A substrate is made of copper having an atomic number of 29. The substrate is formed in the shape of a box without a top, and has a rectangular bottom and sidewalls erected at four sides surrounding the bottom. A scintillator is evaporated onto the bottom. The scintillator includes a non-columnar crystal and a plurality of columnar crystals erected by crystal growth. A photodetector tightly adheres to top surfaces of the sidewalls of the substrate through an O-ring, so as to close the top of the box-shaped substrate. The substrate, the photodetector, and the O-ring seal the scintillator in an air-tight manner.