Pumping out of a vacuum vessel and housing of a scanner is facilitated by providing a hole through a radiation shield. To reduce radiation of heat via this hole to an inner coolant containing vessel a closure member is provided. This is held in spaced apart relationship during pump down by clips. The closure member is provided with a ferrous constituent or member which is then attracted inwards during initialization of the superconducting. This draws the closure member into engagement with the shield to close the hole and to prevent radiation thereby. The clips are profiled to maintain the abutment of the closure member over the hole.