cryogenic pump — A cryopump is a vacuum pump that traps gases and vapours by condensing them on a cold surface. They are only effective on some gases, depending on the freezing and boiling points of the gas relative to the cryopump's temperature. They are sometimes used to block particular contaminents, for example in front of a diffusion pump to trap backstreaming oil, or in front of a McLeod gauge to keep out water. In this function, they are called a cryotrap or cold trap, even though the physical mechanism is the same as for a cryopump. Cryotrapping can also refer to a somewhat different effect, where molecules will increase their residence time on a cold surface without actually freezing. There is a delay between the molecule impinging on the surface and rebounding from it. Kinetic energy will have been lost, the molecules slow down. For example, hydrogen will not condense at 8 kelvins, but it can be cryotrapped. This effectively traps molecules for an extended period and thereby remove them from the vacuum environment just like cryopumping.
References: Wikipedia, the free encyclopedia – http://en.wikipedia.org/wiki/Cryopump