A Current Mode Detector Array for Gamma-Ray Asymmetry Measurements

We have built a CsI(Tl) gamma-ray detector array for the NPDGamma experiment to search for a small parity-violating directional asymmetry in the angular distribution of 2.2 MeV gamma-rays from the capture of polarized cold neutrons by protons with a sensitivity of several ppb. The weak pion-nucleon coupling constant can be determined from this asymmetry. The small size of the asymmetry requires a high cold neutron flux, control of systematic errors at the ppb level, and the use of current mode gamma-ray detection with vacuum photo diodes and low-noise solid-state preamplifiers. The average detector photoelectron yield was determined to be 1300 photoelectrons per MeV. The RMS width seen in the measurement is therefore dominated by the fluctuations in the number of gamma rays absorbed in the detector (counting statistics) rather than the intrinsic detector noise. The detectors were tested for noise performance, sensitivity to magnetic fields, pedestal stability and cosmic background. False asymmetries due to gain changes and electronic pickup in the detector system were measured to be consistent with zero to an accuracy of $10^{-9}$ in a few hours. We report on the design, operating criteria, and the results of measurements performed to test the detector array.Comment: 33 pages, 20 figures, 2 table

Phase coexistence of multiple copper oxides on AgCu catalysts during ethylene epoxidation

Alloy catalysts under reaction conditions are complex entities. In oxidizing atmospheres, multiple phases can coexist on a catalyst s surface as a result of phase segregation and preferential oxidation. Such a scenario can result in unusual substoichiometric and metastable phases that could play important roles in catalytic processes. For instance, AgCu alloys known to exhibit enhanced epoxide selectivity in partial oxidation of ethylene form an oxide like surface structure under reaction conditions. Under these conditions, copper oxides are stable, while silver oxides are not. Consequently, copper segregates to the alloy s surface and forms an oxide overlayer. Little is known about the structure or function of such overlayers, and it is unknown whether they play an active role in the catalyst s enhanced selectivity. In order to develop a clearer picture of such catalysts, the current work utilizes several in situ spectroscopic and microscopic techniques to examine the copper oxide phases that form when AgCu is exposed to epoxidation conditions. It is found that several forms of oxidic Cu coexist simultaneously on the active catalyst s surface, namely, CuO, Cu2O, and some previously unreported form of oxidized Cu, referred to here as CuxOy. Online product analysis, performed during the in situ spectroscopic measurements, shows that increased epoxide selectivity is correlated with the presence of mixed copper oxidation states and the presence of the CuxOy species. These results support previous theoretical predictions that oxidic copper overlayers on silver play an active role in epoxidation. These results furthermore emphasize the need for in situ spectromicroscopic methods to understand the complexity of alloy catalyst

Nab: Measurement Principles, Apparatus and Uncertainties

The Nab collaboration will perform a precise measurement of 'a', the electron-neutrino correlation parameter, and 'b', the Fierz interference term in neutron beta decay, in the Fundamental Neutron Physics Beamline at the SNS, using a novel electric/magnetic field spectrometer and detector design. The experiment is aiming at the 10^{-3} accuracy level in (Delta a)/a, and will provide an independent measurement of lambda = G_A/G_V, the ratio of axial-vector to vector coupling constants of the nucleon. Nab also plans to perform the first ever measurement of 'b' in neutron decay, which will provide an independent limit on the tensor weak coupling.Comment: 12 pages, 6 figures, 1 table, talk presented at the International Workshop on Particle Physics with Slow Neutrons, Grenoble, 29-31 May 2008; to appear in Nucl. Instrum. Meth. in Physics Research

Precision Measurement of PArity Violation in Polarized Cold Neutron Capture on the Proton: the NPDGamma Experiment

The NPDGamma experiment at the Los Alamos Neutron Science Center (LANSCE) is dedicated to measure with high precision the parity violating asymmetry in the $\gamma$ emission after capture of spin polarized cold neutrons in para-hydrogen. The measurement will determine unambiguously the weak pion-nucleon-nucleon ($\pi NN$) coupling constant {\it f$^1_{\pi}$}Comment: Proceedings of the PANIC'05 Conference, Santa Fe, NM, USA, October 24-28, 2005, 3 pages, 2 figure

Parity nonconserving cold neutron-parahydrogen interactions

Three pion dominated observables of the parity nonconserving interactions between the cold neutrons and parahydrogen are calculated. The transversely polarized neutron spin rotation, unpolarized neutron longitudinal polarization, and photon-asymmetry of the radiative polarized neutron capture are considered. For the numerical evaluation of the observables, the strong interactions are taken into account by the Reid93 potential and the parity nonconserving interactions by the DDH model along with the two-pion exchange.Comment: 17 pages, 2 figure

Is nanostructuring sufficient to get catalytically active Au?

Gold nanoparticles on transition metal oxides were synthesized by two different methods precipitation and photo induced decomposition of intermediate gold azido complex. Only samples prepared by precipitation method showed significant CO conversion at low temperature. XPS shows the formation of two Au species Au0 and Au amp; 948; on the surface of active Au TiO2 and Au Fe2O3 samples. The energy shift of Au amp; 948; peak depends on the support and is 0.6 eV and 0.9 eV for Au TiO2 and Au Fe2O3, respectively. TEM images indicate the formation of overlayer on Au particles. These results prove Au activation via a Strong Metal Support Interaction, based on strong influence of the support on the electronic structure of the gold through charge transfer and stabilization of low coordinated Au atom

The Oxidation of Rhenium and Identification of Rhenium Oxides During Catalytic Partial Oxidation of Ethylene An In Situ XPS Study

Rhenium is catalytically active for many valuable chemical reactions, and consequently has been the subject of scientific investigation for several decades. However, little is known about the chemical identity of the species present on rhenium surfaces during catalytic reactions because techniques for investigating catalyst surfaces in-situ – such as near-ambient-pressure X-ray photoemission spectroscopy (NAP-XPS) – have only recently become available. In the current work, we present an in-situ XPS study of rhenium catalysts. We examine the oxidized rhenium species that form on a metallic rhenium foil in an oxidizing atmosphere, a reducing atmosphere, and during a model catalytic reaction (i.e. the partial-oxidation of ethylene). We find that, in an oxidizing environment, a Re₂O₇ film forms on the metal surface, with buried layers of sub-oxides that contain Re⁴⁺, Re²⁺ and Re^δ+ (δ ∼ 1) species at the Re₂O₇/Re interface. The Re²⁺ containing sub-oxide is not a known bulk oxide, and is only known to exist on rhenium-metal surfaces. The Re₂O₇ film sublimes at a very low temperature (ca. 150 ℃), while the Re⁴⁺, Re²⁺ and Re^δ+ species remain stable in oxidizing conditions up to at least 450 ℃. In a reducing atmosphere of H₂, the Re²⁺ species remain on the surface up to a temperature of 330 ℃, while Reδ+ species can be detected even at 550 ℃. Under conditions for partial-oxidation of ethylene, we find that the active rhenium catalyst surface contains no bulk-stable oxides, but consists of mainly Re²⁺ species and small amounts of Re⁴⁺ species. When the catalyst is cooled and inactive, Re₂O₇ is found to form on the surface. These results suggest that Re²⁺ and Re⁴⁺ species may be active species in heterogeneous rhenium catalysts

The oxidation of copper catalysts during ethylene epoxidation

The oxidation of copper catalysts during ethylene epoxidation was characterized using in-situ photoemission spectroscopy and electron microscopy. Gas chromatography, proton-transfer reaction mass spectrometry and electron-ionization mass spectrometry were used to characterize the catalytic properties of the oxidized copper. We find that copper corrodes during epoxidation in a 1:1 mixture of oxygen and ethylene. The catalyst corrosion passes through several stages, beginning with the formation of an O-terminated surface, followed by the formation of Cu₂O scale and eventually a CuO scale. The oxidized catalyst exhibits measureable activity for ethylene epoxidation, but with a low selectivity of < 3%. Tests on pure Cu₂O and CuO powders confirm that the oxides intrinsically exhibit partial-oxidation activity. Cu₂O was found to form acetaldehyde and ethylene epoxide in roughly equal amounts (1.0% and 1.2% respectively), while CuO was found to form much less ethyl aldehyde than ethylene epoxide (0.1% and 1.0 %, respectively). Metallic copper catalysts were examined in extreme dilute-O₂ epoxidation conditions to try and keep the catalyst from oxidizing during the reaction. It was found that in feed of 1 part O₂ to 2500 parts C₂H₄ (P_O2 = 1.2×10^-4 mbar) the copper surface becomes O-terminated. The O-terminated surface was found to exhibit partial-oxidation selectivity similar to that of Cu₂O.With increasing O₂ concentration (> 8/2500) Cu₂O forms and eventually covers the surface

The selective species in ethylene epoxidation on silver

Silver s unique ability to selectively oxidize ethylene to ethylene oxide under an oxygen atmosphere has long been known. Today it is the foundation of ethylene oxide manufacturing. Yet, the mechanism of selective epoxide production is unknown. Here we use a combination of ultrahigh vacuum and in situ experimental methods along with theory to show that the only species that has been shown to produce ethylene oxide, the so called electrophilic oxygen appearing at 530.2 eV in the O 1s spectrum, is the oxygen in adsorbed SO4. This adsorbate is part of a 2D Ag SO4 phase, where the nonstoichiometric surface variant, with a formally S V species, facilitates selective transfer of an oxygen atom to ethylene. Our results demonstrate the significant and surprising impact of a trace impurity on a well studied heterogeneously catalyzed reactio

The electronic structure of iridium and its oxides

Iridium-based materials are among the most active and stable electrocatalysts for the oxygen evolution reaction. Amorphous iridium oxide structures are found to be more active than their crystalline counterparts. Herein, we combine synchrotron-based X-ray photoemission and absorption spectroscopies with theoretical calculations to investigate the electronic structure of Ir metal, rutile-type IrO₂, and an amorphous IrO_x. Theory and experiment show that while the Ir 4f line shape of Ir metal is well described by a simple Doniach–Šunjić function, the peculiar line shape of rutile-type IrO₂ requires the addition of a shake-up satellite 1 eV above the main line. In the catalytically more active amorphous IrO_x, we find that additional intensity appears in the Ir 4f spectrum at higher binding energy when compared with rutile-type IrO₂ along with a pre-edge feature in the O K-edge. We identify these additional features as electronic defects in the anionic and cationic frameworks, namely, formally O^I− and Ir^III, which may explain the increased activity of amorphous IrO_x electrocatalysts. We corroborate our findings by in situ X-ray diffraction as well as in situ X-ray photoemission and absorption spectroscopies