A. Giganon

Development and performance of Microbulk Micromegas detectors

A new Micromegas manufacturing technique, based on kapton etching technology, has been developed recently, resulting in further improvement of the characteristics of the detector, such as uniformity and stability. Excellent energy resolution has been obtained, reaching 11% FWHM for the 5.9 keV photon peak of the 55Fe X-ray source and 1.8% FWHM (with possible evidence of less than 1%) for the 5.5 MeV alpha peak of the 241Am source. The new Microbulk detector shows several advantages like flexible structure, low material and high radio-purity, opening thus new possibilities for both accelerator and low counting-rate experiments. The detector has already been used in CAST and n-TOF, while it is being tested for future neutrinoless double-beta decay experiments like NEXT. Details of the production of several types of Microbulk detectors will be described. First benchmark results will be presented, demonstrating the enhanced performance of Microbulk detectors.

The Micromegas detector of the CAST experiment

A low-background Micromegas detector has been operating in the CAST experiment at CERN for the search for solar axions during the first phase of the experiment (2002?2004). The detector, made out of low radioactivity materials, operated efficiently and achieved a very high level of background rejection (5 ? 10?5 counts keV?1?cm?2?s?1) without shielding.

Micromegas as a large microstrip detector for the COMPASS experiment

Abstract Recent results on the gaseous microstrip detector Micromegas which will be used to track particles in the COMPASS experiment at CERN are presented. Developments concerning its mechanical and electrical design, associated readout electronics and gas mixture were carried out. Particular attention was paid to the discharge phenomenon which affects this type of microstrip detector. The adequacy of the options finally retained, especially the SFE16 readout and the use of a Ne–C 2 H 6 –CF 4 gas mixture, was demonstrated in a set of beam tests performed on a 26×36 cm 2 prototype. Operating at a gain of ∼6400, full efficiency is reached along with a spatial resolution of ∼50 μm and a timing accuracy of 8.5 ns . Discharges are kept at a low rate, less than one per SPS spill in a COMPASS-like environment. Via a decoupling of the strips through individual capacitors their impact is greatly reduced. They generate a dead time on the full detector of ∼ 3 ms , affecting marginally the detection efficiency given their rate. The probability of discharge, at a given value of efficiency, is found to decrease with the mean value of the gas mixture atomic number. In view of these results, the commissioning of Micromegas for COMPASS is foreseen in the near future.

Characterization of microbulk detectors in argon- and neon-based mixtures

A recent Micromegas manufacturing technique, so called Microbulk, has been developed, improving the uniformity and stability of this kind of detectors. Excellent energy resolutions have been obtained, reaching values as low as 11% FWHM at 5.9 keV in Ar+5%iC4H10. This detector has other advantages like its flexible structure, low material budget and high radio-purity. Two microbulk detectors with gaps of 50 and 25 μm have been characterized in argon- and neon-based mixtures with ethane, isobutane and cyclohexane. The results will be presented and discussed. The gain curves have been fitted to the Rose-Korff gain model and dependences of the electron mean free path and the threshold energy for ionization have been obtained. The possible relation between these two parameters and the energy resolution will be also discussed.

New neutron detector based on micromegas technology for ADS projects

A new neutron detector based on Micromegas technology has been developed for the measurement of the simulated neutron spectrum in the ADS project. After the presentation of simulated neutron spectra obtained in the interaction of 140MeV protons with the spallation target inside the TRIGA core, a full description of the new detector configuration is given. The advantage of this detector compared to conventional neutron flux detectors and the results obtained with the first prototype at the CELINA 14MeV neutron source facility at CEA-Cadarache are presented. The future developments of operational Piccolo-Micromegas for fast neutron reactors are also described.

Performances of Anode-resistive Micromegas for HL-LHC

Micromegas technology is a promising candidate to replace Atlas forward muon chambers -tracking and trigger- for future HL-LHC upgrade of the experiment. The LHC accelerator luminosity will be ten times the nominal one, increasing background and pile-up event probability in the same proportion. This requires detector performances which are currently under studies in intensive RD activities. We studied performances of four different resistive Micromegas detectors and with different read-out strip pitches. These chambers were tested using ~ 120 GeV momentum pions with rates from 25 up to 250 kHz/cm2, at H6 CERN-SPS beam line in autumn 2010. We found that the resolution is degraded, if the strip pitch is too wide with respect to the charge distribution at the readout plane. To reduce the systematic effects of the charge sharing we propose a cluster reconstruction algorithm. For narrow strip pitch 500μm we measure a resolution of ~ 90 μm and a efficiency of ~ 98%. The track angle effect on the efficiency was also studied. Our results show that resistive techniques induce no degradation on the efficiency or resolution, with respect to the standard Micromegas. In some configuration the resistive coating is able to reduce the discharge currents at least by a factor of 100 and no HV breakdown was observed.

An ageing study of resistive micromegas for the HL-LHC environment

Resistive-anode micromegas detectors have been in development for several years, in an effort to solve the problem of sparks when working at high flux and high ionizing radiation like in the HL-LHC (up to ten times the luminosity of the LHC). They have been chosen as one of the technologies that will be used in the ATLAS New Small Wheel project (forward muon system). An ageing study is mandatory to assess their capabilities to handle the HL-LHC environment on a long-term period. A prototype has been exposed to several types of irradiation (X-rays, cold neutrons, 60Co gammas and alphas) above the equivalent charge produced in the detector in five HL-LHC running years without showing any degradation of the performance in terms of gain and energy resolution, and with the characterization of the tracking performance in terms of efficiency and spatial resolution, verifying non degradation on the exposed resistive micromegas.

Aging studies of Micromegas prototypes for the HL-LHC

The micromegas technology is a promising candidate to replace the forward muon chambers for the luminosity upgrade of ATLAS. The LHC accelerator luminosity will be five times the nominal one, increasing background and pile-up event probability. This requires detector performances which are currently under study in intensive R&D activities. Aging is one of the key issues for a high-luminosity LHC application. For this reason, we study the properties of resistive micromegas detectors under intense X-ray radiation and under thermal neutrons in different CEA-Saclay facilities. This study is complementary to those already performed using fast neutrons.

New pixelized Micromegas detector for the COMPASS experiment

New Micromegas (Micro-mesh gaseous detectors) are being developed in view of the future physics projects planned by the COMPASS collaboration at CERN. Several major upgrades compared to present detectors are being studied: detectors standing five times higher luminosity with hadron beams, detection of beam particles (flux up to a few hundred of kHz/mm2, 10 times larger than for the present detectors) with pixelized read-out in the central part, light and integrated electronics, and improved robustness. Studies were done with the present detectors moved in the beam, and two first pixelized prototypes are being tested with muon and hadron beams in real conditions at COMPASS. We present here this new project and report on two series of tests, with old detectors moved into the beam and with pixelized prototypes operated in real data taking condition with both muon and hadron beams.

Large bulk-micromegas detectors for TPC applications in HEP

Bulk-MicroMegas detectors are based on a novel technology which is of particular interest for large area Micro Pattern Gas Detectors (MPGD). Their manufacturing process combines detector construction simplicity and robustness, allowing large sensitive areas to be produced at low cost. Such devices provide very good gas gain uniformity and detection coverage with small dead spaces. They are ideally suited in HEP applications where large volume tracking detectors with low material budget are required. This is the case, for instance, of the T2K neutrino experiment in Japan, in which large Time Proportional Chamber (TPC) devices will be used. Bulk- MicroMegas detectors are also being considered for the future Linear Collider Detector. We present in this paper the recent developments and performance of bulk-MicroMegas detectors for the T2K TPC.