Micropatterns in particle physics
Highly ionizing particles produce discharges in all micropattern detectors with a typical gain of several thousand. It is possible to obtain higher gains with gases that permit a lower operating voltage and have higher diffusion, thus lowering the charge density and photon feedback probability. Combining the MSGC with a GEM, reliable operation has been demonstrated up to gains of several 104. Some 200 large detectors are operating at HERA-B at DESY for inner tracking. The DIRAC experiment at the CERN PS also employs MSGC + GEM detectors, which improved momentum resolution by a factor of two.
Two GEMs in tandem provide a robust detector. Large ones are being built for CERN's COMPASS experiment. Adding a third GEM (e.g. the possible LHCb tracker at CERN) offers even more stable operation in a hostile beam environment. At gains of 104, spark probabilities as low as 10-10 have been measured.
For experiments at the future TESLA linear collider, a double or triple GEM configuration is under consideration owing to its fast electron signal, minimal magnetic distortion effects and suppression of ion feedback. Special hexagonal pads are being developed, aiming at unprecedented 50-60 µm resolution in a time projection chamber using charge sharing and induction signals.
A feasibility study aimed at improving the detection of Cherenkov light has been carried out at SLAC, and a cascade of four GEMs in pure ethane has given very high gains.
Beyond high-energy physics
Conventional film radiography has very good spatial resolution but limited dynamic range. For film, the storage and display media are the same, and the film image saturates (additional photons do not cause proportional film darkening). The display contrast is thus fixed at the time of film exposure, and one sees little difference in visible contrast in different parts of a film image that receive widely different signals. On the other hand, in a digital system, the storage medium (computer) does not saturate and has infinite dynamic range. The display media being different from storage, the display contrast can be varied at will.
Digital scanned projection radiography thus offers improved range and adjustable contrast. The image can be enhanced using photon energy information. A GEM + MSGC combination operating in xenon/methane at 4 atm yields good diagnostic X-ray images. Specialized two-dimensional read-out boards have been made with the GEM technology for use in digital absorption radiography (figure 3).
X-ray diffraction studies using MSGCs have yielded the rapid analysis of single-crystal structures using both position and time information from the incident X-rays. Crystal structures of organic molecules can be obtained in a matter of minutes. Fast time-resolved X-ray diffraction measurements offer a time variation of the small angle X-ray scattering pattern of a protein solution within 10 ms. MICROMEGAS detectors have been developed for X-ray imaging (figure 4).
A combination of an X-ray converter, an MSGC and a visible photocathode shows great promise for use in digital mammography. The essential features are a large, flat area and high resolution. With a photocathode (that is sensitive to both ultraviolet and visible light) coupled to a micropattern detector, sealed gas avalanche photomultipliers are being developed for fast imaging, flat read-out devices for scintillator and scintillating fibre arrays, and medical imaging.
Here, single photon detection has been actively pursued. With low preamplification in the drift region, combined with high diffusion, fully efficient single photon detection is possible. Using a CsI photocathode coupled to three or four GEMs in tandem, large gains (105) have been obtained in pure argon, and even larger (106) with an admixture of few percent of methane (figure 5).
Scintillation light and X-rays
With a GEM as amplifier and a CCD camera, X-ray images of individual alpha tracks have been seen via scintillation in argon and CF4 (figure 6). The spectral distribution of the emitted light is analysed in terms of the number of photons emitted per electron in the visible and near-infrared regions (wavelength range 400-1000 nm). The maximum number of emitted photons decreases with pressure.
In X-ray astronomy, measurements of X-ray polarization are useful to investigate features of magnetic fields in pulsars, nebulae, etc. X-ray polarimeters have been developed using glass capillary proportional counters (GCPs) and GEMs.
The emission direction of the primary electron depends on the polarization of the incident X-rays, so information on polarization can be extracted. Polarimeter performance is expressed as a function of detection efficiency and modulation, and hence gas pressure and gas depth.
Exploiting the time resolution and the selective GEM sensitivity to soft X-rays, imaging the dynamics of fusion plasmas has been attempted by a Frascati-Pisa group for the Frascati Tokamak Upgrade. With a GEM and individual pixel read-out, time-resolved plasma diagnostics give information on temperature and turbulence.
Gaseous chambers have matured over the past few decades, resulting in several applications in particle physics and diagnostics. The past 10 years in particular have seen novel developments in micropattern gaseous detectors. Our understanding of the discharge mechanisms in these devices has also increased, leading to design improvements. Progress in rate capability and radiation tolerance has revolutionized the potential applications of these detectors in radiology, diagnostics, astrophysics and other fields. Micropatterns are thus assured of a big future.