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.     

Investigation of the performance of an optimised MicroCAT, a GEM and their combination by simulations and current measurements

A Micro compteur à Trous (MicroCAT) structure which is used for avalanche charge multiplication in gas filled radiation detectors has been optimised with respect to maximum electron transparency and minimum ion feedback. We report on the charge transfer behaviour and the achievable gas gain of this device. A three-dimensional electron and ion transfer simulation is compared to results derived from electric current measurements. Similarly, we present studies of the charge transfer behaviour of a Gas Electron Multiplier (GEM) by current measurements and simulations. Finally, we investigate the combination of the MicroCAT and the GEM by measurements with respect to the performance at different voltage settings, gas mixtures and gas pressures.     

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.     

The green element method

This paper discusses an element-by-element approach of implementing the Boundary Element Method (BEM) which offers substantial savings in computing resource, enables handling of a wider range of problems including non-linear ones, and at the same time preserves the second-order accuracy associated with the method. Essentially, by this approach, herein called the Green Element Method (GEM), the singular integral theory of BEM is retained except that its implementation is carried out in a fashion similar to that of the Finite Element Method (FEM). Whereas the solution procedure of BEM couples the information of all nodes in the computational domain so that the global coefficient matrix is dense and full and as such difficult to invert, that of GEM, on the other hand, involves only nodes that share common elements so that the global coefficient matrix is sparse and banded and as such easy to invert. Thus, GEM has the advantage of being more computationally efficient than BEM. In addition, GEM makes the singular integral theory more flexible and versatile in the sense that GEM readily accommodates spatial variability of medium and flow parameters (e.g., flow in heterogeneous media), while other known numerical features of BEM—its second-order accuracy and ability to readily handle problems with singularities are retained by GEM. A number of schemes is incorporated into the basic Green element formulation and these schemes are examined with the goal of identifying optimum schemes of the formulation. These schemes include the use of linear and quadratic interpolation functions on triangular and rectangular elements. We found that linear elements offer acceptable accuracy and computational effort. Comparison of the modified fully implicit scheme against the generalized two-level scheme shows that the modified fully implicit scheme with weight of about 1·25 offers a marginally better approximation of the temporal derivative. The Newton–Raphson scheme is easily incoporated into GEM and provides excellent results for the time-dependent non-linear Boussinesq problem. Comparison of GEM with conventional BEM is done on various numerical examples, and it is observed that, for comparable accuracy, GEM uses less computing time. In fact, from the numerical simulations carried out, GEM uses between 15 and 45 per cent of the simulation time of BEM.     

Performance of GEM detectors in high intensity particle beams

We describe extensive tests of Double Gas Electron Multiplier (GEM) and Triple GEM detectors, including large size prototypes for the COMPASS experiment, exposed to high intensity muon, proton and pion beams at the Paul Scherrer Institute and at CERN. The measurements aim at detecting problems possibly appearing under these harsh operating conditions, the main concern being the occurrence of discharges induced by beam particles. Results for the dependence of the probability for induced discharges on the experimental environment are presented and discussed. Implications for the application of GEM detectors in experiments at high luminosity colliders are illustrated.     

Solubility measurement,Hansen solubility parameters and solution thermodynamics of gemfibrozil in different pharmaceutically used solvents

Gemfibrozil (GEM) is cholesterol-lowering agent which is being proposed as poorly water soluble drug (PWSD). Temperature based solubility values of GEM are not yet available in literature or any pharmacopoeia/monograph. Hence, the present studies were carried out to determine the solubility of PWSD GEM (as mole fraction) in various pharmaceutically used solvents such as water (H₂O), methanol (MeOH), ethanol (EtOH), isopropanol (IPA), 1-butanol (1-BuOH), 2-butanol (2-BuOH), ethylene glycol (EG), propylene glycol (PG), polyethylene glycol-400 (PEG-400), ethyl acetate (EA), dimethyl sulfoxide (DMSO) and Transcutol^® (THP) at the temperatures ranging from T?=?298.2 K–318.2?K under atmospheric pressure P?=?0.1?MPa. Equilibrium/experimental solubilities of GEM were recorded by applying a saturation shake flask methodology and regressed using ‘van’t Hoff and Apelblat models’. Hansen solubility parameters for GEM and various pharmaceutically used solvents were estimated using HSPiP software. The solid states of GEM (both in pure and equilibrated states) were studied by ‘Differential Scanning Calorimetry’ which confirmed no transformation of GEM after equilibrium. Experimental solubilities of GEM in mole fraction were observed maximum in THP (1.81?×?10^?1) followed by DMSO, PEG-400, EA, 1-BuOH, 2-BuOH, IPA, EtOH, PG, MeOH, EG and H₂O (3.24?×?10^?6) at T?=?318.2 K and similar tendencies were also recorded at T?=?298.2 K, T?=?303.2 K, T?=?308.2 K and T?=?313.2 K. ‘Apparent thermodynamic analysis’ on experimental solubilities furnished ‘endothermic and entropy-driven dissolution’ of GEM in each pharmaceutically used solvent.     

New formulation of the Green element method to maintain its second-order accuracy in 2D/3D

The Green element method (GEM) is a powerful technique for solving nonlinear boundary value problems. Derived from the boundary element method (BEM), over the meshes of the finite element method (FEM), the GEM combines the second-order accuracy of the BEM with the efficiency and versatility of the FEM.The high accuracy of the GEM, resulting from the direct representation of normal fluxes as unknowns, comes at the price of very large matrices for problems in 2D and 3D domains. The reason for this is a larger number of inter-element boundaries connected to each internal node, yielding the same number of the normal fluxes to be determined. The currently available technique to avoid this problem approximates the normal fluxes by differentiating the potential estimates within each element. Although this approach produces much smaller matrices, the overall accuracy of the GEM is sacrificed.The first of the two techniques proposed in this work redefines the present approach of approximating fluxes by considering more elements sharing each internal node. Numerical tests on the potential field exp(x+y) show an increase in accuracy by two orders of magnitude.The second approach is a reformulation of the standard GEM in terms of the flux vector, replacing its normal component. The original accuracy of the GEM is preserved while the number of unknowns is reduced as many as ten-times in the case of a mesh consisting of tetrahedrons. The additional benefit of this novel technique is the fact that the entire flux field is a mere by-product of the basic procedure for determining the unspecified boundary values.     

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.     

The DRM‐MD integral equation method: an efficient approach for the numerical solution of domain dominant problems

This work presents a multi‐domain decomposition integral equation method for the numerical solution of domain dominant problems, for which it is known that the standard Boundary Element Method (BEM) is in disadvantage in comparison with classical domain schemes, such as Finite Difference (FDM) and Finite Element (FEM) methods. As in the recently developed Green Element Method (GEM), in the present approach the original domain is divided into several subdomains. In each of them the corresponding Green's integral representational formula is applied, and on the interfaces of the adjacent subregions the full matching conditions are imposed. In contrast with the GEM, where in each subregion the domain integrals are computed by the use of cell integration, here those integrals are transformed into surface integrals at the contour of each subregion via the Dual Reciprocity Method (DRM), using some of the most efficient radial basis functions known in the literature on mathematical interpolation. In the numerical examples presented in the paper, the contour elements are defined in terms of isoparametric linear elements, for which the analytical integrations of the kernels of the integral representation formula are known. As in the FEM and GEM the obtained global matrix system possesses a banded structure. However in contrast with these two methods (GEM and non‐Hermitian FEM), here one is able to solve the system for the complete internal nodal variables, i.e. the field variables and their derivatives, without any additional interpolation. Finally, some examples showing the accuracy, the efficiency, and the flexibility of the method for the solution of the linear and non‐linear convection–diffusion equation are presented. Copyright © 1999 John Wiley & Sons, Ltd.     

Development and operation of laser machined microwell detectors

Arrays of 100 μm diameter cylindrical wells were laser micromachined on a 200 micrometer Cartesian grid, producing MicroWell Detectors (MWD). The substrate was 125 μm thick polyimide foil, more than twice as thick as a typical GEM or WELL detector. An advantage of the laser micromachining process is that the wells are produced with nearly vertical sidewalls, in contrast to the sloping sidewalls characteristic of conventional chemical etching processes. With the steeper sidewall, active elements may be more closely packed than is possible with wet etching techniques. Thicker substrates can be patterned, increasing the length of the charge multiplication region and reducing the internal capacitance per unit element. A series of prototypes have been produced and tested in a counting gas composed of 85% argon and 15% carbon dioxide, with a maximum measured gas gain of approximately 12 000.     

Subsurface drainage of sloping lands

The problem of groundwater flow over sloping beds is investigated on the assumption that the streamlines are parallel to the sloping bed instead of being horizontal. The sloping bed is bounded below by an impermeable sublayer, and with the upper surface free. In this case, the resulting equation is the non-linear Boussinesq equation incorporating the slope factor. We solve this equation by a modified boundary integral procedure known as the Green element method (GEM). This numerical technique resolves the non-linearity of the governing equation efficiently and straightforwardly. Comparison of the results obtained herewith with those available in literature shows good agreement, and confirms that for a majority of practical cases, GEM simulation can be considered to be satisfactory.     

Hyaluronic acid-coated,prodrug-based nanostructured lipid carriers for enhanced pancreatic cancer therapy

Abstract

Context: Gemcitabine (GEM) and Baicalein (BCL) are reported to have anti-tumor effects including pancreatic cancer. Hyaluronic acid (HA) can bind to over-expressed receptors in various kinds of cancer cells.

Objective: The aim of this study is to develop prodrugs containing HA, BCL and GEM, and construct nanomedicine incorporate GEM and BCL in the core and HA on the surface. This system could target the cancer cells and co-deliver the drugs.

Methods: GEM-stearic acid lipid prodrug (GEM-SA) and hyaluronic acid-amino acid-baicalein prodrug (HA-AA-BCL) were synthesized. Then, GEM and BCL prodrug-based targeted nanostructured lipid carriers (HA-GEM-BCL NLCs) were prepared by the nanoprecipitation technique. The in vitro cytotoxicity studies of the NLCs were evaluated on AsPC1 pancreatic cancer cell line. In vivo anti-tumor effects were observed on the murine-bearing pancreatic cancer model.

Results: HA-GEM-BCL NLCs were effective in entering pancreatic cancer cells over-expressing HA receptors, and showed cytotoxicity of tumor cells in vitro. In vivo study revealed significant tumor growth inhibition ability of HA-GEM-BCL NLCs in murine pancreatic cancer model.

Conclusion: It could be concluded that HA-GEM-BCL NLCs could be featured as promising co-delivery, tumor-targeted nanomedicine for the treatment of cancers.     

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.     

Edge-preserving tomographic reconstruction from gaussian data using a gibbs prior and a generalized expectation-maximization algorithm

The problem of edge-preserving tomographic reconstruction from Gaussian data is considered. The problem is formulated within a Bayesian framework, where the image is modeled as a pair of Markov Random Fields: a continuous-valued intensity process and a binary line process. The a priori information considered here enforces constraints both on the local regularity of the image and on the line configurations. The solution, defined as the maximizer of the posterior probability, is obtained using a Generalized Expectation-Maximization (GEM) algorithm, in which both the intensity and the line processes are iteratively updated. The simulation results show that introducing suitable priors on the line configurations improves the quality of the reconstructed images, and is particularly useful when the data record is small. The relationships with other approaches for managing discontinuities are outlined. A comparison between the GEM algorithm and an algorithm based on mixed-annealing is made on the basis of computer simulations.©1994 John Wiley & Sons Inc     

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.     

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.     

Green element method and singularity programming for numerical well test analysis

One technique available to petroleum reservoir engineers to determine the properties (such as permeability and reservoir size) of oil and gas reservoirs is well test analysis. In a well test a well undergoes a step-change in its flowrate and the resulting variation in the well pressure is carefully measured. Traditionally these pressure responses are interpreted by comparing them to analytical solutions. However these solutions are limited to homogeneous reservoirs of regular shapes. An alternative is to compare the measured data to numerical simulations of the well test. This allows for more complex reservoir geometry and heterogeneity in the reservoir permeability to be included. Traditionally reservoir engineers use finite difference methods for these fluid flow calculations. These are prone to some numerical artifacts that make well test responses difficult to compute accurately.This work explores the advantages of a hybrid boundary element method (BEM) known as the Green element method (GEM) for modeling well tests. BEMs are a natural choice for the problem because they are based on Green's functions, which are an established part of well test analysis [Soc Petrol Engr J (1973) 285]. The classical BEM is limited to single phase flow in homogeneous media. This work presents formulations, which give computationally efficient means to handle heterogeneity. The accuracy of the scheme is further enhanced by incorporating singularity programming.Comparisons of the proposed GEM approach to conventional finite difference simulation, using the same gridding and timestepping, show that finite difference simulations of well test responses do not accurately reproduce the corresponding analytical solutions. GEM can accurately reproduce analytical solutions for the pressure and its derivative even using coarse gridding. It can also efficiently handle heterogeneity.     

Enhancement of the accuracy of the Green element method: Application to potential problems

The 2-D formulation of the Green element method (GEM) which approximates the internal normal directional fluxes by difference expressions in terms of the field variable had been recognized to be fraught with errors that comprise its accuracy. However, this approach is computational attractive because there is only one degree of freedom at every node, the system matrix is slender, and it does require additional compatibility relationships. There have been attempts to reduce the numerical errors of this original GEM formulation by the use of flux-based formulations which essentially retain the internal fluxes but at the expense of those attractive numerical features. Here the original GEM is revisited and shown that, with difference approximation of the internal normal fluxes whose error is of the order of the square of the size of the element, its accuracy is greatly enhanced to a level comparable to the flux-based formulations. This approach is demonstrated on regular domains with rectangular elements and irregular domains with triangular elements using six examples that cover steady, transient, linear and nonlinear potential flow and heat transfer problems in homogeneous and heterogeneous media.     

Bioaugmentation on decolorization of C.I. Direct Blue 71 by using genetically engineered strain Escherichia coli JM109 (pGEX-AZR)

The study showed that Escherichia coli JM109 (pGEX-AZR), the genetically engineered microorganism (GEM) with higher ability to decolorize azo dyes, bioaugmented successfully the dye wastewater bio-treatment systems to enhance C.I. Direct Blue 71 (DB 71) decolorization. The control and bioaugmented reactors failed at a around pH 5.0. However, the bioaugmented one succeeded at around pH 9.0, the influent DB 71 concentration was 150 mg/L, DB 71 concentration was decreased to 27.4 mg/L in 12h. The 1-3% NaCl concentration of bioaugmented reactors had no definite influence on decolorization, DB 71 concentration was decreased to 12.6 mg/L in 12h. GEM was added into anaerobic sequencing batch reactors (AnSBRs) to enhance DB 71 decolorization. Continuous operations of the control and bioaugmented AnSBRs showed that E. coli JM109 (pGEX-AZR) could bioaugment decolorization. The concentrations of activated sludge and GEM were still more than 2.80 g/L and 1.5 x 10(6)cells/mL, respectively, in the bioaugmented AnSBR. All the microbial communities changed indistinctively with time. The microbial community structures of the control AnSBR were similar to those of the bioaugmented one.     

Modified flux-vector-based Green element method for problems in steady-state anisotropic media

In this study we present a new numerical technique for solving problems in steady-state heterogeneous anisotropic media, namely the ‘flux-vector-based’ Green element method (‘$\mathit{q}$-based’ GEM) for anisotropic media. This method, which is appropriate for problems where the permeability has either constant or continuous components over the whole domain, is based on the boundary element method (BEM) formulation for direct, steady-state flow problems in anisotropic porous media, which is applied to finite element method (FEM) meshes. For situations involving media discontinuities, an extension of this ‘$\mathit{q}$-based’ GEM formulation is proposed, namely the modified ‘$\mathit{q}$-based’ GEM for anisotropic media. Numerical results are presented for various physical problems that simulate flow in an anisotropic medium with diagonal layers of different permeabilities or around faults and wells, and they show that the new method, with the extensions proposed, is very suitable for steady-state problems in such media.