A Thallium Beam Frequency Standard

Following a proposal by Kusch a thallium atomic beam frequency standard has been constructed and put into operation. The main characteristic is that its frequency of 21,310 Mc is only very weakly affected by inhomogeneities of the C-field. The construction, which is similar to conventional cesium standards, differs from these in the following main points: a high (1000°K) oven temperature, high (14,000 gauss) A-and B-fields, an oxidized tungsten filament detector. The interaction length is 90 cm; the Q factor of the resonance is 160 million. The measurements show that the uncertainty in the Zeeman-shift is negligible, while the error arising from an unsymmetrical cavity is the same as in the case of cesium.