The Poisoning and Activation of Thoria-coated Filament Emitters

This paper describes an experimental investigation of the activation and poisoning of thoria-coated iridium and tungsten filament emitters. It is shown that activation is by the formation of free thorium. Pulse emissions are derived from both the coating and core surfaces, but under d.c. conditions the coating surface is deactivated and the emission derives mainly from the core surface. Poisoning is caused by oxygen water vapour and carbon dioxide but not by hydrogen and carbon monoxide. The coated tungsten resists poisoning better than the iridium, as free thorium is formed chemically by the former but not by the latter. Taking d.c. emission from either filament produces thorium olectrolytically and this gives an increased resistance to poisoning. Some of the thorium so produced dissolves into the iridium core and subsequently increases the resistance to poisoning. The coated iridium filament can withstand exposure to the atmosphere when hot, without appreciable deterioration.