Measurement of clean-up in gas discharge tubes using radioactive krypton

A radioactive tracer, krypton 85, is used to estimate rare gas clean-up in pre-t.r. tubes, operating at microwave frequencies. Measurements of ß-radiation along the length of the tubes, which are made of silica, indicate the regions in which clean-up occurs and allow an estimate to be made of the gas pressure at intervals during the life of the tubes. It has been established that the rate of change of pressure with time is a constant, dependent upon the power absorbed in the discharge. It does not vary with the gas pressure in the tube nor does it vary with the amount of gas previously absorbed. In one series of experiments, tubes containing 18cm3 of krypton at a pressure of 16mmHg were subjected to an incident mean power level of 3 kW at 3Gc/s. The pressure decreased to about 2mm Hg in 3000 hours, corresponding to a rate of clean-up of 10¿4cm3 per hour at s.t.p. The rate of clean-up under given conditions can be measured by this technique within a few hundred hours, so that ultimate life can be predicted without prolonged life tests.