中子相衬成像技术初步研究

Preliminary study on thermal neutron phase contrast radiography

利用不同材料的中子质量衰减系数不同而获取样品内部的结构潜像,属于吸收散射衬度成像;而相衬成像是基于中子的波动性,当中子波穿过样品时,由于样品对中子的折射,不同厚度、不同材质引入的相位差不同,从而导致波前畸变,畸变的波前在穿过样品后作为次波源互相干涉成像,从而获取样品内部结构潜像。研究工作通过数值模拟方法分析了中子相衬成像过程,并基于反应堆中子照相装置进行了不同条件下的多色热中子相衬成像实验。模拟与实验结果表明,在适当准直比条件下,铝材料边缘将出现明显的边缘增强效应,该效应与准直比、样品与转换屏之间的距离密切相关。

Background： Conventional neutron radiography is an absorption contrast imaging method which is based on wave character. When neutron wavefront traverses the inspected sample, the variations of the thickness, material and neutron refractive index of the sample bring different phasic difference, which would cause a change in a shape of a neutron wavefront, and the aberrance wavefront interference behind the sample leads to enhancement of the image sharpness. Purpose： The aim is to validate thermal neutron to carry out phase contrast imaging. Methods： Numerical simulation of neutron edge enhancement imaging is processed by MATLAB based on reflection theory and experiment is carried out at a reactor using polychromatic thermal neutron beam line. Results： The thermal neutron phase contrast imaging can be realized in CMRR （Chinese Mianyang Research Reactor） neutron radiography device when the L/D is 2 000 and the distance of sample between scintillator is 60 cm. Conclusion： Both the simulation and experiment results show that a proper L/D condition would cause image enhancement at the edge of aluminum materials and the monochromatic neutron source is not necessary. The effect is related to L/D and the distance between sample and scintillator.