Charge transfer and charge broadening of GEM structures in high magnetic fields

We report on the measurements of charge transfer in Gas Electron Multipliers (GEM) structures in high magnetic fields. These were performed in the framework of the R&D work for a Time Projection Chamber at a future Linear Collider. A small test chamber has been installed in the aperture of a superconducting magnet with the GEM structures mounted perpendicular to the B-field direction. The charge transfer is derived from the electrical currents monitored during irradiation with an ⁵⁵Fe source. No significant loss of primary ionisation charge is observed, and an improved ion feedback suppression is achieved for high magnetic fields. Additionally, the width of the charge cloud released by individual ⁵⁵Fe photons is measured using a finely segmented strip readout after the triple GEM structure. Charge widths between 0.3 and 0.5 mm RMS are observed, which originate from the charge broadening inside the GEM amplification. This charge broadening is only partly suppressed at high magnetic fields.     

Further studies on the gain properties of a Gas Electron Multiplier with a Micro-Induction Gap Amplifying Structure (GEM-MIGAS) aimed at low-energy X-ray detection

A Gas Electron Multiplier with Micro-Induction Gap Amplifying Structure (GEM-MIGAS) is formed when the induction gap of the GEM is set between 50 and 100 μm using kapton pillars spaced at regular intervals. This configuration combines the properties of a GEM and Micromegas, allowing operation in tandem to generate high charge gains. We measured the essential operational parameters of this system using argon–isobutane (IB) and helium–IB gas mixtures. The present short induction gap GEM was able to achieve effective gains exceeding 2×10⁴ using argon–IB and 10⁵ using helium–IB mixtures. In view of the high gains achieved, particularly when using helium-based gas mixtures, these studies confirmed the possibility of using the present system for high-performance sub-keV X-ray detection.     

Evaluation of the optimum induction gap for the GEM-MIGAS

A gas electron multiplier with a micro-induction gap amplifying structure (GEM-MIGAS) is formed when a conventional GEM is operated with a short induction gap, typically set at 50 μm. The main aims of this study were to examine the charge amplification in the induction gap of the GEM-MIGAS and then to evaluate the optimum induction gap thickness for achieving the maximum charge amplification in the region. For the present study, the GEM-MIGAS was operated in the gas flow mode using He/iso-C₄H₁₀ (85/15%). It was also possible to determine the optimum gap thickness, where the dependence of the charge amplification was least sensitive to the small variations of the gap thickness and to the ambient variables, such as pressure and temperature. This was accomplished by setting the voltage across the GEM-holes to a small value as to allow the transfer of electrons from the drift region into the induction region, without incurring multiplication within the holes. Through parametric curve fitting of the charge gain-induction gap voltage characteristics, the optimum induction gap for sustaining highest charge gains was evaluated and compared with trends observed using a conventional Micromegas.     

Progress in GEM simulation

The charge induction mechanism and signal formation on the electrodes of the Gas Electron Multiplier (GEM) detector are discussed in detail. The method of simulating the effective gas gain, transparency and charge losses in GEM structures is developed. Results on simulation of various GEM geometries are consistent with published experimental data.     

Ion feedback suppression in time projection chambers

A controlled-voltage Gas Electron Multiplier (GEM) can be used to block the re-injection of positive ions in large volume Time Projection Chambers (TPC). With proper choice of geometry, gas filling and external fields, good electron transmission can be obtained at very low GEM voltages; pulsed ion gating is then much easier than with conventional wire grids, requiring hundreds of volts. Gating schemes suited for the TPC detector planned for the International Linear Collider detector are described. The possibility of GEM-based DC-operated ion filters, exploiting the difference in diffusion properties of ions and electrons, is also discussed.     

Charge-changing Collisions of Stored,Multiply-charged Ions

Abstract

The results of measurements of rate coefficients for electron transfer from several target gases to multiply-charged ions are summarized and discussed. These measurements were carried out primarily in Penning ion traps. The stored ions were produced at low energy by several methods, including electron impact multi-ionization of atoms and molecules, recoil ions from impact of fast, stripped, heavy ions with target gas atoms, and inner shell photoionization of atoms using synchrotron radiation. The measurements typically do not exhibit any simple variation of the rate coefficients with ion charge state at these low energies, consistent with the expected mechanism of electron transfer at avoided crossings of the equipotential surfaces of the quasi-molecule formed during the collision. The magnitudes of the rate coefficients, and their trends with charge, are compared for different ions interacting with the same target, and with calculations of the rates using Langevin theory, which predicts a linear dependence on charge state.     

Development of GEM tracking detector for intermediate-energy nuclear experiments

We have developed a tracking detector with a gas electron multiplier (GEM) for nuclear experiments. The developed GEM detector was installed inside the dipole magnet used for transporting the primary beam to the beam dump and it was used to measure the momentum of charged particles. A sufficiently high spatial resolution was achieved at a high counting rate and a magnetic field for coherent pion production with a 392 MeV proton beam to study the short-range component of the residual nuclear interaction. The detector systems and development procedure are described.     

Study of ion feedback in multi-GEM structures

We study the feedback of positive ions in triple and quadruple Gas Electron Multiplier (GEM) detectors. The effects of GEM hole diameter, detector gain, applied voltages, number of GEMs and other parameters on ion feedback are investigated in detail. In particular, it was found that the ion feedback is independent of the gas mixture and the pressure. In the optimized multi-GEM structure, the ion feedback current can be suppressed down to 0.5% of the anode current, at a drift field of 0.1 kV/cm and gain of 10⁴. A simple model of ion feedback in multi-GEM structures is suggested. The results obtained are relevant to the performance of time projection chambers and gas photomultipliers.     

Operation principles and properties of the multi-GEM gaseous photomultiplier with reflective photocathode

Gas Electron Multipliers with a reflective photocathode deposited on the surface of the first multiplying element are very attractive devices for photon detection and imaging over large area at moderate cost. They combine production and operation simplicity, high sensitivity to single photons, fast time response and accurate localization. In this work we present in detail the mechanisms governing the operation of these photon detectors. The results of electron extraction, transfer, multiplication and detection processes in this multi-element structure are presented and analyzed. We discuss the role of important elements and parameters influencing the detector's operation and performance: the GEM geometry, the choice of the different electric fields and the gas mixture.     

Study of the space charge potential of ion beams

The energy distribution of slow residual gas ions created by an ion beam in collisions with the residual gas molecules inside an ion beam transport system has been measured with an electrostatic spectrometer. For this investigations Ar⁺ ion beams (dc) were used, with energies between 15 and 45 keV. The beam current has been varied from 0.01 up to 1.6 mA. Further parameters were the residual gas pressure, the gas composition, the beam diameter and the potentials of several clearing electrodes in the beam transport system. The experimental results are compared with the calculated values of the space charge potentials of the ion beam. In operating the spectrometer in a gated mode the time could be measured which is necessary to build up the space charge compensation in a pulsed ion beam. Additional residual gas ions produced at different radial distances from the ion beam axis by an electron beam allowed to determine the radial potential distribution.     

Characterization of metal-modified and vertically-aligned carbon nanotube films for functionally enhanced gas sensor applications

Carbon nanotube (CNT) networked films have been grown by radiofrequency plasma enhanced chemical vapour deposition (RF-PECVD) technology onto low-cost alumina substrates, coated by nanosized Fe-catalyst for growing CNTs, to perform chemical detection of hazardous gases, at an operating sensor temperature in the range 25-150 °C. The morphology and structure of the CNT networks have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The carbon nanotubes were “forest-like” with ropes vertically-aligned to the substrate surface. A dense network of bundles of multiple tubes consisting of multi-walled carbon nanostructures appears with a maximum length of 7-10 μm and single-tube diameter varying in the range of 5-35 nm. Surface functionalizations of the vertically-aligned CNT networks with nominally 5 nm thick Pt-, Ru- and Ag-nanoclusters, prepared by magnetron sputtering, provide higher sensitivity for significantly enhanced gas detection of NO₂, H₂, ethanol and toluene up to a low limit of sub-ppm level. The measured electrical conductance of the functionalized CNTs upon exposures of a given oxidizing and reducing gas is modulated by a charge transfer model with p-type semiconducting characteristics. Functionalized CNT gas sensors exhibited better performances compared to unmodified CNTs, making them highly promising candidates for environmental air monitoring applications, at ppb-level of toxic gas detection.     

NiO微球的微波辅助合成及电荷传导能力优化

为了优化电荷传导特性, 提高电极的电化学性能, 本工作采用微波辅助合成了分级多孔结构的氧化镍微球。通过XRD、SEM和TEM对产物的形貌进行了表征。研究结果表明, 开放多孔结构的氧化镍微球是由极薄纳米片自组装而成, 以硫酸镍为镍源, 得到的氧化镍微球的粒径约为2 µm。作为超级电容器电极材料, 在电流密度为0.5 A/g时, 电极的比容量达到455 F/g, 由于NiO微球独特的多孔特性, 使电极表现出良好的阻抗特性, 为法拉第反应过程提供了较多的活性反应点, 从而提高了电极的电容性能。     

Charge transfer and ionisation by intermediate-energy heavy ions

The use of heavy ion beams for microbeam studies of mammalian cell response leads to a need to better understand interaction cross sections for collisions of heavy ions with tissue constituents. For ion energies of a few MeV u(-1) or less, ions capture electrons from the media in which they travel and undergo subsequent interactions as partially 'dressed' ions. For example, 16 MeV fluorine ions have an equilibrium charge of 7(+), 32 MeV sulphur ions have an equilibrium charge of approximately 11(+), and as the ion energies decrease the equilibrium charge decreases dramatically. Data for interactions of partially dressed ions are extremely rare, making it difficult to estimate microscopic patterns of energy deposition leading to damage to cellular components. Such estimates, normally obtained by Monte Carlo track structure simulations, require a comprehensive database of differential and total ionisation cross sections as well as charge transfer cross sections. To provide information for track simulation, measurement of total ionisation cross sections have been initiated at East Carolina University using the recoil ion time-of-flight method that also yields cross sections for multiple ionisation processes and charge transfer cross sections; multiple ionisation is prevalent for heavy ion interactions. In addition, measurements of differential ionisation cross sections needed for Monte Carlo simulation of detailed event-by-event particle tracks are under way. Differential, total and multiple ionisation cross sections and electron capture and loss cross sections measured for C(+) ions with energies of 100 and 200 keV u(-1) are described.     

Film morphology effects on the electrical and optical properties of bulk heterojunction organic solar cells based on MEH-PPV/C60 composite

The influence of film morphology on the electrical behaviour of an MEH-PPV/C₆₀ organic solar cells has been investigated. The dissociation of photogenerated charge pairs in composites of buckminsterfullerenes (C₆₀) in a conjugated polymer matrix (MEH-PPV) forming dispersed heterojunctions was studied at low C₆₀ acceptor concentrations to separate electron transfer from charge transport effects. The motivation of this study was to analyse the strong dependence of organic solar cell efficiencies on the morphology of the composite. Two effects controlling film morphology have been investigated; the first one being the influence of the fullerene concentration and the second one is the effect of the organic solvent used to deposit the photoactive layer. The sample morphology was studied using atomic force microscopy (AFM). Photoluminescence (PL) experiments and current–voltage (I–V) measurements were performed on the deposited photovoltaic film to investigate the influence of dispersion on the charge transfer process between MEH-PPV and C₆₀. An attempt to explain all the results will be presented.     

Influence of Geometry on the Memristive Behavior of the Domain Wall Spintronic Memristors and Its Applications for Measurement

A memristor is characterized by its electrical memory resistance (memristance), which is a function of the historic profile of the applied current (voltage). This unique ability allows reducing charge- and flux-based measurements to straightforward resistance measurements. The memristive measurement seeks a memristor with a constant modulation of the memristance (memductance) with respect to the charge (flux) for charge (flux)-based measurements. In this work the geometry dependent memristive behavior of a spintronic device is studied to demonstrate the possibility of both charge- and flux-based sensing, using spintronic memristors with different device geometries. The dynamic properties of a propagating magnetic domain wall in different geometrical structures make the spintronic memristor suitable for the charge-based capacitance and flux-based inductance measurements.     

Peak-peak repulsion in ion mobility spectrometry

The space charge effect has an important role in instruments dealing with ion packets and charged particles in gas phase such as the mass spectrometer and ion mobility spectrometer (IMS). It has been shown that the space charge is partially responsible for peak broadening in IMS depending on the ion density. Here, we explore the effect of space charge on peak shifting in IMS. We show that the field created by a large peak influences the drift time of a neighboring small peak. An experimental method was introduced to accurately measure the effect of space charge between two peaks. In this method, a double pulse was applied to the shutter grid to create two closed ion packets with a given initial spacing. The final spacing was then measured at the collector through the separation of the two peaks. This study shows that space charge repulsion must be considered for accurate measurements of ion mobilities. The experiments were performed in both normal and inverse modes. A theoretical model was also proposed to describe the repulsion between two ion packets in IMS.     

First microdosimetric measurements with a TEPC based on a GEM

A new type of mini multi-element tissue-equivalent proportional counter (TEPC) based on a gas electron multiplier (GEM) has been designed and constructed. This counter is in particular suitable to be constructed with a small sensitive volume so that it can be used for microdosimetry in intense pulsed radiation fields to measure the microdosimetric spectrum in the beam of, for instance, a clinical linear accelerator. The concept lends itself also for a mini multi-element version of the counter to be used for applications in which a high sensitivity is required. In this paper, we present the first microdosimetric measurements of this novel counter exposed to a 14 MeV monoenergetic neutron beam and a californium (252Cf) source for a counter cavity diameter of 1.8 mm simulating 1.0 microm tissue site size. The measured spectra showed an excellent agreement with spectra from the literature. The specific advantages of the TEPC-GEM are discussed.     

Scanning electrochemical cell microscopy: theory and experiment for quantitative high resolution spatially-resolved voltammetry and simultaneous ion-conductance measurements

Scanning electrochemical cell microscopy (SECCM) is a high resolution electrochemical scanning probe technique that employs a dual-barrel theta pipet probe containing electrolyte solution and quasi-reference counter electrodes (QRCE) in each barrel. A thin layer of electrolyte protruding from the tip of the pipet ensures that a gentle meniscus contact is made with a substrate surface, which defines the active surface area of an electrochemical cell. The substrate can be an electrical conductor, semiconductor, or insulator. The main focus here is on the general case where the substrate is a working electrode, and both ion-conductance measurements between the QRCEs in the two barrels and voltammetric/amperometric measurements at the substrate can be made simultaneously. In usual practice, a small perpendicular oscillation of the probe with respect to the substrate is employed, so that an alternating conductance current (ac) develops, due to the change in the dimensions of the electrolyte contact (and hence resistance), as well as the direct conductance current (dc). It is shown that the dc current can be predicted for a fixed probe by solving the Nernst-Planck equation and that the ac response can also be derived from this response. Both responses are shown to agree well with experiment. It is found that the pipet geometry plays an important role in controlling the dc conductance current and that this is easily measured by microscopy. A key feature of SECCM is that mass transport to the substrate surface is by diffusion and, for charged analytes, ion migration which can be controlled and varied quantifiably via the bias between the two QRCEs. For a working electrode substrate this means that charged redox-active analytes can be transported to the electrode/solution interface in a well-defined and controllable manner and that relatively fast heterogeneous electron transfer kinetics can be studied. The factors controlling the voltammetric response are determined by both simulation and experiment. Experiments demonstrate the realization of simultaneous quantitative voltammetric and ion conductance measurements and also identify a general rule of thumb that the surface contacted by electrolyte is of the order of the pipet probe dimensions.     

Equilibrium and kinetic studies of copper adsorption by activated carbon

Copper adsorption by granular activated carbon is reported in this paper. The experimental section includes titrations of activated carbon, as well as equilibrium and kinetic studies of copper adsorption. The potentiometric titration results show that the point of zero charge is 9.5, and that the surface charge increases with decreasing pH. The adsorption of copper strongly depends on solution pH and increases from 10 to 95% at pH ranging from 2.3 to 8. A dramatic increase in pH and emission of small gas bubbles are observed during the experiments, which may result from adsorption of hydrogen ion and/or reduction-oxidation reactions. The two-pK triple-layer model is employed to describe copper adsorption. KINEQL, an adsorption kinetics algorithm, is used to represent the experimental data, and it is found that the model can describe reasonably well the experimental measurements of surface charge, adsorption equilibrium, and adsorption kinetics. Calculations show that formation of the surface-metal complexes SO⁻Cu²⁺ and SO⁻CuOH⁺ (a hydrolysis product of SO⁻Cu ²⁺) in the outer layer around the surface of carbon results in removal of copper ion. It is also found that mass transfer controls the adsorption rate, and that adsorption occurs in the micropore region where both external mass transfer and diffusion are important.