GB2109624A - Channel plate electron multipliers - Google Patents

Abstract

The dynode plates of a discrete dynode channel electron multiplier should be bonded together in a rigid stack, electrically insulated from one another, at a controlled spacing. To provide barrel-shaped holes in the dynodes for correct electron-optical operation, half dynode pairs 2, 3 are bonded together by the insulating spacer to form a sub-assembly in which one half dynode 3 is coated with the secondary emitter 10 and forms half of one barrel-shaped hole and the other half dynode 2 provides the field controlling part of the following barrel-shaped hole. Glass can be used as the bonding and insulating spacer. In the invention a high melting point glass is fused to the field half-dynode in a layer 13 of controlled thickness and a low melting point glass layer 12 of smaller thickness (as shown) is fused to the emitting half-dynode. The sub- assembly is then formed by heating both half-dynodes to the lower temperature and pressing the glass layers together. Bonding is achieved at a separator thickness largely controlled by the high melting point glass layer, but at a bonding temperature which is low enough to avoid cosmetic disfigurement of the emitting surface 11 and consequent non-uniformity of emission from all channels. In a modification the glass layers on opposed half-dynodes are not in register, in which case the two glass layers may be of equal thickness. <IMAGE>

Description

SPECIFICATION Channel plate electron multipliers The present invention relates to electron multipliers and more particularly to electron multipliers of a laminated channel plate structure which may be used in electronic imaging and display devices and also in photomultiplier tubes.

The laminated channel plate structure comprises a plurality of perforated discrete dynode metal channel plates assembled in a stack with the perforations of the dynode plates aligned to provide channels through the stack. The dynode plates are electrically isolated from one another to an extent sufficient to allow them to be held at different potentials in operation. The plates should be held at controlled separations from one another and are preferably bonded together to make a rigid structure.

Different types of channel plate structures are known for example from British Patent Specifications 1,401,969,1,402,549 and 1,434,053. Figures 5 and 6 of British Patent Specification No. 1 402,549 disclose the use of screen printed glass dots as a means of separating adjacent channel plates of a stack. In order to bond the plates together to form a channel plate structure it is necessary to soften the glass dots by heating them. A problem may arise here in that accurate spacing between the channel plates may be lost due to the glass dots changing shape when soft. In order to avoid this problem it is proposed in that Specification to form spacing separating elements from a high melting point glass, which separating elements may be machined to an accurate thickness after application.Thereafter bonding elements of a low melting point glass are applied to the surface of each channel plate at locations different from the spacing separating elements. The plates are then arranged in a stack with the channels in the plates being aligned as desired and the stack is heated to a temperature to partly melt the low melting point glass bonding elements to bond them to the surface of an adjacent plate. In the case of plates not made of a secondary emissive material, for example mild steel, it is necessary to provide a secondary emissive material, for example, a goldcryolite mixture, such as is disclosed in British Patent Specification No. 1523730, (PHB 32480) as a layer on each steel dynode in each channel.

The heating of the plates, particularly in order to bond high melting point glass separating elements to the plates, produces a non-uniform appearance which can produce a non-uniformity of gain from the secondary emissive material when applied subsequently. This can have the effect that the channels do not behave uniformly over the area of the channel plate structure.

British Patent Specification 1,434,053 discloses half-dynode plates and also a subassembly comprising two half-dynode plates bonded together by an insulating separator. The hole in each half-dynode plate of each subassembly have a concave inner wall, the outside hole diameter being larger than inside hole diameter facing the separator. When two subassemblies are assembled face to face, the metal half-dynodes are then in electrical contact and form a barrel shaped hole. Only one half-dynode of each pair forming a barrel-shaped hole, that farthest from the source of incoming electrons, receives electrons and hence this half-dynode only need be a good secondary emitter.The other half-dynode cooperates with adjacent halfdynodes to produce firstly an electrostatic field at the surface of the emitting half dynodes which accelerates secondary electrons away from the emitting surface to reduce loss by collision with the emitting surface and secondly an electrostatic field between the dynodes which guides secondary electrons from one emitting haifdynode to corresponding areas of the emitting half-dynode of the next dynode.

The present invention takes advantage of the sub-assembly structure to overcome the problem of non-uniform emitting surfaces consequential on heating to relatively high temperatures. The invention provides a channel plate electron multiplier comprising a stack of a plurality of perforated discrete dynode metal plates, the perforations of the dynode plates being aligned to provide channels through the stack, and a separator between each dynode plate which electrically isolates adjacent dynodes from one another to an extent sufficient to allow them to be held at different electrical potentials in operation, each dynode plate comprising two mating ha If- dynode plates each having holes of tapered form and assembled in the stack to provide a dynode plate in which the holes have a larger diameter at the mating surfaces of the half-dynodes than at the outer surfaces of the composite dynode, wherein the stack comprises a plurality of subassemblies, each sub-assembly comprising a first secondary emissive half-dynode of one dynode plate bonded to a second half-dynode of the following dynode plate by the separator, the separator comprising a layer of a first glass bonded to the first half-dynode and a layer of a second glass bonded to the second half-dynode, the softening temperature of the first glass being chosen low enough to prevent disfigurement of the surface of the first half-dynode, to which a secondary emissive layer is subsequently applied, when the first glass is fused to the first halfdynode and to the second glass, and the softening temperature of the second glass being higher than that of the first glass by an amount sufficient to ensure that the second glass does not soften when the first glass is softened to form the subassembly.

The first and second glasses may be applied to their respective half-dynodes in patterns which are mirror images of one another, so that in the sub-assembly the two glasses are in layers on top of one another between the half-dynodes.

The spacing of the half-dynodes is controlled largely by the thickness of the layer of the second glass since this layer is not softened when two half-dynodes are bonded together. The second glass layer thickness is preferably at least twice the thickness of the first glass layer.

A preferred method for providing glass layers of controlled thickness on the half-dynodes is a screen printing process in which the ball-milled glass enamel is applied to the half-dynode in the desired pattern as an ink suspended on a screen.

The two half-dynodes are then fired separately, each at its softening temperature to fuse the glass particles into a layer.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing in which: Figure 1 shows a section a few stages of discrete dynode channel plate electron multiplier, and Figure 2 shows a section of part of an emitting half-dynode bonded to a field half-dynode via glass enamel on the half-dynodes.

In Figure 1 each dynode 1 comprises a field half-dynode 2 in electrically conducting contact with an emitting half-dynode 3. Each half-dynode is a perforated mild steel sheet similar to those produced for shadow-masks for colour television display tubes and is typically 1 50 ,um thick. The perforated holes are dome-shaped, the larger aperture diameter D being typically 420 m, the smaller diameter d being 300 ym. Pairs of field and emitting half-dynodes are bonded together on the sides of small hole diameters via an electrically insulating spacer 4, of thickness t, typically 100 ym. A secondary electron emissive layer 5 is provided on the inside of each hole of each emitting half-dynode by evaporation.A channel plate electron multiplier is assembled as a stack of such half-dynode pairs with the larger diameter apertures in contact, as shown in Figure 1.

Figure 2 shows a bonded pair of half-dynodes in more detail. The half-dynodes 2 and 3 have a 0.1 ,um thick layer 6 of oxidised chromium metal evaporated onto both sides and within the holes of the steel surface which has been roughened by etching to provide a good mechanical key for a subsequently applied glass enamel coating. A method for providing the oxidised chromium metal layer and the benefits to be derived therefrom in improved glass bonding and in chemical isolation of the substrate from the secondary emitter are described in Copending Patent Application 8114676, (PHB 32778).

A layer 12 of a first glass enamel is bonded to surface 14 of the emissive half-dynode 3. A suitable enamel is "Ferro" (Trade Mark) type RW 108 X or B. This enamel is available as a suspension of ball-milled particles in a fluid which is used as an ink in a screen printing process which either applies the suspension overall to surface 14 or in a pattern of patches which lies between the channel apertures. Half-dynode 3 is then fired at a temperature of 500 C to fuse the particles into an enamel layer 30 microns thick bonded strongly to the oxidised chromium layer 6.

At this temperature the cosmetic appearance of the dynode surface remains uniform.

A layer 13 of a second glass enamel having a higher softening temperature than that of the first glass enamel is bonded to surface 15 of the field half-dynode 2. A suitable enamel is "Ferro" (Trade Mark) type M 250P which is likewise prepared as a suspension of ball-milled particles in a fluid. The screen printing process is used to apply this suspension overall to surface 15 or in a matching pattern of patches which is a mirror image of the patches on surface 14. Half-dynode 2 is then fired at a temperature of 720 C to fuse the particles into an enamel layer 70 microns thick bonded strongly to the oxidised chromium layer 6.

The half-dynodes 2 and 3 are then assembled as a pair with the glass enamels in contact in a jig which aligns the holes. Pressure is applied and the pair are fired at a temperature of 5O00C, which is sufficient to soften layer 12 but not layer 13, whereupon the half-dynodes bond together to form a dynode sub-assembly. The spacing of the half-dynodes is controlled largely by the thickness of the glass enamel layer 1 3 of high softening temperature, which layer 1 3 is at least twice the thickness of layer 12. The low softening temperature glass layer is relatively thin initially and yields under pressure during bonding to only a small extent.

The secondary emissive layer 10 may then be evaporated onto the upper surface 9 and inside the holes 11 of half-dynode 3. The secondary emissive layer may be, for example, a goldcryolite mixture as is described in Patent Specification 1,523,730.

A glass-to-glass bond in the sub-assembly is preferred. But alternatively the patterns of the patches of enamels may be non-matching and the two enamels may be of equal thickness. In this arrangement the enamel layers are side by side in the sub-assembly, the separation of the ha If- dynodes now being controlled entirely by the thickness of the high softening temperature layer.

Claims (8)

Claims

1. A channel plate electron multiplier comprising a stack of a plurality of perforated discrete dynode metal plates, the perforations of the dynode plates being aligned to provide channels through the stack, and a separator between each dynode plate which electrically isolates adjacent dynodes from one another to an extent sufficient to allow them to be held at different electrical potentials in operation, each dynode plate comprising two mating half-dynode plates each having holes of tapered form and assembled in the stack to provide a dynode plate in which the holes have a larger diameter at the mating surfaces of the half-dynodes than at the outer surfaces of the composite dynode, wherein the stack comprises a plurality of sub-assemblies, each sub-assembly comprising a first secondary emissive half-dynode of one dynode plate bonded to a second half-dynode of the following dynode plate by the separator, the separator comprising a layer of a first glass bonded to the first ha If- dynode and a layer of a second glass bonded to the second half-dynode, the softening temperature of the first glass being chosen low enough to prevent disfigurement of the surface of the first half-dynode, to which a secondary emissive layer is subsequently applied, when the first glass is fused to the first half-dynode and to the second glass, and the softening temperature of the second glass being higher than that of the first glass by an amount sufficient to ensure that the second glass does not soften when the first glass is softened to form the sub-assembly.

2. A channel plate electron multiplier as claimed in Claim 1, wherein the first and second glasses are applied to their respective ha If- dynodes in patterns which are mirror images of one another so that in the sub-assembly the two glasses are in layers on top of one another between the half-dynodes.

3. A channel plate electron multiplier as claimed in Claim 2, wherein the thickness of the layer of the second glass is at least twice the thickness of the layer of the first glass.

4. A channel plate electron multiplier as claimed in any one of Claims 1 to 3 inclusive wherein said half-dynode plates are mild steel and comprise an evaporated layer of chromium metal laid down on all surfaces of the plate, the exposed surface of the chromium layer being oxidised.

5. A channel plate electron multiplier substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawing.

6. A cathode ray tube including a channel plate electron multiplier as claimed in any one of Claims 1 to 5, a display screen on the output side of said multiplier and an electron gun and electron beam deflector for scanning the input side of said multiplier.

7. A method of manufacturing a channel plate electron multiplier comprising a stack of a plurality of perforated dynode plate subassemblies, each sub-assembly comprising a first secondary emissive half-dynode plate of one dynode bonded to a second half-dynode plate of the following dynode plate by a separator, the separator comprising layers of a first and a second glass bonded to the first and second half-dynodes respectively, said method comprising the steps of applying suspensions of first and second glass particles of different softening temperatures in a fluid by a screen printing process to the first and second half dynode plates respectively, heating the first and second half-dynodes separately to temperatures at which the respective glass particles just fuse into layers, assembling first and second half-dynodes in pairs with their glass layers in contact with each other, heating the subassemblies to at least the lower of the glass softening temperatures, applying pressure to bond the first and second half-dynodes together, applying a layer of secondary emissive material to the first half-dynode plate of each sub-assembly, and assembling a stack of sub-assemblies with the perforations in registration and with the second-half dynode of one sub-assembly facing the first-half dynode of the following subassembly throughout the stack.

8. A method of manufacturing a channel plate electron multiplier substantially as described with reference to Figures 1 and 2 of the accompanying drawing.