Characteristics of High-Energy Radiographic Imaging
In terms of applications, high-energy radiographic imaging possesses the following five characteristics:
High penetration power with large penetration thickness.
Currently, X-ray machines typically penetrate steel to a thickness less than 100mm, while Co60 gamma rays can reach a penetration limit of about 200mm. For industrial applications, high-energy radiation ranges from 1 to 24 MeV, allowing for penetration depths of over 400mm in steel. Therefore, for steelwork with thicknesses exceeding 200mm, high-energy radiation is almost the only choice for radiographic inspection.
2. Small focus point, large focal length, high photo clarity
The high-energy X-ray device is significantly larger in volume than conventional radiographic machines, making heat dissipation a manageable issue, thus allowing for very small focal points. For instance, electron cyclotrons have focal points as small as 0.3 to 0.5 mm, while linear accelerators have focal points of only 1 to 3 mm. Additionally, to ensure a sufficiently large irradiation field, high-energy radiographic imaging requires the use of a large focal length. Both large and small focal lengths are beneficial for enhancing the clarity of the images.
Low thermal radiation, high photo-sensitivity
In the high-energy range, the interaction between ray photons and matter primarily involves Compton scattering and electron-positron pair effects. The scattering ratio decreases as the ray energy increases. On the other hand, due to the high energy of secondary particles produced in the interaction process, the subsequent scattering is mainly concentrated in the direction of the primary ray, with a low total amount of large-angle scattering. Consequently, high-energy ray photography exhibits a small scattering ratio and high sensitivity.
4. High radiation intensity, short exposure time, capable of continuous operation, and high work efficiency
The dosimetry at a distance of 1m from the target is 4~100 Gy/min for the linear accelerator, significantly higher than the dose rates of various gamma-ray sources used in industrial inspections. The ratio of operation to standby for a standard industrial X-ray machine is typically 1:1, whereas the accelerator can run continuously without breaks. Consequently, the exposure time for inspecting workpieces with a linear accelerator is very short, particularly for thick workpieces, resulting in high efficiency. The exposure time for inspecting a 100mm thick steel workpiece is approximately 1 minute, which is unmatched by other equipment.
5. Large photo thickness tolerance
The absorption规律 of matter to high-energy radiation is significantly different from that to low-energy radiation, with the absorption coefficient changing slowly with energy. Generally, within the 1~10 MeV range, the radiation absorption coefficient of matter decreases as the energy increases. In the 10~100 MeV range, the radiation absorption coefficient of matter increases slowly with energy. This change pattern allows high-energy radiation imaging to have a great thickness tolerance.
High-energy radiographic inspection of components with varying thickness, such as crankshafts and turbine blades, can be performed without the need for compensating blocks or special process measures. Even when the thickness of the workpieces differs by a factor of two, it can still meet the required density levels specified by general standards, whereas low-energy radiographic inspection does not offer such thickness tolerance.
