Cherenkov Detectors
A Cherenkov detector is a particle detector based on detecting Cherenkov radiation (visible light or UV photons). In contrast to a scintillation counter, the light production is instantaneous. Typical scintillator detectors have a decay time measured in microseconds, while Cherenkov radiation is nearly instantaneous and, with fast pulse processing equipment, can be measured in picoseconds. Cherenkov counters are used mainly for particle identification, i.e., to determine particle masses. Cherenkov counters contain two main elements:
- a radiator through which the charged particle passes,
- a photodetector (e.g., a photomultiplier tube – PMT).
The Cherenkov radiation, produced in the radiator, is electromagnetic radiation emitted when a charged particle (such as an electron) moves through a dielectric medium faster than the phase velocity of light in that medium. Particles exceeding the phase velocity of light result in polarization along the axis of motion creating a dipole field. When this field collapses, an electromagnetic pulse (Cherenkov radiation) is emitted in the forward direction.
Cherenkov radiation is commonly produced through Compton electrons or pair-production electrons and positrons in dielectric materials. The intensity of light produced by this process is much less than that of luminescence (the basis for scintillation detector operation), requiring more sensitive optical photon detection equipment such as low light photomultiplier tubes (PMT). Cherenkov counters may be classified as either imaging or threshold types, depending on whether they do or do not use Cherenkov angle (θ) information. In the simple case of a threshold detector, the mass-dependent threshold energy allows the discrimination between a lighter particle (which does radiate) and a heavier particle (which does not radiate) of the same energy or momentum. Imaging counters may be used to track particles as well as identify them. Although devices using Cherenkov radiation are often thought of as particle identification (PID) detectors, in practice, they are widely used over a much broader range of applications; including:
- fast particle counters
- hadronic particle identification
- tracking detectors performing complete event reconstruction.
Scintillation Counters
A scintillation counter or scintillation detector is a radiation detector that uses the effect known as scintillation. Scintillation is a flash of light produced in a transparent material by passing a particle (an electron, an alpha particle, an ion, or a high-energy photon). Scintillation occurs in the scintillator, a key part of a scintillation detector. In general, a scintillation detector consists of:
- Scintillator. A scintillator generates photons in response to incident radiation.
- Photodetector. A sensitive photodetector (usually a photomultiplier tube (PMT), a charge-coupled device (CCD) camera, or a photodiode) converts the light to an electrical signal, and electronics process this signal.
The basic principle of operation involves the radiation reacting with a scintillator, which produces a series of flashes of varying intensity. The intensity of the flashes is proportional to the energy of the radiation, and this feature is very important. These counters are suited to measure the energy of gamma radiation (gamma spectroscopy) and, therefore, can be used to identify gamma emitting isotopes.