Application of cosmic ray muon imaging technology in the detection of internal structures of cultural relics

The internal structural conditions of large-scale stone carvings are difficult to assess using conventional nondestructive testing techniques because of their large size, complex geometry, and long service history. In this work, the 3D internal structure of the stone carvings at Mingshan Temple was investigated using cosmic ray muon imaging. Muon flux data acquired from multiple observation points and directions were used to derive path-integrated density distributions. A 3D density inversion model incorporating geometric constraints was then developed, and an iterative reconstruction algorithm with an adaptive-relaxation factor was applied to image the cavity roof structure and the overlying mountain body. A refined 3D model, constructed by integrating unmanned aerial vehicle-derived digital elevation data with interior point-cloud measurements of the grotto, was introduced as a geometric prior to improve reconstruction stability under complex structural conditions. The results reveal a distinct low-density anomaly on the southern side of the roof slab of cave 8, consistent with the gullies observed on site, demonstrating the effectiveness of the proposed approach. These results indicate that cosmic ray muon imaging provides a promising nondestructive tool for internal structural investigation and risk assessment of large-scale stone cultural heritage.