Particle Detectors

Scintillation counters are made from materials which emit light when charged particles move through Figure 1. Schematic diagram of the Geiger counter. Illustration by Hans & Cassidy. Courtesy of Gale Group.
them. To detect these events and to gain information about the radiation, some means of detecting the light must be used. One of the first scintillation detectors was a glass screen coated with zinc sulfide. This sort of detector was used by Ernest Rutherford in the early versions of his classic experiment in which he discovered the nucleus of the atom by scattering alpha particles from heavy atoms such as gold. The scattered alpha particles hit the scintillating screen and the small flashes produced were observed by experimenters in a darkened room using only the human eye.

The modern scintillation counter usually uses what is called a photo multiplier tube to detect the light. Light incident on the photocathode of such a tube is converted into an electrical signal and amplified millions of times after which it can be sent to appropriate counters. Physicists working at particle accelerators often use transparent plastic materials like Lucite or plexiglass to which are added materials to make them scintillate. These plastic scintillators can be cut to convenient shapes, mounted on a photo-multipler tube and placed in particle beams to provide a very fast signal when charged particles pass through them.

A very useful scintillation detector, particularly for the measurement of gamma rays, utilizes a transparent crystal of NaI (sodium iodide) mounted on a photomultiplier tube. These crystals are particularly useful because charged particles produce in them an amount of light directly proportional to their energy over a fairly wide range. A schematic diagram of a gamma ray scintillation spectrometer is shown in Figure 2. Gamma rays have no charge and thus no detector is sensitive to them directly. Fortunately, gamma rays interact with matter and produce charged particles—usually electrons. For the measurement of gamma ray energies, the two most important interactions are the photoelectric effect and the Compton effect. These two processes can combine to produce energetic electrons in the crystal, which scintillates to produce an amount of light directly proportional to the gamma ray energy. These light pulses are converted to electrical pulses in the photomultiplier tube. Figure 2. A gamma ray scintillation spectrometer. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Figure 3. Cerenkov radiation. Illustration by Hans & Cassidy. Courtesy of Gale Group.
These are amplified and sent to a pulse height analyzer which sorts out the pulses and displays a pulse height spectrum. A particular gamma ray shows up as a fairly sharp peak in this pulse height distribution.