Development of a prototype portable muography detector for exploration of underground cavities

A prototype portable muography detector was developed for exploration of underground cavities and some performance tests were conducted. The detector consists of two muon position sensitive detectors (mu-PSDs) having an area size of 140 mm × 140 mm which were placed in a compact light shielding box. Each mu-PSD was fabricated using plastic scintillating fibers (PSFs) connected to multi-pixel photon counters (MPPCs). The pixel size of 8.75 mm × 8.75 mm and the distance of 100 mm between the two mu-PSDs were determined so as to satisfy the solid angle resolution of 30 msr required from subsidence risk cavity exploration. The detector system worked stably by using a Peltier cooling and heating module for temperature control function. An imaging test of interior structure of a seven-story concrete building was successfully performed using the muography detector placed on the basement.