Backscatter Imaging and Transmission X-ray explained ...
X-ray methods make hidden things visible
Backscatter imaging uses X-rays to visualize deviations and anomalies in the structure and surface of a material, as well as up to 30 cm behind the surface. The images are generated in real time.
Transmission versus backscatter
The X-ray backscatter method differs significantly from the transmission method commonly used in medicine or baggage screening. In transmission, the X-ray source and detector are separated. The objects under examination are positioned between the source and the detector. The X-rays emerge from the X-ray tube in a cone shape and penetrate the object and its contents. Since the radiation penetrates different materials to different degrees, the result is a monochromatic image of the contents. Materials that are easy to penetrate appear dark. Materials that are difficult to penetrate appear bright. Whether a material is easy or difficult to penetrate is determined by its density. Metals with high density, such as iron, absorb radiation well and are easily visible in the X-ray image. Plastics, on the other hand, are very difficult to image with transmission X-rays because their density is low and they are penetrated very easily. To image organic materials well, the backscatter X-ray technique is recommended. Backscatter imaging and transmission X-ray methods thus complement each other.
In the backscatter method, the X-ray source and detector are integrated into one device. The object under examination only has to be accessible from one side, which allows a more flexible application. In contrast to transmission, in backscatter the cone-shaped radiation is fanned out by a shutter and then transformed into a moving beam by a rotating disk (chopper). The object is now scanned linearly at a high frequency (~150 Hz). If the backscatter device is moved along the object, the generated beam penetrates the surface and interacts strongly with the material in a material-specific manner. It is then backscattered and enters the detector area of the same device. The software immediately generates a digital, two-dimensional, monochrome grayscale image from the signals and displays it on the screen.
Advantages of backscatter imaging
Compared to transmission, X-ray backscatter imaging reveals other materials. The effect becomes more intense the lower the density of the elements. If the proportion of carbon, oxygen and hydrogen compounds (light elements) is higher, the backscattered radiation also becomes more intense. Therefore, a backscatter X-ray scanner can be used to image e.g. plastics, ceramics, water/moisture, wood, plants, etc. very easily. The method is therefore most effectively used when the different material densities of the irradiated object create a contrast. This opens up many application scenarios application scenarios.