439 research outputs found
Modelling and Measurement of Charge Transfer in Multiple GEM Structures
Measurements and numerical simulations on the charge transfer in Gas Electron
Multiplier (GEM) foils are presented and their implications for the usage of
GEM foils in Time Projection Chambers are discussed. A small test chamber has
been constructed and operated with up to three GEM foils. The charge transfer
parameters derived from the electrical currents monitored during the
irradiation with an Fe-55 source are compared with numerical simulations. The
performance in magnetic fields up to 2 T is also investigated.Comment: 21 pages, 16 figures, submitted to NIM-
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
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
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
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
Charge Transfer Properties Through Graphene for Applications in Gaseous Detectors
Graphene is a single layer of carbon atoms arranged in a honeycomb lattice
with remarkable mechanical and electrical properties. Regarded as the thinnest
and narrowest conductive mesh, it has drastically different transmission
behaviours when bombarded with electrons and ions in vacuum. This property, if
confirmed in gas, may be a definitive solution for the ion back-flow problem in
gaseous detectors. In order to ascertain this aspect, graphene layers of
dimensions of about 2x2cm, grown on a copper substrate, are transferred
onto a flat metal surface with holes, so that the graphene layer is freely
suspended. The graphene and the support are installed into a gaseous detector
equipped with a triple Gaseous Electron Multiplier (GEM), and the transparency
properties to electrons and ions are studied in gas as a function of the
electric fields. The techniques to produce the graphene samples are described,
and we report on preliminary tests of graphene-coated GEMs.Comment: 4pages, 3figures, 13th Pisa Meeting on Advanced Detector
Mitoxantrone in metastatic apudomas: a phase II study of the EORTC Gastro-Intestinal Cancer Cooperative Group.
We performed a phase II study with mitoxantrone in patients with carcinoid tumours, islet cell tumours and medullary carcinomas of the thyroid. Thirty-five eligible patients received mitoxantrone 12 mg m-2 i.v. every 3 weeks. Among 18 previously untreated patients, three responded (17%, 95% CI = 4-41%); no responses were achieved in 17 previously treated patients. Of the 21 patients who had carcinoid tumours, 11 were previously untreated and two achieved a response (18%, 95% CI = 2-52%). Overall response rate was 9% (95% CI = 2-23%). At a median follow-up of 43 months, median overall survival was 16 months. The median survival of 21 patients with a normal alkaline phosphatase was 29 months and 9 months for 14 patients with elevated serum levels (P = 0.005). A similar observation was noticed for gamma-glutamyltransferase (P = 0.007). We concluded that mitoxantrone is not active in APUD tumours. Elevated alkaline phosphatase and gamma-glutamyltransferase are associated with a poor prognosis
A Honeycomb Proportional Counter for Photon Multiplicity Measurement in the ALICE Experiment
A honeycomb detector consisting of a matrix of 96 closely packed hexagonal
cells, each working as a proportional counter with a wire readout, was
fabricated and tested at the CERN PS. The cell depth and the radial dimensions
of the cell were small, in the range of 5-10 mm. The appropriate cell design
was arrived at using GARFIELD simulations. Two geometries are described
illustrating the effect of field shaping. The charged particle detection
efficiency and the preshower characteristics have been studied using pion and
electron beams. Average charged particle detection efficiency was found to be
98%, which is almost uniform within the cell volume and also within the array.
The preshower data show that the transverse size of the shower is in close
agreement with the results of simulations for a range of energies and converter
thicknesses.Comment: To be published in NIM
Recommended from our members
Results from the CERN pilot CLOUD experiment
During a 4-week run in OctoberâNovember 2006, a pilot experiment was performed at the CERN Proton Synchrotron in preparation for the Cosmics Leaving OUtdoor Droplets (CLOUD) experiment, whose aim is to study the possible influence of cosmic rays on clouds. The purpose of the pilot experiment was firstly to carry out exploratory measurements of the effect of ionising particle radiation on aerosol formation from trace H2SO4 vapour and secondly to provide technical input for the CLOUD design. A total of 44 nucleation bursts were produced and recorded, with formation rates of particles above the 3 nm detection threshold of between 0.1 and 100 cmâ3 sâ1, and growth rates between 2 and 37 nm hâ1. The corresponding H2SO4 concentrations were typically around 106 cmâ3 or less. The experimentally-measured formation rates and H2SO4 concentrations are comparable to those found in the atmosphere, supporting the idea that sulphuric acid is involved in the nucleation of atmospheric aerosols. However, sulphuric acid alone is not able to explain the observed rapid growth rates, which suggests the presence of additional trace vapours in the aerosol chamber, whose identity is unknown. By analysing the charged fraction, a few of the aerosol bursts appear to have a contribution from ion-induced nucleation and ion-ion recombination to form neutral clusters. Some indications were also found for the accelerator beam timing and intensity to influence the aerosol particle formation rate at the highest experimental SO2 concentrations of 6 ppb, although none was found at lower concentrations. Overall, the exploratory measurements provide suggestive evidence for ion-induced nucleation or ion-ion recombination as sources of aerosol particles. However in order to quantify the conditions under which ion processes become significant, improvements are needed in controlling the experimental variables and in the reproducibility of the experiments. Finally, concerning technical aspects, the most important lessons for the CLOUD design include the stringent requirement of internal cleanliness of the aerosol chamber, as well as maintenance of extremely stable temperatures (variations below 0.1 _C)
- âŚ