PURPOSE: To prevent droppage of adsorbing material by fixing a planar holding member having many openings for providing a gap against the metal face to be cooled cryogenically through refrigerant through spacer members and holding the adsorbing material between the metal face and the holding member.

CONSTITUTION: Frame-like metal spacer members 14 are secured to the cryo face 5 of a metallic sump 12 for storing the refrigerant 11. A metal mesh 16 is stretched over the spacer members 14 while granular adsorbing material 6 is placed in the gap between the metal mesh 6 and the cryo face 5. The mesh of said metal mesh 16 is set smaller than the size of the adsorbing material 6 while the height of the spacer member 14 is set same or slightly smaller than the size of the adsorbing material 6 thus to hold the adsorbing material 6 between the cryo face 5 and the metal mesh 16 while contacting tightly against the cryo face 5. Consequently, the adsorbing material 6 can be held reliably without employing organic adhesive which will degrade upon irradiation of radiation.