accommodation portion of the sapphire light transmission plate 11 a and the sapphire light transmission plate 11 a is placed and fixed therein the accommodation portion 14 d for the gasket 6 a is formed in the undersurface side of the first fixing flange 14 and the upper half portion of the gasket type seal 6 is inserted and fixed therein as illustrated in fig7 the gasket type seal 6 is configured from the gasket accommodation portion 14 d of the first fixing flange 14 the gasket accommodation portion 17 a on the side of the detector main body 2 the ring like gasket 6 a a ring like retainer 6 b a ring like guide ring 6 c and the like and is configured to be sealed double by the seal surfaces 15 and 15 it is confirmed that the sapphire light transmission plate 11 a constituting the light transmission window formed of the plate member 11 contains a brittle fracture material is a so called single crystal of high purity alumina al 2 o 3 formed with a thickness of 0 8 to 1 5 mm and has excellent abrasion resistance corrosion resistance chemical resistance heat resistance and the like and thus the sapphire light transmission plate 11 a is hardly corroded and degraded by organic raw material gas when used for semiconductor manufacturing and the optical transparency thereof is hardly changed since the gasket type seal.