A centralized detector for detecting radiation

    “The "invisible and intangible" radiation requires people to use radiation detectors to detect various types of radiation, provide characteristics such as radiation type, intensity (quantity), energy, and time, and measure radiation.
1、 Definition of radiation detectors
A device that uses ionization, excitation effects, or other physical and chemical changes caused by radiation in gases, liquids, or solids for nuclear radiation detection is called a radiation detector.
2、 Different operating principles of radiation detectors
1. Utilize the ionization principle generated by radiation in materials to manufacture various gas ionization detectors and semiconductor detectors;
2. Use radiation to excite certain substances, and observe the scintillation detector that emits photons when the invitation is withdrawn;
3. Using the principle of making film sensitive to radiation to produce various nuclear emulsions and films for dose measurement;
4. Utilize the condensation effect of radiation in supersaturated steam to create a Wilson cloud chamber;
5. A bubble chamber that uses radiation to create bubbles in superheated liquid to observe particle tracks.
3、 The basic process of radiation detection
1. The sensitive volume of radiation particles entering the detector;
2. The incident particles release energy in the detector through processes such as ionization, excitation, or nuclear reactions;
3. The detector converts the accumulated energy into some form of output signal through various mechanisms;
4. Detectors are mainly divided into gas detectors, scintillation detectors, and semiconductor detectors based on their detection medium type and mechanism of action.

   4、 Composition and classification of radiation detection devices
Radiation measurement devices usually consist of two parts: a probe and an electronic instrument for analysis and recording;
1. Gas detector
The gas detector uses gas as the working medium, and the output signal is obtained from the ionization effect or nuclear reaction generated by the incident particles in it.

    Total ionization: initial ionization (ion pairs directly generated by incident particles) and secondary ionization (high-speed electrons generated by initial ionization (referred to as δ Electrons are sufficient to ionize gases.
As the voltage applied between the electrodes varies, it results in different working states of the gas detector:
Composite zone: When the applied working voltage is too low, electron ion pairs recombine due to collision with each other; The degree of recombination is related to the applied voltage and the density of ion pairs, and is generally not used as the working area of gas detectors.
Saturation zone: When the applied working voltage is too high, the recombination of electrons and positive ions can be ignored; In this region, changing the applied voltage results in almost no change in the number of ions collected, hence it is called the saturation region; At this point, the logarithm of ions generated is proportional to the energy lost by the incident particle in a sensitive volume, and the detector operating in this state is the ionization chamber; The ionization chamber was the earliest detector used.
Proportional region: As the working voltage increases, the electric field strength is strong enough in a small area near the central anode, so that electrons can undergo new collision ionization under the acceleration of the external electric field. The final number of ions collected, N, is much larger than the number of ions produced by the original ionization, N0. This phenomenon is called "gas amplification", also known as gas amplification or avalanche process. The ratio of N to N0 is called the gas amplification factor, represented by the constant M, which means M=N/N0. Due to the relatively weak field strength near the anode at this time, the avalanche process only occurs in a small area along the anode, and the gas amplification factor is constant at a certain operating voltage. At this point, the logarithm of total ions formed is still proportional to the energy of the incident particles. The proportional counter works in this area.
Limited proportional region: When the working voltage is further increased, it enters the limited proportional region, where a considerable amount of space charge composed of positive ions accumulates within the sensitive volume of the detector. At a certain operating voltage, A no longer maintains a constant, and the A of incident particles with small initial ionization may be slightly larger, known as the finite proportional region. Generally, there are no detectors operating in this area.
G-M counting area: As the working voltage further increases, the avalanche process quickly spreads to the entire anode, and the positive ions formed during the avalanche process tightly surround the anode wire, known as the positive ion sheath. Due to the polarity of the positive ion sheath's charge being the same as the anode charge, it weakens the electric field. When the total charge of the positive ion sheath reaches a certain level, the avalanche process is terminated, and therefore the final total number of ions is independent of the initial ionization. At this point, the incident particle only plays a triggering role, and the size of the output pulse signal is independent of the type and energy of the incident particle. This is the G-M region, which is only used as a counter. The gas amplification factor M increases with the increase of applied voltage. Under a certain applied voltage, the number of ions that are multiplied by any energy or type of radiation is the same.
Continuous discharge zone: Continue to increase the applied voltage, and due to the extremely high electric field strength inside the detector, the inflated body will break down. At this time, regardless of whether nuclear radiation enters the detector, discharge will occur continuously, so this zone is called a continuous discharge zone.
Ionization chamber is one of the earliest gas ionization detectors used. Due to its simple and reliable structure, various geometric shapes can be made, stable and reliable working performance, it is suitable for measuring various rays and can measure exposure, radiation intensity, etc. over a wide range.
Proportional counter tubes are commonly used to measure low energy β Radiation; It has a short resolution time and can perform fast counting, making it suitable for high-intensity measurements; Due to the correlation between the gas amplification factor M and the applied voltage, the proportional counter tube has a high requirement for the stability of the high-voltage power supply. Generally, the long-term stability of the high-voltage power supply is required to not exceed 0.1%.
G-M counter tube is the most widely used type of gas detector, with high sensitivity, large output pulse amplitude, and can be recorded directly without amplification. Therefore, it is convenient to use, easy to make, and low in price. It is widely used for measuring various nuclear radiation. Its detection effect on charged particles reaches almost 100%, but it has a significant impact on γ The detection efficiency of radiation is low, only about 1%. In addition, its output pulse amplitude is the same for different energies and types of rays under a certain voltage, so it cannot be directly used to distinguish the types of rays and measure the magnitude of energy.
2. Scintillation detector
Scintillation detectors generally consist of scintillators and photomultiplier tubes.
Scintillators are light-emitting devices that emit visible light photons called fluorescent photons when charged particles ionize, excite, and de excite the atoms in the detection medium.
Such light intensity is invisible to the naked eye and must be detected by highly sensitive photomultiplier tubes (PMTs).
The photocathode of PMT converts the collected fluorescent photons into photoelectrons. The photoelectrons are collected by the first junction of the photomultiplier tube through focusing, and a considerable pulsating electron flow is formed by the subsequent junction doubling, forming an output signal on the output circuit.
(1) An ideal scintillator should have the following properties:
The efficiency of converting the kinetic energy of charged particles into fluorescent photons is high, that is, high luminescence efficiency;
There is a good linear relationship between the energy loss of incident charged particles and the number of fluorescent photons produced;
The scintillator medium is transparent to the light emitted by itself, that is, its emission spectrum and absorption spectrum should not overlap significantly;
The duration of the flash generated by the incident particles, i.e. the decay time of the scintillator's luminescence, should be as short as possible to generate a fast output signal and obtain a good time response;
Good processing performance with appropriate refractive index.
(2) There are two types of scintillators that are commonly used:
One type is inorganic scintillators, such as NaI (Tl), CsI (Tl), etc. These materials have high density and atomic number, making them suitable for detection γ X-rays and higher energy X-rays;
One type is organic scintillators, such as plastic and organic liquid scintillators, mainly used for β Detection of particles and neutrons.
(4) A photomultiplier tube (PMT) is a type of optoelectronic device:
It mainly consists of a photocathode, a focusing electrode, a dynode (connected electrode), and an anode, sealed inside a glass shell with each electrode led out;
There are many products for photomultiplier tubes, but the main focus is on matching their photocathode and spectral response with the emission spectrum of scintillators;
Has high cathode sensitivity and anode sensitivity;
Lower dark current or noise pulses;
Good craftsmanship and stability.
(5) Main features:
It can be used not only to measure charged particles but also to measure uncharged particles such as neutrons and γ Radiation, etc;
It can be used to measure both radiation intensity and energy spectrum;
The advantages of high detection efficiency and short resolution time.
3. Semiconductor detectors:
The detection medium is semiconductor material, and the incident charged particles lose energy through ionization in the detection medium, while forming electron hole pairs in the detection medium. The electron hole pairs form output signals on the output circuit during the directional drift process relative to the electrode.
The sensitive volume of semiconductor detectors: the depletion region formed in the P-N junction region (adding reverse voltage to the P-N junction will further expand the width of the depletion layer).
To ensure the effective collection of electron hole pairs generated by ionization, it is necessary to choose materials with long lifetime of charge carriers (electrons or holes) in semiconductor materials. Semiconductor silicon and germanium with excellent performance become ideal dielectric materials for semiconductor detectors.
The detector made of general high-purity materials (impurity concentration at the level of 1015 atoms/cm3) is only suitable for applications with a limited width of the P-N junction, which is only a few tenths of a millimeter or 1.2mm α Detection of particles or other heavily charged particles.
With the development of materials and processes, lithium drift detectors Si (Li) and Ge (Li) semiconductor detectors have emerged, resulting in extremely pure semiconductor materials with impurity concentrations of only 1010 atoms/cm3, mainly germanium, known as high-purity germanium semiconductor detectors (generally referred to as HPGe), which can achieve a sensitivity width of several centimeters and a sensitivity volume of over 100 cm3, reaching γ The comparison of the detection efficiency of X-rays with inorganic scintillators;
The energy required to form an electron hole pair in semiconductor materials is only 3eV, which is the ionization energy W=3eV. In gas detectors, the energy required to form an electron ion pair is 30eV. For scintillator detectors, the energy required to form a photoelectron collected by the first dynode of a photomultiplier tube is 300eV; In this way, the number of electron hole pairs formed by incident particles with the same energy in the semiconductor detector that have a decisive effect on the output signal will be greater than the first two types, thus achieving the best energy resolution and distinguishing different incident particles with smaller energy differences than the first two types of detectors.