Possible use of a rubidium maser as a combined frequency standard

The results are reported of an experimental investigation of an optically pumped rubidium maser with its resonator filled with various buffer gases and their mixtures. It is shown that the frequency shift of the rubidium line due to the presence of a buffer gas may be eliminated by filling the resonator with a mixture of nitrogen and methane. The achievement of self-oscillation in a rubidium maser with a zero frequency shift of its line should make it possible to use this maser in conjunction with a rubidium atomic-beam tube in a combined frequency standard.