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Particle Accelerators and Detectors


Detectors

Cloud Chamber The detection of high-energy particle or photon started with its darkening effect on photographic plates. Other methods for detecting radiation were soon developed, including scintillation techniques, electroscopes and the Geiger counters. Then a great breakthrough came in 1912 when the cloud chamber was invented. This device induces visible water droplets to form around the atoms, which have been ionized by the passage of the radiation through air. This provides a plan view of the path of the radiation and so gives a clear picture of what is happening. Figure 09 shows the three components of the cloud chamber - the radioactive source, the chamber, and the magnetic field, which is perpendicular to, and out of the plane of the picture. The magnetic field B, the velocity v, the radius of the

Figure 09 Cloud Chamber [view large image]

circular orbit R, the mass m, and the charge q are related by the formula: R = mv / Bq. If the direction of the initial velocity is not perpendicular to the field, the charged particle will move in a helix.
Bubble Chamber The bubble chamber used in high-energy accelerator experiments is the sophisticated variants of the cloud chamber. It is normally made by filling a large cylinder with a liquid just below its boiling point; at the top of the chamber a camera looks in. The whole chamber is subject to a constant magnetic field. As the particles enter the chamber, a piston suddenly decreases the pressure in the chamber. This brings the liquid to a superheated state, in which a tiny effect, such as the passing of a charged particle near an atom, is sufficient to nucleate a bubble of vaporized liquid. At this moment, the camera records the picture. Figure 10 shows the tracks of the charged particles in the

Figure 10 Bubble Chamber [view large image]

electroweak process + p - + p + + and the subsequent decay of + to +, and to e+. Since is a neutral particle. Although it is shown as a dotted line in the map, it leaves no track in the bubble chamber photograph.
Wire Counter Bubble chambers have largely been replaced by wire chambers, which allow particle energies to be measured at the same time. It consists of very large number of parallel wires (Figure 11a), where each wire acts as an individual detector similar to the proportional counter (the low voltage version of Geiger counter, such that there is no avalanche near the sense wire and thus the current is proportional to the number of electrons produced by the available energy of the incident particle). As in the Geiger counter, a particle leaves a trace of ions and electrons, which drift toward the case or the nearest wire, respectively. By marking

Figure 11a Wire Chamber
[view large image]

off the wires which had a pulse of current, one can see the particle's path. Several planes of wires with different orientations are used to determine the position of the particle very accurately. With the multiwire chamber the data handling capacity increased dramatically.
Drift Chamber A drift chamber has a similar construction, but with the wires in the central plane spaced farther apart. Varying voltages applied to the cathode wires produce a field in which ionization electrons drift at a constant velocity towards the nearest sense wire. The drift time, measured by an electronic "stopwatch" started by a signal from a scintillator, is directly related to the distance between the track of the particle and the wire that produces a signal. This greatly increases the accuracy of the path reconstruction. Figure 11b shows a schematic diagram of a drift chamber and the actual construction of the

Figure 11b Drift Chamber
[view large image]

wires that make up the drift chamber. See also bottom right diagram in Figure 11a.


The ALEPH installed in the now de-activated LEP at CERN is a good example to illustrate the construction of the modern particle detector. The various parts are shown in Figures 11c, 12, 13 and 14a with a brief description in the followings:
Detection Display Figure 15 shows an example of particles generated after a collision in diagram a, and the actual display observed in diagram b:
1. Magnetic field bends the low and medium energy charged particles into curved trajectories.
2. The filled-in dots represent the charged particles detected by the ionization detectors.
3. The smaller size ellipses show the showers (secondary particles) detected by the ECAL.

Figure 15 Detection Display [view large image]

4. The larger size ellipses show the showers detected by the HCAL.
5. Three additional dots are from the muon chamber at the outer fringe of the detector.
6. The neutrino, which disappears without a trace, can be accounted for from missing energy.

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