US 2702259 A
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United States Patent MANUFACTURE OF ELECTRODES WHICH ARE SENSITIZED SO AS TO BE EMITTERS 0F PHOTQ- ELECTRONS OR SECDNDARY ELECTR'GNS Alfred Herman Summer, Iver, England, assignor to Electric & Musical Industries Limited, Hayes, England, a British company No Drawing. Application August 9, 1951, Serial No. 241,151
1 Claim. ((11. 117211) This invention relates to the manufacture of electrodes which are sensitized so as to be emitters of photoelectrons or secondary electrons.
One form of electrode which has been proposed hitherto for use as a semitransparent photoelectric cathode in electron discharge devices, comprises a layer of bismuth on to which is evaporated a layer of silver and which isthen exposed successively to oxygen, caesium and again to oxygen. Such an electrode may be formed, for example by evaporating a layer of bismuth on to a suitable transparent support, for example on the inner surface of a glass envelope, until the light transmission through the support is reduced to about 60 to 70 per cent of its original value. Thereafter, silver is deposited by evaporation on to the layer of bismuth until the total light transmission is reduced to about 50 to 60 per cent. The silver layer is then oxidized by exposing the support in an electric discharge in oxygen until the light transmission increases to the value prior to the silver deposition, the assumption being that silver oxide is completely trans parent. Thereafter the support is exposed to caesium vapour by conventional methods while baking the support at about 150 C. and finally, after cooling the support, superficial oxidation of the layers is effected. This method has however, the disadvantage that consistent sensitivities are not obtained. Occasionally a sensitivity of the order of 40 ,ua/L. (micro-amperes per lumen of incident light) is obtained, but sensitivities between 15 and 25 a/L. are more usual and, for reasons which are not apparent, sensitivities as low as 5 to 8 a./l. are quite frequent. There is also the disadvantage that the sensitivity has a tendency to decrease after the envelope of the device has been sealed off the pump, in some cases to a value as low as half the initial value, this decrease being particularly marked for red light.
The object of the present invention is to reduce the above-mentioned disadvantages.
According to the present invention there is provided a method of manufacturing an electrode which is sensitized to be an emitter of photoelectrons or secondary electrons comprising the steps of depositing successively on a suitable support a layer of bismuth, a layer of silver, a layer of bismuth and a further layer of silver, oxidizing the last-mentioned layer of silver and subsequently sensitizing the electrode by exposure to an alkali metal.
In one example of manufacturing a semi-transparent photocathode in accordance with the present invention, a layer of bismuth is deposited on a transparent support in an evacuated envelope by evaporation until the lighttransmission through the support is reduced to about 75 per cent. A layer of silver is then deposited by evaporation on to the bismuth layer until the light transmission is reduced to about per cent. Thereafter a second layer of bismuth is deposited until the light-transmission is about 55 per cent and then a second layer of silver is deposited until the light-transmission is about 50 per cent. Subsequently oxygen is introduced into the envelope and if necessary an electric discharge is initiated and maintained in the oxygen until only the top layer of the silver is oxidized, as indicated by a restoration of the light-transmission through the support and the deposited layers to about 55 per cent of the value before the deposition of the top layer of silver. Caesium vapour is thereafter introduced and the electrode is sensitized While the envelope is being baked, in known manner. Superficial oxidation may then be effected after cooling the envelope, although in accordance with the present invention satisfactory results have been obtained without such superficial oxidation.
Phitoelectric cathodes manufactured in this way have been foundto have sensitivities about 40 a/L. and the results appear to be reproducible. Sensitivities up to a/L. have been obtained. Moreover, the sensitivity and in particular the red response has been maintained under test over several Weeks, and no fatigue has been observed. The mechanism of the improvement which is obtained as a result of the present invention is not fully understood.
What I claim is:
A method of manufacturing an electrode sensitized to be an emitter of photoelectrons or secondary electrons, comprising the steps of evaporating a layer of bismuth on to a transparent support until the light transmission is reduced to the order of per cent, evaporating a layer of silver on the bismuth layer until the light transmission is reduced to the order of 65 per cent, evaporating a layer of bismuth on the silver layer until the light transmission is reduced to the order of 55 per cent, evaporating a layer of silver on the latter layer of bismuth until the light transmission is reduced to the order of 50 per cent, exposing the metal evaporated on said support to oxygen until the light transmission is increased to the order of 55 per cent, and thereafter exposing the metal evaporated on said support to caesium.
References Cited in the file of this patent UNITED STATES PATENTS 2,285,058 Samson June 2, 1942 2,285,062 Sommer June 2, 1942 2,297,467 Gorlich Sept. 29, 1942 2,391,280 Teal Dec. 18, 1945 2,548,514 Brarnley Apr. 10, 1951