What Is a Radiation Detector?
A radiation detector is a device that detects and measures radiation indirectly by using the physical and chemical reactions caused by the interaction of radiation and matter.
Humans cannot sense radiation directly with the five senses. Therefore, detection and measurement are performed using ionization and excitation caused by radiation. For example, ions, free electrons, and electromagnetic waves such as fluorescence are generated, which are converted into electric current signals. Based on this current signal, the radiation dose can be displayed on a meter or made audible as a sound.
There are many other applications, such as detectors based on electron emission, detectors that use heat generation, detectors based on the practical use of neutron material activation, and detectors based on the detection of Cherenkov light.
Uses of Radiation Detectors
Radiation detectors are widely used in radiation decontamination sites, yards, and factories. There are different types of radiation, such as alpha, gamma, beta, and X-rays, and they emit high to low doses. Therefore, the detector itself must be carefully selected according to the situation.
By measuring the air dose rate, it is possible to determine how much radiation is flying around in space. Also, by detecting radiation emitted from the surface of an object, it is possible to determine whether the object is contaminated and to identify the source of contamination. In addition, radiation detectors can be used to measure the level of radiation exposure of a person.
Principle of Radiation Detectors
There are two main types of radiation detectors: those that use the ionizing effect of radiation on gas molecules and those that use the excitation of electrons in materials, mainly solids and liquids.
The former is called a gas detector and the latter a scintillation detector.
1. Gas Detector
In a gas detector, the detector is filled with a gas such as inert gas or air, and when radiation passes through it, the molecules ionize to produce cations and electrons. The ionization of these gas molecules is used to measure the amount of radiation. There are several types of expectation detectors, such as ionization chambers, GM countertube, and proportional countertube.
Ionization Chamber
In an ionization chamber, cations and electrons are attracted to electrodes respectively and converted into electrical signals for measurement. The number of cations and electrons ionized by the energy of the radiation is converted directly into an electrical signal, so the signal intensity is almost proportional to the energy of the radiation. In other words, it is possible to determine the energy of the radiation. The disadvantage, however, is that the sensitivity is low because the ionization is directly observed.
GM counter
In a GM counter, a gas is charged in the same way as in an ionization chamber, but by applying a high voltage between the electrodes, the electrons generated by ionization move at high speed, causing other gas molecules to become ionized. This allows a strong signal to be obtained.
As a result, one pulse is run between the electrodes for each ionization. A strong signal is obtained, but the disadvantage is that no information about the energy of the radiation is obtained because the signal is a pulse.
Proportional counter
When the voltage applied between the electrodes of a detector filled with gas is appropriately adjusted, ionization of other gas molecules occurs following ionization by radiation, producing a strong signal, and a signal proportional to the number of molecules first ionized can also be obtained. The type that measures these conditions is a proportional counter.
2. Scintillation Detector
Scintillation detectors utilize the effect of radiation on electrons in orbit around an atomic nucleus, which is then excited and transferred to an outer orbital. An example of an instrument is a scintillation survey meter.
A material that has the property of emitting light via excitation by radiation is called a scintillator. Sodium iodide (NaI) crystals are used as solid crystal scintillators. When radiation is absorbed by the scintillator, the atoms become unstable due to electronic excitation and then return to their original stable state. During this process, the atoms emit energy as light.
This weak light (photon) is amplified by a photomultiplier tube and converted into an electric current for measurement. Since the number of photons emitted is proportional to the energy of the radiation, scintillation detectors can determine the energy of the radiation.
Since NaI crystals are hygroscopic, they are sealed to prevent exposure to air. On the other hand, an incident window is provided where the radiation enters the detector. The incident window is made of a very thin metal with a very low atomic number, such as beryllium or aluminum, about 100 μm in thickness.
How to Select Radiation Detectors
When selecting radiation detectors, it is important to check the following items:
1. Type of Radiation
There are different types of radiation: alpha rays, beta rays, neutron rays, gamma rays, and X-rays. The structure and principle of radiation detectors determine the type of radiation that can be detected and the expected sensitivity, so it is important to select a detector with an understanding of these factors.
2. Displayed Value
The selection of a detector should be based on the suitability of the displayed value (whether it is a simple count or a 1 cm dose equivalent, etc.) for the intended use.
3. Radiation Permeability
Since radiation must reach the site of ionization (gas or solid scintillator) to be detected, understanding radiation permeability allows for confident operation. For example, NaI scintillation survey meters are for gamma and X-ray measurements. This is because they cannot detect radiation that cannot penetrate the thin metal window (alpha and beta rays). After all, they must be sealed around a scintillator that is hygroscopic.
Some GM counters are capable of measuring beta radiation, while others are not. The type that can measure beta rays is the one with a large window and a very thin mica window. Beta rays can pass through this mica window. GM counters that can measure both beta and gamma radiation have a metal cap, but the metal cap must be removed for beta measurement. Beta rays do not penetrate the metal cap.
Other Information on Radiation Detectors
1. Purpose of Radiation Measurement
There are two main purposes of radiation measurement.
- To measure the radiation dose specific to a radiation field, such as the type of radiation, its energy, or the number of particles, to control it when handling radiation.
- To understand the physical, chemical, and biological effects of radiation, or to make effective use of radiation, the absorbed dose is measured by multiplying the radiation dose in a radiation field by a coefficient resulting from the interaction between radiation and matter.
Radiation safety management is an extension of the latter. To evaluate the effects of radiation on the human body, the effective dose is calculated based on the latter absorbed dose, adding the biological effects of each type of radiation and an evaluation of the sensitivity of the body part receiving the radiation.
2. Scintillation-based High-energy X-ray Detectors
Scintillation detectors using solid scintillator crystals are used to measure high-energy X-rays and even higher-energy gamma rays. The scintillation detector is characterized by its ability to detect X-rays in proportion to their energy since the scintillator efficiently receives and detects X-rays.
This feature is different from gas detectors, which cannot catch high-energy X-rays. In addition, the time from the time the X-rays enter the detector to the time they are converted into an electrical signal and output is very short, making it suitable for measurements when there are many incident X-ray photons. In the research field, position-detecting high-energy X-ray detectors have also been developed, which utilize the advantages of the scintillation type to acquire a two-dimensional image of X-rays.