A New Secondary Cathode

One of the main difficulties in the use of secondary-emission multiplier stages in thermionic valves is the instability of the secondary cathode, which is usually characterized by a gradually decreasing secondary-emission ratio during its useful life. In this paper, experiments are described showing that this instability is due to the bombardment of the secondary cathode by the primary electron beam, and it is shown that this bombardment causes disintegration of the surface layer. A new type of secondary cathode, formed from a mixture of magnesium oxide and barium carbonate, has been found to have a stable life of over 1 000 hours, when operating at 400° C and with a primary bombarding current density of 20 mA/cm2. The use of this secondary cathode has made possible the development of a single-stage thermionic electron-multiplier valve, with a mutual conductance of 20 mA/volt.