缪子对小尺寸中低原子序数物质三维成像技术的模拟研究

Simulation Study of 3D Imaging Method for Low and Medium Atomic Number Objects at Small Scale by Cosmic-ray Muon

缪子成像是一种利用天然本底辐射中极具穿透力的缪子射线通过散射或透射技术对高原子序数Z或隐藏物体进行无损成像的技术。当前的研究对象是大尺寸的高Z物质,而针对小尺寸的中低Z物质的成像技术在国内鲜见报道。本文提出了一种针对小尺寸中低Z物质的三维成像技术,利用Geant4软件模拟了缪致次级粒子的产生过程,模拟结果表明次级粒子主要为电子和γ光子,通过缪致次级粒子与入射缪子符合探测技术筛选入射缪子径迹,利用有限角度成像算法对待测物体进行了三维图像重建,结果表明,该技术适合于小尺寸的中低Z物质成像,尤其对小尺寸中Z物质的重建效果明显,重建图像可区分0.8 cm的凹槽。

Cosmic-ray muon imaging is a new non-destructive technique that uses highly penetrating natural muon to reconstruct objects with high atomic number(Z)or subjects hidden in something by applying muon scattering or transmission technique,respectively.Up till now,muography mainly focused on objects with high Z or large volume.But few studies have been reported on objects at small scale with low or medium Z in China.A method that aims to reconstruct 3D image for objects at small scale as well as low or medium Z has been brought out in this paper,which uses the amount of muonic secondary particles and coinciding muon trajectories in different directions to reconstruct objects.A simulation about how muonic secondary particles generate and their performances based on Geant4 toolkit was carried out.The results of this simulation show that the secondary particles are mainly composed of secondary electrons and gamma photons.And as the thickness of the objects increases along one certain direction,the amount of secondary particles increases approximately linearly.Therefore,the amount of them carries the shape and location information of objects.Furthermore,the energy of muonic secondary particles is mainly beyond 0.1 MeV,so secondary particles can be distinguished with the background noise in the experimental environment.After selecting coincide muon trajectory through injected cosmic-ray muon and its secondary particles coincide detection technique,the 3D images of three 3 cm×3 cm×3 cm cubes made up from different materials(iron,water,and lead)were reconstructed based on limited-angle computed tomography algorithm.It can be seen from the imaging results that the reconstructed image of iron has best quality.Both water and lead have some voids in their reconstructed images,but the voids of water is apparently less than those of lead.Thus,this imaging method is fit for imaging small scale objects with low or medium Z and the imaging effect of objects at the small scale with medium Z is especially good.A reconstruction of complex objects which is defined as“USC”letter model has also been made.The reconstructed image can match with the size and location of the geometry model in Geant4 and can also distinguish the intervals of 0.8 cm in the letter“S”.A much better imaging accuracy will be achieved by optimizing the mathematical model of the algorithm.This method extends the application area of muon tomography.