Electronegative Impurities Research Articles

A gas mixture regeneration system for a position-sensitive thermal neutron detector with a 3He-based gas converter

A gas mixture regeneration system is designed for a two-dimensional position-sensitive thermal neutron detector based on a multiwire proportional chamber with a 3He-based converter. The basic characteristics of the system are given, and processing steps aimed at decreasing the gas liberation from the detector structure are described. The influence of the electronegative impurity accumulation on the detector characteristics and operation of the gas regeneration system are studied. The system is designed on request of the Laboratory of Neutron Physics, JINR (Dubna), for modernizing the ДH-2 diffractometer operating on beams of the ИБP-2 reactor.

A regenerable filter for liquid argon purification

A filter system for removing electronegative impurities from liquid argon is described. The active components of the filter are adsorbing molecular sieve and activated-copper-coated alumina granules. The system is capable of purifying liquid argon to an oxygen-equivalent impurity concentration of better than 30 parts per trillion, corresponding to an electron drift lifetime of at least 10 ms. Reduction reactions that occur at ∼ 250 ∘ C allow the filter material to be regenerated in situ through a simple procedure. In the following work we describe the filter design, performance, and regeneration process.

Quantitative calculations of charge-carrier densities in the depletion layers atYBa2Cu3O7−δinterfaces

Charge redistribution at high-${T}_{c}$ superconductor interfaces and grain boundaries on one hand is problematic for technological application. On the other hand, it gives rise to a great perspective for tailoring the local electronic states. For prototypical (metallic) interfaces, we derive quantitative results for the intrinsic doping of the ${\text{CuO}}_{2}$ planes, i.e., for the deviation of the charge-carrier density from the bulk value. Our data are based on ab initio supercell calculations within density-functional theory. A remarkable hole underdoping is inherent to the clean interface, almost independent of the interface geometry. On the contrary, cation substitution as well as incorporation of electronegative impurities can compensate the intrinsic charge transfer and provide access to an exact adjustment of the superconductor's doping. The effects of oxygen deficiency are discussed.

Tailoring the intrinsic doping at YBa2Cu3O7-metal-contacts

Charge redistribution at interfaces between high-Tc superconductors and metals is problematic for technological applications due to local variations in the electronic structure. In particular, the interface affects the charge density in the superconducting CuO2-planes. For obtaining quantitative insight into effects of interface doping, we address YBCO-metal-contacts by means of first principle supercell calculations within density functional theory. On the one hand, we find that the CuO2-planes are intrinsically electron-overdoped, i.e. hole-underdoped. On the other hand, very strong effects on the near-contact electronic states are also caused by electronegative impurities incorporated into the metal. Doping of such impurities consequently paves the way for a controlled re-extracting of charge from intrinsically doped interfaces and, therefore, for a tailoring of the interface properties.

Development of a High Pressure Xe Ionization Chamber for Environmental Radiation Spectroscopy

A High Pressure Xenon ionization chamber is a promising radiation detector for environmental radiation measurement due to its radiation hardness, its physical rigidity, and its capability of operation at a high temperature up to about 170 °C. A cylindrical high pressure xenon ionization chamber, which was configured with a shielding mesh to improve its energy resolution, was designed on the basis of an electron transfer simulation code (EGSnrc) to extract an optimal density of Xe gas and a thickness of the chamber wall. An electron drift simulation code, Garfield, which was coupled with a Maxwell electric filed calculator, was also employed for the electron drift simulations due to the geometry of the shielding mesh. Shielding inefficiency was also calculated. A spherical ionization chamber was also designed and fabricated to monitor environmental radiation. A noble gas system was constructed to create a noble gas with a high purity and to inject the noble gas up to 60 atm. The combination of an oxygen absorbent (Oxisorb), a molecular sieve, and a high temperature getter can minimize the electro-negative impurities, such as the O2 and N2 gas, to below about several ppb levels. Preliminary tests such as leakage currents, saturation currents, and gas leak test were performed. The performance of the two fabricated ionization chambers at a low dose rate was tested by using a conventional shadow technique with a NIST certified 33.52 MBq 226Ra source in the calibration room at KAERI.

Performance of a high-pressure xenon ionization chamber for environmental radiation monitoring

High-pressure xenon (HPXe) ionization chambers are ideal for use in uncontrolled environments, as a detector's response has been shown to be uniform over large temperature ranges ( 20 – 170 ∘ C ) . A cylindrical HPXe ionization chamber, which was configured with a shielding mesh to improve its energy resolution, was designed on the basis of an EGSnrc simulation code to extract an optimal density of Xe gas and a thickness of the chamber's wall. A Garfield, which was coupled with a Maxwell electric field calculator, was also employed for the electron drift simulations due to the geometry of the adapted shielding mesh. Shielding inefficiency was also calculated. A spherical ionization chamber was also designed to monitor an environmental radiation level and the responses for low dose rates with the fabricated HPXe ionization chamber were compared. A noble gas system was constructed to create a noble gas with a high purity and to inject the noble gas at up to 60 atm. The combination of an oxisorb, a molecular sieve, and a high-temperature getter can minimize the electro-negative impurities, such as the O 2 and N 2 gas, to below about several ppb levels. Preliminary tests such as leakage currents, saturation currents, and gas leak tests were performed. The performance of the two fabricated ionization chambers at a low dose rate was tested by using a conventional shadow technique with a NIST certified 0.906 mCi 226Ra standard source in a calibration room at KAERI.

A setup for measurements of free-electron lifetime in a gas phase

A setup for measuring the lifetime of free electrons in a neutral buffer gas that contains an electronegative impurity is described. This setup is intended for prompt diagnostics of carcinogenicity of chemical compounds and consists of an ionization chamber with laser photoionization of the cathode, a gas system, and a system for collection and processing of experimental data based on an HBJI45 amplitude-to-digital converter. The techniques for preparing the tested gaseous mixtures and processing the experimental results are described. The results of measurements of the lifetimes and calculated free-electron capture constants for a CO2 + O2 calibrating mixture and mixtures of CO2 with organic molecules are presented. The setup allows measurements of the lifetimes of free electrons in mixtures in the range 0.5–70 μs at measurement errors of 0.2–6%, respectively, and study of two or three potentially carcinogenic substances a week.

Change in detector properties caused by electronegative impurities

Detector properties (energy resolution, gas gain, drift-time measurements) depend quite critically on the concentration of impurities. The most frequent impurities in the working gas are water vapour and oxygen. Systematic measurements of the detector properties as a function of both H 2 O vapour and O 2 concentration have been made. Ar/CO 2 (80/20) and Ar/CO 2 /CF 4 (70/10/20) have been selected as the working gases. The first mixture is commonly used, the second one is very promising due to its fastness. The concentration of H 2 O vapour and O 2 was varied from 0% to 1.9% and 3%, respectively.

A purity monitoring system for liquid argon calorimeters

For liquid argon calorimeters electronegative impurities dissolved in the medium degrade the detector response and deteriorate the energy resolution, especially at high energies. A concept for a purity monitoring system for liquid argon calorimeters has been developed and is presented here. Special combined monitors of 241Am- and 207Bi-cells are used to monitor the concentration of impurities. The working principle as well as results from test measurements are discussed.

High-temperature simultaneous removal of acetaldehyde and ammonia gases using corona discharge

The electrostatic precipitator (ESP) is the most prevalent application for corona as ion source. ESP is widely used in industrial dust collection and home ventilation systems for cleaning gas streams. However, ESP has not been used to remove gas pollutants from a gas stream. Electron attachment is one of the promising techniques for gas purification. Electron attachment reactions occur when low-energy electrons generated in a corona-discharge reactor are captured by electronegative impurities such as acetaldehyde and fine particles to produce negative ions and charged particles. The negative ions and charged particles drift in the electric field to the anode (reactor wall) and are removed by deposition there. Moreover, O¯, O radical, ozone, and OH¯ are also generated when the carrier gas is humid air. These anions and radicals also contribute to the removal efficiency. Though acetaldehyde (CH3CHO) and ammonia (NH3) are emitted at high temperature from a crematory furnace, there is insufficient report on the effect of high temperature and simultaneous removal. The present research experimentally investigates the technical feasibility of using corona discharge reactions to treat high-temperature exhaust gas containing acetaldehyde and ammonia. The simultaneous removal efficiency of these gases from nitrogen and air is obtained experimentally. The experimental results reveal that the presence of oxygen does significantly enhance the overall removal efficiency. Similarly, it is found that the presence of water vapor also enhances the removal efficiency of the target gases from the nitrogen–oxygen mixture. The generation of undesirable byproducts such as NOx is also investigated. The present technique should be applicable to the simultaneous removal of different nanoparticles at high efficiency.

The ICARUS project: a 3000t LAr TPC for neutrino physics and a search for nucleon decays

The ICARUS collaboration has developed in an intensive R&D program the novel liquid argon Time Projection Chamber (LAr TPC) technique. The final detector is planned to contain 3000 t of LAr, divided into five 600 t (T600) modules, and will be operated at the Gran Sasso underground laboratory in Italy. The physics program of the collaboration is to study neutrino oscillations with the long base line neutrino beam from CERN to the Gran Sasso laboratory (CNGS), atmospheric, solar and supernova neutrinos, and a sensitive search for nucleon decays. The technique was first developed with several small scale prototype detectors and resulted in a full scale T600 detector consisting of two identical 300 t half-modules. The maximal drift length of the electrons in LAr is 1.5 m requiring a contamination of the LAr with electronegative impurities <0.1 ppb (O2 equivalent). The first 300 t half-module was taken in operation in the year 2001 at Pavia (Italy) and tested in a three months run taking cosmic ray data. Results of this test run are presented, demonstrating the excellent calorimetric and imaging properties of the detector.

The ICARUS project: a 3000 t LAr TPC for neutrino physics and a search for nucleon decays

The ICARUS collaboration has developed in an intensive R&D program the novel liquid argon Time Projection Chamber (LAr TPC) technique. The final detector is planned to contain 3000 t of LAr, divided into five 600 t (T600) modules, and will be operated at the Gran Sasso underground laboratory in Italy. The physics program of the collaboration is to study neutrino oscillations with the long base line neutrino beam from CERN to the Gran Sasso laboratory (CNGS), atmospheric, solar and supernova neutrinos, and a sensitive search for nucleon decays. The technique was first developed with several small scale prototype detectors and resulted in a full scale T600 detector consisting of two identical 300 t half-modules. The maximal drift length of the electrons in LAr is 1.5 m requiring a contamination of the LAr with electronegative impurities < 0.1 ppb (O 2 equivalent). The first 300 t half-module was taken in operation in the year 2001 at Pavia (Italy) and tested in a three months run taking cosmic ray data. Results of this test run are presented, demonstrating the excellent calorimetric and imaging properties of the detector.

On the appearance of induced magnetic moment of β-FeSi 2 by doping halogen or oxygen atoms

We have theoretically examined whether magnetic moment of iron atom can be induced or not when highly electronegative elements such as halogen or oxygen atoms are doped. Based on the impurity Anderson model, we evaluated the green function of d electrons. Through the analysis of green function, it is disclosed that a magnetic moment can be induced by impurity potential caused from doping elements, although intrinsic β-FeSi 2 is non-magnetic. In particular, high electronegativity of doping atoms and/or strong Coulomb repulsive interaction between conduction electrons and doped atoms easily induce magnetic moment from non-magnetic state. In the viewpoint of chemical bonding, such an induced magnetic moment appears as a result of decomposition of covalent FeSi bonds by a strong electronegative impurity potential, because the broken bonds create the unpaired 3 d electrons.

Current distribution in B- and N-doped carbon nanotubes

Using density functional theory and Keldysh nonequilibrium Green's functions, we investigate electron current density distribution in molecular electronic devices. In particular, we present the current distribution in pristine (5,5) armchair carbon nanotube as well as in nanotubes with substitutional doping of boron and nitrogen impurity atoms. The presence of impurity breaks the uniformity of current distribution around the carbon rings. For the more electronegative impurity of nitrogen, the current density is attracted toward the side of the tube where the N atom is located; but for the less electronegative impurity of boron, the opposite happens. Accordingly there appears a chiral flow of current in the B- and N-doped armchair nanotube near the impurity.

Long-range coupling of adsorbates and its modifications on anisotropic metal surfaces

Coupling over large distances up to nine lattice constants of ( n×1) chain structures, formed by alkali and alkaline earth metals on anisotropic surfaces like Mo(1 1 2), occurs mainly by adsorbate-induced Friedel oscillations of the electron system at the surface. We briefly review evidence for this mechanism. It causes wave-like modulations in the adsorbate-induced lateral relaxations in the systems Li/Mo(1 1 2) and Sr/Mo(1 1 2), found by LEED-IV analysis. Within this coupling mechanism the depinning transitions of the low coverage p(8×1) and p(5×1) chain structures of the Sr/Mo(1 1 2) system can be described semi-quantitatively. The collective nature of coupling is demonstrated by adding small amounts of electronegative impurities (oxygen atoms) at concentrations far below one atom per unit cell. These impurities are able to change periodicities and symmetries as well as stabilities of structures, and partly suppress phase transitions. The complexity of these phenomena goes beyond descriptions by the simple one-dimensional coupling mechanism.

States of electronegative impurities in liquid alkali metal

States of electronegative impurities in liquid alkali metal

Ammonia-treated activated carbon as support of Ru-Ba catalyst for ammonia synthesis

Ammonia-treated activated carbon has been studied as a support of Ru-Ba catalyst for ammonia synthesis. It is shown that the introduction of nitrogen leads to a decrease of ammonia synthesis activity for the catalysts with a low Ba/Ru molar ratio, while no significant changes are obtained for the catalysts with a high Ba/Ru molar ratio, confirming that electronegative impurities suppress the activity in ammonia synthesis and consume part of the promoters.

A Method for Rapid Measurements of Working-Gas Properties in a Pulse Ionization Chamber

A method for measuring the properties of a working gas (the electron-drift velocity, the electron diffusion, and the probability of electron capture by electronegative impurities) is described. The method is based on the digital processing of the anode and cathode pulses of an ionization chamber after passing through a waveform digitizer. The measured values of the drift velocity, the diffusion coefficient, and the electron-capture probability are presented for various argon–methane mixtures. The results may be used in designing, adjusting, and operating ionization chambers, as well as in carrying out original investigations of the phenomena occurring during the drift of electrons in various gas mixtures.

The role of negative ions in experiments with complex plasma

The influence of negative ions on the state of an rf gas-discharge dusty (complex) plasma containing electronegative gaseous impurities was investigated. A simple one-dimensional argon-discharge model allowing for the impurity-induced plasmachemical reactions was taken as an example to show that the addition of even a minor amount of molecular oxygen changes appreciably the plasma composition and plasma transport properties, as well as the microparticle charges. In turn, these changes have a strong effect on the microparticle force balance and on the formation of various dusty structures in the discharge.

Interpretive approach to collective effects of electrons in multicenter problems

Interpretive theoretical tools prove valuable in guiding the analysis of experiments in the realm of atomic clusters. Here, we review basic elements of an analytic approach that makes it possible to find and visualize the effective electrostatic potential and Coulomb correlations in multicenter problems. To illustrate the utility of these concepts we apply them to exploring molecular-doped metallic clusters. This study is aiming at a systematic, visual assessment of changes induced in screening, Coulomb correlation and effective potential by varying the charge of the electronegative impurity and its position in the cluster cage.