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Atomic Hydrogen Masers at the CSIRO

Two active hydrogen masers constructed at the CSIRO Division of Applied Physics have been contributing to the laboratory's frequency and time scales since 1982. Design and performance details are discussed. Vacuum-pump lifetimes necessitate servicing of the masers every few years and these operations can cause frequency drifts of 2 × 10-15 per day due to contamination of the maser bulb surface. However, recovery, to 2 × 10-14 per year occurs over a year or two. By staggering service operations, continuous frequency and time scales are achieved which compare favourably with such scales in other countries.