Scintillation detectors are one of the types of measuring equipment designed to detect elementary particles. Their feature is that reading occurs through the use of photosensitive systems. For the first time, these instruments were used in 1944 to measure uranium radiation. There are several types of detectors depending on the type of working agent.
Appointment
Scintillation detectors are widely used for the following purposes:
- registration of environmental pollution;
- analysis of radioactive materials and other physicochemical studies;
- application as an element for launching more complex detector systems;
- spectrometric study of substances;
- a signaling component in radiation protection systems (for example, dosimetric equipment designed to notify the entry of a ship into the zone of radioactive contamination).
Counters can produce both high-quality registration of radiation and measure the magnitude of its energy.
Device detectors
The principal arrangement of the scintillation radiation detector is shown in the figure below.
The main elements of the equipment are the following:
- photomultiplier;
- a scintillator designed to translate the excitation of the crystal lattice into visible light and transmit it to the optical transducer;
- optical contact between the first two devices;
- Voltage regulator;
- electronic system for recording electrical impulses.
Types
There is the following classification of the main types of scintillation detectors by the type of substance that fluoresces under the influence of radiation:
- Inorganic alkali halide counters. They are used to register alpha, beta, gamma and neutron radiation. Several types of single crystals are produced in industry: sodium iodide, cesium, potassium and lithium, zinc sulphide, tungstates of alkaline earth metals. They are activated with special impurities.
- Organic single crystals and clear solutions. The first group includes: anthracene, tolan, trans-stilbene, naphthalene and other compounds, the second - terphenyl, mixtures of anthracene with naphthalene, solid solutions in plastics. They are used for temporary measurements and for registration of fast neutrons. Activating additives are not added to organic scintillators.
- Gaseous medium (He, Ar, Kr, Xe). Such detectors are mainly used to detect fission fragments of heavy nuclei. The radiation wavelength is in the ultraviolet spectrum, therefore, they require appropriate photodiodes.
For scintillation neutron detectors with kinetic energies of up to 100 keV, zinc sulfide crystals activated by a boron isotope with a mass number of 10 and 6 Li are used. When alpha particles are detected, zinc sulphide is applied in a thin layer on a transparent substrate.
Among organic compounds, scintillation plastics are most widely used. They are solutions of luminescent substances in high molecular weight plastics. Most often, scintillation plastics are made on the basis of polystyrene. Thin plates are used to detect alpha and beta radiation, while thick ones are used for gamma and x-rays. They are available in the form of transparent polished cylinders. Compared to other types of scintillators, plastic ones have a number of advantages:
- short flash time;
- resistance to mechanical damage, moisture;
- constancy of characteristics at high doses of radiation;
- low cost;
- ease of manufacture;
- high registration efficiency.
Photomultipliers
The main functional component of this equipment is a photomultiplier. It is a system of electrodes mounted in a glass tube. To protect it from external magnetic fields, it is placed in a metal casing of a material having high magnetic permeability. Due to this, shielding of electromagnetic interference occurs.
In a photomultiplier, a light flash is converted into an electric pulse, and an electric current is also amplified as a result of secondary electron emission. The magnitude of the current depends on the number of dynodes. The focusing of electrons occurs due to the electrostatic field, which depends on the shape of the electrodes and the potential between them. Knocked-out charged particles are accelerated in the interelectrode space and, getting to the next dynode, cause another emission. Due to this, the number of electrons increases several times.
Scintillation detector: principle of operation
Counters work as follows:
- The charged particle enters the working substance of the scintillator.
- The ionization and excitation of the molecules of the crystal, solution or gas.
- Molecules emit photons and, after a millionth of a second, return to their equilibrium state.
- In a photomultiplier, a flash of light “builds up” and hits the anode.
- In the anode circuit, amplification and measurement of electric current occurs.
The principle of operation of the scintillation detector is the phenomenon of luminescence. The main characteristic of these devices is conversion efficiency - the ratio of the energy of a flash of light to the energy lost by a particle in the active substance of a scintillator.
Advantages and disadvantages
The advantages of scintillation radiation detectors include the following:
- high detection efficiency, especially with respect to short-wave gamma rays with high energy;
- good temporal resolution, that is, the ability to give a separate image of two objects (it reaches 10 -10 s);
- simultaneous measurement of energy of registered particles;
- the possibility of manufacturing counters of various shapes, the simplicity of a technical solution.
The disadvantages of these counters are low sensitivity to particles with low energy. When they are used as part of spectrometers, the processing of the obtained data is much more complicated, since the spectrum has a complex form.