A method has been developed for the determination of low-level sulfur in steels by radiochemical neutron activation analysis.
During sample irradiation, 35S is produced by the 34S(n,γ)35S reaction. Irradiated steels are mixed with sulfur carrier and dissolved in HCl/HNO3. Sulfur is reduced to H2S by reaction with HI/H3PO2/HCl. The evolved H2S is absorbed in dilute NaOH, which is mixed with scintillation cocktail for the measurement of 35S by liquid scintillation counting. Sulfur carrier yield is determined by iodometric titration. Chlorine is also determined
by RNAA in order to correct for 35S produced via the 35Cl(n,p)35S reaction. Sulfur has been determined at mass fractions as low as ≈5 mg/kg in ultra-high-purity iron using this method.
An RNAA procedure has been developed for measurement of low-level phosphorus in metals. Samples are irradiated at a neutron flux of 2.7·1013 n·cm–2·s–1 then mixed with carrier and dissolved in acid. After chemical separation and purification of the phosphorus and gravimetric determination of carrier yield, 32P is determined using a beta proportional counter. The detection limit for a 0.1 g sample irradiated for 30 minutes is 5 g/kg. The method has been used to determine 6.4±0.6 mg/kg phosphorus in SRM 2175 refractory alloy.
The viability of radiochemical neutron activation analysis as a method for certification of phosphorus in steels was tested
by analysis of SRM low alloy steels. Preliminary results are generally in agreement with certified values. The limit of detection,
as defined by Currie1, was determined to be 5 μg/kg.
Analytical bias due to neutron scattering and absorption in cold neutron prompt gamma-ray activation analysis (CNPGAA) is largely eliminated for homogeneous samples when element ratios are measured. Application of sensitivity ratios (measured relative to titanium) to the multielement analysis of the Allende meteorite increases both the speed and accuracy of the measurement. Greater measurement accuracy is achieved for some samples when ratios of element concentrations are reported. Problems are encountered when applying the ratio method to measurement of elements which deviate from 1/v behavior, and when gamma-ray attenuation or sample heterogeneity are significant.
Prompt gamma-ray activation analysis (PGAA) is an important nuclear analytical technique that complements conventional neutron activation analysis (NAA). When a target is placed in a beam of neutrons, gamma-rays emitted upon neutron capture are measured by a shielded germanium detector, yielding quantitative elemental analysis. The radiation is penetrating and the analysis both nondestructive and independent of the chemical form of the element(s) being measured. The technique is most useful for measurement of light elements (H, B, C, N, Si, P, S, Cl) which can not be easily measured by other methods. Best sensitivity is achieved with neutron beams from research reactors. Although sample preparation is minimal, care must be taken to select proper standards and blanks, and numerous corrections must sometimes be applied to the data from the complex spectra. PGAA has proven useful for multielement analysis of a wide variety of different materials spanning a broad range of scientific disciplines. Of particular importance has been the measurement of hydrogen in materials.
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.
An instrument for prompt gamma-ray activation analysis is now in operation at the NIST Cold Neutron Research Facility (CNRF). The cold neutron beam is relatively free of contamination by fast neutrons and reactor gamma rays, and the neutron fluence rate is 1.5·108 cm–2·s–1 (thermal equivalent). As a result of a compact target-detector geometry the sensitivity is better by a factor of as much as seven than that obtained with an existing thermal instrument, and hydrogen background is a factor of 50 lower. We have applied this instrument to multielement analysis of the Allende meteorite and other materials.
Determination of53Mn in meteorites by neutron activation analysis requires a thermal neutron flux high enough to ensure adequate production of54Mn from53Mn with a sufficiently low fast neutron component to minimize its production through fast neutron reactions. Thermal and fast neutron fluxes were mapped as a function of sample position within the NIST research reactor in order to determine the optimum position for irradiation of53Mn.
The cold neutron capture prompt -ray activation analysis (CNPGAA) spectrometer located in the Cold Neutron Research Facility (CNRF) at NIST has proven useful for the analysis of hydrogen and other elements in a wide variety of materials. Modifications of the instrument and the CNRF have resulted in improved measurement capabilities for PGAA. The addition of an atmosphere-controlled sample chamber and Compton suppression have reduced -ray background and increased signal-to-noise ratio. More recent revisions are expected to yield still further improvement in analytical capabilities. Replacement of the D2O ice cold source with a liquid H2 moderator is expected to yield a 5–10 fold increase in neutron capture rate, and improved neutron and -ray shielding will result in further reduction of the background. Other modifications to the instrument allow easier sample mounting and more precise positioning of samples in the neutron beam. Significant improvements in detection limits and analytical accuracy are expected.