Process for stabilizing a microchannel plate

#This invention relates to microchannel plates and, more particularly, to a process for stabilizing the characteristics of a microchannel plate.

#Devices which amplify light, commonly termed "low light level sensing devices", are old and known. These devices find application in military or security installations, for example, applications in which one desires to obtain a visible image of a scene that is in substantially complete darkness, such as a scene illuminated only by starlight. One type of low light level sensing device includes as a component an element which is termed a "microcapillary multiplier matrix", or microchannel electron multiplier plate, and also known as simply a "microchannel plate". This element is in the form of a wafer or plate of glass which contains a very large number of minute passages arranged parallel that extend through to the front and back surfaces. For example, there is on the order of 1,000,000 such passages in a one-inch diameter wafer. The passages in the microchannel plate are lined with or contain a material having a high enough coefficient of secondary emission to be used as a continuous strip electron multiplying device so that for each electron entering the front of a passage a larger number of electrons is emitted through the other end.

#In a given low light level sensing device the microchannel plate is arranged between a photocathode and a phosphor display screen in an evacuated envelope or housing and conventional bias voltages are applied to the elements. Then when an image is cast upon the photocathode, electrons are emitted by the photocathode in the mosaic of the image and these electrons are directed through suitable accelerating voltages to the microchannel plate. The mosaic electrons accordingly enter the passages in the plate and a larger number of electrons exit from the rear of the microchannel plate in the same mosaic and thence are accelerated by a suitable electric field onto the phosphor display screen. As a result, the brightness of the scene as it appears on the phosphor display screen is greater than the original scene presented to the photocathode.

#In the drawings:

#FIG. 1 illustrates schematically the apparatus for performing the process of my invention; and

#FIG. 2 is a graph showing the gain and stability of a microchannel plate processed in accordance with my invention.

#The equipment and supplies useful to the practice of my novel process is schematically illustrated in FIG. 1. This includes a conventional canister 1 of hydrogen gas (H.sub.2) under pressure, a canister 3 of nitrogen gas (N.sub.2) under pressure, and a canister 5 of hydrogen sulfide gas (H.sub.2 S) under pressure. These gases are of high grade commercial quality and contain no more than 10 parts per million of any impurity such as water vapor or CO or CO.sub.2, and are commercially available. Each of the gas canisters 1, 3 and 5 includes a regulator valve 7, 9 and 11, respectively. The output of canister 1 is connected by suitable piping 13 to the inlet of flowmeter 15. Likewise the output of the canister 3 is connected by piping 17 to a flowmeter 19 and the output of canister 5 is connected by pipe 21 to a third flowmeter 23. The outlets of the respective flowmeters 15, 19 and 23 are connected in common to pipe 25 and pipe 25 is in turn connected to the inlet 28 of a chamber 31, which I refer to as an oven, shown schematically and in section to better illustrate its elements. The oven includes a cylindrical chamber 31, suitably of Pyrex or quartz and capable of withstanding the high temperatures hereinafter described; a gas outlet 32, located at the lower end of the chamber; and a cover 33. Fastening means, not illustrated, hold the cover in place. Support rods 35 and 37 are attached at one end to the underside of cover 33 and at their other end support a tray 39 in a lower portion of the chamber. A microchannel plate 41 is illustrated seated on tray 39 within the oven for processing. A series of heat baffle plates 41, 43, 45, 47 and 49 are installed in the chamber and evenly distributed above and below the tray 39 so as to maintain some symmetry of heat at the tray location. A pipe 51 connected to outlet 32 is included to permit exhaust gases and waste products to be transmitted to any suitable disposal, not illustrated, such as a burner. An electric heater 53, symbolically indicated as a section of a resistance wire coil, is located external of the metal chamber and in abutting relationship with the chamber walls for conducting heat thereto. The heating coil is connected to a suitable source of electric current, not illustrated. A thermocouple 55, symbolically illustrated, which monitors oven temperature, is installed within the chamber by suitable supports, not illustrated. The thermocouple is connected via an electric lead 57 to suitable translating and indicating apparatus 59, symbolically illustrated as a temperature indicator.

#In operation the microchannel plate is installed on the tray 39 and cover 33 is lowered into position onto the cylindrical oven 31. The cover is fastened in place to close the heating chamber leaving the microchannel plate positioned within the oven chamber. The valve 9 on the nitrogen container 3 is opened to allow gas to flow into inlet 29, through the oven and out the outlet. Electrical current is supplied to the electric heater and the heat generated in the resistance wire 53 is conducted to the chamber walls and hence to within the chamber raising the atmosphere in the chamber temperature to within the range of 350.degree.-450.degree. C. as hereinafter discussed. The nitrogen gas is a neutral substance which is used essentially to purge air from the oven chamber and although not absolutely necessary to the practice of the invention, it is preferred. After the nitrogen has been flowing for a period of approximately 10 minutes the valve is closed to stop the flow of nitrogen. Next, valve 7 of hydrogen canister 1 is opened and hydrogen flows through pipe 13, flowmeter 15, pipe 25 and into chamber 31. The flow rate is adjusted to that desired and monitored on flowmeter 15. In one example the flow rate is 0.1 cubic feet per minute. The hydrogen is heated as it flows through the chamber to a temperature of approximately 415.degree. Centigrade, in one example, and approximately at atmospheric pressure. The hydrogen is incident upon the microchannel plate and enters the passages therein. The hydrogen chemically reacts with the lead oxide in the passage walls in a manner more fully discussed hereinafter. This reaction creates water vapor, H.sub.2 O, as a waste gas and reduces some of the lead oxide (PbO) in the microchannel plate to free lead (Pb). The hydrogen and such water vapor as is generated in the reaction is exhausted through outlet 51 to the sink. At the temperature and flow rate specified in the example the hydrogen is allowed to flow for approximately 12 hours. Valve 11 of hydrogen sulfide gas canister 5 is then opened and adjusted to the desired flow rate, which in the example is approximately 0.05 cubic feet per minute, and this is indicated on flowmeter 23. The hydrogen sulfide gas flows through pipe 21, flowmeter 23, mixes with the hydrogen gas in pipe 25, and both gases enter oven inlet 29. Both gases are heated as they flow from the upper end of the chamber through the oven and in this example are raised in temperature to approximately 415.degree. Centigrade as indicated by thermocouple 55 and indicated on indicator 59 at approximately atmospheric pressure. The hydrogen sulfide enters the passages in the microchannel plate and reacts chemically with the exposed chemical elements lining the passages. As is more fully explained hereinafter, I believe the H.sub.2 S reacts with the lead, Pb, created as a result of the previous reduction reaction with hydrogen to create another substance, lead sulfide, PbS, while the hydrogen gas acts, I believe, simply as a carrier at this stage. The mix of gases, including hydrogen and hydrogen sulfide, continue through the oven and exit at outlet 51 to the sink. The lead sulfide created in the process is volatile, relatively, and vaporizes and likewise exits as a gas out outlet 51 to the waste disposal system, where the gases are conveniently burned off. At the flow rates and temperature given in the preceding example and in this step of the process the hydrogen sulfide and hydrogen flows for a period of approximately 12 hours.

#At the conclusion of this process valves 7 and 11 are closed and the electric heater is turned off. If desired, the valve 9 of nitrogen container 3 may be opened to allow nitrogen gas to purge any residual gases in the chamber at this time.