Muon radiography simulation for underground palace of Qinshihuang Mausoleum

Abstract

Muon radiography is a nondestructive imaging technology based on the naturally existing cosmic ray muons. Because cosmic ray muons have the strong ability to penetrate, muon radiography in which the absorption of muons through matter is utilized, is especially suitable for the imaging of large-scale objects. While the traditional geophysical technologies used in archeology have some limitations, muon radiography is expected to become a powerful supplement in the nondestructive detection of large-scale cultural relics. Based on Monte Carlo simulation method Geant4, the muon radiography of the underground palace of Qinshihuang Mausoleum is studied in this work. A model of the underground palace of Qinshihuang Mausoleum is set up with GEANT4 program according to the data acquired by the previous archaeological study of Qinshihuang Mausoleum’s inner structure, as well as a reference model without these inner structure. By investigating the differences between the muon fluxes obtained from the two models, the muon radiography image of the inner structure of the model can be obtained. During the simulation, the cosmic ray muon source is generated by sampling according to an empirical formula summarized by Reyna, which can accurately describe the energy spectrum and angular distribution of cosmic ray muons at sea level. In addition, two viewpoints are selected in order to determine the three-dimensional position of the chamber. The simulation data are processed by using an image reconstruction algorithm which can be described as the following three steps. Firstly, the counts of muons in different directions are converted into muon flux. Secondly, the muon flux of the reference model is deducted from that of the Qinshihuang Mausoleum model, and the angular coordinates of the chamber walls are determined. Finally, combined with the wall’s angular coordinates obtained from the two viewpoints and the relative position between the two viewpoints, the chamber size and its position are reconstructed according to the geometric relationship. The errors of the reconstructed chamber center position and the length of chamber walls are both approximately 7%. In this article, we conduct only a preliminary study of muon radiography applied to the nondestructive detection of Qinshihuang Mausoleum, but the results show that muon radiography can be a promising tool for the archeological study of Qinshihuang Mausoleum. In the follow-up study, more factors will be taken into consideration, including the details of Qinshihuang Mausoleum model, and the improvement of image reconstruction algorithm.