Evolution and absorption of gases in electron tubes

Abstract

Gas-evolution effects in vacuum tubes have been determined by use of the gas-collection method of Morrison, in which evolved gases in an operating tube are collected with a mercury pump, stored in its fore-vacuum system, and measured with a mass spectrometer after many hours of tube operation. The speed of absorption of the tube for various gases has also been determined by simultaneously leaking gases into the tube and using the same collection system, In this way the partial pressures of gases and their internal evolution and absorption rates in the operating tube were found. These techniques were applied to a practical vacuum tube, the 21CYP22 colour-television picture tube. Typical partial pressures of gases determined during electrical operation of the tube were: hydrogen, 10 -8 Torr; nitrogen (including a small fraction of carbon monoxide), 10 -9 Torr; and oxygen, 2 × 10 -11 Torr. Oxygen is present only when the tube is scanned, and is evolved by non-metallic bulb surfaces which are bombareded with high-voltage electrons. Self-absorption of the tube for oxygen is very rapid (pumping speed is 3000 l./sec). Thus, oxygen partial pressure is maintained at such a low level during operation of this tube that there is no effect on performance of the oxide cathode.