A particle detector is used to detect a passing particle and obtain information such as its position, arrival time and momentum. In experimental physics, the particle is usually coming from a particle accelerator but it can also come from space or from a nuclear reactor. The Micromegas detector detects particles by amplifying the charges that have been created by ionisation in the gas volume. In a Micromegas detector, this gas volume is divided in two by a metallic micro-mesh placed between 25 μm and 150 μm of the readout electrode. The micro-mesh is the key element since it allows, at the same time, a high gain of 104 and a fast signal of 100 ns.
While passing through the detector, a particle will ionise the gas atoms by pulling up an electron creating an electron/ion pair. When no electric field is applied, the ion/electron pair recombines and nothing happens. But here, within an electric field in the order of 400 V/cm the electron will drift toward the amplification electrode and the ion toward the cathode. When the electron arrives close to the mesh, it enters an intense electric field. Accelerated by this field, the electron reaches enough energy to produce ion/electron pairs that will also ionise the gas, creating pairs; it is the avalanche effect. By this means, several thousand pairs are created from hundreds of primary charges, which originate from the interactions with the impinging particle. The primary charges need to be multiplied to create a significant signal. A last, we read the electronic signal on the readout electrode by a charge amplifier. The readout electrode is usually segmented in strips and/or pixels in order to obtain the position of the impinging particle in the detector. The amplitude and the shape of the signal, read via the electronic on the readout electrode, gives information on the time and energy of the particle.
The signal is induced by the movement of charges between the micro-mesh and the readout electrode. The 100 nanoseconds signal consists of an electron peak and an ion tail. Since the electron mobility in gas is over 1000 times faster than the ion mobility, the electronic signal is much shorter than the ionic one. That is why it is used to measure precisely the time. The ionic signal carries more than half of the signal and is used to reconstruct the energy of the particle.
In 1991, to improve the detection of hadrons at the Hadron Blind Detector experiment, I. Giomataris and G. Charpak reduced the amplification gap of a parallel plate detector in order to speed up the signal. A 1 mm amplification gap prototype was built for the HDB experiment but the gain was not uniform enough to be used in the experiment. The millimetre gap was not enough controlled and created large gain fluctuations. Nevertheless, the benefits of a reduce amplification gap has been demonstrated and the Micro-Mesh gaseous structure or Micromegas concept was born in October 1992, shortly before the announcement of the Nobel prize attribution to Georges Charpak for the invention of the wire chambers. Georges Charpak used to say that this detector and some other new concepts belonging to the family of micro-pattern gaseous detectors will revolutionise nuclear and particle physics just as his detector did.
Starting in 1992 at CEA Saclay and CERN, the Micromegas technology has been developed to provide more stable, reliable, precise and faster detectors. In 2001, twelve large Micromegas detectors plane of 40 x 40 cm2 were used for the first time in a large scale experiment at COMPASS situated on the Super Proton Synchrotron accelerator at CERN. Since 2002 they have been detecting millions of different particles per seconds and still continue today. Another example of the development of the Micromegas detectors is the invention of the “bulk” technology. The “bulk” technology consists of the integration of the micro-mesh with the printed circuit board in order to build a monolithic detector. Such a detector is very robust and can be produced within an industrial process allowing public applications. For instance, by modifying the micro-mesh in order to make it photo-sensitive to UV light, the Micromegas can be used to prevent forest fires.
Micromegas detectors in experimental physics
The Micromegas detectors are now used in several experiments :
Hadronic physics: COMPASS, NA48, and projects for the ILC-TPC and CLAS12 at J-lab are under active study
