An apparatus is described which allows direct measurements of the reflection coefficient of cryopumps, the complement of the sticking coefficient. The reflected fraction of a gas flow directed towards a cryopump is measured directly. This gives accurate results especially for high sticking coefficients. Measured values quest; 0.995 of the sticking coefficient of nitrogen gas on a 16 K cryopump are presented as a function of the gas flow per unit area of the pump. The density of the condensed layer is nearly the same as for solid nitrogen ice.
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