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 security 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.
Worldwide, various forms of terrorist incidents have occurred frequently and public safety 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, and X-ray CT. 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 (ρ) 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.
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 irradiates 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 being processed by the computer image technology, the dangerous goods and prohibited articles in the baggage package can be checked out.
When X-rays pass through a substance, its attenuation has the following rules:
(1) In the formula, I0 is the incident intensity of X-rays; t is the thickness of the test object; μ is the total attenuation coefficient of the test object; I is the outgoing X-ray intensity. According to the interaction of X-rays and matter, it can be divided into photoelectric effect, Compton scattering effect, Rayleigh scattering effect and positive and negative electron pair effect according to its principle of action. When X-rays pass through a substance, the total attenuation coefficient μ is:
(2) In the formula, Σ is the macroscopic interface; N is the number of nuclear atoms per unit volume; σ is the total attenuation cross section; σph is the photoelectric absorption cross section; σc is the Compton scattering cross section; σr is the Rayleigh scattering cross section; σK is the electron pair cross section A is atomic mass number; NA is Avogadro's constant; ρ is material density. For substances with small atomic number (such as: most of the explosives, drugs) can be approximated as: σph is a function of atomic number Z and incident X photon energy hν, σc is only a function of hν, σr is a function of Z and hν, When the incident photon energy reaches megaelectron volts, electron pair effects will occur. Generally, the X-ray photon energy used in the safety inspection equipment is in the order of thousands of electron volts, and no electron pair effect will be produced. The σK term can be ignored. Therefore, the total attenuation coefficient μ is a function of Z, ρ, hν.