Subatomic Physics

4.6. Wire Chambers 65
voltage supply, and a high-voltage pulse (10–20 kV) is applied to the plates within
less than 50 ns. The resulting sparks are recorded on stereophotographs.
The standard spark chamber
arrangement of the type just
discussed has been used in
many experiments, and chambers
have been designed to
solve many problems. Thin
plates are employed if only the
direction of charged particles
is desired; thick lead plates
are used if gamma rays are
to be observed or if electrons
have to be distinguished from
muons. The electrons produce
showers in the lead plates and
can thus be recognized.
A
Incoming Incoming
particle particle
Helium-neon gas
High-voltage pulse
Logic
B
C
Outgoing
particle
Outgoing
particle
Figure 4.14: Spark chamber arrangement. The spark chamber
consists of an array of metal plates in a helium–neon
mixture. If the counter-and-logic system has decided that
a wanted event has occurred, a high-voltage pulse is sent
to alternate plates, and sparks are produced along the ionization
trails.
Spark chambers have been replaced in high-energy experiments by silicon semiconductor
detectors and by drift chambers, but they are still used in some experiments
because they are simple and inexpensive.
4.6 Wire Chambers
Bubble and spark chambers share one disadvantage: Events must be photographed
and then evaluated later. In experiments where a large amount of data is collected
this approach is cumbersome. (6)
Wire chambers (multi-wire proportional counters), pioneered by Charpak, avoid
this disadvantage. Wire chambers have very good time resolution, very good position
accuracy, and are self-triggered. Their use has spread from high-energy physics
to many other fields such as nuclear medicine, heavy ion astronomy, and protein
crystallography. A cross section through a wire chamber is sketched in Fig. 4.15. A
chamber may be a few m long and high. Tungsten wires of diameter 2a(≈ 20µm)
are stretched in one direction and a voltage of a few kV is applied between the
anode wires and the cathode surfaces. The resulting field lines are indicated for two
wires in Fig. 4.15. An ionizing particle passing through the chamber creates ion
pairs. Electrons produced close to the wire are accelerated towards the wire with an
energy sufficient to produce additional pairs and an avalanche results which leads
to a negative pulse on the wire. In many wire chambers, each wire is connected to
6 G. Charpak and F. Sauli, Annu. Rev. Nucl. Part. Sci. 34, 285 (1984).