PROGRESS TOWARDS A CESIUM ATOMIC FOUNTAIN FREQUENCY STANDARD. William M. Klipstein, Building 221, Room A167, National Institute of Standards and Technology, Gaithersburg, Maryland, USA. (301-975-4208; email:

One of NIST's primary responsibilities is to provide the nation's standards of time and frequency. We have been developing a fountain of laser--cooled cesium atoms for use as the next generation atomic frequency standard. In our fountain, atoms in a slowed atomic beam are first collected in a Magneto-Optic Trap (MOT) and then laser-cooled to a few microKelvin in optical molasses. The cooled atoms are then launched vertically into a "moving molasses" by shifting the frequencies of the vertical cooling beams. The atoms then travel through a microwave cavity tuned to the 9.2 GHz cesium hyperfine. After roughly 0.5 seconds of free flight under the influence of gravity, the atoms fall back through the microwave cavity and into an optical state-detection region which detects the number of atoms making the hyperfine transition. The increased Ramsey interaction time improves the short--time precision as compared to traditional atomic beam experiments, while many systematic shifts which limit the accuracy of an atomic beam clock are reduced by the low atomic velocity and the retrace of the atomic trajectory through the microwave cavity. To date we have trapped, cooled and launched atoms, and detected their return to the detection region. We will discuss the progress towards a working fountain being assembled in our laboratory.