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

248 28. Detectors at accelerators
When an ionizing particle passes through the gas, it creates electron-ion
pairs, but often the ejected electrons have sufficient energy to further ionize
the medium. As shown in Table 28.5, the total number of electron-ion
pairs (NT ) is usually a few times larger than the number of primaries
(NP ).
The probability for a released electron to have an energy E or larger
follows an approximate 1/E2 dependence (Rutherford law), taking into
account the electronic structure of the medium. The number of electronion
pairs per primary ionization, or cluster size, has an exponentially
decreasing probability; for argon, there is about 1% probability for primary
clusters to contain ten or more electron-ion pairs [59].
Once released in the gas, and under the influence of an applied electric
field, electrons and ions drift in opposite directions and diffuse towards
the electrodes. The drift velocity and diffusion of electrons depend very
strongly on the nature of the gas. Large drift velocities are achieved by
adding polyatomic gases (usually CH4, CO2, orCF4) having large inelastic
cross sections at moderate energies, which results in “cooling” electrons
into the energy range of the Ramsauer-Townsend minimum (at ∼ 0.5 eV)
of the elastic cross-section of argon. In a simple approximation, gas kinetic
theory provides the drift velocity v as a function of the mean collision
time τ and the electric field E: v = eEτ/me (Townsend’s expression). In
the presence of an external magnetic field, the Lorentz force acting on
electrons between collisions deflects the drifting electrons and modifies the
drift properties.
If the electric field is increased sufficiently, electrons gain enough energy
between collisions to ionize molecules. Above a gas-dependent threshold,
the mean free path for ionization, λi, decreases exponentially with the field;
its inverse, α =1/λi, is the first Townsend coefficient. In wire chambers,
most of the increase of avalanche particle density occurs very close to
the anode wires, and a simple electrostatic consideration shows that the
largest fraction of the detected signal is due to the motion of positive
ions receding from the wires. The electron component, although very fast,
contributes very little to the signal. This determines the characteristic
shape of the detected signals in the proportional mode: a fast rise followed
by a gradual increase.
28.6.2. Multi-Wire Proportional and Drift Chambers : Revised
March 2010 by Fabio Sauli (CERN) and Maxim Titov (CEA Saclay).
Multiwire proportional chambers (MWPCs) [65,66], introduced in the
late ’60’s, detect, localize and measure energy deposit by charged particles
over large areas. A mesh of parallel anode wires at a suitable potential,
inserted between two cathodes, acts almost as a set of independent
proportional counters. Electrons released in the gas volume drift towards
the anodes and produce avalanches in the increasing field.
Detection of charge on the wires over a predefined threshold provides
the transverse coordinate to the wire with an accuracy comparable to that
of the wire spacing. The coordinate along each wire can be obtained by
measuring the ratio of collected charge at the two ends of resistive wires.
Making use of the charge profile induced on segmented cathodes, the
so-called center-of gravity (COG) method, permits localization of tracks
to sub-mm accuracy.
Drift chambers, developed in the early ’70’s, can be used to estimate the
longitudinal position of a track by exploiting the arrival time of electrons
at the anodes if the time of interaction is known [69]. The distance
between anode wires is usually several cm, allowing coverage of large areas
at reduced cost.