PURPOSE: To reduce the absorption X-rays by isolating rare gas for a counter operation and a counter connection part from each other double with an X-ray transmission window made of a high polymer organic film and an X-ray transmission window of beryllium thin plate and controlling the degree of vacuum of the intermediate part.

CONSTITUTION: The ultra-high vacuum side of the X-ray transmission window 1 uses the beryllium thin plate whose both surfaces are coated with a collodion film and is adhered to a housing 2 with epoxy resin. The X-ray counter side of the X-ray transmission window 3 uses 'Capton(R)' film and its airtightness is held with a double 'Biton(R)' O-ring 4, housing 5, and cap 6, minimizing the leak of the thin gas for X-ray counter operation. the space between the X-ray transmission windows 1 and 3 is evacuated by a noble pump.