Advanced detector systems: What do they have to offer for activation analysis?

Neutron activated reference materials have been analyzed using a standard Ge(Li)-detector with 17% relative efficiency, a very large Ge-detector with 96% relative efficiency, and a well-type Ge detector. Sensitivities are presented, and usefulness of these systems for NAA is compared on the basis of performance, economics and complexity.     

Locating the more acidic hydroxyl group on dihydroxy compounds o,o'-dihydroxyazo-dye metal-ion indicators

In o,o'-dihydroxyazo-dye metal-ion indicators, one hydroxyl group is 10(4) times more acidic than the other. The location of the more acidic group has been a mystery. By methylation of the acidic group, reduction of the monomethylated compound, isolation and characterization of the split methoxyamine and hydroxyamine, the location of the acidic group has been established. Indicators examined include Calmagite, Eriochrome Black T, Eriochrome Blue Black R and 4-(2-pyridylazo)resorcinol (PAR).     

Crystalline calmagite and a study of sulphonation effects on azo dye metal-ion indicators

Calmagite was introduced in 1960 as a stable substitute for Eriochrome Black T in EDTA titrations of calcium plus magnesium. The colour changes were the same but somewhat sharper. Indicator stock solutions were stable indefinitely. The new indicator was designed as an indicator and not as a wool dye. Azo dye impurities which may be present in some commercial supplies tend to lengthen the end-points. A simple means of removing these impurities to yield crystalline indicator has been developed. The effects of sulphonation on the performance of azo dyes as indicators have been studied.     

Amide-water interactions in cosolvent systems. I. Solubilities and apparent molar volumes of methyl paraben

Solubility and apparent molar volume data are used to demonstrate effects of amide alkylation on amide-water interactions at 25° C. Precise measurements were made of the apparent molar volumes of the amides in binary amide-water mixtures using a dilatometric technique. The results show that the apparent molar volumes of alkyl-substituted amides in water pass through a minimum at an amide concentration which varies inversely with the degree of alkylation. Further studies showed that the solubilities of methyl paraben (methyl-p-hydroxybenzoate) and naphthalene in various amide-water solvent systems increased in characteristic fashion with amide alkylation.     

Electron transport across the alkanethiol self-assembled monolayer/Au(111) interface: role of the chemical anchor

Alkanethiol self-assembled monolayers (SAMs) on Au(111) are model systems for molecular electronics. We probe the role of the chemisorption bond on electron dynamics at the SAM/Au interface using time-resolved two-photon photoemission. Formation of the Au-S bond is evidenced by a localized sigma resonance, which broadens and shifts upward in energy when the lying-down chemisorbed molecules stand up. The localized chemisorption bond does not affect the electronic coupling between delocalized image resonances and the metal substrate. Instead, lifetimes of image resonances are decreased due to scattering with S atoms within the thiol or thiolate monolayer.     

Induced radioactivity of LDEF materials and structural components

We present an overview of the Long Duration Exposure Facility (LDEF) induced activation measurements. The LDEF, which was gravity-gradient stabilized, was exposed to the low Earth orbit (LEO) radiation environment over a 5.8 year period. Retrieved activation samples and structural components from the spacecraft were analyzed with low and ultra-low background HPGe gamma spectrometry at several national facilities. This allowed a very sensitive measurement of long-lived radionuclides produced by proton- and neutron-induced reactions in the time-dependent, non-isotropic LEO environment. A summary of major findings from this study is given that consists of directionally dependent activation, depth profiles, thermal neutron activation, and surface beryllium-7 deposition from the upper atmosphere. We also describe a database of these measurements that has been prepared for use in testing radiation environmental models and spacecraft design.     

QA/QC Procedures,Validation Method,Traceability and Intercomparisons

Summary This work provides a systematic experimental approach to the demonstration of viability of instrumental neutron activation analysis (INAA) in chemical metrology. The practical approach was derived from a complete survey of uncertainty components that affect the INAA measurement process. These uncertainty components were classified by their magnitude and origin and subsequently minimized by appropriate steps in the INAA process. The process was tested with the INAA determination of Cr in SRM 1152A stainless steel; the Cr value is certified at 17.76% with an estimated uncertainty of 0.04% (0.23% relative). The INAA results from this procedure are in agreement with these specifications. Similar procedures have been applied to INAA multi-element determinations in a high temperature alloy. Agreement with available consensus values was demonstrated in the alloy. The guidelines on the determination of uncertainty were fully met, providing through INAA a valuable independent non-destructive tool in chemical measurements of metrological value such as required in the CCQM key comparisons.     

Neutron captue prompt gamma-ray activation analysis at the NIST Cold Neutron Research Facility

An instrument for neutron capture prompt gamma-ray activation analysis (PGAA) has been constructed as part of the Cold Neutron Research Facility at the 20 MW National Institute of Standards and Technology Research Reactor. The neutron fluence rate (thermal equivalent) is 1.5·10⁸ n ·cm^–2·s^–1, with negligible fast neutrons and gamma-rays. With compact geometry and hydrogen-free construction, the sensitivity is sevenfold better than an existing thermal instrument. Hydrogen background is thirtyfold lower.