多重库仑散射对小尺度物体缪子透射成像精度的影响

Influence of multiple Coulomb scattering on accuracy of muon transmission imaging of small-scale matter

缪子透射成像方法是一种基于宇宙线缪子穿过目标物体前后的通量变化,进而获得其内部密度结构的无损探测成像方法.缪子透射成像方法假设缪子在低原子序数物质中沿直线运动,但实际上多重库仑散射作用会使缪子路径一定程度上偏离直线,有可能对成像精度造成影响.为此,本文使用Geant4软件包开展了缪子透射成像蒙特卡罗模拟,针对数米尺度多种密度结构的模型,定量分析了打开和关闭多重库仑散射物理过程时对成像精度的影响.结果表明:对于数米尺度标准岩石物质,缪子透射成像方法能够很好地恢复内部密度异常几何特征;但多重库仑散射作用对目标物体内部区域近垂向缪子通量造成的偏差可达5%,而在目标物体边界区域的偏差可达13%.因此,需要宇宙线缪子透射成像中考虑多重库仑散射作用的影响,以获得数米尺度目标物体更准确的绝对密度值成像结果.

The muon transmission imaging method is a non-destructive detection imaging method and can obtain the internal density structure of the target object by analyzing the flux change of cosmic ray muon before and after passing through the target object.This method assumes that muon travels along a straight line in a lowatomic number material.However,the multiple Coulomb scattering causes the muon deviate from the straight line to a certain extent when penetrating the material,which may have a certain influence on the accuracy of muon transmission imaging.This study uses the Geant4 software package to carry out Monte Carlo simulation of muon transmission imaging.Object models with multiple density structures and several meters are used to analyze the effect of multiple scattering on the accuracy of transmission imaging.In the experiment,we set up a model in which muons of different energy vertically pass through a rock with a certain thickness,we can intuitively see the influence of scattering on the penetration path of muons.By setting up rock-water block models with thickness in a range of 0.8,2.4 and 4.0 m,the effect of Coulomb scattering on the transmission imaging of small-scale material muons is analyzed.The results suggest that the muon transmission imaging method can well restore the geometric shape and spatial distribution characteristics of density anomalies for objects with several-meter scale for standard rock materials with a scale of several meters.However,the flux deviation caused by multiple Coulomb scattering on the muons in the near-vertical direction can reach 5%,and up to 13%in the boundary areas of the standard rock and air.We limit the scattering angle of the muon,and select the muon with a scattering angle of less than or equal to 1°for imaging.The results of transmission imaging by using the selected muon have improved.The image does not have the illusion of an abnormally increased flux around the model caused by scattering,but the muon flux in the model area is reduced even more,thereby affecting the accuracy of restoring the absolute density of an object using flux differences.Therefore,the effects of multiple Coulomb scattering should be considered for recovering more accurate absolute density in small-scale muon transmission imaging study.