Zworykin and Malter have recently discovered that it is possible to obtain a much higher ratio of secondary electrons from a target electrode when the sensitive material, such as caesiated silver, is separated from the metal backing plate by a very thin layer of insulating material. The target is first subjected to an initial bombardment of electrons, which appear to have a polarising effect, so that the composite surface becomes abnormally sensitive. So much so that it will release as many as 3,000 secondary electrons for each primary electron which strikes against it. This so-called 'anomalous' emission compares with a ratio of nine or ten which was previously considered to be the best obtainable.
According to the invention, a target electrode, capable of giving anomalous secondary emission, is prepared by depositing a very thin layer (of the order of one thousandth of an inch) of an alkaline-earth borate upon a metal backing plate, and then covering the borate with an equally thin layer of caesium.
Marconi's Wireless Tefegraph Co., Lid. (assignees of F R Pidre). Convention date (USA), January 30th, 1937. No. 491287.
From Wireless World February 2, 1939.