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In recent years, various types of terrorist incidents have occurred worldwide and public security has become the focus of attention of the international community. In order to deal with increasingly rampant terrorist activities, governments of all countries have issued corresponding policies, including strengthening security measures for public places such as airports, stations, and docks, and have focused on strengthening the inspection of explosives, drugs, and other contraband. However, due to the large variety of explosives and the wide variety of material forms, it is undoubtedly necessary to increase the technical requirements for security inspection equipment by accurately and quickly checking out contraband such as explosives.
At present, international research on detection technologies for explosives and other contraband products mainly focuses on X-ray detection technology, neutron detection technology, electromagnetic measurement technology, and vapor particle detection technology. Among them, X-ray detection technology is a relatively mature and widely used technology, mainly including X-ray transmission, dual-energy X-ray detection, X-ray scattering, X-ray CT and so on. These technologies implement the inspection of prohibited items by extracting the characteristic physical quantity information of the tested object.
The characteristic physical quantity of the extracted sample is mainly the density (Ï) and effective atomic number (Zeff) information of the sample. In theory, the density and effective atomic number of a known substance can accurately determine the type of substance. However, there are various shortcomings and deficiencies in various current security inspection methods. Comparing various security inspection methods and realizing multi-level inspection are commonly used methods in security inspection. This article elaborates the basic principles of X-ray security inspection technology and introduces the commonly used X-ray security inspection methods.
principle
X-ray radiation is a kind of high-energy electromagnetic radiation. It is produced by high-energy electrons in the material for deceleration or by the transition of atomic orbital electrons. Because of its strong penetrability, it can penetrate packages, luggage, and other items. Comes as a radiation source for safety inspections.
After X-ray generation, the fan-shaped beam formed by the collimator illuminates the object to be detected. After interaction with the object to be inspected, part of the energy of the fan beam is absorbed by the substance, and part of the energy is scattered by the substance due to different types of Since the material absorbs and scatters X-rays differently, the transmitted X-ray beam has different energy when it reaches the detector. The detector converts the detected X-ray energy into carriers and distinguishes the changes in X-ray energy within a small size range. After processing, the energy received by the detector is displayed in gray-scale images. After processing by the computer image technology, the dangerous goods and prohibited articles in the baggage package can be checked out.
Commonly used X-ray safety inspection technology
The single-energy X-ray X-ray transmission method distinguishes different X-ray attenuation coefficients of different substances, and simply measures the incident X-ray energy I0 and the outgoing X-ray energy I to obtain a product of μt. The use of this method allows for clear imaging of substances with a high effective atomic number (eg, metals, etc.), and has a good inspection effect, but cannot be detected for contraband (such as explosives) hidden behind substances with a large absorption coefficient. The ability is very limited.
The dual-energy X-ray method further develops a dual-energy X-ray detection technique, which can obtain information on the effective atomic number of the test object, and can separate the organic matter from the inorganic matter.
By using high-energy and low-energy X-rays to irradiate substances, the ratio K of the attenuation of the high-energy X-rays to the attenuation of the low-energy X-rays is measured, thereby extracting the effective atomic number Zeff of the test object. The dual-energy X-ray detection method can obtain the information of the effective atomic number of the test object, and can separate the organic matter from the inorganic substances, such as separating the common inorganic substances such as glass in the baggage, thereby greatly improving the detection accuracy. The detection speed is quick and the cost is low, becoming the mainstream product of security inspection equipment. However, the dual-energy X-ray detection technology still has difficulties in distinguishing the organic substances (such as food, plastics, fabrics and other living organics) allowed in the baggage and the prohibited organic substances (such as TNT, C4, heroin, cocaine, etc.).
The X-ray backscattering method has developed an X-ray backscattering technique in view of the fact that prohibited items are mostly hidden in the outer layer of the baggage and in the dark compartments. It is a relatively new X-ray security inspection technology. It mainly detects the Compton scattering signal of matter to X-rays. Therefore, it has a good inspection effect on articles with low Zeff, high density and being placed on the outer layer of baggage. Compared with the dual-energy X-ray inspection method, the X-ray backscattering method obtains more abundant and sensitive information on the outer layer of baggage, which can further improve the inspection accuracy, especially for plastic explosives with lower Zeff and higher density. well.
In recent years, X-ray backscatter technology has also been applied to the safety inspection of large items. The use of vehicles to smuggle explosives, drugs, and smuggling of people is often used to transform vehicles and to create thick grids, sandwiches, etc., because iron plates absorb X-rays more strongly, and explosives, drugs, etc. hidden behind heavy iron plates are used. Organics are obscured by traditional X-ray inspection and are difficult to find. However, the low effective atomic number of organic matter (drugs, explosives, etc.) has a strong X-ray scattering effect, and X-ray backscattering methods can be used to detect low-atomic-number organics (drugs, explosives, etc.), especially when The organic matter in the shallow area (sandwich, etc.) of the test object detects strong scattering signals. Figure 1 shows the multi-view X-ray backscattered image of the vehicle under inspection. It can clearly see the contraband hidden in the front and rear compartments of the car and the smugglers hidden in the roof sandwich. However, the X-ray backscattering method still cannot obtain effective atomic number and density information at the same time, and it cannot check the internal and partial plastic explosives in baggage.
In the security check, due to the presence of complicated and overlapped, blocked, and mixed articles in the packaged baggage, it is difficult for the security inspection. At present, the combination of X-ray dual energy and backscattering technology can provide security inspectors with more information to improve inspection accuracy, improve detection accuracy, and reduce false alarm rate. Has been applied to the container inspection, vehicle inspection, baggage inspection, human inspection and other security inspections.
in conclusion
At present, a variety of X-ray detection technologies are widely used in various fields of security inspections, but there are no security inspection products that can independently provide effective atomic number and density information of substances. Each detection technology has certain advantages, limitations and pertinence. In short, a variety of technology integration and multi-level inspection is one of the directions for the development of security inspection equipment.
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The safety inspection of baggage items carried by persons passing through important entrances and exits has become a widely used safety measure in the world. In recent years, driven by the needs of aviation safety, X-ray safety inspection technology has developed rapidly and has become a widely used security inspection technology in the world. This article describes the basic principles of X-ray safety inspection techniques and the commonly used X-ray screening methods.