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

Abstract

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.