When it comes to radiation detection instruments, there are three main types of detectors that are most commonly used, depending on the specific needs of the device. These are gas ionization based detectors, scintillation detectors and semiconductor detectors. Gas-filled radiation detectors work based on the ionization effect that occurs when radiation passes through air or a specific gas. If a high-voltage potential difference is applied to a chamber containing air or a specialized gas, ionizing radiation causes the generation of positively charged ions and free electrons that were released from gas atoms during the ionization process.
The electrical potential difference in the chamber will cause positive ions to accumulate at the cathode and free electrons to accumulate at the anode of the detector. This accumulation of charged particles results in a small flowing current, which the detector can detect and display as an output signal or a “count”. The current level depends on the level of radiation incident on the detector. It is important to note that gas-filled radiation detectors do not detect all particles, as some of the radiation may pass through and not produce enough ionization in the chamber to be detected.
In addition, these detectors generally do not report on the load, energy level, or type of incident radiation. The most common type of gas-filled radiation detector is the Geiger-Muller or GM detector. For more information on these types of radiation detectors, see our related guide About Gas-Filled Radiation Detectors. Scintillation radiation detectors use the level of light energy that occurs when radiation interacts with a material to ultimately establish the level of radiation. Flashes of twinkling light can be brief in duration, allowing the device to detect large quantities of particles in a short period of time.
Scintillating materials can be solid or liquid, and when incoming radiation hits the material, the photons are released in a device called a photomultiplier tube. The photomultiplier tube consists of a series of plates, called dynodes, each with a more positive electrical potential than the previous plate in the tube. As the radiation releases a photon in the tube, it hits the initial dynode and releases an electron through the photoelectric effect. The released electron is attracted to the higher positive potential of the next plate in the photomultiplier, which causes the release of more electrons to pass further through the tube to the successive dynodes. This process is repeated with the amplification in the number of electrons released at each successive dynode.
The result of this action is that the photomultiplier tube generates an output pulse that is proportional to the amount of light energy entering the tube, which in turn is directly proportional to the amount of radiation energy that entered the scintillation radiation detector. Scintillation radiation detectors have the ability to provide a wealth of data on incident radiation, including energy, charge, particles, and source direction. Solid-state radiation detectors operate using an ionization principle within a semiconductor device that contains two types of semiconductor material and an n-type and a p-type. These materials consist of atoms that have charge carriers that can be considered electrons or holes, which are the absence of electrons. N-type material has more electrons than holes, whereas p-type material has more holes than electrons. When these type semiconductor materials are united within a solid-state radiation detector, a depletion zone is created by the migration of electrons from the n region to the p region.
Incident radiation hitting the semiconductor material produces free electrons and holes, and the number of electron-hole pairs is proportional to the level of radiation. As these charge carriers travel within the detector, they form a current pulse that can be used to establish the level of radiation present. Solid-state radiation detectors can be created using silicon, germanium, cadmium telluride, and cadmium-zinc telluride. Germanium-based detectors must be cooled to cryogenic temperatures, while detectors based on cadmium telluride and cadmium and zinc telluride can operate at room temperature. The interactions of alpha, beta and gamma radiation with matter produce positively charged ions and electrons.
Radiation detectors are devices that measure this ionization and produce an observable output. Early detectors used photographic plates to detect traces left by nuclear interactions. Fog cameras, used to discover subnuclear particles, needed photographic recording and tedious measurement of fingerprints from photographs. Advances in electronics, particularly with regards to transistors allowed for development in electronic detectors.
Scintillator-type detectors use vacuum tubes to perform initial conversion from light into electrical pulses; amplification and storage follows advances in transistor electronics. Miniaturization in electronics has revitalized types of gas-filled detectors; these were developed as single-element detectors but have now been converted into multi-element detectors with more than a thousand elements. Advances in materials - particularly ultra-pure materials - as well as manufacturing methods have been fundamental for creation of new and better detectors. Scintillation detectors are also known as sodium iodide detectors; they consist of salt crystal (usually sodium iodide) attached to photomultiplier tube.
When particle of radiation hits crystal it causes excitation and release photons which produces small flash visible light; flash reflects through lens making contact with photocathode - specialized device producing electrons when coming into contact with light - drawn through photomultiplier picked up by anode forming electromagnetic pulse; pulse detected displayed by meter. Scintillation detectors represent best means detecting gamma or X radiation second most common type detector after G-M tubes; they have ability distinguish between alpha beta gamma configure correspondingly different sounds through meter. Beta detectors often use liquid scintillation medium achieve reasonably high efficiencies even lower energy emission levels; solid state detectors modern semiconductor instruments use reverse biased p-n junction diode.
