Cryogenic hydrogen maser at 10 Kelvin

We have constructed and are evaluating an atomic hydrogen maser that oscillates at temperatures ranging from 8.5 to 11.5 kelvins. The principal scientific purpose for which the maser was constructed is to study the effects of hyperfine interactions during collisions on the radiative frequency shifts produced by the collisions. An equally important technological objective is to explore the reductions of short term frequency fluctuations postulated for low temperature operation. Our maser oscillates strongly using atomic beam intensities about 10¹² state selected atoms s^-1, with output power (at temperatures above 10 K) essentially undiminished by collisions between atoms or with the storage surface. Preliminary frequency measurements indicate that the short term stability of our cryogenic maser is at least as good as the measurement limitations imposed by the short term frequency stability of the conventional hydrogen masers that we use as frequency references