Heterogeneous electron-transfer rate constants for M2(O2CR)4(0)/+, where M = Mo, W, Ru, or Rh and R = alkyl or aryl

By the use of Nicholson's method, the heterogeneous electron-transfer rate constants (ks) for the oxidation of a series of M2(O2CR)4 complexes have been determined in benzonitrile, where the metal M = Mo, W, Ru, or Rh and R = alkyl or aryl. For R = tBu, the values of ks follow the order M = Mo > W > Ru > Rh. No simple influence of R on ks was observed, although added ligands that are known to reversibly bind to the dinuclear center were shown to influence the E1/2 values in order of their basicity and to suppress the rate of electron transfer. The reported data are compared with those obtained for Cp2Fe0/+, Cp2*Fe0/+, and Ru(bpy)2(2)+/3+ and with earlier work on dirhenium multiply bonded compounds.     

Delayed neutron activation analysis for safeguards

Delayed neutron activation analysis (DNAA) presents a fast, accurate, and reliable method for quantification of fissile material. The method has relatively few sources of error and may be accomplished nondestructively. The need for a fast, accurate screening of materials stems from the necessity to protect cleanroom facilities from widely varying fissile quantities in samples and from desired gains in efficiency of mass spectrometric analysis by assisting in spike level selection and by removing from the sample set those materials that are not of interest. During the last several years, many different materials have been screened or analyzed in support of international safeguards, internal process control for actinide separations, and in uranium contamination assessments. Swipes from a variety of sources have been analyzed, either before or after dissolution, and comparison of the DNAA results to mass spectrometry results is generally favorable. A facility characterization of the High Flux Isotope Reactor was performed using filter paper swipes to demonstrate the utility of the DNAA technique.     

Effects of microchannel geometry on pulsed flow mixing

Although the mixing of reagents is often crucial in many microfluidic devices, good mixing in these laminar, low Reynolds number, flows remains a challenge. It was shown in Refs. [Glasgow, I., Aubry, N., 2003. Lab on a Chip 3, p. 114; Glasgow, I., Batton, J., Aubry, N., 2004. Lab on a Chip 4, p. 558] that pulsing can induce mixing at the confluence of two inlet microchannels in an efficient manner. In this paper, we show that this mixing is affected by both the geometry of the confluence and the inclusion of features in the channels, which induce secondary flow. More specifically, we study mixing in 200 μm wide by 120 μm deep channels, at flow rates from 48 nl s⁻¹ to 4.8 μl s⁻¹, corresponding to Reynolds numbers of 0.3–30. For the parameter values studied, the pulsed flow technique is more effective at mixing than the secondary flow induced by the channel geometry features, and combining both methods leads to even better mixing. In addition, pulsing the reagents such that they pass multiple times through the spatial features, which induce secondary flow leads to mixing over shorter distances.     

A road to stable dispersions: polymer-particle interactions

Dispersion stability is an essential requirement for an ideal binder for new magnetic media formulations. In this paper, an attempt is made to establish a dispersion model to enhance the degree of dispersion and its stability through the use of polymer-particle interactions. Dispersion properties have been enhanced by incorporating various functional groups in the polymer chain, and polymer design has been optimized using a statistical model of ideal polymer compositions and properties. Applications of these results are discussed.     

The simultaneous determination of 235U and 239Pu using delayed neutron activation analysis

Delayed neutron activation analysis (DNAA) remains one of the most sensitive methods of nondestructively determining fissile materials in a variety of sample matrices, provided that the samples contain only a single fissile component. This has historically been the limiting factor in many applications of DNAA, and often chemically destructive methods of analysis have needed to be utilized for many real-world samples. This work seeks to develop a method that will allow for DNAA to be utilized on samples containing multiple fissile components. Initial efforts, presented here, show that using a multivariate linear regression model to describe the delayed neutron emission profile of an irradiated sample allows for the concurrent determination of fissile nuclides in samples containing both ²³⁵U and ²³⁹Pu, without chemical separations and using only a single counting step.     

Comparison of cryoscopic determinations of purity of benzene by thermometric and calorimetric procedures

Workers at the National Bureau of Standards and elsewhere have been conscious of the discordant results sometimes yielded by two methods for determining purity employing the same physical principle. A comparison was made on samples of benzene by the thermometeric method, by which a stirred sample is frozen at nearly constant rate, and by the calorimetric method, by which a frozen sample is melted in stages by the addition of accurately controlled increments of energy under adiabatic conditions.Benzene, purified by single crystal formation and of very high purity, was contaminated in known amounts by n-heptane and samples of the same level of purity determined by both procedures. Great care was taken to submit samples of the same composition to the different groups employing the two different methods. Further, the actual degree of contamination of the benzene was unknown to both groups until final values of the purity were submitted. Results of this comparison showed that the large divergence between the two methods that had characterized earlier comparisons is not displayed. The differences between the analyses on all three samples are roughly of the order of the statistically estimated uncertainties of the averages and less than deviations between individual experiments by one method. Comparison of the methods with absolute values of purity could not be made because of contamination of the highly purified sample of benzene with supposedly chemically adsorbed water from the borosilicate glass walls.     

Neutron activation by neutrons produced via proton-induced spallation in a liquid-mercury target: Measurements and assessment of uncertainties

A preliminary test of a liquid mercury target for the production of neutrons by spallation was undertaken at the Alternating Gradient Synchrotron facility at Brookhaven National Laboratory. Neutron activation of elemental foils placed on the target demonstrates that a range of neutron energies does exist, as expected, and that the neutron flux is at a maximum 10–20 cm from the front of the target, moving deeper with increasing proton energy. Uncertainties in the activity calculations are in general significantly <10%. Impurities in some of the foils are a significant source of interference for some reactions, although there is no interference for most of the reactions. The presence of many interference-free reactions, along with the low uncertainties indicates that the foils will be useful benchmarks to validate the neutronics codes utilized in the target design.     

Acceleration of free electrons in a symmetric evanescent wave

The possibility of accelerating free electrons in a vacuum gap between closely spaced dielectric materials is explored. Plane waves impinging symmetrically on the gap from either side at oblique incidence produce an evanescent wave with net electric field along the direction of propagation. Near the critical angle, the evanescent wave propagates at the vacuum speed of light. A theoretical development and numerical simulations show that free electrons in the gap can be accelerated and accumulate energy indefinitely. This approach lies outside the purview of the Lawson-Woodward theorem, which does not apply in the vicinity of a medium. Damage thresholds of materials restrict the light intensity to far below that achievable by current high-power lasers. This limits the particle energy that might be achieved from an accelerator based on this approach.     

Average energy flow of optical pulses in dispersive media

The arrival time of a light pulse at a point in space is defined using a time expectation integral over the Poynting vector. The delay between pulse arrival times at two distinct points is shown to consist of two parts: a spectral superposition of group delays (inverse of group velocity) and a delay due to spectral reshaping via absorption or amplification. The result provides a context wherein group velocity is always meaningful even for broad band pulses and when the group velocity is superluminal or negative. The result imposes luminality on sharply defined pulses.     

Characterization of fluorescence of ANS-tear lipocalin complex: evidence for multiple-binding modes

ANS is widely used as a probe for locating binding sites of proteins and studying structural changes under various external conditions. However, the nature of ANS-binding sites in proteins and the accompanying changes in fluorescence properties are controversial. We examined the steady-state and time-resolved fluorescence of the ANS-protein complexes for tear lipocalin (TL) and its mutants in order to discern the origin of lifetime components via analysis that included the multiexponential decay and the model-free maximum entropy methods. Fluorescence lifetimes of ANS-TL complexes can be grouped into two species, 14.01-17.42 ns and 2.72-4.37 ns. The log-normal analyses of fluorescence spectral shapes reveal the heterogeneous nature of both long- and short-lifetime species. The constructed time-resolved emission, amplitude (TRES) and area normalized (TRANES), and decay-associated spectra are consistent with a model that includes heterogeneous modes of ANS binding with two separate lifetime components. The two lifetime components are not derived from solvent relaxation, but rather may represent different binding modes.