Subatomic Physics

4.8. Time Projection Chambers 67
Figure 4.16: Drift chambers can be used to track charged particles. In this picture a scientist checks
part of the TWIST apparatus, (8) which used 44 drift chamber planes to do precision measurements
of muon decay. The drift chambers are circular planes whose edges can be seen in the photograph.
[Courtesy Bob Tribble.]
Drift chambers are still very popular because they are reliable and not expensive.
Figure 4.16 shows a picture of the TWIST apparatus which used drift chambers to
track electrons from muon decay. (8)
A drift chamber can be planar or cylindrical. In the latter form it can be made
into a time projection chamber.
4.8 Time Projection Chambers
Wire chambers have one major disadvantage: they only yield information about one
spatial direction. To determine both coordinates, a second wire chamber must be
used. This requirement makes the experimental arrangement complicated and reduces
the solid angle subtended by the detector. Time projection chambers (TPCs),
invented in 1974 by David Nygren, avoid this limitation and are nearly ideal detectors:
TPCs have large solid angles, give excellent spatial resolution in three dimensions,
yield charge and mass information, and allow good pattern recognition. (9,10)
TPCs can be as small as a grapefruit or weigh as much as 10 tons. The main
features are illustrated in Fig. 4.17. The drift chamber is filled with a gas, usually a
mixture of Ar and CH4 because it is inexpensive and allows high electron mobility.
Uniform electric (E) and magnetic (B) fields are applied parallel to the axis (beam
pipe). A charged particle passing through the chamber produces ion pairs along its
8 TWIST Collaboration, Phys. Rev. Lett. 94, 101805 (2005).
9 R. J. Madaras and P. J. Oddone, Phys. Today 37, 38 (August 1984).
10 “The Time Projection Chamber”, ed. J.A. MacDonald, AIP Conference Proceedings No. 108,
American Institute of Physics, New York, 1984.