Subatomic Physics

64 Detectors
Figure 4.13: Neutrino interaction in a hydrogen bubble chamber. A neutrino enters from the right
and interacts with the proton of a hydrogen atom to yield a muon (the long track that extends
to the top left), a positive pion (the short top track), and a proton (the short bottom track).
[Courtesy Argonne National Laboratory.]
3.6 m (12 ft) hydrogen bubble chamber of the Argonne National Laboratory which
contained about 20,000 liters of hydrogen. A superconducting magnet produced a
field of about 18 kG in the chamber volume of 25 m 3 .
Because of their slow response, bubble chambers are seldomly used in present
days for experiments with high counting rates. However, they are still being used for
applications with low counting rates in combination with CCD cameras. (5) Some
versions can be triggered using the spike in pressure when a pulse develops. In the
next section we explain in more detail what triggering means.
4.5 Spark Chambers
Whereas spark chambers are no longer state-of-the-art, they illustrate the basic
principles of a triggerable detector clearly. Spark chambers are based on a simple
fact. If the voltage across two metal plates, spaced by a distance of the order
of cm, is increased beyond a certain value, a breakdown occurs. If an ionizing
particle passes through the volume between the plates, it produces ion pairs, and
the breakdown takes the form of a spark that follows the track of the particle. Since
the ions remain between the plates for a few µs, the voltage can be applied after
passage of the particle: A spark chamber is a triggerable detector.
The elements of a spark chamber system are shown in Fig. 4.14. The problem
to be studied in this simplified arrangement is the reaction of an incoming charged
particle with a nucleus in the chamber, giving rise to at least two charged products.
Thus the signature of the desired events is “one charged in, two charged out.”
Three scintillation counters, A, B, and C, detect the three charged particles. If the
particles pass through the three counters, the LOGIC circuit activates the high-
5 See, for example, W.J. Bolte et al., Journal of Physics: Conference Series 39, 126 (2006);
http://collargroup.uchicago.edu/
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