US2151797A - Photoelectric tube - Google Patents

Description

March 28, 1939.

c. H. PRESCOTT, JR

PHOTOELECTRIC TUBE HYOROGEN SULPHIDE Filed June 26, 1937 INVENTOR C.H-. PRESCOTT JR.

3 5? fl/(W ATTORNEY Patented Mar. 28, 1939 UNITED STATES PATENT OFFICE PHOTOELECTRIO TUBE New York Application June 26, 1937, Serial No. 150,456

2 Claims.

This invention relates to photoelectric tubes and more particularly to cathodes therefor and methods of making such cathodes.

An object of the invention is to provide an improved composite electron emitting cathode.

Photoelectric tubes are now used for a great variety of purposes among which may be mentioned the matching of colors and television of colored objects. For these purposes it is sometimes advantageous to utilize a photoelectric tube having a spectral response characteristic approximating that of the human eye. As is Well known, the average human eye is most responsive to greenish-yellow light, that is, to light of wave-lengths in the neighborhood of 5500 A. U.

(Angstrom units).

A photoelectric cathode manufactured in accordance with this invention is stable under relatively high temperatures and in a glass envelope has a spectral response approximately like that of the human eye. A typical cathode according to this invention comprises a roughened silver plate carrying a thin layer of an intimate mixture of finely divided caesium, caesium sulphide and metallic silver on the exposed surface of which caesium is adsorbed.

A typical process for producing sucha cathode comprises oxidizing by a glow discharge in oxygen. a substantially pure silver plate which is mounted in an evacuated glass bulb reducing the silver oxide by heat to leave a roughened silver surface, producing a thin layer of silver sulphide by a glow discharge in hydrogen sulphide, and finally exposing the silver sulphide to caesium 5 vapor under suitable temperature conditions. An unexpectedly stable photoelectric cathode results from this process which cathode has a spectral sensitivity approximating that of the human eye. The invention will now be described more in detail having reference to the accompanying drawing.

Fig. 1 shows the structure of a completed photoelectric tube according to this invention.

Fig. 2 illustrates a pumping station suitable for processing four tubes concurrently.

Fig. 3 is a schematic electric circuit used in processing such tubes.

The photoelectric tube illustrated in Fig. 1 comprises a glass bulb 5 having a reentrant stem 30 6 sealed therein. The stem 6 is provided with a press 7 in which are sealed cathode support wires 8, 8a and 8b, an anode wire 9, lead-in wires l0 and II connected to support wire 8 and anode wire 9, respectively. A cathode l2 comprises a 55 semi-cylindrical silver plate, the straight edges of which are rolled around the support wires 8 and 8a and to the lower rear edge of which support wire 8b is connected. Also supported by wires sealed into the press I is a heavy copper wire l3 which partially encircles the stem 6 and 5 between the ends of which is a metallic capsule M. This capsule l4 encloses a mixture for producing caesium vapor when heated. A nickel shield i 5 is supported, also from wires sealed into the press 1, between the capsule H and the 10 press l and the concave surface of cathode I 2. The stem 6 is provided with an exhaust tube It.

A coil I1 is arranged to induce current in the copper wire l3 and capsule It for heating the mixture within the capsule to its reaction teml5 perature. This coil ll is used only during the processing of the cathode l2 and is shown in partial section in Fig. 1. A portion of the glass bulb 5 is shown removed for clearness of illustration. p v

The electrode and accessory structure carried by the stem 6 is fabricated before the stemis sealed into the glass tube 5. The cathode plate It is formed from a polished silver sheet of a high degree of purity which has a clean mirror finish. The formed cathode is washed and reduced in hydrogen. The concave surface of the cathode has an area of about 2 square inches. The anode 9 is a nickel wire.

The caesium producing mixture within the capsule M is in the form of a compressed pellet which is composed of approximately 5 milligrams of caesium chromate, Cs2CrO4, 32 milligrams of chromic oxide, CraOa, and 13 milligrams of powdered aluminum, Al. 'These in- 85 gredients are carefully prepared, finely pulverized and thoroughly mixed in proper proportions before being formed into pellets.

The pumping station illustrated in Fig. 2 is adapted for the processing of four tubes concurrently. The tubes 5 are sealed to a glass header 2G by exhaust tubes [6. This header 20 runs through an oven 2! which comprises a base 22 carrying end supports 23, on the upper ends of which is a metallic cap 24. An electric heater 25 between the supports 23 is surmounted by a removable cover 26 carrying four chimneys 27. This cover is provided with handles 28. The heater 25, cover 26 and chimneys 21 may be m raised as a unit so that the chimneys surround the bulbs 5. Each chimney is provided with a slidable baffle (not shown) to control the convection air currents flowing therethrough. With the cover 26 and chimneys 21 removed, the heater cc 25 may be raised up against the cap 24 to complet'ely surround the bulbs 5.

The header 2!! is connected to a pumping apparatus comprising a liquid air trap 29 two McLeod gauges 30 and 3|, a mercury vapor pump 32, a mercury cut-off 33, and a vacuum pump .(not shown) connected to tube 34. Between the mercury cut-off 33 and the liquid air trap 23 three gas supply units 35, 36 and 31 are connected to the pumping apparatus. Each unit comprises a flexible coiled glass tube 38 associated with a gas container 33 through a mercury seal surrounding a pair of porous plugs of Lavite" which when brought together permit gas to pass from the container 33 to the bulbs through the glass coil 38. An ionization manometer 40 is connected to the other end of the header 20.

An electrical circuit used during the processing of four tubes while they are sealed on the pumping station is illustrated in Fig. 3. This circuit comprises conductors 50 adapted to be connected to a direct current source (riot shown) and a potentiometer 5| for determining the potentials to be impressed across electrodes 9 and I2 of tubes 5. With the switch 52 in its lower position and jack 55 closed, a potential may be impressed on any one of tubes 5 by closing the lower contacts of switch 53 which is individual to such tube. With the switch 52 in its upper position, the potential determined by potentiometer 5| is impressed through the upper contacts of all of switches 53 in series on condensers 54 which have a total capacitance of 5 mlcrofarads. Condensers 54 can then be discharged through any one of tubes 5 by closing the lower contacts of the corresponding switch 53. The voltage determined by potentiometer 5| is indicated by voltmeter 56. The current flowing through tubes 5 during a later stage of the processing is indicated by an ammeter 53 which may be connected into the circuit by inserting plug 51 into jack 55.

The processing of four tubes after the untreated assemblies have been sealed onto the header 20 will now be described.

The tubes 5 are sealed onto the header 2|! with the concave sides of the cathode plates l2 facing directly forward, with the bulbs 5 centered with respect to the high frequency coils I, and at such a height that the circular portion of the wire I3 is approximately midway of the coil longitudinally.

The vacuum pump is turned on. The liquid air trap is cooled with a mixture of Cellosolve acetate" and solidified carbon dioxide. When the pressure has fallen sufliciently, the manometer 40 is turned on. The heater 25, without the cover 25 and chimneys 21, is raised to engage the cap 24. The current is turned on to bring the oven temperature up to 400 C. and maintained until the manometer shows a pressure of 5X10- mm. of Hg (millimeters of mercury) or lower when the current is shut off and the oven allowed to cool. Slow leaks anywhere in the system are indicated if the pressure is not less than 2 mm. of Hg when the tubes are cooled to room temperature. If leaks are indicated they should be eliminated before the processing proceeds.

I This treatment removes occluded gases from the tubes but does not cause any chemical reaction in the caesium pellet. The pumping station is then flushed with oxygen from gas supply unit 35 until a clear green haze is obtained in the manometer 40 which indicates suflicient purity of the oxygen.

The mercury cut-off 33 is now closed to cut off the vacuum pumps and the manometer 40 turned oil. Additional oxygen is admitted to a pressure of 2.2 mm. of Hg as indicated by McLeod gauge 33. is set in its lower position and the potentiometer 5| adjusted so that voltmeter 56 shows a voltage of 700 volts. Switches 53 are then operated in succession to close their lower contacts so that each cathode H in succession is subjected to positive ion bombardment. The first effect of such bombardment is to produce a heavy layer of silver oxide on the concave surface of the cathode I 2. followed by a heating up of the silver plate and reduction of the layer of silver oxide. The cathodes are allowed to cool, the oxygen pressure again adjusted to 2.2 mm. of Hg and the oxidation, reduction and cooling repeated. The mercury cut-oi! 33 is then opened and the oxygen pumped out.

This treatment leaves the'concave surface of each cathode |2 clean and slightly rough so that it has a uniform matte finish.

After a good vacuum has again been obtained, as indicated by the manometer 40, the mercury cut-01f 33 is again closed and the manometer turned ofi. Hydrogen sulphide gas through gas supply unit 35 is admitted to a pressure of 2.0 mm. of Hg. Switch 52 is thrown to its upper position after potentiometer 5| has been adjusted so that voltmeter 56 shows a voltage of 650 volts. Each tube 5 is then subjected to a series of discharges from condensers 54 by the operation of closing switches 53 on their lower contacts. This operation is hereinafter called tapping. The switch 52 in its upper position is therefore said to be in the "tapping position. Each switch 53 is tapped fifty times in four groups of twenty taps each and one group of ten taps. The first group of taps is applied to the four tubes in succession, then the second group, and so on until each tube Switch 52 of the electrical circuit of-Fig. 3

has received the required number of taps or discharges. The mercury cut-oil isagain opened and the hydrogen sulphide gas pumped out. The manometer is again turned onand the pumping continued until the pressure is down to 3X 10- mm. of Hg or less.

This treatment with hydrogen sulphide gas produces a thin layer of silver sulphide on the concave surface of each cathode l2. At each tap of a switch 53 a glow discharge occurs in the associated tube 5 and the cathode |2 of that tube is bombarded with a definite number of positive ions. The amount of silver sulphide which is formed is therefore closely controlled.

The cathodes l2 are now in condition to be treated with caesium vapor under suitable temperature conditions. A source of high frequency current (not shown) is connected to the coils II in succession to flash" the caesium capsules, that is, to induce sufllcient current in the copper wire l3 and capsule H to start a. chemical reaction of the ingredients of the caesium pellet. The exothermic reaction which follows develops a large amount of heat and causes the immediate and complete expulsion of all of the caesium. The high frequency source is disconnected as soon as the reaction starts. The caesium travels in straight lines from the capsule l4 and is condensed on the glass walls of the bulb 5. The shield I5 prevents the hot caesium vapor from impinging on the cathode surface. This shield I5 is so shaped that it lies substantially parallel to the magnetic lines of force produced by the high frequency current in the coil I I and therefore is not heated to any great extent by eddy currents induced therein.

Immediately after the caesium capsule has been flashed stem heaters (not shown) are inserted around the stems of the tubes 5. Thermocouples are placed against each bulb 5 at the surface pposite the stem to indicate temperatures. The cover 26 with the chimneys 27 are placed on the heater 25 and the whole raised until a chimney 27 surrounds each bulb 5. The stem heaters are turned on first and after two minutes the heater Z is turned on to quickly bring up the temperature of the convection air currents flowing past the bulbs 5. When the hottest bulb reaches a temperature of 150 C. the heater is regulated to hold this temperature for five minutes. At the end of this five-minute period the heater 25 is again regulated to raise the temperature of the air currents to quickly bring the temperature of the hottest bulb 5 up to 225 C. The heater 25 is then regulated to hold this temperature until the cathode surface of each tube has reached the proper sensitivity.

The sensitivity may be indicated by the ammeter 58 when connected into the circuit of Fig. 3 by the insertion of plug 5'5 into jack 55. Each cathode l2 may be illuminated by a source of light (not shown). The potentiometer 55 is adjusted to give a voltage of 50 volts as shown on voltmeter 56. Switch 52 is closed in its lower position. The sensitivity of each tube 5 will be indicated by the ammeter 58 if the corresponding switch 53 is closed in its lower position. The tubes 5 are tested from time to time while illuminated and the hot air treatment is continued until the photoelectric current tends to decrease. As each tube reaches its proper sensitivity the air current for that tube is cut off. When all of the tubes have reached the proper sensitivity, the chimneys 2! are lowered and the bulbs allowed to cool to room temperature with the pumping apparatus still on.

When the bulbs 5 are cool and the pressure is down to 2 10 mm. of Hg, mercury cut-off 33 is closed and argon is admitted from the gas supply unit 3i. The argon is admitted until the pressure assumes a steady state at the desired value, when the tube is sealed off from the header 20 by sealing of the exhaust tube IS. The pressure of argon is dependent upon the amount of gas ampli- [ication desired in the completed tube. A suitable pressure is 5 10 mm. of Hg.

Stable cathodes may be produced by varying considerably the specific process described in detail hereinbefore. However, in all such variabions the silver surface is so treated with hydrogen sulphide gas as to produce a layer of silver iulphide on the cathode, which layer is subjected :o heated caesium vapor to reduce at least some if the silver sulphide to form caesium sulphide ind finely divided metallic silver.

The remaining free caesium is in a finely n vided state. In fact it may be so finely divided that the particles are approaching atomic dimensions, so that the behavior of an atom will depend more upon the other ingredients in the cathode layer than upon neighboring atoms of caesium. The cathode, therefore, may be said to comprise a mixture of finely divided caesium sulphide and metallic silver throughout which mixture is adsorbed free caesium. Some of the caesium also may well be alloyed or absorbed in the free silver particles or underlying rough silver surface.

The roughened silver surface may be produced in other ways than by oxidation and reductior Jf the silver oxide. The treated silver surface may also consist of a layer of silver on a plate of another metal, such as copper or nickel.

Stable cathodes may also be formed by subjecting the sulphided silver surface to the vapor of others of the alkali metals, rubidium, lithium, potassium and sodium.

The preferred form of the invention, however, is that described in detail hereinbcfore since that process produces an electron emitting cathode which is stable at relatively high temperatures and has a spectral response closely approximating that of the human eye.

What is claimed is:

1. A method of producing a light-sensitive electron emitting cathode within a closed container,

which comprises mounting an element having a polished silver surface within said container, producing a glow discharge on said surface in an atmosphere of oxygen to produce a heavy layer of silver oxide, heating said element to reduce said silver oxide, cooling said element, producing a glow discharge on the resulting surface of said element in an atmosphere of hydrogen sulphide gas at low pressure to produce a thin layer of silver sulphide, stopping the glow discharge, removing the residual gas from the container, producing caesium vapor within said container, and heating the container to cause the caesium to diffuse into the layer of silver sulphide.

2. A method of producing a light-sensitive elec--- tron emitting cathode within a closed container, which comprises mounting an element having a polished silver surface within said container, producing a glow discharge on said surface in an atmosphere of oxygen to produce a heavy layer of silver oxide, continuing the discharge until the layer of silver oxide is reduced leaving a clean rough silver surface, cooling said element, producing a glow discharge on the resulting surface of said element in an atmosphere of hydrogen sul= phide gas at low pressure to produce a thin layer of silver sulphide, stopping the glow discharge, re= moving the residual gas from the container, producing caesium vapor within said container, and heating the container to cause the caesium to diffuse into the layer of silver sulphide.

.S H. PRESCQTI. Ja.

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