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.
Scintillation counters are widely used in radiation protection, an assay of radioactive materials, and physics research because they can be made inexpensively yet with good efficiency and can measure both the intensity and the energy of incident radiation. Hospitals worldwide have gamma cameras based on the scintillation effect; therefore, they are also called scintillation cameras.
The advantages and disadvantages of scintillation counters are determined by the scintillator. The following features are not general for all scintillators.
Advantages of Scintillation Counters
- Efficiency. The advantages of a scintillation counter are its efficiency and possible high precision and counting rates. These latter attributes result from the extremely short duration of the light flashes, from about 10-9 (organic scintillators) to 10-6 (inorganic scintillators) seconds.
- Spectroscopy. The intensity of the flashes and the amplitude of the output voltage pulse are proportional to the energy of the radiation. Therefore, scintillation counters can be used to determine the energy and the number of the exciting particles (or gamma photons). For gamma spectrometry, the most common detectors include sodium iodide (NaI) scintillation counters and high-purity germanium detectors. The NaI(Tl) scintillator has a higher energy resolution than a proportional counter, allowing more accurate energy determinations. On the other hand, if a perfect energy resolution is required, we have to use a germanium-based detector, such as the HPGe detector.
Disadvantages of Scintillation Counters
- Hygroscopicity. A disadvantage of some inorganic crystals, e.g., NaI, is their hygroscopicity, a property that requires them to be housed in an airtight container to protect them from moisture.
- NaI(Tl) has no beta or alpha response and poor low-energy gamma response.
- Liquid scintillators are relatively cumbersome.
Semiconductor Detectors
A semiconductor detector is a radiation detector based on a semiconductor, such as silicon or germanium, to measure the effect of incident charged particles or photons. Semiconductor detectors are widely used in radiation protection, an assay of radioactive materials, and physics research.
Advantages of HPGe Detectors
- Higher atomic number. Germanium is preferred because its atomic number is much higher than silicon, which increases the probability of gamma-ray interaction.
- Germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium.
- Very good energy resolution. The FWHM for germanium detectors is a function of energy. For a 1.3 MeV photon, the FWHM is 2.1 keV, which is very low.
- Large Crystals. While silicon-based detectors cannot be thicker than a few millimeters, germanium can have a depleted, sensitive thickness of centimeters. It, therefore, can be used as a total absorption detector for gamma rays up to a few MeV.
Disadvantages of HPGe Detectors
- Cooling. The major drawback of HPGe detectors is that they must be cooled to liquid nitrogen temperatures. Because germanium has a relatively low band gap, these detectors must be cooled to reduce the thermal generation of charge carriers to an acceptable level. Otherwise, leakage current-induced noise destroys the energy resolution of the detector. Recall that germanium’s band gap (a distance between valence and conduction band) is very low (Egap= 0.67 eV). Cooling to liquid nitrogen temperature (-195.8°C; -320°F) reduces thermal excitations of valence electrons so that only a gamma-ray interaction can give an electron the energy necessary to cross the band gap and reach the conduction band.
- Price. The disadvantage is that germanium detectors are much more expensive than ionization chambers or scintillation counters.
Advantages of Silicon Detectors
- Compared with gaseous ionization detectors, the density of a semiconductor detector is very high, and charged particles of high energy can give off their energy in a semiconductor of relatively small dimensions.
- Silicon has a high density of 2.329 g/cm3, and the average energy loss per unit of length allows for building thin detectors (e.g., 300 µm) that still produce measurable signals. For example, in the case of a minimum ionizing particle (MIP), the energy loss is 390 eV/µm. The silicon detectors are mechanically rigid, so no special supporting structures are needed.
- Silicon-based detectors are very good for tracking charged particles, and they constitute a substantial part of the detection system at the LHC in CERN.
- Silicon detectors can be used in strong magnetic fields.
Disadvantages of Silicon Detectors
- Price. The disadvantage is that silicon detectors are much more expensive than cloud or wire chambers.
- Degradation. They also suffer degradation over time from radiation. However, this can be greatly reduced thanks to the Lazarus effect.
- High FWHM. In gamma spectroscopy, germanium is preferred due to its atomic number being much higher than silicon, increasing the probability of gamma-ray interaction. Moreover, germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium. This also provides the latter with a better resolution in energy.