamination in a timely and cost effective manner.

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Abstract  

We have developed a sequential extraction technique for determining the geochemical partitioning of Am, Pu, and U in soils and sediments. Stable element analyses were combined with radiometric measurements to determine the most probable geochemical host phases of these actinides in reference sediment IAEA-135.241 Am results indicate an association with carbonate minerals and organic matter. The extraction profile of238U was similar to that of refractory elements Al, Ti, and K.239/240Pu data suggest a fractionation of Pu into Fe-bearing phases of varying solubility. The reproducibility of the method was quite good (replicates agreed to within 10% at a 95% confidence level).

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