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An analytical expression has been developed to calculate the neutron self-shielding in a cylindrical sample using the elemental thermal neutron absorption cross sections, σ abs , and the newly-defined epithermal neutron absorption cross-sections, σ abs,ep . The σ abs,ep were measured experimentally for 13 nuclides and calculated from resonance parameters for 76 nuclides. Agreement between the two was good to about 20% in most cases. A spreadsheet program was written to use these nuclear parameters to perform iterative self-shielding corrections of concentrations measured by NAA. In cases with up to 30% self-shielding, the correction factors had uncertainties varying from 2% to 3%. [ABSTRACT FROM AUTHOR]

Neutron activation analysis using the single comparator method or the k 0 method may be less accurate in the cases of non-1/ v nuclides if irradiations are done at varying reactor temperatures. We propose a method to correct this effect, using available reactor moderator temperature readings. It was verified by the irradiation of two non-1/ v nuclides, 176Lu and 151Eu. Irradiations were carried out in two irradiation sites at two reactor powers and at estimated neutron temperatures varying from 18 to 46 °C. In all cases the measured activities showed a variation with temperature consistent with published tables of the variation of the Westcott g-factor. The method should be accurate to 1% for most small reactors and eliminates the need to irradiate temperature monitors. [ABSTRACT FROM AUTHOR]

The recently discovered universal functions for thermal and epithermal neutron self-shielding were adapted to NAA of cylindrical samples, expressing the magnitude as the product of a nuclear factor, a geometrical factor and the amount of the neutron absorbing element. The theory was tested and the nuclear factors were measured for 1 ml samples containing the halogens Cl, Br and I. Tests on samples containing these elements at a priori unknown concentrations, irradiated in a mixed thermal and epithermal neutron spectrum, showed that self-shielding as high as 30% could be corrected with an accuracy of about 1%, except in cases with significant epithermal shielding of one element by another. [ABSTRACT FROM AUTHOR]

Abstract: The accuracy of k 0-based neutron activation analysis (k 0-NAA) using 14 nuclides produced by (n,γ) reactions with high Q 0 values was recently brought into question. In this work we have re-measured Q 0 for these 14 cases by the Cd-ratio method and by the two-channel method, and we have determined the k 0 values for the most intense gamma-rays. Five of the proposed new Q 0 values, 15.0±0.4 for 76As, 39.6±0.5 for 115Cd, 12.3±0.2 for 134mCs, 15.1±0.3 for 134Cs and 36.9±1.4 for 182Ta, will significantly improve k 0-NAA using reactor neutron spectra with a large epithermal component. Several new k 0 values are significantly different from those commonly used. [Copyright &y& Elsevier]

Abstract: Analytical expressions recently developed for calculating thermal and epithermal neutron self-shielding for cylindrical samples used in neutron activation analysis were verified using three different irradiation sites of a SLOWPOKE reactor. The amount of self-shielding varied by less than 10% from one site to another. The self-shielding parameters varied with the size of the cylinder as r(r+h), for h/r ratios from 0.02 to 6.0, even in slightly non-isotropic neutron fields. A practical expression, based on the parameters of the neutron spectrum and the well-known thermal neutron absorption cross-section and the newly defined epithermal neutron absorption cross-section, is proposed for calculating the self-shielding in cylindrical samples. [Copyright &y& Elsevier]

Neutron activation allows the analysis of wet and dry biological tissues without sample preparation. To eliminate the need to prepare standards of each element to match each sample, we have standardized for a small number of typical sample types and irradiation and counting conditions. Then, for each sample analyzed, the concentration calculation includes corrections to the neutron flux and the counting efficiency, which depend on sample size and composition. The effects corrected include the neutron flux gradient in the irradiation site, the moderation of the neutrons with the hydrogen content of the sample and the variation of counting efficiency with sample volume. [ABSTRACT FROM AUTHOR]

Neutron activation analysis of toenails is often used to monitor body selenium in studies looking for an association between selenium deficiency and an increased cancer risk. In this study, 87 toenail samples were analyzed for Se four times, using neutron activation with 17.4-second 77mSe, to determine the possible systematic effects of long-term storage, the washing procedure, and irradiation in a nuclear reactor. The mean Se concentration found was 0.92 mg/g, standard deviation 0.14 mg/g. The results showed that the Se concentrations are unaffected by washing and neutron irradiation, but the samples lost moisture during storage causing a 2% increase in the mean Se concentration. [ABSTRACT FROM AUTHOR]