1. X-ray Security Screening: More Than Just 'Taking a Photo'
When we place our carry-on luggage into the security scanner, the conveyor belt sends the bag into what appears to be an ordinary 'box.' Seconds later, a colorful image appears on the screen. This is not a simple black-and-white photo, but a 'see-through image' formed as X-rays penetrate the object.
X-rays have penetrating capabilities, and materials of different densities absorb X-rays to varying degrees, creating contrast on the detector. However, traditional single-energy X-rays can only distinguish the shape and density of objects, not directly tell us 'what material this is.' This led to the development of dual-energy X-ray technology.
2. Dual-Energy Imaging: 'Weighing' and 'Identifying' Materials
Dual-energy X-ray systems alternately emit X-rays at high and low energy levels. As the rays pass through an object, the absorption ratios of high-energy and low-energy rays differ based on the atomic number of the material. By comparing the attenuation coefficients at the two energy levels, the system can calculate the effective atomic number (Zeff) of the object.
Organic materials (such as plastics, explosives, and food) are primarily composed of low-atomic-number elements like carbon, hydrogen, and oxygen, resulting in a low Zeff and typically appearing orange on the screen. Inorganic materials (such as metals, ceramics, and glass) contain higher-atomic-number elements, yielding a high Zeff and appearing blue or green. Mixtures display colors in between. This allows security officers to quickly determine whether a liquid is water or a flammable solvent, or whether a solid object is a metal knife or a plastic toy.
3. CT Tomography: From 2D to 3D Recognition
While dual-energy imaging is effective, it can be prone to misjudgment with overlapping objects. CT-based security scanners further address this challenge. They acquire projection data from multiple angles through rotational scanning and use algorithms to reconstruct three-dimensional tomographic images of the inspected object.
In CT images, each voxel has its own attenuation coefficient and effective atomic number, allowing the system to automatically calculate the object's density and Zeff distribution. Using material identification algorithms, it can automatically flag suspicious items. For example, explosives typically have specific density and Zeff ranges, which the CT system can precisely lock onto and highlight, greatly reducing reliance on the operator's experience.
4. How Material Identification Technology Protects Public Safety
Whether at airports, train stations, or major conference venues, dual-energy and CT security scanners work tirelessly. They can identify not only metallic prohibited items like knives and firearms but also non-metallic dangerous goods such as liquid explosives and improvised explosive devices.
In recent years, the integration of artificial intelligence has made material identification even smarter. Deep learning algorithms can learn the shapes and material characteristics of contraband from vast image datasets, assisting security officers in rapid screening and reducing missed detections and false alarms.
Of course, X-ray security screening technology has its limitations, such as the shielding effect of high-density materials like lead and potential misjudgment of complex-shaped objects. Therefore, security checkpoints still require manual re-inspection and complementary technologies like millimeter-wave and ion mobility spectrometry to form a multi-layered protection network.
In the future, with the development of new detectors and spectral imaging technologies, X-ray security screening will be able to identify materials more precisely at the molecular level, making security protection more efficient and reliable. Every smooth passage through security is a testament to how technological advances silently safeguard public safety.