Particle Physics Booklet - Particle Data Group - Lawrence Berkeley ...

28. Detectors at accelerators 249
28.6.4. Micro-Pattern Gas Detectors : Revised March 2010 by
Fabio Sauli (CERN) and Maxim Titov (CEA Saclay)
By using pitch size of a few hundred μm, an order of magnitude
improvement in granularity over wire chambers, these detectors offer
intrinsic high rate capability (> 106 Hz/mm2 ), excellent spatial resolution
(∼ 30 μm), multi-particle resolution (∼ 500 μm), and single
photo-electron time resolution in the ns range.
The Gas Electron Multiplier (GEM) detector consists of a thin-foil
copper-insulator-copper sandwich chemically perforated to obtain a high
density of holes in which avalanches occur [86]. The hole diameter is
typically between 25 μm and 150 μm, while the corresponding distance
between holes varies between 50 μm and 200 μm. The central insulator
is usually (in the original design) the polymer Kapton, with a thickness
of 50 μm. Application of a potential difference between the two sides of
the GEM generates the electric fields. Each hole acts as an independent
proportional counter. Electrons released by the primary ionization particle
in the upper conversion region (above the GEM foil) drift into the holes,
where charge multiplication occurs in the high electric field (50–70 kV/cm).
Most of avalanche electrons are transferred into the gap below the GEM.
Several GEM foils can be cascaded, allowing the multi-layer GEM
detectors to operate at overall gas gain above 104 in the presence of highly
ionizing particles, while strongly reducing the risk of discharges.
The micro-mesh gaseous structure (Micromegas) is a thin parallel-plate
avalanche counter. It consists of a drift region and a narrow multiplication
gap (25–150 μm) between a thin metal grid (micromesh) and the readout
electrode (strips or pads of conductor printed on an insulator board).
Electrons from the primary ionization drift through the holes of the mesh
into the narrow multiplication gap, where they are amplified. The small
amplification gap produces a narrow avalanche, giving rise to excellent
spatial resolution: 12 μm accuracy, limited by the micro-mesh pitch, has
been achieved for MIPs, as well as very good time resolution and energy
resolution (∼ 12% FWHM with 6 keV x rays) [89].
The performance and robustness of GEM and Micromegas have
encouraged their use in high-energy and nuclear physics, UV and visible
photon detection, astroparticle and neutrino physics, neutron detection
and medical physics.
28.6.5. Time-projection chambers : Written September 2007 by D.
Karlen (U. of Victoria and TRIUMF, Canada)
The Time Projection Chamber (TPC) concept, invented by David
Nygren in the late 1970’s [74], is the basis for charged particle tracking
in a large number of particle and nuclear physics experiments. A uniform
electric field drifts tracks of electrons produced by charged particles
traversing a medium, either gas or liquid, towards a surface segmented into
2D readout pads. The signal amplitudes and arrival times are recorded to
provide full 3D measurements of the particle trajectories. The intrinsic 3D
segmentation gives the TPC a distinct advantage over other large volume
tracking detector designs which record information only in a 2D projection
with less overall segmentation, particularly for pattern recognition in
events with large numbers of particles.
Gaseous TPC’s are often designed to operate within a strong magnetic
field (typically parallel to the drift field) so that particle momenta can
be estimated from the track curvature. For this application, precise
spatial measurements in the plane transverse to the magnetic field are
most important. Since the amount of ionization along the length of the
track depends on the velocity of the particle, ionization and momentum