Subatomic Physics

Chapter 4
Detectors
What would a physicist do if he were asked to study ghosts and telepathy? We
can guess. He would probably (1) perform a literature search and (2) try to design
detectors to observe ghosts and to receive telepathy signals. The first step is of
doubtful value because it could easily lead him away from the truth. The second
step, however, would be essential. Without a detector that allows the physicist to
quantify his observations, his announcement of the discovery of ghosts would be
rejected by Physical Review Letters. In experimental subatomic physics, detectors
are just as important and the history of progress is to a large extent the history
of increasingly more sophisticated detectors. Even without accelerators and using
only neutrinos or cosmic-ray particles, a great deal can be learned by making the
detectors bigger and better. In the following sections, we shall discuss different
types of detectors. Many beautiful and elegant tools are not treated here; however,
once the ideas behind typical instruments are understood, it is easy to pick up more
details concerning others. We also add a brief section about electronics because it
is an integral part of any detection system.
4.1 Scintillation Counters
The first scintillation counter, called spinthariscope, was constructed in 1903 by
Sir William Crookes. It consisted of a ZnS screen and a microscope; when alpha
particles hit the screen, a light flash could be seen. In 1910, Geiger and Marsden
performed the first coincidence experiment. As Fig. 4.1 shows, they used two
screens, S1 and S2, and two observers with microscopes M1 and M2. If the radioactive
gas between the two screens emitted two alpha particles within a “short” time
and if each hit one screen, each observer would see a flash. They probably shouted
to indicate the time of arrival. The human eye is slow and unreliable and the scintillation
counter was abandoned for many years. It was reintroduced in 1944 with a
photomultiplier replacing the eye. The basic arrangement for a modern scintillation
counter is shown in Fig. 4.2.
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