Experimental Research on Material distinguishment Based on Muon Discrete Energy

Cosmic-ray muon is a kind of charged particle with strong penetrating ability and sensitivity to high-Z materials. In recent years, muongraphy emerged as a novel technique by using cosmic-ray muon to image unknown volume, widely applied to nuclear security, geosciences, archaeology and civil engineering. Muon has multiple Coulomb scattering with traversed matter and the scattering angle is relative to material Z and muon energy. If we could measure muon scattering angle and muon energy, we could identify the category of material interacted with muon. However, it is difficult to measure the energy of each muon in real time, most of the research use the mean muon energy ~4GeV to alternate each muon energy which cause obscure of image and reduction of material identification accuracy. This article introduces a method, exploiting the discrete muon energy to fit the scattering angle distribution derived from continuous muon energy, estimating the weight of each discrete muon energy. Then measure the scattering angle of the material to be identified, histogram the scattering angle and couple it with the calibrated scattering angle distribution by discrete muon energy. The last step is calculating the radiation length according to the above equation. Radiation length is a characteristic quantity of the material, used to identify materials in this method. We carried out the materials identification experiment based on the apparatus of cosmic-ray muon tomography made by CIAE. The system contains 6 layers, consist of several drift tube detectors planted in two orthogonal coordinate for tracking muon trajectory. Five materials were selected to distinguish in the experiment: C, Al, Fe, Pb, W. Measure the scattering angle formed by muons passing through these materials respectively, calculate their radiation length and identify them in pairs. In order to evaluate material identification accuracy, we plotted ROC curve between each pair of materials. The result reveals that Al-Fe, Fe-Pb can be effectively distinguished at 95% confidence level, under 1400 valid muon events. Considering about difference in material’s density also influences identification accuracy, we simulated Pb discriminated against several other materials. It is concluded that this method can distinguish materials with radiation length greater than 0.7cm. Compared with the identification method of unknowing muon energy, this method improves the accuracy of Pb-W identification by 18.5%.