GB2045808A - Method of forming a secondary emissive coating on a dynode - Google Patents

Abstract

A method of forming an emissive coating on a dynode substrate 1. The method comprises the steps of vapour depositing a composite coating 10,11 consisting of magnesium and aluminium onto the dynode substrate 1. A 50 to 500 ANGSTROM thick layer 12 of aluminium is vapour deposited over the composite coating 11,12 and the aluminium layer 12 is oxidised. The coated dynode is then activated by heating it in a partial pressure of from 5 x 10<-6> to 4 x 10<-4> Torr oxygen at a temperature between 270 and 400 DEG C. The resulting secondary emissive coating contains from 1.5 to 90% by weight of magnesium. The coated dynodes are used in channel electron multipliers which are suitable for use in electron display tubes such as image intensifiers or colour television display tubes. <IMAGE>

Description

SPECIFICATION Method of forming a secondary emissive coating on adynode The present invention relates to a method of forming a secondary emissive coating on a dynode, to a dynode coated by such a method, to a channel electron multiplier comprising a stack of such dynodes, and to an electron display tube, for example a cathode-ray tube or an image intensifier, including such a channel electron multiplier.

United Kingdom Patent Specification 1,401,969, 1,402,549 and 1,434,053 describe different types of channel electron multipliers, which each consist essentially of a stack of perforate metal electrically conductive layers each having a regular array of apertures with the apertures of each of said layers aligned with those of the other conductive layers in the stack so as to define the channels, and separating means disposed between each pair of adjacent conductive layers, which separating means do not obstruct the channels. When the conductive layer material is not sufficiently secondary emissive for a particular application, the secondary emissive properties of the conductive layers can be enhanced by providing a coating of a more emissive material at least on the exposed surfaces of the conductive layers inside the channels.This may be done on all the conductive layers, but it may be preferable to apply the emissive coating only to the first few conductive layers located on the input side of the channel electron multiplier.

United Kingdom Specification 1,523,730 describes dynodes suitable for use in channel electron multipliers, the dynodes consisting of substrates bearing secondary emissive coatings of cermets of specified compositions, each containing an alkali metal fluoride. However, the secondary emissive coefficients of these cermets are not appreciably more than 4.

An article "Growth of MgO films with high secondary emission on Al-Mg alloys" by B. Goldstein and J. Dresser in Surface Science, Vol. 71 No.1(1978), pages 15-26, discloses the formation of secondary emissive layers by the oxidation and activation of high purity sheet Al-Mg alloys having Mg-contents of from 0.1 to 3% by weight. The magnesium concentration in the surface oxide layers was increased by heating oxidized alloy sheet at temperatures of the orderof450"C, and values of the secondary emission coefficient (6) of from 10 to 15 were obtained by this method. These alloys are not suitable for making dynodes of channel electron multipliers since an etching technique is not at present available which is suitable for etching the geometries desired for dynodes in these magnesium alloys.

During the investigations which led to the present invention, it was found that secondary emissive coatings formed by evaporating aluminiummagnesium alloys containing from 0.1 to 3% by weight of magnesium could not be activated readily.

The present invention provides a method of forming a secondary emissive coating on a dynode, the method comprising the steps of vapour depositing a composite coating at least 200 A thick consisting of magnesium and aluminium onto the dynode, vapour depositing from 50 to 500 A of aluminium over the composite coating, oxidizing the exposed aluminium layer, and activating the coated dynode by heating it in a partial pressure of from 5 x 106to 4 x 10-4 Torr oxygen at a temperature between 270 and 400"C, wherein the secondary emissive coating contains from 1.5 to 90% by weight of magnesium.

The composite coating may consist of a magnesium layer disposed on a subjacent aluminium layer which abuts the dynode. The composite coating may be formed by vapour depositing a layer of aluminium onto the dynode, and vapour depositing magnesium and aluminium simultaneously onto the aluminium layer abutting the dynode.

The purpose of depositing an aluminium layer over the magnesium-containing layer is to provide a barrier between the magnesium in this magnesiumcontaining layer air, since the coated dynode will be exposed to air when it is removed from the vapour deposition atmosphere and before the coating is activated. If the surface skin of the coating contained a significant quantity of magnesium, the magne sium-containing layer is difficult to oxidise - this is probably due to the formation of a magnesium hydroxide layer which must be decomposed before a MgO layer can be produced.When the magnesium-containing layer is covered by an aluminium layer, at least the outer thickness of this aluminium layer is converted by oxidation into aluminium oxide, and magnesium diffuses into this aluminium oxide layer during the activation step, and is oxidised when it reaches the surface of the coating.

Dynodes coated by a method according to the invention are used to make channel electron multipliers which comprise a stack of coated dynodes separated from each other by separating means disposed between each pair of adjacent dynodes, each dynode having a regular array of apertures, wherein the apertures of the respective dynodes are aligned so as to form the channels, wherein the supporting means do not obstruct the channels, wherein the separating means are electrically insulating or have a higher electrical resistivity than that of the dynodes. The dynodes consist of single sheets or of two mating sheets which are in electrical contact with each other.The dynode material may be coated so as to improve adhesion with interdynode insulating material, for example glass, and to act as a diffusion barrier to impurities, for example, sulphur, so that the impurities do not poison the emissive coating.

Preferably, the dynodes consist of mild steel, since well-estabiished etching techniques can be used to produced desired geometries of the dynodes in sheet mild steel.

The magnesium and aluminium may be vapour deposited by evaporation, since these metals are easily evaporated. The magnesium-aluminium layer may be, for example from 1000 to 2000 A thick.

When the dynode consists at least substantially of mild steel, an aluminium layerfrom 100 to 1000A thick may be disposed between the mild steel and a magnesium-comprising layer so as to reduce the rate of diffusion of magnesium into the mild steel.

An emissive coating consisting solely of magnesium is more difficult to activate, needing an activation temperature of 300"C, than an emissive coating produced by a method according to the invention. It is possible to activate the coatings formed by the method according to the present invention by heating in the above-specified atmosphere for 3 hours at 270"C and this is appropriate when dynodes are activated inside an electron-display tube having an envelope with a pressure-bonded seal comprising a lead sealing member.

It is possible to activate the dynodes outside an electron tube - this avoids heating other components of the tube in an oxidising atmosphere, but there is then the risk of contaminating the activated secondary emissive surfaces.

When the metals used to form the emissive coating are evaporated onto dynodes, the dynodes may be at room temperature and the pressure in the evaporation chamber is preferably from 1 to 3 x 10-5 Torr, the atmosphere in the evaporation chamber then consisting mainly of water vapour.

Although carbon contamination of the dynode surface has the effect of degrading the secondary emission coefficient of the emissive coating if this contamination is not removed, the effect of surface contamination of the dynode with carbon on the activated dynode is reduced to a low level since this contamination is reduced during the activation process in 15 minutes from 30% of a monolayer to less than 5% of a monolayer.

It was found that the maximum inthe6-voltage curve for emissive coatings formed by a method according to the invention is a higher voltages (about 600 volts) than was the case for gold-cryolite cermet layers of United Kingdom Patent Specification 1,523,730. This feature may be advantageous with respect to space charge problems on account of the higher currents flowing through the dynodes.

The emissive coatings formed by the method according to the invention are more stable to electron bombardment than are the cermets formed with alkali metal fluorides, and there is no risk of fluorine contamination of electron tube components when using aluminium-magnesium emissive coatings.

Two embodiments of the present invention will now be described with reference to the Examples and to the drawings in which: Figure 1 is a schematic side-sectional elevation of an apparatus used to evaporate a secondaryemissive coatng on a dynode substrate by a method according to the invention.

Figure 2 is a side-sectional elevation of part of a channel elctron multiplier produced from dynodes coated by a method according to the invention, Figure 3 is a side-sectional elevation of part of another channel electron multiplier produced from dynodes coated by a method according to the invention, and Figure 4 is a diagrammatic longitudinal section of a channel plate cathode-ray tube including a channel electron multiplier as described with reference to Figure 2 or to Figure 3.

Referring to Figure 1, a dynode substrate 1 is mounted on a rotatable work-holder 2 (the means used for rotating the work-holder 2 are not shown for the sake of clarity) inside an evaporation vessel 3 mounted on a pump table 4. The evaporation vessel 3 contains a magnesium source consisting of a molybdenum boat 5 having a perforated cover 6, the boat 5 containing a charge of magnesium, and an aluminium source which is a tungsten helix 7 which supports pieces of aluminium wire (not shown). The dynode substrate 1 is disposed at a distance d (20 cms) above the molybdenum boat 5 and the tungsten helix 7, and the distance s between the centres of the molybdenum boat 4 and the tungsten helix 7 is 2 cms. The aluminium and magnesium sources are heated by passing current from respective power supplies 8 and 9 through the helix 7 and through the boat 5, respectively.It appears, that provided that the ratio d: s is at least 10:1,the composition of a magnesium-aluminium alloy deposited on the dynode substrate 1 by simultaneously evaporating magnesium and aluminium is homogeneous over the area of the dynode substrate 1.

Example 1 A mild steel plate 1 which had been plated with 1,um of nickel was placed on the work-holder 2 in the apparatus described with reference to Figure 1. The work-holder 2 was rotated at 30 r.p.m. Pressure in the apparatus was reduced to 2 x 10-5 Torr, and the aluminium source 7 was energised and formed a 100 A thick aluminium layer lOon the mild steel plates in 2 minutes. Evaporation from the aluminium source 7 was continued and the magnesium source 6 was energised, a 500 A thick layer 11 consisting of 40% by weight aluminium and 60% by weight magnesium was deposited in 3 minutes. Magnesium deposition was then stopped, and a 75 thick layer 12 of pure aluminium was deposited over the magnesium-aluminium layer 11.The coated plate 1 was then left in air at atmospheric pressure at 20"C for 60 horus so as to convert the surface aluminium layer 12 into aluminium oxide. The coated plate was then activated by heating at 4 hours in a partial pressure of 4 x 10-5 Torr oxygen. The secondary emission coefficient (6) of the activated coating was 5.6 at 500eV.

Example 2 Mild steel plates which have been plated with 1 um nickel were placed in the apparatus described with reference to Figure 1. Pressure in the apparatus was reduced to 2 x 1 10-5 Torr, and the aluminium source 7 was energised to deposit a 150 A thick layer of aluminium on the mild steel plates in 2 minutes.

Aluminium deposition was terminated, and 500 A of magnesium was deposited over the aluminium layer. 200 of aluminium was then deposited over the magnesium layer. Afirstsetofth coated pltes were oxidised by leaving them in air at atmospheric pressure at 20"C for 60 hours. A second set of the coated plates were oxidised by heating in air at atmospheric pressure at 200"C for 1 hour. Both sets of plates were activated by heating for 4 hours in a partial pressure of 4 x 10-5 Torr oxygen at 270"C.

The secondary emission coefficient (6) at 500 eV of the first set of plates was 5.85 and was 6.15 for the second set of plates.

Channel electron multiplier Figure 2 shows part of a channel electron multiplier 13 built up from dynodes 14, 15, 16 and 17. Each of these dynodes comprises a nickel-plated perforated steel plate, the perforations constituting channels 18 each bearing a secondary emissive coating 22 formed by a method according to the present invention. The channels 18 of the dynodes 14, 15, 16 and 17 are aligned with each other and converge in the directions of electron multiplication. The dynodes 14to 17 are separated by spherical separating elements 19 in the form of ballotini which are bonded by glass enamel 20 to adjacent dynodes. By way of illustraton the density of the elements 19 at the imperforate edges of the dynodes 14 to 17 is greater than in the centre thereof.Although the elements 19 are shown positioned between each channel opening of a dynode, they could be spaced apart by integral multiples of the distance between the centres of adjacent channels 18 of a dynode.

Each channel 15 bears a secondary emissive coating 22 formed by a method according to the invention.

As the illustrated separating elements 19 are electrically insulating, it is necessary that each dynode be biassed separately by a power supply 21.

Figure 3 shows an alternative embodiment of a channel plate structure 13 to that shown in Figure 1.

Dynodes 23 to 26 each comprise two, juxtaposed, mating perforated metal plates 28,29. Each of the channels 18 in the plate 29 and the top surface of each of the dynodes 23 to 26 bear a secondary emissive coating 22 formed by a method according to the invention. A single perforated metal plate 27 is disposed above the dynode 23. The separating elements 19 comprise ballotini arranged at suitable intervals between the channels. Once again taps of the power supply 21 are connected to respective dynodes.

Channel plate cathode-ray tube Figure 4 diagrammatically illustrates a channel plate cathode-ray tube 30 comprising a metal, for example mild steel, cone 31 having a substantially flat plate glass screen 32 closing the open end of the cone 31. A channel electron multiplier 13 as described with reference to Figure 2 is disposed at a small distance, for example 10 mm, from the screen 32. An electron gun 33 is disposed adjacent the closed end of the cone 31 and a deflection coil assembly 34 is disposed adjacent to, but spaced from, the electron gun 33.

In operation a low energy electron beam 35 from the electron gun 33 is deflected in raster fashion across the input side of the channel electron multi plier structure 13 by the coil assembly 34. The beam undergoes electron multiplication in the channel electron multiplier 13 and the output electrons are applied substantially normally to the screen 32.

Claims (9)

1. A method of forming a secondary emissive coating on a dynode, the method comprising the steps of vapour depositing a composite coating at least 200 A thick consisting of magnesium and aluminium onto the dynode, vapour depositing from 50 to 500 A of aluminium over the composite coating, oxidising the exposed aluminium layer, and activating the coaed dynode by heating it in a partial pressure of from 5 x 106to4 x 10-4 Torr oxygen at a temperature between 270 and 400"C, wherein the secondary emissive coating contains from 1.5 to 90% by weight of magnesium.

2. A method as claimed in Claim 1, wherein the composite coating consists of a magensium layer disposed on a subjacent aluminium layer which abuts the dynode.

3. A method as claimed in Claim 1, wherein the composite coating is formed by vapour depositing a layer of aluminium onto the dynode, and then simultaneously depositing magnesium and aluminium onto this aluminium layer.

4. A method as claimed in Claim 3, wherein the dynode consists at least substantially of mild steel, a 100 to 1000 thick aluminium layer is vapour deposted onto the dynode, and a coating from 1000 to 2000 thick of magnesium and aluminium is vapour deposited onto this aluminium layer.

5. A method of forming a secondary emissive coating on a dynode, substantially as herein described with reference to Example 1 or Example 2.

6. A dynode bearing a secondary emissive coating formed by a method as claimed in any of Claims 1 to 4.

7. A channel electron multiplier comprising a stack of dynodes bearing secondary emissive coatings as claimed in Claim 6 separated from each other by separating means disposed between each pair of adjacent dynodes, wherein each dynode substrate comprises a perforate electrically conductive sheet having a regular array of apertures, wherein the apertures of the respective dynodes are aligned so as to form the channels, wherein the separating means do not obstruct the channels, wherein the separating means are electrically insulating or have a higher electrical resistivity than that of the dynodes, and wherein the secondary emissive coatings extend at least over the walls of the apertures in the electrically conductive sheet.

8. A channel electron multiplier comprising a stack of dynodes bearing secondary emissive coatings as claimed in Claim 6 separated from each other by separating means disposed between each pair of adjacent dynodes, including dynodes comprising two perforate electrically conductive mating sheets having a regular array of apertures, which two mating sheets are in electrical contact with each other, wherein the apertures of the respective dynodes are aligned so as to form the channels, wherein the separating means do not obstruct the channels, wherein the separating means are electrically insulating or have a higher electrical resistivity than extend at least over the walls of the apertures in the electrically conductive mating sheets.

9. An electron display tube including a channel electron multiplier as claimed in Claim 7 or Claim 8.