Measurements performed in the past to determine sensitivity enhancements (later identified as neutron density increases) in PGNAA as a function of hydrogen concentration in slab-shaped samples are described. The results are compared to the results of Monte Carlo computations. It is concluded that, like H2O, D2O can also cause substantial neutron density increases. In one concentrated salt solution, however, D2O seems to cause a neutron density decrease that cannot be explained from the macroscopic neutron scattering and absorption cross sections in the model used.
We have investigated the effects of scattering and absorption in neutron beam activation analysis experiments, both by direct measurement and by Monte Carlo simulation. Significant sensitivity enhancements occur for thin disks placed at 45° to the beam but very much smaller effects occur for spheres. The agreement between measurement and calculation is generally good.
Background enhancements in neutron capture prompt -ray activation analysis were determined over a large range of total scattering cross sections by irradiating graphite, S, Be, paraffin, urea and H2O targets. Relative to irradiations using no target, B, Na, Cl, Al, and Pb backgrounds were 7–12 times greater with 2 ml of H2O, but N and Fe backgrounds were only 1.2 and 1.75 times greater, respectively. For biological targets, background count rates can be determined as functions of the 2223 ke V H photopeak count rate.
The NIST program for environmental Standard Reference Materials (SRM) includes materials covering a range of matrices, mass
fraction values and analytes. For many SRMs, mass fraction data are accumulated, incidentally, over time, as these are used
routinely for quality assurance purposes. Although these are not formal stability studies, data generated may be useful in
assessing stability. To evaluate the potential for assessing material stability from incidental use of SRMs, results of neutron
activation analysis performed from 1992 through 2008 were compiled for SRM 1547 Peach Leaves and SRM 1566a Oyster Tissue.
Results indicate that incidental use of SRMs yields useful information on SRM stability.
The effects of neutron scattering by hydrogen within targets for cold neutron prompt -ray activation analysis (CNPGAA) have been characterized. For most targets studied, the probability for neutron absorption, and hence CNPGAA sensitivities (counts·s–1·mg–1), decrease with increasing H content and with target thickness. Comparisons with results from thermal neutron PGAA indicate that the effects of cold neutron scattering differ from those of thermal neutron scattering. CNPGAA sensitivities for l/v
nuclides show similar sensitivity decreases, while Sm sensitivities show smaller decreases.
The thermal neutron prompt gamma-ray activation analysis (PGAA) facility, operated by the US Food and Drug Administration and National Institute of Standards and Technology Center for Neutron Research, has been redesigned to lower background radiation levels and improved analytical capabilities. Analysis of 22 element standards and food and botanical certified reference materials revealed significant sensitivity increases and lower limits of detection for H, B, C, N, Na, Al, P, S, Cl, K, Ca, Fe, and Cd. Mass fractions for these elements, as well as Mg, Al, Si, Ti, Mn, Fe, Cu, I, Zn, Sm, and Gd, were determined for 6 dietary supplements.
The National Institute of Standards and Technology (NIST) recently released the second renewal of its Trace Elements in Coal Fly Ash Standard Reference Material (SRM 1633b). This new material is currently certified for 23 major, minor and trace elements, and concentrations of an additional 24 elements are provided for information only purposes. Current plans are to certify the concentrations of a number of rare earths upon completion of additional analytical work now in progress. Instrumental neutron activation analysis (INAA) has played a major role in the certification of this new material in view of its potential for accuracy, multielemental capability, ability to assess homogeneity, high sensitivity for many elements, and essentially blank-free nature. For an element to be certified in a NIST SRM its concentration is usually determined by at least two independent analytical techniques. INAA has provided analytical information for 15 of the 23 elements certified, as well as for 22 of the 24 elements listed for information only. In addition, INAA has provided much of the homogeneity information for this SRM. This paper will describe these analytical procedures, and highlight those designed to optimize and assess the accuracy of the INAA measurements.
Authors:R. Zeisler, W. James, E. Mackey, R. Spatz, and R. Greenberg
The National Institute of Standards and Technology (NIST) is preparing a freeze-dried powdered bovine liver tissue Standard
Reference Material (SRM) to replace SRM 1577b Bovine Liver as the stock of this material was exhausted during 2006. Like the
original SRM 1577 issued in 1972, this renewal focuses on the key elements for diagnostic, nutritional, and toxicological
measurements that are important to medical, veterinary, and environmental sciences investigations. NIST’s approach for value
assignment included extensive characterization by neutron activation analysis (NAA). Difficulties in the determination of
some elements present at very low levels were overcome by use of radiochemical separations. Twentyone elements were characterized
in SRM 1577c by NAA. The previous materials, SRM 1577 and 1577b, served as quality control.
Authors:R. Paul, R. Lindstrom, C. Brocker, and E. Mackey
A new instrument for cold neutron prompt gamma-ray activation analysis (CNPGAA) is being designed and constructed at the NIST
Center for Neutron Research (NCNR). The new instrument is expected to have lower gamma-ray and neutron background and better
detection limits for most elements than the current cold neutron PGAA instrument. Other advantages over the current facility
will include the ability to analyze larger samples and greater overall measurement capability due to the addition of scanning
stages, cryostats, and sample changers.
Authors:H. Chen-Mayer, E. Mackey, R. Paul, and D. Mildner
We report the first quantitative results using a focused cold neutron beam for prompt gamma activation analysis (PGAA). We have measured the prompt gamma signal from known Fe, Cr, Ti, B, and Cd specimens in the focusing geometry, from which we determine the sensitivities for these elements by the method of standard addition. Furthermore, we show results of measurements for homogeneous standard reference materials (boron in SRM 611 glass, and iron and chromium in a steel alloy SRM 160b) to verify the sensitivities determined. Finally, we present a position-dependent study of the Cr to Fe mass ratio in an industrial material, taking advantage of the narrowly focused beam. Existing problems for achieving routine quantitative analysis using the focused beam and suggestions for future directions are discussed.