456 research outputs found
Simulation of gain stability of THGEM gas-avalanche particle detectors
Charging-up processes affecting gain stability in Thick Gas Electron
Multipliers (THGEM) were studied with a dedicated simulation toolkit.
Integrated with Garfield++, it provides an effective platform for systematic
phenomenological studies of charging-up processes in MPGD detectors. We
describe the simulation tool and the fine-tuning of the step-size required for
the algorithm convergence, in relation to physical parameters. Simulation
results of gain stability over time in THGEM detectors are presented, exploring
the role of electrode-thickness and applied voltage on its evolution. The
results show that the total amount of irradiated charge through electrode's
hole needed for reaching gain stabilization is in the range of tens to hundreds
of pC, depending on the detector geometry and operational voltage. These
results are in agreement with experimental observations presented previously
A dynamic method for charging-up calculations: the case of GEM
The simulation of Micro Pattern Gaseous Detectors (MPGDs) signal response is
an important and powerful tool for the design and optimization of such
detectors. However, several attempts to simulate exactly the effective charge
gain have not been completely successful. Namely, the gain stability over time
has not been fully understood. Charging-up of the insulator surfaces have been
pointed as one of the responsible for the difference between experimental and
Monte Carlo results. This work describes two iterative methods to simulate the
charging-up in one MPGD device, the Gas Electron Multiplier (GEM). The first
method uses a constant step for avalanches time evolution, very detailed, but
slower to compute. The second method uses a dynamic step that improves the
computing time. Good agreement between both methods was reached. Despite of
comparison with experimental results shows that charging-up plays an important
role in detectors operation, should not be the only responsible for the
difference between simulated and measured effective gain, but explains the time
evolution in the effective gain.Comment: Minor changes in grammatical statements and inclusion of some
important information about experimental setup at section "Comparison with
experimental results
Simulation of VUV electroluminescence in micropattern gaseous detectors: the case of GEM and MHSP
Electroluminescence produced during avalanche development in gaseous
avalanche detectors is an useful information for triggering, calorimetry and
tracking in gaseous detectors. Noble gases present high electroluminescence
yields, emitting mainly in the VUV region. The photons can provide signal
readout if appropriate photosensors are used. Micropattern gaseous detectors
are good candidates for signal amplification in high background and/or low rate
experiments due to their high electroluminescence yields and radiopurity. In
this work, the VUV light responses of the Gas Electron Multiplier and of the
Micro-Hole Strip Plate, working with pure xenon, are simulated and studied in
detail using a new and versatile C++ toolkit. It is shown that the solid angle
subtended by a photosensor placed below the microstructures depends on the
operating conditions. The obtained absolute EL yields, determined for different
gas pressures and as functions of the applied voltage, are compared with those
determined experimentally.Comment: Accepted for publication in Journal of Instrumentatio
Charge Transfer and Charge Broadening of GEM Structures in High Magnetic Fields
We report on measurements of charge transfer in 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 into 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
Fe source. No severe loss of primary ionisation charge is observed,
but an improved ion feedback suppression is achieved for high magnetic fields.
Additionally, the width of the charge cloud released by individual Fe
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 readout. This charge
broadening is only partly suppressed at high magnetic fields.Comment: 11 pages, 9 figure
Cluster ions in gas-based detectors
Avalanches in gas-based detectors operating at atmospheric pressure and using Ar-CO2 or Ne-CO2 as drift medium produce in a first instance mainly Ar+, Ne+ and CO2+ ions. The noble gas ions transfer their charge to CO2 in a few ns. A few ns later, the CO2+ ions have picked up CO2 molecules, forming cluster ions, in particular CO2+ center dot (CO2)(n). Since the cluster ions are slower than the initial ions, the signals induced by ion motion are altered. The effect is shown to be present in constant-field detectors and TPC readout chambers, and is expected to affect devices such as Micromegas and drift tubes.Enerji ve Tabii Kaynaklar Bakanlığı - Turkey (2013 TAEK CERN-A5.H2.P1.01-23)Federal Ministry of Education & Research (BMBF)Helmholtz AssociationFundacao para a Ciencia e a Tecnologia (FCT) (SFRH/BD/52333/2013
The “invisible cholecystectomy”: A transumbilical laparoscopic operation without a scar
Background Looking to further reduce the operative trauma of laparoscopic cholecystectomy, we developed, in patients with no history
of cholecystitis and a normal BMI, a scarless operation through the umbilicus. The operative technique, along with the results
of the first 10 patients operated in this way, are fully described.
Methods 10 female patients underwent transumbilical scarless laparoscopic cholecystectomy.
Through the umbilicus, two trocars of 5 mm were introduced parallel to another with a bridge of fascia between them (one for
the 5-mm laparoscope and the other for the grasper). With the help of one 1-mm Kirschner wire, introduced at the subcostal
line and bent with a special designed device, the gallbladder was pulled up and the triangle of Callot was dissected free,
clipped, cut, and the gallbladder was subsequently resected. Finally the gallbladder was taken out through the umbilicus and
the umbilicus reconstructed.
Results 10 female patients, mean age 36 years (range: 31–49), mean body mass index (BMI) 23 (range: 20–26), after one attack (six
patients) or a second attack (four patients) and cholelithiasis confirmed by ultrasonography with no suspicion of inflammation
were included in this preliminary study. Mean operative time was 70 minutes (range: 65–85) with no conversions; hospital stay
was less than 24 hours with no complications.
Conclusion Looking to reduce operative trauma and improve the cosmetic result following laparoscopic cholecystectomy, a transumbilical
operative technique has been developed. Results of the operative procedure in a selected group of patients are encouraging
with no signs of inflammation and normal BMI. The umbilicus can be developed as a natural port for performing various operative
procedures with the help of the traction produced by thin Kirschner wires
TPC track distortions III: fiat lux
We present a comprehensive overview and final summary of all four types of static track distortions seen in the HARP TPC, in terms of physical origins, mathematical modelling, and correction algorithms. 'Static'Â distortions are defined as not depending on the event time within the 400 ms long accelerator spill. Calculated static distortions are compared with measurements from cosmic-muon tracks. We characterize track distortions by the r phi residuals of cluster positions with respect to the transverse projection of a helical trajectory constrained by hits in the RPC overlap regions. This method provides a fixed TPC-external reference system (by contrast to the co-moving coordinate system associated with a fit) which solely permits to identify individually, and measure quantitatively, the static TPC track distortions arising from (i) the inhomogeneity of the solenoidal magnetic field, (ii) the inhomogeneity of the electric field from the high-voltage mismatch between the inner and outer TPC field cages, (iii) the anode-wire durchgriff, and (iv) a homogenous ion-charge density in the drift volume. Five voltage levels are identified which were set incorrectly during data taking with the HARP TPC, and unfortunately conspired toward large static and dynamic track distortions. The observed time development of static distortions after a 83mKr calibration lends decisive support to our conclusions on static TPC distortions
Modelling the behaviour of microbulk Micromegas in Xenon/trimethylamine gas
We model the response of a state of the art micro-hole single-stage charge
amplication device (`microbulk' Micromegas) in a gaseous atmosphere consisting
of Xenon/trimethylamine at various concentrations and pressures. The amplifying
structure, made with photo-lithographic techniques similar to those followed in
the fabrication of gas electron multipliers (GEMs), consisted of a 100 um-side
equilateral-triangle pattern with 50 um-diameter holes placed at its vertexes.
Once the primary electrons are guided into the holes by virtue of an optimized
field configuration, avalanches develop along the 50 um-height channels etched
out of the original doubly copper-clad polyimide foil. In order to properly
account for the strong field gradients at the holes' entrance as well as for
the fluctuations of the avalanche process (that ultimately determine the
achievable energy resolution), we abandoned the hydrodynamic framework,
resorting to a purely microscopic description of the electron trajectories as
obtained from elementary cross-sections. We show that achieving a satisfactory
description needs additional assumptions about atom-molecule (Penning) transfer
reactions and charge recombination to be made
Charge Transfer Properties Through Graphene Layers in Gas Detectors
Graphene is a single layer of carbon atoms arranged in a honeycomb lattice
with remarkable mechanical, electrical and optical properties. For the first
time graphene layers suspended on copper meshes were installed into a gas
detector equipped with a gaseous electron multiplier. Measurements of low
energy electron and ion transfer through graphene were conducted. In this paper
we describe the sample preparation for suspended graphene layers, the testing
procedures and we discuss the preliminary results followed by a prospect of
further applications.Comment: 2014 IEEE Nuclear Science Symposium and Medical Imaging Conference
with the 21st Symposium on Room-Temperature Semiconductor X-Ray and Gamma-Ray
Detectors, 4 pages, 8 figure
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