Image Calibration Based on Dynamic No-Load Data for the 60Co Gantry-Movable Dual-Projection Radiography Inspection System

The 60Co gantry-movable dual-projection radiography inspection system is newly developed, aiming to the inspection of vehicles entering and exiting the nuclear facilities. It adopts two Co-60 radioactive sources and two arrays of gas ionization chambers corresponding to the two sources, respectively. They can move synchronously with the gantry driven by the mechanical and control subsystem. So, dual projections could be obtained through one scan from two different directions. Compared to a single projection, the dual projections make it easier to found hidden objects and distinguish whether a dark area is due to overlapping objects or because there are well-shielded prohibited items such as nuclear materials. Therefore, it is helpful to found well-shielded nuclear materials and prevent them from being stolen. However, problems also come due to the using of two radioactive sources and the moving gantry. For the former one, it will bring about scattering effect between two sources, while for the latter one, the signals of the detectors would fluctuate as the gantry moves, owing to the vibration of collimator and gantry as well as the non-synchronous movement of sources and detectors. So, the radiography projections are needed to be corrected. In response to the second question, the no-load data is repeatedly measured when the gantry is in different positions, then a method of image calibration based on the dynamic no-load data is proposed for this inspection system to replace the correction with average no-load data. Result shows that the corrected no-load image turns smoother, meaning that the method of dynamic correction could effectively improve the radiation image.

Study on Scattering Correction of the 60Co Gantry-Movable Dual-Projection Digital Radiography Inspection System

Nuclear safety and security is getting more and more attention all over the world. Vehicles in and out of nuclear facilities need to be strictly inspected to prevent carrying of nuclear materials, explosives or other dangerous items. A new-type vehicle inspection system — 60Co gantry-movable dual-projection digital radiography inspection system is developed in Tsinghua University under the support of China’s Nuclear Energy Development Project, which uses two 60Co as radiation sources. The radiation sources are arranged at the bottom and side of the vehicles to be detected and the ionization chamber detectors are set in the two side of the gantry correspondingly. With moving of the sources and gantry synchronously, the system can obtain both side-view and upward-view images of the vehicles simultaneously [1]. However, there is a problem that the scattered rays from source of one projection can enter the detector array of another projection to form an interference signal. Experiments show that this kind of scattering noise can account for 20%, resulting in blurring and even artifacts, especially in thicker areas of the image. This problem needs to be solved urgently. According to the characteristics of the Compton scattering effect, it is inferred that there is a certain non-linear mapping relationship between the scattering distribution of the detector array of one projection and the mass distribution in the plane of the other projection. This paper attempts to use BP neural network to learn this mapping relationship to quantitatively remove this kind of scattering noise. The results show that this method has certain effects on the removal of artifacts and blur caused by scattering. This method has the advantage of being fast and more targeted, compared with traditional image post-processing methods.