Comparison of the3He/16O and1H/18O prompt nuclear reactions for lattice location measurements of oxygen in niobium

Experimental data are presented for comparison of the determination of oxygen by the¹⁶O(³He, p)¹⁸F,¹⁶O(³He, α)¹⁵O and¹⁸O(p, α)¹⁵N prompt nuclear reactions, and of their use in the lattice location, by the ion-channelling technique, of oxygen atoms in single crystal targets of elements such as niobium, where oxygen contents of ≈0.1 atomic % or more can be obtained. Both reaction cross-sections and lattice-defect production rates are considered in the comparison. Details are given of an arrangement for automatic crystallographic angular scanning of nuclear reaction and backscattering yields in channelling/lattice location measurements.     

Oxygen depth profiling by activation with the16O(3He,p,)18F reaction

A technique has been developed for oxygen depth profiling in a thickness range of 5 to about 100 mg cm⁻² by activation with the¹⁶O(³He, p)¹⁸F reaction. An apparatus was set up for the activation of oxygen with a uniform probability along the depth, and a method has been devised for step-wise etching of the activated sample. This technique has been applied to the study of oxygen behaviour in heat treatment of silicon under various conditions.     

Active3He activation analysis for carbon and oxygen

The surface and body concentrations of carbon and oxygen have been measured in various materials using prompt³He activation techniques. This target excitation functions were determined for (³He, p) reactions with carbon and oxygen over a³He energy range from 2.5 to 9.0 MeV. These data were used in the calculations of activation curves for various combinations of incident particle energy, impurity distributions, and material. Information on a computer code that was used in the analysis of the data is presented. Work done under the auspices of the U.S. Atomic Energy Commission.     

Spin-orbit and spin-spin couplings in He2 and He2−

The spin-orbit and the spin-spin coupling constants of the ⁴Π_g state of the He₂ ⁻ ion, of the parent a³Σ_u ⁺, and of the b³Π_g states of He₂ have been evaluated by a multireference configuration interaction method. The theoretical spin-spin splittings of the a³Σ_u ⁺ state and the R-dependent spin-spin function are found to be in excellent agreement with experiment, with deviations in the range of a few MHz. The theoretical spin-orbit constants and splittings of the b³Π_g state are larger than the experimental values by about 370 MHz. The spin-orbit coupling constant of the ⁴Π_g state of He₂ ⁻ is␣estimated to be three times smaller than in the b³Π_g state, but one of the intramultiplet off-diagonal spin-spin interactions is predicted to give a large contribution to the fine structure of the metastable ion. The theoretical fine structure constants for the He₂ ⁻ ion are expected to␣aid future spectroscopic investigations of the fine structure splittings of the negative ion. Received: 14 April 1998 / Accepted: 27 July 1998 / Published online: 19 October 1998     

On the determination of carbon and oxygen impurities in silicon by3He activation analysis

Carbon and oxygen impurities in silicon have been determined by 7.00 MeV³He activation analysis. The detection limits obtained for interference-free conditions are 0.1 ppb (wt) for carbon and 1.0 ppb (wt) for oxygen in silicon.     

Helium-3 activation analysis of oxygen in silicon nitride films on silicon wafers

Oxygen in silicon nitride films on silicon wafers was analyzed by activation with the¹⁶O(³He, p)¹⁸F reaction. By³He bombardment of samples propertly arranged under consideration of the¹⁸F recoil effect, total oxygen was reliably determined and its predominant part was estimated to be located whether on film surface, in film interior, or on film-substrate interface. Sample films with 0.1 to 2 μm thicknesses were found to contain 0.2 to 2 μg/cm² of oxygen in locations varying with preparation conditions. This method has been compared with ESCA and other methods for surface analysis.     

Gas-phase reactions in plasmas of SF6 with O2 in He

Processes which occur in microwave discharges of dilute mixtures of SF₆ and O₂ in He have been examined using a flow reactor sampled by a mass spectrometer. Two classes of experiments were performed. In the first set of experiments, mixtures containing 6×10¹¹ cm^–3 SF₆, 6×10¹⁶ cm^–3 He, and O₂ in the range (0–3.6)×10¹³ cm^–3 were passed through a 20-W 2450-MHz microwave discharge. The gas mixtures arriving at a sample point downstream from the discharge were examined for SF₆, SF₄, SOF₂, SOF₄, SO₂F₂, SO₂, F, and O. In the second class of experiments, rate coefficients were measured for the reactions of SF₄ with O and O₂ and for the reaction of SF with O. The rate coefficient for the reaction of SF with O was found to be (4.2±1.5)×10^–11 cm^–3 s^–1. SF₄ was found to react so slowly with both oxygen atoms and oxygen molecules that only upper limits could be placed on the rate coefficients for these reactions. These values were 2×10^–14 cm³ s^–1 and 5×10^–15 cm³ s^–1 for reactions with O and O₂ respectively. The observed distribution of products from the discharged mixtures is discussed in terms of the measured rate coefficients.     

Density of He adsorbed in micropores at 4.2K

The density of He adsorbed in the cylindrical micropores of zeolites NaY and KL has been determined by He adsorption at 4.2K. He adsorption isotherms were then compared with N₂ adsorption isotherms at 77K. Crystallographic considerations of the micropore volumes gave the density of the He adsorbed layer, which is necessary for assessment of ultramicroporosity of less-crystalline microporous solids, such as activated carbons. The determined density of He adsorbed in the cylindrical micropores of the zeolite was in the range 0.22 to 0.26 gml^–1, greater than that of He adsorbed on a flat surface (0.202 gml^–1). A value for the density of He between 0.20 to 0.22 gml^–1 is recommended for evaluation of ultramicroporosity of a slit-shaped microporous system such as activated carbon.     

Comparison of He and BF3 neutron detectors used to detect hydrogenous material buried in soil

Thermal neutrons detectors have been used for a long time and continue to be used to detect hydrogenous material. In this work, BF₃ and ³He detectors ability have been compared with each other to detect Polyethylene (PE) sample that was buried in soil. It was found that neither BF₃ nor ³He could detect PE sample without shields. This research shows that a thickness of 5 cm graphite is suitable shield for ³He detector that has been used to detect buried PE sample.     

CEPA calculations on open-shell molecules. III. Potential curves for the six lowest excited states of He2 in the vicinity of their equilibrium distances

CEPA-PNO and PNO-CI calculations have been performed for the potential energy curves of the He ₂ ⁺ ground state and the six lowest excited states of He₂ in the range of 1.4 a₀ ≤R ≤ 3.5 a₀. The calculated equilibrium distances as well as the spectroscopic constants are in very good agreement with molecular constants as derived experimentally from the rotation-vibration spectrum of He₂ by Ginter, except for thec ³∑ _g ⁺ state. This latter discrepancy is probably due to an “obligatory” hump in thec ³∑ _g ⁺ state occurring at 3.5 a₀ which cannot be properly treated in our calculation. The relative energetic positions of the six lowest states and their ionization energies are reproduced by our calculations with an accuracy of 0–400 cm⁻¹. Extrapolation of our results to infinite basis sets leads to estimates of the dissociation energies of He₂ excited states which cannot be measured spectroscopically because of the humps in all these states.     

Microanalysis of “biometallic” compounds using nuclear and atomic activation

A novel method for the characterization of metal containing biological compounds has been developed which utilizes both nuclear activation [e.g.,¹²C(³He, α)¹¹C] and atomic activations (X-ray emission) induced by 8 MeV³He bombardment. Detection limits for carbon (≈1 μg), oxygen (≈1μg), and metals (1 to 10 ng for elements between P and Zn) were obtained under routine experimental conditions. The metal stoichiometry of six different compounds (alkaline phosphatase, amylase, carbonic anhydrase, diamine oxidase, my oglobin, vitamin B₁₂) were determined with typical relative precisions of ±25% for a 40 μg sample. A comparison of these ratios with carbon and metal measurements obtained by other methods showed a relative accuracy of 1 to 20%.     

Determination of cadmium in oyster tissue using isotope dilution inductively coupled plasma mass spectrometry: comparison of results obtained in the standard and He/H2 cell modes

An inductively coupled plasma quadrupole mass spectrometer equipped with an octopole collision/reaction cell was used for the determination of cadmium in oyster tissue samples using isotope dilution inductively coupled plasma mass spectrometry. The oyster samples in question were found to contain Mo and Zr. In our feasibility study on a Cd standard solution (10 μg L⁻¹) containing a matrix of Mo (1000 μg L⁻¹) or Zr (250 μg L⁻¹), the potentially interfering species (MoO⁺ or ZrO⁺) present at the analytical mass of cadmium concerned (m/z 111, 112 or 114) was reduced effectively through the use of a mixture of He and H₂ as cell gases. The accuracy of the method was validated by the analysis of a matrix-matched certified reference material (CRM) of NIST SRM 1566b. The CRM was analyzed under the standard and He/H₂ cell modes. Two isotopic pairs of ¹¹⁴Cd/¹¹¹Cd and ¹¹²Cd/¹¹¹Cd were selected for quantification purposes. The recoveries of cadmium obtained in the two cell modes were compared with each other. The validated method was applied successfully to the APMP.QM-P5 pilot study for international comparability purposes.     

Interaction dipole moment in Rg–Xe (Rg = He, Ne, Ar, and Kr) heterodiatoms from conventional ab initio and density functional theory calculations

We have obtained interaction dipole moment curves for the rare gas heterodiatoms Rg...Xe (Rg = He, Ne, Ar, and Kr) from conventional ab initio and density functional theory calculations with flexible Gaussian-type basis sets. All methods seem to reproduce fairly similar dipole moment curves for all pairs. Our best values for the interaction dipole moment (at the respective experimental equilibrium separation R _e) were obtained at the coupled-cluster theory with single, double, and perturbatively linked triple excitations level of theory: μ_int(RgXe)/eα₀ = − 0.0025(He), − 0.0047(Ne), − 0.0055(Ar), and − 0.0037(Kr). The same trend (in absolute terms) is observed at the MP2 level of theory for the derivative of the dipole moment at R _e, as (dμ_int (RgXe)/dR) _e /e = 0.0043 (He), 0.0082 (Ne), 0.0091 (Ar), and 0.0059 (Kr). Around R _e , μ_int(HeXe) ≡ μ_HeXe varies at the MP2 level of theory as [μ_HeXe(R) − μ_HeXe(R _e)]/ea₀ = 0.0043(R − R _e) − 0.0033(R − R _e)² + 0.0018(R − R _e)³ − 0.0005(R − R _e)⁴.     

Determination of oxygen in gallium arsenide by3He activation analysis using substoichiometric precipitation

Distillation of fluoride as fluorosilicic acid and substoichiometric precipitation as lanthanum fluoride was studied for the determination of oxygen in gallium arsenide by³He activation analysis. Fluoride could be separated substoichiometrically with a good reproducibility by two distillations and successive precipitation with lanthanum. The oxygen concentration in gallium arsenide prepared by the horizontal Bridgman technique was 10 to 36 ppb and increased a little along the front, middle and tail portions in the crystal.     

The electrochemical reduction reaction of dissolved oxygen on Q235 carbon steel in alkaline solution containing chloride ions

Cyclic voltammetry, electrochemical impedance spectroscopy, and rotating disk electrode voltammetry have been used to study the effect of chloride ions on the dissolved oxygen reduction reaction (ORR) on Q235 carbon steel electrode in a 0.02 M calcium hydroxide (Ca(OH)₂) solutions imitating the liquid phase in concrete pores. The results indicate that the cathodic process on Q235 carbon steel electrode in oxygen-saturated 0.02 M Ca(OH)₂ with different concentrations of chloride ions contain three reactions except hydrogen evolution: dissolved oxygen reduction, the reduction of Fe(III) to Fe(II), and then the reduction of Fe(II) to Fe. The peak potential of ORR shifts to the positive direction as the chloride ion concentration increases. The oxygen molecule adsorption can be inhibited by the chloride ion adsorption, and the rate of ORR decreases as the concentration of chloride ions increases. The mechanism of ORR is changed from 2e⁻ and 4e⁻ reactions, occurring simultaneously, to quietly 4e⁻ reaction with the increasing chloride ion concentration.     

Analysis of palladium concentrations in airborne particulate matter with reductive co-precipitation, He collision gas, and ID-ICP-Q-MS

The concentration of platinum group elements (PGE) in the environment has increased significantly in the last 20 years mainly due to their use as catalysts in automotive catalytic converters. The quantitation of these metals in different environmental compartments is, however, challenging due to their very low concentrations and the presence of interfering matrix constituents when inductively coupled plasma-mass spectrometry (ICP-MS) is used for analysis. Previously, the research focus was on the analysis of platinum (Pt) and rhodium (Rh). However, due to the increasing use of palladium (Pd) in automotive catalytic converters, quantitation of this element in airborne particulate matter (PM) is also needed. Compared to Pt and Rh, measurements of Pd using ICP-MS are plagued by greater molecular interferences arising from elements such as copper (Cu), zinc (Zn) strontium (Sr), yttrium (Y), and zirconium (Zr). The aim of this study was to evaluate the applicability of reductive co-precipitation procedures using both mercury (Hg) and tellurium (Te) for the pre-concentration of Pd from airborne PM. Furthermore, helium (He) was tested as a collision gas for isotope dilution-inductively coupled plasma-quadrupole-mass spectrometry (ID-ICP-Q-MS) to measure Pd in the Hg and Te precipitates. Airborne PM samples (PM₁₀) were collected from Neuglobsow (Brandenburg, north-eastern Germany) and Deuselbach (Rhineland-Palatinate, south-western Germany), considered to represent background levels, and from the city Frankfurt am Main (Hesse, Germany), a high-traffic area. Samples were first digested with aqua regia in a high-pressure asher (HPA) at 320 °C and 130 bar prior to the application of reductive co-precipitation procedures. The method was validated with road dust reference material BCR-723 and the CANMET-CCRMP reference material TDB-1 and WPR-1. In airborne PM collected at the background areas Neuglobsow and Deuselbach, Pd was detected with median concentrations values of 0.5 and 0.6 pg/m³, respectively. Much higher median concentration values of 14.8 pg Pd/m³ (detection limit = 0.01 pg Pd/m³) were detected in samples collected in the city of Frankfurt am Main. Results have shown that Hg co-precipitation depletes the concentrations of interfering matrix constituents by at least one order of magnitude more, compared to Te co-precipitation, making it a more effective method for the isolation and pre-enrichment of Pd from airborne PM prior to analysis. The use of a He gas flow of 120 ml/min in the plasma further minimized interferences, particularly those arising from CuAr⁺, YO⁺, and ZrO⁺ during the determination of Pd. The results demonstrate that Hg co-precipitation and the use of He collision gas, in combination with isotope dilution, are highly effective methods for the quantitation of Pd in airborne PM using ICP-MS.      

Experimental and theoretical studies of the Bi-excited collision systems He*(23 S) + He*(23 S, 21 S) at thermal and subthermal kinetic energies

We have carried out a comprehensive experimental and theoretical investigation of the autoionizing collision systems He*(2³ S, 2¹ S) + He*(2³ S). We present high resolution electron energy spectra, obtained with a single He* beam (average relative collision energy 〈E _rel〉=1.6 meV) and with crossed He* beams (〈E _rel〉> =61 meV). The spectra show substantial structure, and under single beam conditions fast oscillations due to the interference of incoming and outgoing heavy particle waves in the entrance channels are observed. Accurate ab initio potential curves for the seven lowest He*—He*(Σ) molecular states have been obtained from a Feshbach projection scheme, and width functions for He*(2³ S) + He*(2³ S) have been derived by Stieltjes imaging. Based on these ab initio data, detailed quantum mechanical calculations of the electron spectra have been carried out and provide a thorough understanding of the experimentally observed spectral features. Good overall agreement of the calculated spectra with the experimental data is observed. The close coincidence in the positions of the experimental and theoretical peaks, especially for He*(2³ S) + He*(2³ S), underlines the reliability of the ab initio potentials. In the He*(2¹ S) + He*(2³ S) electron spectrum, the dominant peak is traced to be due to autoionization from the 2³Σ⁺ _g molecular state accessed via an avoided crossing. We also present a detailed discussion of the total ionization cross sections σ_tot and of the fraction σ_AI/σ_tot for associative ionization together with a critical comparison with previous work. The ionization probabilities for close collisions in entrance channels, from which autoionization is spin-allowed, are near unity, and therefore the absolute values and the collision energy dependence of the total cross sections simply reflect the long-range behaviour of the excited state potentials.     

Evaluation and application of argon and helium microstrip plasma for the determination of mercury by the cold vapor technique and optical emission spectrometry

The suitability of a 2.45-GHz atmospheric pressure, low-power microwave microstrip plasma (MSP) operated with Ar and He for the determination of Hg by continuous-flow cold vapor (CV) generation, using SnCl₂/HCl as the reducing agent, and optical emission spectrometry (OES) using a small CCD spectrometer was studied. The areas of stability for a discharge in the Ar and in the He MSP enclosed in a cylindrical channel in a quartz wafer were investigated. The excitation temperatures as measured for discharge gas atoms (Ar I, He I), and the electron number densities at 35–40 W and 15–400 mL min⁻¹ were found to be at the order of 3,200–5,500 K and 0.8 × 10¹⁴–1.6 × 10¹⁴ cm⁻³, respectively. The relative intensity of the Hg I 253.6-nm line and the signal-to-background ratio as a function of the forward power (35–40 W) as well as of the flow rate of the working gas (15–400 mL min⁻¹) were evaluated and discussed. For the selected measurement conditions, the Ar MSP was established to have the lower detection limit for Hg (0.6 ng mL⁻¹) compared with the He MSP. The linearity range is up to 300 ng mL⁻¹ and the precision is on the order of 1–3%. With the optimized CV Ar MSP-OES method a determination of Hg in spiked domestic and natural waters at concentration levels of 20–100 μg L⁻¹ and an accuracy of 1–4% could be performed. In an NIST domestic sludge standard reference material, Hg (3.64 μg g⁻¹) could be determined with a relative standard deviation of 4% and an agreement better than 4%.     

Spectroscopy with sound on superfluid 3He B

Collective modes of superfluid ³He B have been studied by means of sound attenuation and sound velocity studies. A new sound attenuation peak has been observed in superfluid ³He B at ?ω ≈ Δ(T). This peak has been identified with the real squashing mode. Pulse time of flight methods has been used in these studies. At high sound amplitudes, interesting transient effects have been observed which have been tentatively identified with solitonlike behavior.     

Thermodynamic interaction functions for 4He and 3He quantum liquids

The method of interaction functions is applied to quantum liquids. The interaction energies at 0 K are found as 14.3 cal/mole for ⁴He and 11.0 cal/mole for ³He. The interaction energy as a function of volume for all temperatures along the saturation line is deduced as an expression common to the two isotopes. The two isotopes have a region of anomaly in the interaction entropy, where the liquid appears less ordered than an ideal gas. These anomalies can be eliminated for ³He by allowance for the spin entropy and for ⁴He via the difference between the concentrations of the component with zero entropy in the liquid and Bose gas.