GB2181296A

GB2181296A - Electron multipliers

Info

Publication number: GB2181296A
Application number: GB08622659A
Authority: GB
Inventors: Ronald Albert Sink
Original assignee: Deutsche ITT Industries GmbH; ITT Industries Inc
Current assignee: TDK Micronas GmbH; ITT Inc
Priority date: 1985-09-30
Filing date: 1986-09-19
Publication date: 1987-04-15
Anticipated expiration: 2006-09-19
Also published as: GB8622659D0; AU589448B2; GB2181296B; AU6277886A

Abstract

A microchannel electron multiplier is formed by placing into a glass tube (22) a plurality of bundles of optical fibres (16), each having an etchable glass core and a glass cladding which is non-etchable when subjected to the conditions used for etching the core material. The fibre bundles located around the inside surface of the glass tube are replaced by support fibres (24) having both a core and a cladding of a material which is non-etchable under the above-described conditions. The assembly of the tube, bundles and support fibres is heated to fuse the tube, bundles and support fibres together. The etchable core material is then removed and the assembly sliced into wafers. The inner surface of each of the claddings which bound the channel formed by the removal of the core material is rendered electron-emissive by reduction of the lead oxide by hydrogen gas. Metal films are deposited onto the opposed surfaces of each of the wafers to form contacts. <IMAGE>

Description

SPECIFICATION Electron multipliers This invention relates to electron multipliers, and more particularly, to a channel-type electron multi- plierand an image tube orthe like incorporating such an electron muiltiplier.

Microchannel plate electron multiplier devices provide exceptional electron amplification but are generally limited in application because oftheirdeli- cate glass structure. The device basically consists of a honeycomb configuration of continuous pores through a thin glass plate. Secondary emissive prop erties are imparted to the walls either by chemically treating the glass walls ofthe pores or coating an em issivelayerthereon. Electrons transported through the pores subsequently generate large numbers of free electrons by multiple collisions with the electron emissive internal pore surface.

However, there are problems associated with the forming of the microchannel plates. In one method employed, a plurality of optical fibres are enclosed within an envelope structure and the structure and fibres are heated to fuse the fibres together. Problems arose because the fibres would become distor tsd and/or broken during the fusion process.

U.S. Patent No. 4,021,216 discloses oneattemptto solve this problem and is directed to a lineararray of electron multiplier microchannels sandwiched between a pair of glass plate support members. The present invention takes a different approach to this problem.

It is an object of the present invention to provide a method offorming microchannel plates which overcomes the disadvantages ofthe prior art.

It is another object of the present invention to provide a microchannel plate in which the area surrounding the edges ofthe plate is substantially free from distortions.

It is a further object of the present invention to provide a microchannel plate in which broken channel walls are substantially eliminated.

According to one aspect ofthe invention there is provided a method of forming a microchannel plate comprising the step of fusing together an assembly of a plurality of optical fibres, at least a portion of each of which has a cladding layer bounding an opening, a plurality of support rods substantiallysur- rounding the optical fibres and a tube enveloping the optical fibres and the support rods.

According to another aspect ofthe invention there is provided a method offorming a microchannel electron multiplier comprising the steps offorming a fused assembly comprising a plurality of bundles of fused optical fibres, a plurality of support rods substantiallysurrounding the bundles offused optical fibres to cushion the optical fibres, and a tubeenclosing the support rods, each of the optical fibres having a core and a cladding surrounding the core, removing the core from at least a portion ofthe optical fibres to form a channel therethrough, and treating the inner surface of at least some ofthe claddingsto render the surface electron-emissive.

According to a further aspect of the invention there is provided a microchannel electron miltipliercomprising a plurality of optical fibres, some of which include a cladding layer bounding an internal space from which core material has been removed, a tube surrounding the optical fibres, and a plurality of support rods substantially surrounding the optical fibres ata region along the inside periphery ofthetube.

Embodiments ofthe invention will now be described bywayofexamplewith referencetotheaccompanying drawings, in which Figure lisa perspective view of a clad fibre having a circular configuration.

Figure 2 is a perspective view of a clad fibre bundle having a hexagonal configuration, Figure 3 is a cross-sectional view of a glasstube packed with multi fibres and support fibres after etching, Figure 4 is a perspective view of a section of the microchannel plate after etching and slicing, and Figure is a perspective view of a microchannel.

In Figure 1 there is shown a starting fibre 10 forth microchannel plate ofthis invention. The fibre 10 includes a glass core 12 and a glass cladding 14surrounding the core. The core 12 is made of a material that is etchable in an appropriate etching solution in such a waythatthe core can be subsequently removed during the process according to this embodi ment.The glass cladding 14 is madefrom a glass which has a softening temperature substantiallythe same as that ofthe glass core 12. The glass material of the cladding 14 is differentfrom thatofthe core 12 in that it has a higher lead content which renders it nonetchable underthose conditions used for etching the core material.Thus, the cladding 14 remains afterthe dissolution or etching of the glass core 12 and becomes a boundary for the channel which is left.A suitable cladding glass is a lead-type glass such as Corning Glass 8161. The lead oxide is subsequently reduced in the final stages of the manufacturing pro cess to make the inner surfaces of each ofthefibres 10 capable ofthe emission of secondary electrons.

The optical fibres 10 are formed in the following manner. An etchable glass rod and a cladding tube coaxially surrounding the rod are suspended vertically in a draw machine which incorporates a zone furnace. The temperature of the furnace is elevated to the softening temperature ofthe glass. The rod and tube fuse together and are drawn into the single fibre 10. The fibre 10 is fed into a traction mechanism wherethe speed is adjusted untilthedesiredfibre diameter is achieved. The fibre 10 is then cut into shorter lengths of approximately 18 inches.

Several thusands ofthe cut lengths ofthesingle fibre 10 are then stacked into a graphite mould and heated to the softening temperature ofthe glass in order to form a hexagonal array 16 as shown in Figure 2 wherein each of the cut lengths of the fibre 10 has a hexagonal configuration. The hexagonal configuration provides a better stacking arrangement.

The hexagonal array 16, which array is also known as a multi-assembly or bundle, includes several thousand single fibres 10 each having the core 12 and the cladding 14. This multi-assembly 16 is suspended vertically in a draw machine and drawn to again decrease the fibre diameter while still maintaining the hexagonal cnfiguration ofthe individual fibres.the multi-assembly 16 isthen cut into shorter lengths of approximately 6 inches.

Several hundredofthecutmulti-assemblies 16 are packed into a precision inner diameter bore glass tube 22 as shown in Figure 3. The glass tube 22 has a high lead cntent and is made of a glass material which issimilarto the glass cladding 14 and is thus non-etchable by the process used herein to etch awaythe glass core 12. The tube 22 has a coefficient of expansion which is approximately the same as that of the fibres 10. The lead glass tube 22 will eventually become the solid rim border of the microchannel plate.

lnordertoprotectthefibres l0ofeachmulti- assembly 16 during processing to form the microchannel plate, a plurality of support structures are positioned in the glass tube 22 to replace those multi-assemblies 16 which form the outer layer of the assembly. The support structures may take the form of hexagonal rods of any material having the neces sarystrength and the capabil ity to fuse with the g lass fibres. The material should have a temperature coefficient close enough to that of the glass fibres to prevent distortion ofthe latterduringtemperature changes.In one embodiment, each support structure may be a single optical glass fibre 24 of hexagonal shape and a cross-sectional area approximately as large as that of one of the multi-assemblies 16, the single fibre having a core and a cladding which are both non-etchable underthe aforementioned conditions where the cores 12 are etched. The optical fibres 24 are illustrated in Figure 3. Both the rod which forms the core and the glass tube which forms thecladding ofthesupportopticalfibres24aremade ofthesamehigh lead content glass material asthe glass cladding 14 of thefibres 10.These support fibres 24 will form a cushioning layer between the tube 22 and the multi-assemblies 16 sothatduring a later heating step, distortion ofthe area adjacent the inner surface ofthe glass tube 22 is substantiallyel- iminated. The glass rod and tube which will form the core and the cladding ofthe supportfibre 24 are sus pended in a drawfurnaceand heatedto fuse the rod and the tube together and to soften the fused rod and tube sufficiently to form a fibre.The so formed supportfibres 24 are then cut into lengths of approximately 18 inches and subjected to a second draw to achieve the desired geometric configuration and smaller outside diameter which is substantially the same as the outside diameter of each ofthe multiassemblies 16. The support structures may be formed from one optical fibre orany numberoffibres uptoseveral hundred.Thefinal geometricconfiguration and outside diameter of one support structure should be substantially the same as one multi assembly 16. The multiple fibre supportstructure may be formed in a mannersimilarto thatofthe multi-assembly 16.

Each multi-assembly 16 which forms the outer most layer offibres in the tube 22 is replaced by a support optical fibre 24. This is preferably done by positioning one end of a supportfibre 24 against one end of a multi-assembly 16 which is to be replaced and pushing the supportfibre24againstthe multiassembly 16 until the multi-assembly 16 is out of the tube 22. The assembly formed when all of the outer multi-assemblies 16 have been replaced by the support fibres 24 is called a boule.

The boule 30 is inserted into a lead glass envelope tube (not shown) which has one open end. The en velopetube has a softening point similarto that of the supportfibres24and a multi4ibre array 16. The boule 30 is then suspended in a furnace and the open end of the lead glass envelope tu be connected to a vacuum system. The temperature of the furnace is elevated to the softening point ofthe material ofthe multi-assembly 16 and the supportfibres 24. The multi-fibre assemblies 16 fuse together, and the sup- portfibres 24 fuse to the multi-assemblies 16 and to the glass tu be 22.

During this heating step, the supportfibres 24 act as a cushion between the rim ofthe glass tube 22 and the multi-assembly 16. This cushioning provides structural support so that the individual fibres 10 do notdistortduringthe heattreatment. In addition,the cushining effect ofthe supportfibres 24 makes itpossibleto use a higher heat during fusion withoutcausing distortion ofthefibres 10. During the heating step the lead giass envelope adheresto the glass tube 22 but does notform a good interface therewith. In order to prevent problems during later stages of processing, the lead glass envelope is ground away after the heat treatment. The fused boule 30 is then sliced intothincross-sectional plates.The plantar end surfaces are ground and polished.

In orderto form the microchannels, the cores 12of the fibres 10 are removed, preferably by etching with dilute hydrochloric acid.Afteretching,the high lead contentglasscladdings 14 will remain to form the microchannels 32 as illustrated in Figure 4. Also, the support fibres 24 remain solid and thus provide a good transition from the solid rim ofthe tube 22 to the microchannels 32.

After etching, the plates are placed in an atmosphere of hydrogen gas whereby the lead oxide ofthe non-etched lead glass is reduced to render the cladding electron emissive. In this way, a semiconducting layer is formed in each ofthe glass claddings 14, which layer extends inwardlyfrom the surfacewhich bounds the microchannel 32. Because the support fibres 24 are not eteched and remain solid, the active area ofthemicrochannel plateisdecreased.lnthis way also there are less channels to outgas. Addition- ally,whilethe plate must be madeto a predetermined outside diameter so that it can be accommodated in an image intensifiertube, the area along the rim of the plate is not used since it is blocked by internal structures in the tube. Therefore, reducing the active area ofthe plate at the rim is advantageous since the microchannels in that area are not needed.

Thin metal layers are applied as electrical contacts to each ofthe plantar end surfaces ofthe microchannel plate which provide entrance and exit paths for electrons when an electric field is established across the microchannel plate by means of the metallised contacts. The metal ofthe contacts may be nickel chromium.

Figure 5 illustrates one completed microchannel 40 showing metal cntact layers 42 and a semiconducting layer44which surrounds the channel. A primary electron 46 is multiplied during its passage through the channel 40 into the output electrons 48 by means ofthe semiconducting layer44 and the potential difference between the contact layers 42.

Claims

1. A method of forming a microchannel plate comprising the step of fusing together an assembly of a plurality of optical fibres, at least a portion of each ofwhich has a cladding layer bounding an opening, a plurality ofsupport rods substantiallysur- rounding the optical fibers and a tube enveloping the optical fibres andthe support rods.

2. A method as claimed in claim 1 wherein the fusing step includes forming each ofthe support rods to have substantially the same cross-sectional area as one of the optical fibres.

3. A method as claimed in claim 1 wherein the fusing step includes making each ofthe support rods from a plurality of fused optical fibres, each optical fibre having a core and a cladding surrounding the core.

4. A method as claimed in claim 1 wherein the fusing step includes making each ofthe support rods from an optical fibre having a core and a cladding surrounding the core.

5. A method offorming a microchannel electron multiplier comprising the steps offorming a fused assembly comprising a plurality of bundles offused optical fibres, a plurality of support rods substantially surrounding the bundles offused optical fibres to cushion the optical fibres, and a tube enclosing the support rods, each ofthe optical fibres having a core and a cladding surrounding the core, removing the core from at least a portion ofthe optical fibres to form a channel therethrough, and treating the inner surface of at least some ofthe claddingsto renderthe surface electron-emissive.

6. A method as claimed in claim Swhereinthe forming step includes placing a plurality of bundles ofthe opticalfibres intothetube and replacing at least some of the bundles which are positioned along the periphery ofthe plurality by a support optical fibre and heating togetherthe tube, the bundles of optical fibres and support fibres to form an assembly, the support fibres preventing distortion of the optical fibres.

7. A method as claimed in claim 5 wherein there moving step includes etching.

8. Amethod claimed as claimed inclaim5furthercom- prising, before the removing step, slicing the assembly to form wafers having opposed surfaces.

9. A method as claimed in claim 8furthercom- prising the step of applying electrodes to the opposed surfaces.

10. Amethod as claimed in claim 6whereinthe placing step includes making the optical fibre claddings of a lead oxide material.

11. A method as claimed in claim 10 wherein the treating step includes reducing the lead oxide material ofthe optical fibre cladding in hydrogen gas to form an electron-emissive layer.

12. A microchannel electron multipliercomprising a plurality of optical fibres, some of which include a cladding layer bounding an internal space from which core material has been removed, a tube surrounding the optical fibres, and a plurality of support rods substantially surrounding the optical fibres art a region along the inside periphery of the tube.

13. A microchannel electron multiplier as claimed in claim 12 wherein said cladding layer has an electron-emissive region.

14. A microchannel electron multiplier as claimed in claim 12 wherein each ofthe support rods comprises a plurality of optical fibres, each fibre having a core and a cladding surrounding the core and being a solid tube.

15. Amicrochannel electron multiplier as claimed in claim 12 wherein each ofthe support rods comprises an optical fibre having a core and a cladding surrounding the core and being a solid tube.

16. Amicrochannel electron multiplieras claimed in claim 13 wherein the optical fibres, the support optical fibres and the tube have substantially the same softening temperature.

17. Amicrochannel electron multiplier as claimed in claim 12 wherein the optical fibre cladding layer,thetube and the support optical fibres are formed of a high lead content glass.

18. A microchannel electron multiplier as claimed in claim 12 wherein the cross-sectional area of each of the support rods is substantially the same as that of one of the optical fibres.

19. Amethod offorming a microchannel plate substantially as described with reference to the accompanying drawings.

20. A method of forming a microchannel electron multiplier substantially as described with reference to the accompanying drawings.

21. A microchannel electron multipliersubstantially as described with reference to the accompanying drawings.