[en] Compositional analysis provides essential information for understanding the physics of high-temperature superconductors and in developing the production processes that will be required for their industrial application. Knowledge of the stoichiometry of sample materials, especially the oxygen content, is particularly important. Although oxygen is difficult to measure by activation with reactor neutrons, it may be determined by difference if the concentrations of the other elements are known. Prompt and delayed neutron activation analysis has been applied to the measurement of the metals in YBa₂Cu₃O₇ superconductors, starting materials, and intermediate products. These measurements show that neutron activation can deliver accurate and timely nondestructive determinations of the stoichiometry of high-temperature superconductor materials

[en] The use of external beams of reactor neutrons has brought new capabilities to nuclear analytical chemistry in recent years. Bringing the neutrons to the sample rather than [as in conventional neutron activation analysis (NAA)] inserting the sample into the neutron field allows the secondary particles induced by neutron reactions to be detected without the time required to remove the sample from the reactor. Since most of the secondary particles from neutron-induced reactions are emitted in a very short time after capture, the intrinsic measurement sensitivity of prompt methods per neutron per gram of target is better than conventional neutron activation for the measurement of most elements. The qualifying phrase is, of course, severely limiting; external neutron beams typically have 10^-5 of the flux density of the neutron fields in the irradiation position of a research reactor with proportionally lower analytical sensitivity. Neutron-induced prompt analytical techniques have found numerous applications since the advent of high-resolution, high-efficiency detectors for gamma rays and charged particles. The combination of clean, intense neutron beams, high-efficiency and high-resolution detectors, high-rate counting electronics, and computer-based data acquisition and processing systems will lead to major advances in the applications of neutron beams to chemical analysis

[en] The presence of excess hydrogen in a structural titanium alloy reduces its mechanical strength. If the hydrogen concentration exceeds ≅100 mg/kg, titanium hydride precipitates as a separate phase, preferentially at grain boundaries. As a result, controlling the hydrogen concentration is crucial. Standard methods for analysis are destructive, using high temperature in inert gas or in vacuum to extract the hydrogen. These methods are rapid but assume that all the hydrogen present is extracted, and they also require materials with known composition for calibration. In a related area, quantitative nondestructive assay by neutron radiography or tomography also requires artifact standards with known hydrogen content. Methods have been set in place at the National Institute of Standards and Technology to produce standard reference materials (SRMs) and other standards to satisfy these needs. The reaction Ti + H₂ = TiH₂ is reversible. From the free energy of formation (105 kJ/mol), one may calculate that the equilibrium pressure of H₂ is below 10^-8 Pa (lt10^-13 atm) at room temperature, and 5 MPa (50 atm) at 800 C. The reaction is rapid above 500 C. Specimens with known hydrogen content are prepared by degassing a weighed quantity of titanium alloy in vacuum at high temperature and reacting it with a known volume of hydrogen gas in a closed vessel. The composition is stable at room temperature. The accuracy of the concentration thus achieved is limited by the pressure and volume measurements in the reaction system (accuracy >0.5%) and by any residual hydrogen left after bakeout. For additional certainty, the hydrogen concentration measured volumetrically is confirmed by several methods of lower accuracy or sensitivity. For high concentrations (above approximately100 mg/kg), weighing the sample before and after treatment gives easily measurable changes in good agreement with the volume dosing. Prompt-gamma activation analysis with cold neutrons (CPGAA) is specific for hydrogen, but limited counting statistics make measurements below 100 mg/kg imprecise. More sensitivity, but more matrix dependence, is obtained from neutron incoherent scattering (NIS). This method can be used to measure as little as a few tens of mg/kg, which is useful to confirm the efficiency of the initial bakeout

[en] Samples of ²⁴Na, ⁴²K, ⁷⁶As and ¹⁹⁸Au were produced by irradiation in the National Institute of Standards and Technology (NIST) reactor, and examined for impurities before and after measurement. Half-life measurements were carried out in the NIST 4πγ pressurized ionization chamber. The results are compared to presently accepted values and previous NIST measurements

No abstract available

[en] The quality of silicon and other electronic materials needs to be carefully controlled by appropriated method of chemical analysis. Sufficiently sensitive analytical techniques as well as suitable certified reference materials are, however, lacking to the analytical community. It is argued that semiconductor grade polycrystalline silicon can be a useful standard material because of its high purity. (author). 4 refs

[en] Counting of radioactive sources on the surface of the vacuum container of a Ge(Li) detector may lead to errors because of large efficiency gradients. In order to explore the radial dimension of this problem, several point sources were stepped across in contact with the cap of several detectors, and curves of absolute efficiency against radius were measured. The insensitive core of an open-end coaxial detector reduced the central point-source efficiency at 122 keV at the cap to 20% less than a comparable closed-end detector. In compensation, however, there was a reduction in the radial efficiency gradient. The radial efficiency gradient was approximately proportional to the radius, with the central flattening for the open-end detector superimposed on the trend. Open-end detectors are favoured over closed-end coaxial ones for the most accurate work, although the differences in the two configurations are not marked. Differences are greatest for low energy and close geometry. (T.G.)

[en] A project to determine the cosmic-ray production rate and the natural levels of 35-day half-life ³⁷Ar in the atmosphere has been underway at the National Bureau of Standards for about the past year. The prime objective of this project is to determine the spatial dependence of ³⁷Ar production in the atmosphere, and the spatial distribution of the naturally-produced ³⁷Ar (observed concentrations). The results of this study are to be used, in cooperation with L. Machta (National Oceanographic and Atmospheric Administration), to derive information about atmospheric mixing. The purpose of this communication, however, is to present a general description of the various components of the measurement system. As the lowest concentrations of interest are but approximately equal to 10^-3 dpm (³⁷Ar)/l-Ar, very high sensitivity measurement techniques are required. Among the techniques which we have adopted are: quantitative separation of the noble gases from about 1 m³ of air, using a CaC₂ reactor; gas chromatographic separation of the argon fraction; isotopic enrichment (by a factor of approximately equal to 100) of purified argon; use of specially selected low-level gas proportional counters together with massive shielding and anticoincidence meson cancellation; and the application of pulse discrimination based upon both amplitude (energy) and pulse shape. Finally, on-line computer techniques are being applied for data acquisition and system control

[en] Nuclear methods of chemical analysis have played a prominent role in the accurate determination of the composition of Standard Reference Materials and other standards. The characteristic sensitivity, multielement capability, and freedom from chemical biases unique to nuclear methods assure them a continuing role in the standards laboratory. (author). 31 refs

[en] Measurement of small quantities of aluminum in silicon of semiconductor or solar grade is complicated by production of ²⁸Al by an (n,p) reaction on the matrix, in competition with the (n,γ) reaction on the trace aluminium sought. In selecting the optimum irradiation facility, the conflicting requirements of high thermal flux for the greatest signal and of low fast flux for the smallest interference must be balanced. The present work outlines a simple experimental procedure for assessing and correcting for interferences. No explicit knowledge of fast fluxes or cross sections is required; these are often poorly known. The procedure is generally applicable to other cases of competing reactions