Subatomic Physics

66 Detectors
Figure 4.15: Cross section through a multi-wire proportional counter. Typical dimensions are
l =8mm,s= 2 mm. Field lines are shown for two wires.
a separate amplifier and pulse shaper; the output pulse indicates position and time
of the particle.
4.7 Drift Chambers
Drift chambers (7) are like wire detectors, but can provide much better spatial resolution
(≤ 200µm) at lower cost because fewer wires are required. Drift chambers
use a low electric field (∼ 1keV/cm) to make electrons drift to one or more anode
wires. To produce a relatively constant electric field strength, potential wires are
introduced between neighboring anode wires. Close to the anode wires, the electric
field gets very large and an avalanche results. The drift time is used to define the
position of the particle. The drift velocity is given by
vD = eτE
2m
, (4.11)
where e is the charge of the particle, τ is the mean collision time, E is the electric
field intensity, and m is the mass of the particle. The distance traversed to reach
the avalanche region is
∆x =
� t1
vDdt , (4.12)
t0
where t0 is the creation time and t1 is the arrival time of the electron. For electrons,
for which the chamber is most useful, the approximate drift speed is about
50 mm/µs and the drift distance of the order of 5-10 cm.
7W. Blum and L. Rolandi, Particle Detection with Drift Chambers, Springer Verlag, New York,
1993.