Micromegas, a multipurpose gaseous detector

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Abstract

In this paper the advantages of a new gaseous detector. Micromegas, are described. Some of its properties are given and the consequences are out-lined in a series of experiments related to charged particle localization, neutron imaging, gamma or X-ray imaging.

Introduction

The Vienna Conference is a favorable opportunity for appreciating the state of the art in the development of gaseous detectors. Ten years of intensive research aiming at matching the needs of high luminosity colliders has helped in the emergence of more than a dozen of new detectors. Some have similar structures, some rely on different approaches. While the choice of the detectors for the internal trackers for LHC is now a closed matter, with the selection of the semi-conductor detectors, chosen mostly out of reasons of prudence and not cost, the diversified research has resulted in new instruments which can be of decisive importance in many fields: physics, astrophysics, biology, medicine.

We would like to bring the attention of the physics community to a new gaseous detector, Micromegas [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. The research effort started in Saclay, with various steps pioneered in Nantes, College de France, Lausanne, Mulhouse [15], [16], [17], [18], [19] and at Biospace, a firm investigating the applications of Micromegas in medicine and biology.

The design of Micromegas is offering, for several applications, substantial advantages in energy, space and time resolution, microscopic granularity on large surfaces, insensitivity to discharges, simplicity of construction and capacity to identify and reduce some sources of background, which can be of great interest in the search for rare events. We will describe some of the properties of this structure and their advantageous consequences in a series of experiments related to charged particle localization, neutron imaging, gamma or X-ray imaging. We refer the reader to more detailed publications and wish to give here a general view of the possible impact of the Micromegas detector.

Section snippets

Principle

Micromegas (Fig. 1) is a gaseous parallel plate detector in which several innovative properties rely on a narrow amplification space, typically 50–100μm, between two parallel electrodes, the cathode and anode conducting plates [1]. The cathode is made of a thin metallic micromesh, few microns thick, the anode microelements (strips or pads) of a conductor, printed on a insulator board. The technological challenge in such a detector is to keep the small gap constant over the active area. The

Charged particle detection

For the tracking of charged particles in High Energy Physics, a spatial resolution as good as possible is needed, often in a high-rate environment for which an occupancy of the detector as low as possible is also needed. The conversion gap appears then to be the main source of degradation of the detector performances.

Several groups, using various Micromegas configurations in terms of strip pitch, amplification gap and operating gas, have investigated the space resolution of the detector, with a

Low background applications

Some of the features specific to the Micromegas detectors can be exploited in the search for rare-events in neutrino and astroparticle physics [10]. The goal is to detect sporadic signals from a large amount of competing backgrounds. In particular, in the CAST experiment at Cern [23], the expected signal comes from solar axions conversion into detectable low-energy photons in the keV energy range. Background rejection will be essential to achieve the intended sensitivity. A Micromegas detector

Neutrons detection

The use of Micromegas as a neutron profiler, with high transparency and low efficiency, has been demonstrated on a neutron beam [12]. The converter was a thin solid target (6Li or 10B) deposited on the drift electrode with detection of produced ions and proton recoils in the gas. A high rejection of background gamma rays has been observed. A space resolution better than 400μm was obtained with a 3mm drift gap and charge preamplifiers. The efficiency is limited by the converters. For the

Imaging of photons from visible light to high-energy gammas

The main problem is the high-efficiency conversion of the radiations to free electrons. The converters can be solid or gaseous. They have to be compatible with the amplifying gaseous filling and be free of the secondary effects due to photon or ions produced in the electron avalanches. The qualities displayed by Micromegas in position, time and energy resolution would permit various improvements in the detection of electromagnetic radiations. The considerable variability in gas mixtures or

Conclusions

Micromegas is a gaseous detector based on amplification of electron avalanches in short gaps of the order of 100μm at atmospheric pressure. It has specific properties of stability with respect to the gap length or gas pressure connected with the region of electric field at which it operates. It combines a good energy resolution, 5.4% FWHM at 22keV X-rays, excellent position resolution that can reach 12μm, high rate capability well adapted to particle physics experiments. A simple

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