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Register a new userMicromegas TPC studies at high magnetic fields using the charge dispersion signal
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The International Linear Collider (ILC) Time Projection Chamber (TPC) transverse space-point resolution goal is 100 microns for all tracks including stiff 90 degree tracks with the full 2 meter drift. A Micro Pattern Gas Detector (MPGD) readout TPC can achieve the target resolution with existing techniques using 1 mm or narrower pads at the expense of increased detector cost and complexity. The new MPGD readout technique of charge dispersion can achieve good resolution without resorting to narrow pads. This has been demonstrated previously for 2 mm x 6 mm pads with GEMs and Micromegas in cosmic ray tests and in a KEK beam test in a 1 Tesla magnet. We have recently tested a Micromegas-TPC using the charge dispersion readout concept in a high field super-conducting magnet at DESY. The measured Micromegas gain was found to be constant within 0.5% for magnetic fields up to 5 Tesla. With the strong suppression of transverse diffusion at high magnetic fields, we measure a flat 50 micron resolution at 5 Tesla over the full 15 cm drift length of our prototype TPC.
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The Time Projection Chamber (TPC) for the International Linear Collider will need to measure about 200 track points with a resolution close to 100 $mu$m. A Micro Pattern Gas Detector (MPGD) readout TPC could achieve the desired resolution with existing techniques using sub-millimeter width pads at the expense of a large increase in the detector cost and complexity. We have recently applied a new MPGD readout concept of charge dispersion to a prototype GEM-TPC and demonstrated the feasibility of achieving good resolution with pads similar in width to the ones used for the proportional wire TPC. The charge dispersion studies were repeated with a Micromegas TPC amplification stage. We present here our first results on the Micromegas-TPC resolution with charge dispersion. The TPC resolution with the Micromegas readout is compared to our earlier GEM results and to the resolution expected from electron statistics and transverse diffusion in a gaseous TPC.
Since the summer of 2003, a large Micromegas TPC prototype (1000 channels, 50 cm drift, 50 cm diameter) has been operated in a 2T superconducting magnet at Saclay. A description of this apparatus and first results from cosmic ray tests are presented. Additional measurements using simpler detectors with a laser source, an X-ray gun and radio-active sources are discussed. Drift velocity and gain measurements, electron attachment and aging studies for a Micromegas TPC are presented. In particular, using simulations and measurements, it is shown that an $Argon-CF_4$ mixture is optimal for operation at a future Linear Collider.
We present ion backflow measurements in a Micromegas (MICRO-MEsh GASeous detector) TPC device developed for the next high energy electron-positron linear collider under study and a simple explanation for this backflow. A Micromegas micro-mesh has the intrinsic property to naturally stop a large fraction of the secondary positive ions created in the avalanche. It is shown that under some workable conditions on the pitch of the mesh and on the gas mixture, the ion feedback is equal to the field ratio (ratio of the drift electric field to the amplification field). Measurements with an intense X-ray source are in good agreement with calculations and simulations. The conclusion is that in the electric field conditions foreseen for the Micromegas TPC (drift and amplification fields respectively equal to 150-200 V/cm and 50-80 kV/cm) the expected ion backflow will be of the order of $2 - 3 x 10^-3$. In addition, measurements have been done in a 2T magnetic field: as expected the ion backflow is not altered by the magnetic field.
Directional Dark Matter Detection (DDMD) can open a new signature for Weakly Massive Interacting Particles (WIMPs) Dark Matter. The directional signature provides in addition, an unique way to overcome the neutron and neutrino backgrounds. In order to get the directional signature, the DDM detectors should be sensitive to low nuclear energy recoils in the keV range and have an angular resolution better than $20^{circ}$. We have performed experiments with low energy ($<30,mathrm{keV}$) ion beam facilities to measure the angular distribution of nuclear recoil tracks in a MIMAC detector prototype. In this paper, we study angular spreads with respect to the electron drift direction ($0^{circ}$ incident angle) of Fluorine nuclear tracks in this low energy range, and show nuclear recoil angle reconstruction produced by a monoenergetic neutron field experiment. We find that a high-gain systematic effect leads to a high angular resolution along the electron drift direction. The measured angular distribution is impacted by diffusion, and space charge or ion feedback effects, which can be corrected for by an asymmetry factor observed in the flash-ADC profile. The estimated angular resolution of the $0^{circ}$ incident ion is better than $15^{circ}$ at $10$ keV kinetic energy and agrees with the simulations within $20$%. The distributions from the nuclear recoils have been compared with simulated results based on a modified Garfield++ code. Our study shows that protons would be a more adapted target than heavier nuclei for DDMD of light WIMPs. We demonstrate that directional signature from the Galactic halo origin of a Dark Matter WIMP signal is experimentally achievable, with a deep understanding of the operating conditions of a low pressure detector with its diffusion mechanism.
We report on the construction, tests, calibrations and commissioning of an Optical Readout Time Projection Chamber (O-TPC) detector operating with a CO2(80%) + N2(20%) gas mixture at 100 and 150 Torr. It was designed to measure the cross sections of several key nuclear reactions involved in stellar evolution. In particular, a study of the rate of formation of oxygen and carbon during the process of helium burning will be performed by exposing the chamber gas to intense nearly mono-energetic gamma-ray beams at the High Intensity Gamma Source (HIgS) facility. The O-TPC has a sensitive target-drift volume of 30x30x21 cm^3. Ionization electrons drift towards a double parallel grid avalanche multiplier, yielding charge multiplication and light emission. Avalanche induced photons from N2 emission are collected, intensified and recorded with a Charge Coupled Device (CCD) camera, providing two-dimensional track images. The events time projection (third coordinate) and the deposited energy are recorded by photomultipliers and by the TPC charge-signal, respectively. A dedicated VME-based data acquisition system and associated data analysis tools were developed to record and analyze these data. The O-TPC has been tested and calibrated with 3.183 MeV alpha-particles emitted by a 148Gd source placed within its volume with a measured energy resolution of 3.0%. Tracks of alpha and 12C particles from the dissociation of 16O and of three alpha-particles from the dissociation of 12C have been measured during initial in-beam test experiments performed at the HIgS facility at Duke University. The full detection system and its performance are described and the results of the preliminary in-beam test experiments are reported.
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