INTRODUCTION

Cæsium atoms at a temperature of only 3$\mu$K can now be produced by laser cooling. These atoms have a velocity of only about 10 mms$^{-1}$ - a few times the recoil velocity from a single photon. Such ultracold atoms allow much longer measurement times than have previously been possible, together with smaller systematic shifts. These features have been demonstrated for RF and microwave transitions by Kasevich et al. [1] [online] who made the first working atomic fountain using sodium, and Clairon et al. [2] [3] [4] who made a cæsium atomic fountain. The advantages of this technique were first considered by Zacharias who attempted to use velocity selection to implement the Ramsey separated oscillatory field method in a fountain by allowing the atoms to interact twice with the same cavity, once on the way up and again on the way down [5]. This symmetric configuration reduces the cavity phase shift which is one of the most troublesome problems in present primary frequency standards. In this paper, we report on our work to make a frequency standard following this approach. Firstly we give an overall view of the operation of the fountain and then, in the following sections, a more detailled description of those aspects which are specific to our system.