COLLISIONAL FREQUENCY-SHIFT WITH A PRIMARY FREQUENCY STANDARD

Collisional frequency shift can be a serious problem affecting the ato mic fountain cesium standard. In this paper, I show that it can also b e a problem for a traditional cesium standard employing an atomic beam that passes through two small, widely separated holes. The atomic wav e packet is broadened perpendicular to the atomic beam. Atomic relativ e motion is parallel to the atomic beam and the wave packet is broaden ed transversally to relative motion. The collisional cross section is then much larger than that in a room temperature cell, as the collisio nal cross section is a value measured on a plane that is transverse to the relative motion. This cannot be derived from the previous formula for the collisional cross section, based on assumptions that the angu lar momentum of relative motion is deterministic. Note that the collis ional cross section does not reach infinity even if the transverse vel ocity component is definitely zero.