GB2113000A - Improvements relating to fast focussed electron multiplier tubes - Google Patents

Abstract

Fast focussed electron multiplier tubes include arrays of electron emitting dynodes, which typically possess coated secondary electron emitting surfaces. A manufacturing method is disclosed which achieves efficient in-situ coating of these surfaces without subsequent undue disturbance of the electron tube optics. Two facing dynode assemblies (6,6') include trough-shaped dynodes (20,22). At least one dynode (20) includes a wire (not shown) spanning the length of the trough- shape and located on an equipotential (31) between facing dynodes (20,22) such that the tube function is substantially unaffected thereby. A spot (10) of secondary emitting material is provided on the wire and when a radio frequency field is applied to the tube, currents developed in the wire cause the spot to evaporate thereby coating the facing dynodes (20,22). <IMAGE>

Description

SPECIFICATION Improvements relating to fast focussed electron multiplier tubes This invention relates to fast focussed electron multiplier tubes and the manufacture thereof.

Focussed electron multiplier tubes typically include a photocathode disposed at an entrance to an array of electron multiplying dynodes, which may possess coated secondary electron emitting surfaces. The chemicai composition of these surfaces affects substantially the tube performance. A typical composition is described in G.B. patent no: 1307453, comprising antimony activated with potassium and cesium. A preferred method of tube manufacture involves application of antimony to the dynodes in the tube, subsequently activating with potassium and cesium. It will be apparent that a means for in-situ antimony coating, whilst being efficient, should not cause undue disturbance to the electron optics of the system, or removal of the means would be necessary.

It is an object of this invention to provide a focussing dynode array including means for in tube deposition of an electron emissive material, the electron optics of the array being substantially unaffected by the presence of the means.

According to one aspect of the invention there is provided an electron multiplier tube including a focussing dynode assembly comprising arrays of shaped focussing dynodes, a first array being arranged to face a second array, an axis of which lies substantially parallel to an axis of said first array; at least one of said dynodes being a coating dynode having associated therewith a secondary electron emitting material arranged to lie on an equipotential coupling facing dynodes of said arrays and to be susceptible of indirect heating means.

In a preferred embodiment the dynodes have trough shaped emitting sections, the at least one coating dynode having a wire, spanning said trough lengthwise, and being located on said equipotential, and said material being located thereon. The dynode includes a further electrical loop sufficient to allow said wire to heat in the presence of a radio frequency heating means.

The invention also encompasses a method of manufacturing a focussed electron multiplier tube.

For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example only, to the accompanying drawings, in which: Figure 1 illustrates a shaped focussing dynode being a coating dynode.

Figure 1 a illustrates a side view of Figure 1 along arrow A.

Figure 2 illustrates two facing arrays of shaped focussing dynodes.

Figure 3 shows a cross-section of Figure 2 along the plane indicated by arrows B, and illustrates the equipotential pattern.

Referring to Figures 1 and 1 a, a shaped focussing dynode comprises an emitting section 1 being trough shaped and having side members 2 and 3, which lie in slots 4 and 5 appropriately positioned an an electrically insulating member 6.

The extensions locate with and may pass through slots in a brnr-' strip 7 disposed on the side of the insulating member opposite to the emitting section 1. The emitting section side members and broad strip may comprise any suitable electrical conductor, the emitter section is preferably formed from 0.005" thick beryllium copper. The extensions 2 and 3 may be electrically connected with the strip 7 by any suitable means, for example welding, and the insulating member 6 may be formed from a suitable electrical insulator, for example ceramic, or may have an electrically insulating coating. Electrical connection to the strip 7 may be made by a wire 8, which may form an integral part of the strip.

The dynode described above is disclosed in our published patent application No. 2050048A. The structure of the dynode produces a loop around which eddy currents may circulate. A wire 9 may be fixed by suitable means, for example spot welding, across the length and hence spanning and running alongside, the emitting section.

Placement of the wired dynode structure in a radio frequency (RF) field of said 2 MHz, can produce heating of the wire sufficient to evaporate a spot 10 of secondary electron emitting material located thereon which evaporation coats the emitting section 1. Preferably the spot 10 consists of 0.1 mg of antimony and the wire upon which it is located is a 0.005" thick platinum coated molybdenum. It will be apparent therefore, that utilization of the wired dynode structure or coating dynode, inside a photo multiplier tube allows in-situ coating of the emitting section, because the necessary RF field can be generated outside the tube.

An array of focussing dynodes may be constructed and two arrays positioned facing one another within an electron multiplier tube, the axes of their insulating members substantially parallel, producing thereby a potential field pattern resulting in amplification of the signal generated by a photocathode of the tube (not shown).

Figure 2 illustrates a typical arrangement wherein one of the dynodes 20 is a coating dynode, that is to say, it has a wire 9 welded across and running alongside the emitting section of the dynode 20 and a spot 10 of secondary electron emitting material, for example antimony located thereon. It will be apparent that any of the dynodes may be coating dynodes. A cross-section of the arrangement illustrated in Figure 3, portrays a number of equipotential field lines 30, 31, 32, 33 between the two arrays. Evidently an equipotential line 31 couples opposing dynodes 20 and 22. Location therefore of wire 9, on this dynode coupling equipotential will not substantially affect the field pattern, which pattern is critical to the effective amplication resulting from the two arrays. Therefore, antimony spot 10 may be caused to evaporate by an independent radio frequency means external to the electron multiplier tube as disclosed hereinabove, coating thereby dynodes 20 and 22, the residual wire 9 not affecting the tube equipotential field pattern.

Clearly a focussing dynode assembly including at least one coating dynode allows a photomultiplier tube to be constructed permitting in tube coating of emitting sections of the assembly without later disturbance of the tube performance characteristics.

It will be understood that the embodiments illustrated shows an application of the invention in one form only for the purposes of iilustration only.

In practice the invention may be applied to many different configurations, the detailed embodiments being straightforward for those skilled in the art to implement.

Claims (8)

1. The electron multiplier tubes including a focussing dynode assembly comprising arrays of shaped focussing dynodes, a first array being arranged to face a second array, an axis of which lies substantially parallel to an axis of said first array; at least one of said dynodes being a coating dynode having associated therewith a secondary electron emitting material arranged to lie on an equipotential coupling facing dynodes of said arrays and to be susceptible of indirect heating means.

2. A focussing dynode assembly according to claim 1 wherein each of said dynodes comprise a trough shaped electron emitting section located on one face of an electrically insulating sheet, said emitting section incorporating side members arranged to protrude through slots in said insulating sheet to locate with an electrical linking member on the second face of said sheet, forming thereby an electrically conducting loop.

3. An assembly according to claim 1 or 2 wherein the coating dynode includes a wire, spanning said trough lengthwise, and being located on said equipotential, and said secondary electron emitting material being disposed thereon.

4. An assembly according to any of claims 1 to 3 wherein said secondary electron emitting material comprises antimony.

5. An assembly according to any preceding claim wherein said indirect heating means comprises a radio frequency heating means.

6. A method of manufacturing an electron multiplier tube including the step of suitably locating, an assembly according to any of claims 1 to 5 and subsequently applying a radio frequency field to cause heating and evaporation of said secondary electron emitting material.

7. A method of manufacturing electron multiplier tubes as herein described.

8. A focussing dynode assembly as herein described with reference to and as illustrated by Figures 2 and 3.