GB816452A - Improvements in or relating to electron beam tubes - Google Patents

Abstract

816,452. Cathode ray tubes. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. Nov. 16, 1956 [Nov. 25, 1955], No. 35099/56. Class 39(1). In a cathode ray receiving tube surfaces liable to be struck at grazing incidence by electrons of the beam, i.e. at the junction between the tube neck and flared portion, in wide angle deflection tubes, are coated with an element or mixture of high atomic number or high average atomic number such as Ag, Au, or Mo, (high means >25 as opposed to graphite whose atomic number is 12); the emission of low velocity secondaries (energy up to 50 e v) from such a surface particularly for grazing incidence is less than that (60 degrees or more to the normal) for graphite so that fewer positive and negative ions are generated by collision between secondaries and residual gas molecules and consequently the screen and other parts of the tube are not subjected to such heavy ion bombardment. The remainder of the inner wall of the envelope except the face plate has a conductive coating, e.g. of silver in contact with the first conductive coating and an atomic number of L25. While more high velocity scattered electrons are generated than for graphite, these are not so effective in producing positive and negative ions by collision and those ions which are produced are more diffuse. If the deflection angle of the tube is less than 90 degrees, as shown, the high atomic number coating does not have to extend into the flared part of the envelope, only a small part of the neck at the end adjacent the flared portion is therefore coated. The coating can be replaced by an annular insert. The positive ion bombardment (e.g. of H,O ions) of a non-aluminised screen causes an "X burn " i.e. a cross at the centre whose arms extend along the picture diagonally. Secondary emission from a non-aluminised fluorescent screen and the share charge thereof causes the screen potential (originally equal to cooling potential) to drop by a few hundred volts thus causing an accelerating and focusing field between coating and screen for the positive ions, the focusing action producing the sharp cross. In the form shown a silver coating 24 is in conductive contact with graphite coating 26 on either side thereof, the graphite being put on in an aqueous suspension ; potential is applied to the coatings through a bulb terminal and, vice contact springs 28, to the gunanode. The deflection angle can be 90 degrees in which case the high atomic number coating extends from A to B (the flared part of the envelope being of rectangular cross section) In manufacture the coating material together with a binder of methyl methacrylate or nitrocellulose is painted on the required surface the binder being decomposed or removed in subsequent bobing and degassing. The surface of the coating can be roughened to reduce the secondary emissivity still further by embedding particles of high atomic number material in the surface by sintering or by sand blasting a smooth surface, or by evaporation of metal in an inert atmosphere.