Absolute Frequency of the Hydrogen Maser Using a Flexible Storage Bulb

Abstract

The hydrogen maser is a very highly stable frequency source having a fractional stability over periods longer than one second of a few parts in 1014. Unfortunately, difficulties in measuring the wall shift of the hydrogen maser cause uncertainties of several parts in 1013 in the absolute frequency of the hydrogen maser. A new technique for measuring the wall shift shows promise for considerably improving the accuracy of wall shift measurements, while greatly decreasing the time required for the measurements. Wall shift measurements are usually performed by making oscillation frequency measurements on two similarly coated storage bulbs which have different volume-to-area ratios, and thus different mean-free paths between wall collisions for the atoms. In the new technique described here a flexible storage bulb is used instead of the usual rigid bulb, and the mean-free path is changed by squeezing the flexible bulb instead of substituting a smaller rigid bulb for the original bulb. Since the same surface is used for both measurements, anomalous wall shifts caused by ``bad spots'' in a coating do not affect the accuracy of the wall shift measurements. This paper reports measurements made with the hydrogen maser on several varieties of flexible storage bulbs. An FEP Teflon bulb, and a polyethylene bulb coated with Fluorolube were the most promising of the flexible storage bulbs used. It is expected that further development of the flexible storage bulb technique will lead to accuracies for the absolute frequency of the hydrogen maser which approach its present frequency stability.