Dissociator for a pulsed source of polarized protons

The results of our investigation of a pulsed dissociator to be used in a polarized proton source are presented. Supersonic flow of hydrogen (2.8 ± 0.6) × 10²⁰ atoms·sr^?1·s^?1 was obtained at a Mach number M = 3.1 ± 0.2.     

Spin exchange rate constant for collisions of metastable helium atoms with rubidium atoms

The spin exchange rate constant C _se in the He(2³S₁)-Rb(5²S_1/2) system has been measured for the first time in experiments on the optical orientation of metastable helium atoms in the presence of rubidium atoms. In the temperature interval T = 293–348 K, this value is C _se = (1.8 ± 0.4) × 10^?9 cm³ s^?1. The chemiionization rate constant, which has been simultaneously measured in collisions of these particles, is C _ci = (3.1 ± 0.6) × 10^?9 cm³ s^?1.     

The mechanism of N-H bond decomposition in silicon nitride films

Thin dielectric films produced by the ammonolysis of monosilane in the temperature range 750–900 °C have 5–30% of their nitrogen atoms and 0.5–5% of their silicon atoms chemically bonded with hydrogen.We investigated the process of N-H bond decomposition under heat treatment at 900–1100 °C in vacuum and in different ambient gases by means of multiple internal reflection spectroscopy in the IR. It was found that in vacuum or in an inert gas ambient the activation energy of the process is 65 ± 2 kcal mol^?1, which is close to the N-H bond dissociation energy. When hydrogen or ammonia are added to the inert gas the activation energy of the process increases; pureammonia (P_NH3 = 1.01 × 10³ hPa) or hydrogen (P_h2 = 1.01 × 10³ hPa) suppress the decomposition entirely. The mechanism of the process and some features of the degradation of MNOS memory devices are discussed.     

Hydrogen dissociation below 1 K

We report on an investigation of hydrogen dissociation at dilution refrigerator temperatures. The objective of this study is to develop a low-temperature hydrogen atom source for experiments on spin-polarized atomic hydrogen. We find that, in a discharge tube coated with a solid H₂ layer and a superfluid He film, hydrogen dissociation is mediated by a glow discharge supported by He vapor. The He discharge provides the energetic particles needed for removing H atoms from the solid H₂ coating. In order to strike a He discharge, the He vapor pressure has to correspond to a wall temperature of 0.60–0.75 K. The best results are obtained close to the low-temperature threshold for ignition, or at even lower temperatures if the rf discharge pulse is preceded by a heat pulse adjusted to flash-evaporate the required initial He density. Hydrogen atom yields of 10¹³ H/discharge pulse are typical, and scale with the pulsed rf energy as long as the discharge volume is not overheated and remains in good thermal contact with its cooling jacket. In these conditions the H flux from the dissociator is proportional to the average rf power with a dissociation efficiency of 3%. With a dilution refrigerator circulation of 0.3 mmole/sec of³He, H fluxes of 5×10¹³ H/sec are obtained in steady-state discharge conditions.Deceased March 1986.     

Atomic Hydrogen in Thick H2 Films at Temperatures 0.05�C2?K

We describe experiments on hydrogen atoms stabilized in a 100 ??m thick H₂ film at temperatures between 0.05 and 2 K. The molecular hydrogen matrix was condensed directly from natural hydrogen gas. The H atoms are produced with a plasma discharge at temperatures below 1 K and studied with electron spin resonance. H densities of 2×10¹⁹ cm^?3 in solid H₂ were reached. As observed earlier in thin H₂ films, we found a high stability of atomic populations and strong deviation from Boltzmann statistics of lowest two hyperfine states at the lowest temperatures. In thick films we found that the ESR resonance lines consisted of two closely spaced components with different widths indicating separate regions of high and low concentrations of H atoms in the H₂ matrix. Upon warming, the two components show very different rates of recombination with the higher density component having a faster recombination rate at T>1 K. We discuss the atomic interactions and mobility, and also the structure of the samples of H atoms in the H₂ matrix.     

Intense polarized atomic deuteron source

An intense polarized deuteron gas jet, to be used as a target in an electron storage ring, is described.With a gas flow of 0.3 1 Torr/s through a nozzle, the flux of polarized atoms from a separation magnet is Q = 1.5 × 10¹⁶ atoms/s. At a distance of 45 cm from the magnet the jet width, at half-height, is 6 mm and the target thickness t = 2 × 10¹¹ atoms/cm². The degree of tensor polarization is P_zz = ?0.75 ± 0.10 for the 3 → 5 transition and P_zz = 0.80 ± 10 for the 2 → 6 transition.     

Progress Toward Redetermining the Boltzmann Constant with a Fixed-Path-Length Cylindrical Resonator

A single, fixed-path-length cylindrical-cavity resonator was used to measure c ₀ = (307.825 2 ± 0.001 2) m · s^?1, the zero-density limit of the speed of sound in pure argon at the temperature of the triple point of water. Three even and three odd longitudinal modes were used in this measurement. Based on the ratio M/?? ₀ = (23.968 644 ± 0.000 033) g · mol^?1, determined from an impurity and isotopic analysis of the argon used in this measurement and the measured c ₀, the value k _B = 1.380 650 6 × 10^?23J · K^?1 was obtained for the Boltzmann constant. This value of k _B has a relative uncertainty u _r(k _B) = 7.9 × 10^?6 and is fractionally, (0.12 ± 8.1) × 10^?6 larger than the value recommended by CODATA in 2006. (The uncertainty is one standard uncertainty.) Several, comparatively large imperfections of our prototype cavity affect the even longitudinal modes more than the odd modes. The models for these imperfections are approximate, but they suggest that an improved cavity will significantly reduce the uncertainty of c ₀.     

Grain boundary self-diffusion in evaporated Au films at low temperatures

Direct self-diffusion measurements in vapor-deposited polycrystalline Au films have been made using ¹⁹⁵Au radioactive tracer and an r.f. back-sputtering technique for serial sectioning. A temperature range of 117°–177°C was investigated. It has been demonstrated that self-diffusion in thin Au films at these low temperatures takes place by rapid transport of the tracer atoms along the grain boundaries. The grain boundary self-diffusion parameters are Q_b=1.0±0.1 eV and δD_b⁰ = 9 × 10^?10 cm³/sec, which compare well with those in bulk polycrystalline Au.     

Effet Hall dans les films d'argent déposés par voie chimique

Study of the Hall effect on silver films prepared by chemical reduction shows an electronic conductivity. At 25°C the Hall coefficient R_H is ?(12 ± 1) × 10^?11 m³ C^?1 and the number of conduction electrons n = 0.89 electrons per atom agrees within 10% of the result obtained from measurement of the conductivity.The mobility of the electron transport in the annealed films, μ_Hr(25°C) = 4.85 × 10^?3 m²V^?1s^?1 is six to seven time greater than that of the same unannealed films, μ_Hi(25°C) = 0.75 × 10^?3 m² V^?1 s^?1.     

Quantum isotope effect in “hydrogen solution in palladium-hydroxide proton conductor” heterojunctions

A hydrogen isotope effect in electrode potentials has been observed for the first time at 368 ± 0.5 K in solid hydroxide protonic PdH_x|KOH · nH₂O (x < 0.02, n = 0.5 and 1) heterojunctions obtained upon electrochemical activation of Pd|KOH · nH₂O|M heterostructures, where M = Pd or C (graphite). The chemical composition of the second (M) electrode did not influence the isotope shift of the electrode potential upon the complete replacement of protium by deuterium, which amounted to 105 ± 15 and 120 ± 20 mV for n = 0.5 and 1, respectively. The isotope shift in the electrode potential of the “hydrogen solution in palladium-hydroxide proton conductor” heterojunction implies the presence of quantum effects. The observed isotope shift is correlated with the isotope effects in the zero-point energies of (i) protons involved in the O-H chemical bonds in the structural units of electrolyte and (ii) hydrogen atoms dissolved in palladium.     

Nitrogen Vacancies on 2D Layered W2N3: A Stable and Efficient Active Site for Nitrogen Reduction Reaction

Electrochemical nitrogen reduction reaction (NRR) under ambient conditions provides an avenue to produce carbon‐free hydrogen carriers. However, the selectivity and activity of NRR are still hindered by the sluggish reaction kinetics. Nitrogen Vacancies on transition metal nitrides are considered as one of the most ideal active sites for NRR by virtue of their unique vacancy properties such as appropriate adsorption energy to dinitrogen molecule. However, their catalytic performance is usually limited by the unstable feature. Herein, a new 2D layered W₂N₃ nanosheet is prepared and the nitrogen vacancies are demonstrated to be active for electrochemical NRR with a steady ammonia production rate of 11.66 ± 0.98 µg h^?1 mg_cata^?1 (3.80 ± 0.32 × 10^?11 mol cm^?2 s^?1) and Faradaic efficiency of 11.67 ± 0.93% at ?0.2 V versus reversible hydrogen electrode for 12 cycles (24 h). A series of ex situ synchrotron‐based characterizations prove that the nitrogen vacancies on 2D W₂N₃ are stable by virtue of the high valence state of tungsten atoms and 2D confinement effect. Density function theory calculations suggest that nitrogen vacancies on W₂N₃ can provide an electron‐deficient environment which not only facilitates nitrogen adsorption, but also lowers the thermodynamic limiting potential of NRR.     

A two species trap for chromium and rubidium atoms

Abstract

We realize a combined trap for bosonic chromium (⁵²Cr) and rubidium (⁸⁷Rb) atoms. Initial experiments focus on exploring a suitable loading scheme for the combined trap and on studies of new trap loss mechanisms originating from simultaneous trapping of two species. By comparing the trap loss from the ⁸⁷Rb magneto-optical trap (MOT) in the absence and presence of magnetically trapped ground state ⁵²Cr atoms we determine the scattering cross-section of σ _inel,RbCr = 5.0 ± 4.0 × 10^?18 m² for light-induced inelastic collisions between the two species. Studying the trap loss from the Rb magneto-optical trap induced by the Cr cooling-laser light, the photoionization cross-section of the excited 5P_3/1 state at an ionizing wavelength of 426 nm is measured to be σ _P = 1.1 ± 0.3 × 10^?21 m².     

X-Ray Diffraction Studies of the Recrystallization of C60-Based Polymers

Abstract

A time-dependent x-ray scattering study is presented of the kinetics of the C₆₀-based star polymer containing 6 urethane-connected polyether and the star model polymers containing 4, 3, and 2 urethane-connected polyether arms. By varying the temperature from 40°C to -40°C, the order-disorder phase transition (recrystallization) of the polymers has been observed. Two crystalline domains with the periodicities of 4.5±0.6 Å and 3.7±0.4 Å were detected. The domain sizes ranging from 70 Å to 250 Å, the transition temperatures from -15°C to 25°C and the maximum percentage of recrystallization from 13% to 34% increase as the number of polyether arms increases. The x-ray scattering peaks of the order phase grow up as I₁ +I₂(l - e^?t/t0), where I₁ and I₂ are the initial intensity and the proportional intensity modulus, respectively. The relaxation time t₀ of the order phase is varying as a power law with respect to the reduced temperature T_R(=(T - T_C)/T_C): t₀ = (35±10)·|T_R|^?1.42±0.07 seconds for the C₆₀-based star polymer containing 6 urethane-connected polyether; t₀ = (315 ± 64)·|T_R|^?1.68±0.08 seconds for the star model polymer containing 4 urethane-connected polyether arms.     

Structural relaxation and electrical conductivity of molybdovanadate glass

⁵⁷Fe Mössbauer spectrum of conductive barium iron vanadate glass with a composition of 20BaO·10Fe₂O₃·70V₂O₅ (in mol%) showed paramagnetic doublet peak due to distorted Fe^IIIO₄ tetrahedra with isomer shift (δ) value of 0.37 (±?0.01) mm s^?1. Mössbauer spectra of 20BaO·10Fe₂O₃·xMoO₃·(70???x)V₂O₅ glasses (x?=?20–50) showed paramagnetic doublet peaks due to distorted Fe^IIIO₆ octahedra with δ’s of 0.40–0.41 (±?0.01) mm s^?1. These results evidently show a composition-dependent change of the 3D-skeleton structure from “vanadate glass” phase, composed of distorted VO₄ tetrahedra and VO₅ pyramids, to “molybdate glass” composed of distorted MoO₆ octahedra. After isothermal annealing at 500 °C for 60 min, Mössbauer spectra also showed a marked decrease in the quadrupole splitting (Δ) of Fe^III from 0.70 to 0.77 to 0.58–0.62 (±?0.02) mm s^?1, which proved “structural relaxation” of distorted VO₄ tetrahedra which were randomly connected to FeO₄, VO₅, MoO₆, FeO₆ and MoO₄ units by sharing corner oxygen atoms or edges. DC-conductivity (σ) of barium iron vanadate glass (x?=?0) measured at room temperature was 3.2?×?10^?6 S cm^?1, which increased to 3.4?×?10^?1 S cm^?1 after the annealing at 500 °C for 60 min. The σ’s of as-cast molybdovanadate glasses with x’s of 20–50 were ca. 1.1?×?10^?7 or 1.2?×?10^?7S cm^?1, which increased to 2.1?×?10^?2 (x?=?20), 6.7?×?10^?3 (x?=?35) and 1.9?×?10^?4 S cm^?1 (x?=?50) after the annealing at 500 °C for 60 min. It was concluded that the structural relaxation of distorted VO₄ tetrahedra was directly related to the marked increase in the σ, as generally observed in several vanadate glasses.     

Dielectric and photoconducting properties of Ga2Te3 and In2Te3 crystals

Single crystals of defect III–VI semiconductors Ga₂Te₃ and In₂Te₃ have been grown by the Bridgman method. Capacitance vs frequency measurements have been carried out from which the low frequency dielectric constants ?₅ have been determined to be 10.95 ± 0.26 and 12.3 ± 0.13 respectively. These values are compared with the high-frequency dielectric constants ?₆₀ calculated from the Phillips' model. Dark conductivity and photoconductivity have been studied as a function of annealing upto 210°C, maxinum photosensitivity being obtained for both crystals for T_anneal = 80°C. This behaviour has been related to lattice ordering through x-ray diffraction studies. Measurements of photo conductive gain indicate carrier life-times of 2 × 10^?4s and 5 × 10^?4s respectively at room-temperature.     

Ferroelasticity in the semiconductor PtGeSe

Orthorhombic PtGeSe, with a = 6.00984 ± 0.00004, b = 6.06174 ± 0.00005 and c = 5.98187 ± 0.00005 Å at 298°K (λCuKα₁ = 1.540562 A), is predicted on the basis of atomic coordinates reported by Entner and Parthé (Acta Cryst. (1973) B29, 1557) to be ferroelastic. Crystals with volume as large as 10 mm³ have been grown at about 1500°K from the elements by unidirectional solidification. The twinning in many as-grown crystals corresponds to interchange among the principal lattice vectors. Uniaxial stress of about 16 MNm^?2 is sufficient to eliminate all but one lattice vector along the stress direction and interchange lattice vectors in single domains. The ferroelastic behavior is accounted for by three independent atomic mechanisms. Some of the resulting atomic displacements, for Ge and Se, are as large as 2 Å. The resistivity of PtGeSe is 8.18×10^?3 ohm-cm at 300°K, 3.21×10^?1 ohm-cm at 4.2°K.     

Crystal data and thermal expansion of tricalciumborate

Single crystals of tricalciumborate were grown by the flux method. Ca₃B₂O₆ crystallizes in the rhombohedral system with the unit cell parameters $\text{a}{}_{\mathrm{R}}\text{= 6.3577(7)}\text{A}\text{?}$, α_R = 85.68(8)°, Z = 2. The equivalent hexagonal lattice parameters are a_H = 8.640(1), $\text{C}{}_{\mathrm{H}}\text{= 11.854(1)}\text{A}\text{?}$. The anisotropic thermal expansion parameters α_a = (1.00 ± 0.14) × 10^?5, α_c = (3.60 ± 0.47) × 10^?5 were determined by X-ray methods in the temperature interval 25 – 1000°C.     

Lattice parameter study of silicon uniformly doped with boron and phosphorus

Powder and single-crystal X-ray techniques have been employed to obtain precise lattice parameters of silicon uniformly doped with boron or phosphorus. Good agreement is found between the two methods. Previous accurate determination of the CuKα₁, effective wavelength has yielded λ=1.540621±0.000006 Å. Particular care has been devoted to the chemical and electrical characterization of the alloys, whose maximum dopant concentrations were 8×10¹⁹ atoms cm^?3 for P and 4.4×10²⁰ atoms cm^?3 for B. A linear dependence of lattice parameter on concentration has been found for P in the whole examined range, while for B a deviation from the linear trend starts at about 2.25×10²⁰ atoms cm^?3. Tetrahedral radii are found to be 1.176 Å for pure Si, 1.07 Å and 0.91 Å respectively for dissolved substitutional P and B. Values of the linear lattice contraction coefficient, volume size factor, Vegard's law factor and elastic strain energy in both alloys are reported and discussed. The deviation from linear trend in borondoped alloys is analysed and it is shown that the phenomenon is insensitive to heattreatments and does not depend on the degree of ionization of boron atoms.     

Aerodynamic quality management for the NACA 23012 airfoil model using the surface high-frequency discharge

The effect of the surface capacity HF discharge on airfoil flow-around has been studied in the situation when the oncoming flow velocity is 20 m/s and the Reynolds numbers are Re = 10⁵. The power delivered to discharge was modulated with a frequency of 3 × 10²–2 × 10⁴ Hz, which corresponds to a Strouhal number of St = 1.2–80, and the average electric power (W _av) was 50–400 W. It has been indicated that the aerodynamic drag decreased and the lift increased at stall and post-stall angles of attack when the HF dielectric barrier discharge was turned on. A nonstationary stochastic change in the C _x and C _y aerodynamic characteristics was observed at a stall angle in the St = 4–10 range of Strouhal numbers when the power was insufficient (W _av ≈ 100 W).     

Signal and noise characteristics of a terahertz frequency-selective YBa2Cu3O7−x Josephson detector

The performance characteristics of a frequency-selective Josephson detector based on a symmetrical [001]-tilt YBa₂Cu₃O_7?x bicrystal junction have been studied in the terahertz frequency range. At an external radiation frequency of 0.692 THz and a detector temperature of 55 K, the volt-watt responsivity reaches (7 ± 2) × 10⁴ V/W, which corresponds to theoretical estimates calculated with allowance for thermal fluctuations in the junction. The noise equivalent power (2.9 ± 0.9) × 10^–13 W/Hz^1/2 and the power dynamic range D = 47 ± 3 dB of the detector are determined by the excess 1/f type noise in the junction. It is shown that NEP values up to 5 × 10^?15 W/Hz^1/2 and D above 60 dB can be reached with high-frequency modulators or pulsed sources of terahertz radiation.