Research On Key Problems Of γ-photon 3D Imaging Technology In Industrial Inspection

The industrial complex parts such as the engine parts and nozzle pneumatic runner in aviation propulsion system have high material density and complex internal cavity structure.The non-destructive test for internal cavity structure and inner wall defects in complex parts has become an urgently needed solving problem in the field of testing methods.Conventional testing methods are subject to factors such as the material density and composition of complex parts,which cannot achieve undisturbed and nondestructive high-resolution detection.Based on the positron annihilation theory and γ-photon detection mechanism,the key problems of γ-photon scattering and γ-photon three-dimensional image reconstruction noise interference are studied facing the detection characteristics of practical industrial complex parts,which aims to explore a new testing method for detecting inner cavity of complex parts in aviation propulsion system.In addition,γ-photon has the advantages of strong penetrating ability and strong anti-interference ability,which ensures the technical stability of γ-photon 3D imaging technology in industrial inspection applications.During the process of γ-photon penetrating from high-densed complex parts to the detector,γ-photon will have scattering events due to the Compton scattering effect and Rayleigh scattering effect.The reconstructed image has artifacts,edge blur,etc.,thus the detection resolution is reduced.Aiming at the problem of single scattered γ-photons,this paper proposes a trajectory mapping basedγ-photon single scattering correction method,which based on γ-photon trajectory mapping attenuation theory.The Lo R projection is mapped into the current reconstructed image,and the single scattering angle is combined to determine the trajectory of single scattered γ-photon.The number of single scattered γ-photon is determined the attenuation of the scattered γ-photons their motion trajectories.Aiming at the problem of multiple scattered γ-photons,this paper proposes a spatial distribution fitting based γ-photon multiple scattering correction method,which based on γ-photon Compton scattering theory.It regards the convolution of the distribution function of the single scatteredγ-photons and a Gaussian kernel function as the distribution of multiple scattered γ-photons.The kernel function coefficients are determined by the spatial distribution fitting of the scattered γ-photons,thereby the number of multiple scattered γ-photons can be calculated.A γ-photon 3D imaging detection comparision experiment is conducted to verify the effectiveness of trajectory mapping basedγ-photon single scattering correction method and spatial distribution fitting based γ-photon multiple scattering correction method.The experiment is to detect the cylinder block structure and piston nut shape in industrial hydraulic parts as the object and the data used for image reconstruction is theγ-photons before and after γ-photons single/multiple scattering correction.The classical γ-photon image reconstruction OSEM algorithm belongs to the unconstrained optimization algorithm.In the image reconstruction process,it is assumed that each image pixel passed by Lo R contributes to the generation of Lo R,and the different pixels belonging to the same Lo R interact with each other.This causes statistical noise between the pixels on the Lo R and the noise is amplified and propagated,which makes the reconstructed image has too smooth edges,which reduces the signal-to-noise ratio and contrast of the reconstructed image.Aiming at the γ-photon image reconstruction noise interference problem,an improved OSEM algorithm of ToF based data recombination is proposed in this paper,which based on the mapping principle of γ-photon ToF information and positron annihilation position.The solution constrained image reconstruction model with ToF information as index is established.Under the premise of not increasing the dimension of the system matrix,the Lo R data is recombined using the ToF information as the index and the solution constrained improved OSEM algorithm is proposed to reconstruct the image.The weight coefficient with ToF information is used to adjust the system matrix components and the system matrix is sparsed to reduce its size.By which the contribution of image pixels to Lo R is consistent with the distribution probability of the positron annihilation “Position-ToF” curve of the image pixels with ToF information,the purpose of eliminating stastical noise propagation is achieved,thereby improving the signal-to-noise ratio of the reconstructed image.The effectiveness of the improved OSEM algorithm of ToF based data recombination algorithm is verified by comparing the results of γ-photon 3D imaging detection of the cylinder block structure and piston nut shape in industrial hydraulic components with the classical OSEM algorithm.This paper takes the physical γ-photon detection system and system in GATE simulation software as the experimental platform,and uses the industrial metal casting with internal cavity structure as the detection object.Through the γ-photon 3D imaging detection experiment of the inner cavity in the metal casting,the parameters such as the inner cavity diameter and spiral distance were measured.The γ-photon scattering correction experiment and image reconstruction experiment from the improved OSEM algorithm of ToF based data recombination algorithm are performed to quantitatively measure and analyze the parameters of the inner cavity.The research works in this paper solve the key technical problems of γ-photon 3D imaging technology in the industrial detection,and provide a new theoretical method for γ-photon 3D imaging technology in the non-destructive detection of the internal cavity structure and inner wall defects of industrial complex parts,making it a new industrial testing method.