F. Kunne

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.

Development of a fast gaseous detector : "micromegas"

Abstract Several 15×15 cm 2 gaseous Micromegas chambers (MICROMEsh GAseous Structure) which consist of a conversion gap and an amplification gap separated by a thin grid have been extensively tested in low-intensity 10 GeV /c pion beam and high-intensity (up to 5×10 5 Hz / mm 2 ) 100 GeV /c muon beam. The detector behaviour has been studied with respect to many parameters: conversion gaps of 1 and 3xa0mm, amplification gaps of 50 and 10xa0μm, an external magnetic field and many different filling gases. So far no effect of the magnetic field up to 1.3xa0T has been observed. The gas mixture argon + cyclohexane appears to be very suitable with gains above 10 5 and a full-efficiency plateau of 50xa0V at 340xa0V. With a conversion gap as small as 1xa0mm and an electronics with a threshold at 5000 electrons the efficiency reaches 96%. With the addition of CF 4 a time resolution of 5xa0ns (RMS) has been obtained. A spatial resolution better than 60 μ m has been observed with anode strips of 317 μ m pitch and was explained by transverse diffusion in the gas. Simulations show that with a pitch of 100 μ m and the appropriate gas a resolution of 10 μ m is within reach. This development leads to a new generation of cheap position-sensitive detectors which would permit high-precision tracking or vertexing close to the interaction region, in very high-rate environments.

Fast photon detection for particle identification with COMPASS RICH-1

Particle identification at high rates is an important challenge for many current and future high-energy physics experiments. The upgrade of the COMPASS RICH-1 detector requires a new technique for Cherenkov photon detection at count rates of several

Pattern recognition and PID for COMPASS RICH-1

10^6

Micromegas, a microstrip detector for Compass

per channel in the central detector region, and a read-out system allowing for trigger rates of up to 100 kHz. To cope with these requirements, the photon detectors in the central region have been replaced with the detection system described in this paper. In the peripheral regions, the existing multi-wire proportional chambers with CsI photocathode are now read out via a new system employing APV pre-amplifiers and flash ADC chips. The new detection system consists of multi-anode photomultiplier tubes (MAPMT) and fast read-out electronics based on the MAD4 discriminator and the F1-TDC chip. The RICH-1 is in operation in its upgraded version for the 2006 CERN SPS run. We present the photon detection design, constructive aspects and the first Cherenkov light in the detector.

The fast readout system for the MAPMTs of COMPASS RICH-1

Abstract A package for pattern recognition and PID by COMPASS RICH-1 has been developed and used for the analysis of COMPASS data collected in the years 2002–2004, and 2006–2007 with the upgraded RICH-1 photon detectors. It has allowed the full characterization of the detector in the starting version and in the upgraded one as well as the PID for physics results. We report about the package structure and algorithms, and the detector characterization and PID results.

New pixelized Micromegas detector for the COMPASS experiment

Abstract Recent results obtained with a gaseous microstrip detector Micromegas developed for the tracking in the high-rate environment of the COMPASS experiment at CERN are presented. A 26×36 cm 2 prototype equipped with the low-noise preamplifier SFE16 was tested in a high-energy hadron beam at CERN. With a gas mixture based on neon, the full efficiency of the Micromegas prototype is obtained at a gain of 6400; the spatial resolution is 50xa0μm and the time jitter 8.5xa0ns. We have studied the problem of discharges that affect this kind of microstrip detector, and found that, at fixed gain, the probability of discharge is higher for heavier gas mixtures in the detector. In the conditions of the COMPASS experiment, discharges are kept at a low rate.

Long term experience and performance of COMPASS RICH-1

A fast readout system for the upgrade of the COMPASS RICH detector has been developed and successfully used for data taking in 2006 and 2007. The new readout system for the multi-anode PMTs in the central part of the photon detector of the RICH is based on the high-sensitivity MAD4 preamplifier-discriminator and the dead-time free F1-TDC chip characterized by high-resolution. The readout electronics has been designed taking into account the high photon flux in the central part of the detector and the requirement to run at high trigger rates of up to 100 kHz with negligible dead-time. The system is designed as a very compact setup and is mounted directly behind the multi-anode photomultipliers. The data are digitized on the front-end boards and transferred via optical links to the readout system. The readout electronics system is described in detail together with its measured performances.

Micromegas: Large-Size High-Rate Trackers in the High Energy Experiment COMPASS

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.

Tracking with 40×40 cm2 MICROMEGAS detectors in the high energy, high luminosity COMPASS experiment

COMPASS RICH-1 is a large size gaseous Imaging Cherenkov Detector providing hadron identification in the range from 3 to 55 GeV/c, in the wide acceptance spectrometer of the COMPASS Experiment at CERN SPS. It uses a 3 m long C4F10 radiator, a 21 m2 large VUV mirror surface and two kinds of photon detectors: MAPMTs and MWPCs with CsI photocathodes, covering a total of 5.5 m2. It is in operation since 2002 and its performance increased thanks to progressive optimization and to a major upgrade of its photon detection system, implemented in 2006; a new upgrade is foreseen for 2016, with the use of MPGD-based photon detectors. The main characteristics of COMPASS RICH-1 components are described and the most critical aspects related to the C4F10 radiator gas system, to the mirrors and their alignment, as well as the performance of the photon detectors are presented and discussed. The response of the MWPCs and the observed evolution of the effective quantum efficiency of the CsI photocathodes is analyzed. The properties and performance of the MAPMTs with individual fused lens telescopes are presented together with the readout characteristics. The PID performance of COMPASS RICH-1 is discussed and the future upgrade program is mentioned.