An Improved Cæsium Frequency and Time Standard

Abstract

PRELIMINARY measurements have been completed with the longer and more precise standard mentioned at the conclusion of our earlier work¹. The beam chamber, including the diffusion pump, is 5.2 m. long and the distance between the cavity resonators in which the transitions are excited is 2.77 m. The width of the resonance curve is 52 c./s. and at the resonance frequency of approximately 9,200 Mc./s. this corresponds to a Q factor of 1.8 × 10⁸. The theoretical line width is 55 c./s., given by the formula δF = 0.65 α/L, where α is the most probable velocity of the atoms and L is the distance between the cavities. The difference between the experimental and theoretical values could easily be due to some velocity selection. The resonance curve is reproduced in Fig. 1, in which the experimental points are individual measurements. They illustrate the steadiness of the conditions and the signal-to-noise ratio. The pressure was 3 × 10⁻⁷ mm. mercury, and was obtained after 2 days pumping with a 9-in. 3-stage fractionating oil-diffusion pump, and with the help of liquid-nitrogen traps.