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We have developed a portable muography detector for internal infrastructure condition exploration and feasibility study for underground cavity detection [1]. The detector comprises two muon position sensitive detectors (mu-PSDs), with a detection area of 132 \times 132 mm$^2$ each. In a test measurement of a seven-story building from underground, we found this simple system could identify thick concrete walls and thick iron skins within two weeks. As a result, we have begun developing an upgraded detector for the next phase.
The detection concept of the second detector is similar to the first but has a four-fold larger detection area than that of the prototype. The mu-PSD is made up of scintillating fiber (BCF-10, Saint Gobain) connected to MPPC (S13360-3075CS, Hamamatsu). The data acquisition system consists of two EASIROC modules [2], driving a single mu-PSD. The coincidence events between the two modules are selected by a NIM coincidence module and saved in a front-end laptop.
A six-days open-sky measurement was conducted to measure the characteristics of the second detector. After atmospheric pressure correction, the average total event rate was 16354 counts per hour, with a standard deviation of only 74 counts, and the entire detection system is sufficiently stable enough for long-term measurement. Additionally, the detector measured lead blocks in the vertical direction to have a single pixel size on the resultant muography image with 5% absorption of cosmic-ray muons. The results showed that the detector has an angulare resolution of seven msr of angular resolution and the lead block absorbed 4.5$\pm$0.8% of the cosmic-ray muons which is in good agreement with the expected performance of the detector.
[1] K. Chaiwongkhot et al., IEEE Trans. Nucl. Sci. 65, 2316 (2018).
[2] I. Nakamura et al., Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. 787, 376 (2015).
