D-c electric-potential method applied to thermal/mechanical fatigue crack growth

The paper describes a d-c electric potential system for measuring crack length under thermal/mechanical fatigue-crack-growth (TMFCG) test conditions. A programmable d-c current supply and precision multimeter produce reliable electric-potential readings. H.H. Johnson's formula is used to calculate crack length from electric potential for the center-crack-tensionM(T) geometry. Calibration constants for the formula are determined from an initial optical crack-length measurement. The resolution of the system is 1.0 microvolt which corresponds to a crack extension of approximately 0.002 mm for the center-crack-tension geometry using a current of 10.00 amps. Good crack-length accuracy and low data scatter are achieved by taking special precautions to minimize or eliminate errors in potential measurement due to thermal effects. Material resistivity changes are identified as the cause of short and long term changes in the measured electric potential for uncracked specimens. Crack-length accuracy is discussed in terms of short-term scatter and longterm drift.     

Development of a fast-pneumatic sample transfer system for DT neutron activation analysis at the University of Michigan

The development of an automated pneumatic transfer system used to quickly acquire data from materials irradiated with a deuterium–tritium (DT) neutron generator is described in this paper. This system was designed to gather data on short-lived activation and fast-fission products, and was used to characterize the generator’s neutron field. The average sample transit time between irradiation and data acquisition is 363.9 ms at an average velocity of 30.92 m/s (101.3 ft/s). The neutron flux profile as a function of depth into the sample capsule is shown to decrease exponentially, having a maximum flux value of 5.662 × 10⁸ ± 0.056 × 10⁸ n/cm² s. The average DT neutron energy in the system’s sample geometry was determined to be 14.250 ± 0.011 MeV using a unique zirconium–niobium “sandwich” technique. A flux surface equation is also presented as a function of accelerator voltage and deuterium beam current. Methods of analysis are discussed with a proof of a linear flux profile assumption for thin foils.     

Flavin–cyclodextrin conjugates: effect of the structure on the catalytic activity in enantioselective sulfoxidations

A series of flavin–cyclodextrin conjugates has been prepared and tested in the enantioselective oxidations of prochiral aromatic and aliphatic sulfides with hydrogen peroxide. The newly prepared conjugates contain isoalloxazinium or alloxazinium moieties attached to the primary rim of α- and β-cyclodextrins at the C-6 positions. In addition, flavinium units were attached to the secondary rim of the β-cyclodextrin macrocycle. The relationship between the structural features and the catalytic performance of the conjugates, including those recently reported by us, was analyzed. The rate and enantioselectivity of the sulfoxidations catalyzed by flavin–cyclodextrin conjugates are influenced mainly by the size of the cyclodextrin cavity, the type of flavin unit (alloxazine or isoalloxazine), and by the relative orientation of the flavin and cyclodextrin moieties.     

Improving neutron activation analysis accuracy for the measurement of gold in the characterization of heterogeneous catalysts using a TRIGA reactor

To measure the gold content of a catalyst accurately, neutron activation analysis (NAA) is one of the methods of choice. NAA is preferred for such heterogeneous catalysts because: (1) it requires minimal sample preparation; (2) NAA provides consistent and accurate results; and (3) in most cases results are obtained much quicker than competing methods. NAA is also used as a referee for the other elemental techniques when results do not fall within expected statistical uncertainties. However, at very high gold concentrations, applying NAA to determine the gold in a heterogeneous catalyst is more challenging than a routine NAA procedure. On the one hand, the neutron absorption cross section for gold is very high, resulting in significant self-shielding related errors. On the other hand, gold exhibits low energy resonance neutron absorptions. In this application the self-shielding minimization effort was handled more rigorously than the classic suppression of neutron flux within a specimen. This non-routine approach was used because: (1) for most applications, high accuracy, <3 % relative, is desired, (2) the low energy resonances of gold make its neutron reaction rate complex and (3) the TRIGA reactor flux profile used in this study contains both thermal and significant epithermal neutron fluxes. Accuracy and precision, using this new approach, are expected to improve from 15 % to better than 3 % relative uncertainty. This has been accomplished through a rigorous assessment of the observed effects of low energy resonance on the neutron flux spectral shape within the sample and designing an experiment to minimize the effects.     

Studies Directed Toward the Regioselective Carboxylation of the 4-Oxo-4,5,6,7-Tetrahydrothieno-[3,2-C]pyridine Ring System

Carboxylation of 4-oxo-4,5,6,7-tetrahydro[3,2-c]pyridine was achieved using a metallation/carbonylation strategy with desired regioselectivity provided by the existing oxo directing group.     

Astronomical gamma ray telescopes in the pair production regime

Most of the gamma ray telescopes used up to now for energies above about 50 MeV have been very similar in design. Anticipated requirements for future telescopes will probably necessitate the use of a considerable amount of new technology.