This paper describes some highlights from the author’s efforts to improve neutron activation analysis (NAA) detection limits
through development and optimization of radiochemical separations, as well as to improve the overall accuracy of NAA measurements
by identifying, quantifying and reducing measurement biases and uncertainties. Efforts to demonstrate the metrological basis
of NAA, and to establish it as a “Primary Method of Measurement” will be discussed.
Neutron activation analysis (NAA) is used extensively at the National Bureau of Standards as one of the analytical techniques in the certification of Standard Reference Materials (SRMs). Characteristics of NAA which make it valuable in this role are: accuracy; multielemental capability; ability to assess homogeneity; high sensitivity for many elements, and essentially blank-free nature. Examples of recent SRM analyses illustrating these characteristics are described.
Errors in preparing standards, especially multielemental standards, are extremely important if accurate results are desired from neutron activation analysis (NAA). It is often convenient to prepare standards for NAA from single or multi-element solutions which are then deposited onto (or into) a suitable matrix, such as filter paper or quartz vials. There are many potential sources of error in preparing single-element standards including: impurities and non-stoichiometric composition of the element or compound used to prepare the standard solutions; evaporative losses of solvent; inaccuracy of calibration, and imprecision of the pipettes used; moisture content of elements or compounds used; contamination from reagents, equipment, laboratory environment, or final matrix of the standard; instability of standard solutions (i.e., to losses via precipitation or adsorption), and losses of volatile elements during dissolution and/or irradiation. Additional sources of error in preparing multielement standards includes: instability of mixed, multielement solutions, and cross-contamination of one element by the addition of a second element. Procedures previously used by the author at NIST to prepare multielement standards with concentrations accurate to about one percent are described. Additional techniques needed to prepare multielement standards with accuracies better than 1 percent will be discussed.
A neutron activation analysis scheme based upon a radiochemical separation of the activation products has been developed.
The method utilizes the inherent sensitivity of the activation reaction198Pt(n, γ)199Pt and counting of the daughter nuclide199Au. This nuclide is radiochemically separated from interfering activities by homogeneous precipitation as elemental gold.
The remaining interference of the secondary reaction197Au(n,γ)198 Au(n,γ)199Au from gold in the samples is quantitatively assessed and corrected. During this process accurate gold concentrations in
the samples are obtained at ultratrace levels. The analysis scheme is applied to gold and platinum determinations in biological
Standard Reference Materials and human liver specimens. Gold and platinum are determined at concentrations of 5·10−11 g/g, and at higher levels.
Chromium is one of the most difficult elements to accurately determine at the naturally occuring, ultratrace levels normally found in uncontaminated biological samples. In view of the importance of Cr, both as an essential and as a toxic element, efforts have focused on developing a simple, yet reliable, radiochemical procedure for Cr determination using neutron activation analysis. A number of problem areas have been identified in earlier methods, and an improved radiochemical separation procedure, based upon the liquid/liquid extraction of Cr(VI) into a solution of tribenzylamine/chloroform, has been developed. The fast neutron interference from Fe has been evaluated for the highly thermal RT-4 facility of the NBS Research Reactor, and Cr concentrations have been determined in samples of whole human blood collected under clean conditions and in two certified reference materials.
High precision gamma spectrometry measurements have been made on five sets of uranium isotope abundance reference materials for nondestructive assay (NDA). These sets are intended for international safeguards use as primary reference materials for the determination of the235U abundance in homogeneous uranium bulk material by gamma spectrometry. The measurements were made to determine the count rate uniformity of the235U 185.7 keV gamma-ray as well as the235U isotope abundance for each sample. Since the samples were packaged such that the U3O8 is infinitely thick for the 185.7 keV gamma-ray, the measured count rate was not dependent on the material density. In addition, the activity observed by the detector was collimated to simulate calibration conditions used to measure bulk material in the field. The sample-to-sample variations observed within the 5 sets of samples ranged between 0.005–0.11% (1s) with standard deviations of the mean ranging from 0.01–0.02%. This observed variation appears to be due predominantly to counting statistics and not to material inhomogeneity and/or packaging. The results of this study indicate that accuracy of235U determinations via gamma spectrometry, in the range of few hundredths of a percent (2), is achievable. The main requirement for achieving this level of accuracy is a set of standards whose235U isotope abundances are known to within 0.01% (2).
A procedure is described for the preconcentration of 100 ml of estuarine and seawater into a solid sample using Chelex-100
resin. This solid sample weighs less than half a gram and contains the transition metals and many other elements of interest,
but is essentially free from the alkali metals, the alkaline earth metals, and the halogens. The concentrations of Co, Cr,
Cu, Fe, Mn, Mo, Ni, Sc, Th, U, V and Zn have been determined in seawater when this procedure was coupled to neutron activation
Authors:R. Lindstrom, R. Zeisler, and R. Greenberg
The basic assumptions of activation analysis are that the induced radioactivity is proportional to the amount of analyte,
and that the quantity of radioactivity can be related simply to the number of counts observed. Quantitative measurement of
activity (and of its uncertainty) is not always simple, especially when accuracy better than a few percent is sought. Recent
work with 77Ge and 76As has demonstrated that the accuracy of half-lives in the literature is sometimes insufficient. Despite these and other problems,
quantitative understanding and documentation of uncertainties can be accomplished, providing demonstrable quality assurance
and supporting claims of traceability to the Système International.
Using a new parallel beam apparatus, the dynamic mechanical properties of poly-(methyl methacrylate) were determined over a wide range of molecular weights (1500< <
<600 000). Results showed that the modulus (25 °C) was only slightly dependent on chain length, and equalled 2.3×109 Pa for the highest molecular weight scanned. Simultaneous acquisition ofα- andβ-relaxations indicated a decrease inTα in accordance with Gibbs' relation, whileTβ was invariant. BothTα∞=111° andTβ∞=40° corroborated previous results from several sources, including dynamic mechanical measurements. Such modulus and glass transition data are essential to the calculation of fracture toughness and to the assessment of radiation damage of acrylic, respectively.
Standard reference material (SRM) 2134 Arsenic Implant in Silicon was produced at the National Institute of Standards and Technology (NIST) as a calibrant for secondary ion mass spectrometry. Instrumental neutron activation analysis was used as a primary method for certification of the arsenic implanted dose. A complete evaluation of all sources of uncertainty yielded an expanded relative uncertainty for the mean value of this SRM to be 0.38% at approximately the 95% level of confidence. No evidence indicating significant heterogeneity among samples was observed.