Comparison between Various Designs of Micropattern Detectors

In this chapter a comparison between various designs of micropattern detectors is given, describing their specific advantages and disadvantages, which finally determines the fields of their applications. It is shown that at low counting rates the maximum achievable gas gain is determined by the Raether limit, which is about 106-107 electrons, depending on the design. At high counting rates, the maximum achievable gain additionally drops due to the contribution of several other effects (e.g. avalanches overlapping in space and time). Typically, micropattern detectors have a position resolution of ~30 µm, energy resolution of ~ 20% FWHM for 6 keV X-rays, and a time resolutions of ~1 ns. Some advanced designs offer even better characteristics. The diversity of micropattern detectors makes them attractive for many applications. For example, in measurements requiring simultaneously excellent time and position resolutions, mutigap multistrip detectors can be used in high energy physics applications, and hole-type structures are advantageous for the detection of visible photons. In some commercial applications, where reliability and robustness are important, spark protected detectors with resistive electrodes could be useful.

Recent Developments of Micropattern Detectors

In this chapter, the exciting developments in micropattern detectors in recent years are described. This includes GEM and MICROMEGAS detectors combined with micropixel readout, some peculiar designs of GEM and GEM-like detectors sensitive to UV and visible light, large area (>1m2) GEM and MICROMEGAS prototypes developed for the upgrades of the experiments at the large hadron collider, etc. A special focus is put on a new generation of spark-proof micropattern detectors, using resistive electrodes instead of traditional metallic ones. These detectors operate as ordinary micropattern detectors. However, in the case of occasional sparks, their current is limited by the resistivity of the electrodes so that the energy of the discharge is reduced by several orders of magnitude. Various designs of such detectors have been developed and successfully tested, including resistive GEM, resistive MICROMEGAS, resistive MSGC, etc. Among this family of detectors, a special place belongs to resistive parallel-plate micropattern detectors allowing one to achieve at the same time excellent spatial (38 µm) and time (77 ps) resolutions. Finally, the potential of multilayer detector technology for further optimization of the detector operation is discussed.

Some Interesting Work on Capillary Glass Detectors

In this chapter, the progress of the development of glass capillary plates is described. In some applications, capillary plates have advantages over GEM or other gaseous detectors. For example, they are compatible with vacuum technology allowing them to be used in sealed gaseous detectors. Prototypes of capillary plates combined with photocathodes sensitive to ultraviolet and visible light were the first to be developed and successfully tested. These detectors resemble vacuum imaging microchannel plates, widely used in many applications. However, the glass capillary plates operate in gas atmosphere and in avalanche mode. This offers a possibility to build large area position-sensitive photomultipliers since at atmospheric pressure there are no serious mechanical constrains on the window. Since glass has a high density, the capillary plate can also be used as efficient convertors of X-rays, and be used at the same time as a multiplication structure for the created primary electrons. Such a device is attractive for X-ray and gamma ray imaging and the first successful tests of a prototype of such a detector are described.

Microstrip Gas Counters

In this chapter, the first micropattern gaseous detector, the microstrip gas counter, invented in 1988 by A. Oed, is presented. It consists of alternating anode and cathode strips with a pitch of less than 1 mm created on a glass surface. It can be considered a two-dimensional version of a multiwire proportional chamber. This was the first time microelectronic technology was applied to manufacturing of gaseous detectors. This pioneering work offers new possibilities for large area planar detectors with small gaps between the anode and the cathode electrodes (less than 0.1 mm). Initially, this detector suffered from several serious problems, such as charging up of the substrate, discharges which destroyed the thin anode strips, etc. However, by efforts of the international RD28 collaboration hosted by CERN, most of them were solved. Although nowadays this detector has very limited applications, its importance was that it triggered a chain of similar developments made by various groups, and these collective efforts finally led to the creation of a new generation of gaseous detectors-micropattern detectors.

MICROMEGAS

This chapter describes another type of micropattern detector invented in 1995 by G. Charpak and collaborators, a micromesh gaseous detector. This detector is in fact a parallel-plate avalanche counter with a very small gap (50-100 µm) between a special cathode mesh and the anode plate. This feature offers excellent position resolution, down to 30 µm in conventional gas mixtures and close to 14 µm in some special gas mixture in which diffusion of electrons is very low. Initially, small prototypes of MICROMEGAS were made by hand. Later, microelectronic technology was used in their manufacturing, allowing the building of individual modules with active areas up to 40x40 cm2. The results from detailed studies of maximum achievable gain, rate characteristics, time-, position-, and energy resolutions of this detector are presented in this chapter, as well as a comparison with classical parallel-plate avalanche counters. Nowadays, this detector conquers more and more applications in high-energy physics and other applications.

Other Early Designs of Micropattern Detectors Developed Between 1998 and 2003

This chapter focuses on the intense developments of micropattern detectors that happened between 1998 and 2003. In this period, many new designs were invented and manufactured by means of a photolithographic technology. These detectors include microwire detectors, microslit detectors, LEAK multiplication structures, microgap parallel-plate chambers, micro-hole strip plate gaseous detectors, etc. Some of them remain simply as interesting exercises demonstrating the great capability of microelectronic technique. Some of them are used in practice, for example in 2 dimensional and 3 dimensional mammographic scanners. These scanners are based on microgap parallel-plate chambers and give high quality X-ray images at a reduced radiation dose delivered to the patients. Early versions of the LEAK detector were intensively used in plasma diagnostics. Micro-hole strip plate gaseous detectors are currently used in some prototypes of photodetectors. This chapter also describes an MSGC type MWPC invented by Charpak et al. in an attempt to overcome the problems associate with the MSGC (i.e. charging up effects and poor rate characteristics).

Pixel and Microdot Detectors

This chapter describes pixel, microdot, and micropixel detectors. Their invention was inspired by A. Oed's work on MSGCs. In these detectors avalanche multiplication occurs near small anode dots/pixels instead of near anode strips. This naturally segments the detector area into independent active cells, or pixels, which can be attractive in some applications. For two-dimensional position measurements, each anode and cathode row is connected to the readout line. These readout lines are placed perpendicular to the anode/cathode rows. If necessary, each pixel can be connected to its own preamplifier. One of the advantages with this pixel geometry is that it allows gas gains that are ten times higher than what is achievable with MSGCs. This is due to that the electric field lines near the anode dots are radial, which is favorable for quenching surface streamers. Although up to now the microdot and micropin detectors remain in a stage of laboratory prototypes, this interesting concept recently gained new momentum after the development of microdot detectors with resistive electrodes. These innovative detectors satisfy the requirements of some application such as noble liquid Time Projection Chambers.

Applications

In this chapter, some applications of micropattern detectors are described. Their main application is tracking of charged particles in high-energy physics. However, currently there are a lot of research and developments going on, which may open new exciting fields of applications, for example in dark matter search, medical applications, homeland security, etc. The authors start with the traditional applications, which are in high-energy physics and astrophysics. Later, the focus shifts to promising developments oriented towards new applications. These innovative applications include: imaging of charged particles and energetic photons with unprecedented high 2-D spatial resolution (e.g. in mammography), time projection chambers capable operating in a high flux of particles (e.g. ALICE upgraded TPC), and visualization of ultraviolet and visible photons. Finally, a short description of the international collaboration RD51 established at CERN is given in order to promote the development of micropattern detectors and their applications.

Operation of Micropattern Gaseous Detectors

This chapter is dedicated to the physic of the operation of micropattern detectors. The authors analyze in more detail what causes discharges in these detectors. The chapter shows that, at low counting rates, the breakdowns appear due to the Raether limit and in some specific cases due to surface streamers. In some particular detectors (e.g. combined with high-efficient photocathodes or operating in very clean noble gases) the discharges may appear via a feedback mechanism. At high counting rates, the maximum achievable gain drops with the counting rate due to avalanches overlapping in space and time, and also due to a contribution from explosive electron emission. Detailed studies of the problems that micropattern detectors, in particular GEM, may experience while operating in cascade mode are presented. A better understanding of these effects has allowed researches to make a further step in the development of micropattern gaseous detectors in recent years.

The Capability of Key Micropattern Detectors to Suppress Ion Back Flow

In this chapter, the authors discuss the unique ability of some micropattern detectors, in particular, GEM, MHSP, COBRA, and MICROMEGAS, to suppress the positive ion flow from the multiplication region of the detector back to the drift space. This effect is based on how electrons and ions move in these detectors. It will be shown that an optimized cascaded detector can so efficiently block the ion back flow that only 10-3% of the avalanche ions can penetrate back into the drift region. This feature makes these detectors attractive for applications such as photodetectors combined with highly efficient solid photocathodes or time projection chambers for tracking high fluxes of charged particles when the penetration of the avalanche ions back to the drift region may strongly disturb the detector operation. For example, in the case of photodetectors, the ions cause undesirable feedback preventing high gain operation necessary for single photoelectron detection. In the case of the time projection chambers, positive ions may disturb the uniformity of drift filed and thus affect the particle identification.