GB1601406A

GB1601406A - Photocathode for electroradiographic and electrofluoroscopic devices

Info

Publication number: GB1601406A
Application number: GB1340378A
Authority: GB
Original assignee: Siemens Corp
Current assignee: Siemens Corp
Priority date: 1977-04-06
Filing date: 1978-04-05
Publication date: 1981-10-28
Also published as: FR2386845A1; FR2386845B1

Description

(54) PHOTO CATHODE FOR ELECTRORADIOGRAPHIC AND ELECTROFLUOROSCOPIC DEVICES (71) We, SIEMENS AKTIENGESELLS CHAFT, a German company of Berlin and Munich, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a photo cathode for electroradiographic and electrofluoroscopic devices and also to methods for the manufacture of such a photocathode.

According to one aspect of the present invention, there is provided an electroradiographic or electrofluoroscopic device having a photocathode for emitting photoelectrons when acted upon by X-rays of a suitable wavelength, the photocathode comprising a stack of sheets which are spaced apart and contain material of high atomic number, each sheet having a multiplicity of perforations and incorporating an electrically insulating layer sandwiched between first and second layers of electrically conductive foil.

According to another aspect of the present invention, there is provided a method of manufacturing a photocathode comprising a stack of sheets which are spaced apart and contain material of high atomic number, each sheet having a multiplicity of perforations and incorporating an electrically insulating layer sandwiched between first and second layers of electrically conductive foil, in which method each sheet is produced by covering the unperforated portions on one side of a first layer of perforated electrically conductive foil with an electrically insulating layer and subsequently covering the unperforated portions of the side of the electrically insulating layer facing away from the first layer of perforated electrically conductive foil with an electrically conductive layer to produce a second layer of perforated electrically conductive foil.

According to a further aspect of the present invention, there is provided a method of manufacturing a photocathode comprising a stack of sheets which are spaced apart and contain material of high atomic number, each sheet having a multiplicity of perforations and incorporating an electrically insulating layer sandwiched between first and second layers of electrically conductive foil, in which method each sheet is produced by covering each side of a highly electrically insulating plastics layer with a respective electrically conductive layer, providing the resultant first and second layers of electrically conductive foil with perforations such that the perforations in the first layer are in register with the perforations in the second layer, and removing those parts of the plastics layer separating the perforations of the first layer from the corresponding perforations of the second layer.

By a "highly electrically insulating plastics layer" is meant a layer having a dielectric strength of at least 104 V/cm.

According to a still further aspect of the present invention, there is provided a photocathode comprising a stack of sheets which are spaced apart and contain material of high atomic number, each sheet having a multiplicity of perforations and incorporating an electrically insulating layer sandwiched between first and second layers of electrically conductive foil.

In order that the present invention may be more fully understood, three methods of manufacturing a photocathode according to the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 diagrammatically illustrates a portion of an X-ray photocathode according to the present invention; Figures 2 and 3 illustrate a first method of producing parts of such a photocathode.

Figures 4 and 5 illustrate a second method of producing these parts; and Figures 6 to 15 illustrate a third method of producing these parts.

In the field of medical engineering, attempts are being made to replace the X-ray film which is still generally used in X-ray diagnostics by an image receiving means which is more reasonably priced and more sparing of raw materials. Taking as a starting point the method usually adopted in xerographic copier technology of developing and fixing an electrostatic image by means of a contrast powder, an attempt is being made in electroradiography, as the corresponding radiographic methods are collectively termed, to translate the information content of an Xray beam which has penetrated the exposed subject into electrical charges and then to collect and fix these charges on a paper or plastics sheet.As well as the requirements known from copier technology, there is additionally the requirement in medical electroradiography for high sensitivity since the devices developed for diagnostic purposes already have a sensitivity equivalent to that of the X-ray films used with intensifying screens, and furthermore since the patient should not be expected to bear a radiation dose which is any higher than hitherto.

Because of this requirement for sensitivity of the method, so-called xeroradiography which has been developed from zerography is out of the question for general application in medical diagnostics.

A further method, so-called high-pressure ionography, operates on the principle of an ionisation chamber. The charge carriers produced during the passage of X-rays through a gas in a chamber are collected on a foil sheet. This known method has good sensitivity and definition, but technology it represents an unsatisfactory solution. This is because, in order to obtain sufficiently high absorption of the X-rays in the gas volume, a gas of a high atomic number, e.g. the expensive gas xenon, must be used, and the gas, moreover, has to be present at a high pressure, of 5 bars for example. This places considerable demands on the chamber construction. Moreover, the chamber must be opened after each irradiation to remove the charged foil sheet. The technology required is consequently relatively expensive and the whole exposure operation takes a fairly long period of time.

A further method is so-called low pressure ionography (see Phys. Med. Biol. 18 (1973) pages 695 to 703). In this method, the external X-ray photoelectric effect for X-rays passing through a photocathode constituted by a solid body is utilized to produce electrical charge carriers in the form of photoelectrons. The emitted photoelectrons are then multiplied so strongly in a gas chamber by means of a Townsend-discharge that an electrostatic image capable of being developed is formed on a paper or plastics sheet by these photoelectrons. If an electroluminescent screen is used instead of this sheet to collect the photoelectrons, an image which varies with time can be represented with this method. A method of this type is called electrofluoroscopy. An X-ray image intensifier may utilize this method.When a suitable filler gas is used, which can be present under atmospheric pressure in the chamber, multiplication factors of 104 are readily obtained.

There is, however, a considerable disparity between the penetration depth of the X-rays and the range of the emitted photoelectrons.

Because of this disparity which is about 100:1, solid plane photocathodes can produce only a quantum yield of about 0 5 to 1%. By quantum yield is meant the number of photoelectrons emitted per impinging Xray quantum. With the quantum yield of the known photocathodes, the requirements of medical engineering as regards the sensitivity and resolving power cannot therefore be met.

Figure 1 shows, in section, the diagrammatic structure of a portion of an X-ray photocathode according to the present invention, this photocathode being suitable for an X-ray diagnostic system operating on the basis of low-pressure ionography and having a relatively high quantum yield when exhibiting the external X-ray photoelectric effect. The photocathode of Figure 1 should be disposed in a chamber (not shown in the figure) containing a suitable filler gas such as argon at atmospheric pressure. The photocathode comprises an entry window 2 through which the X-rays represented by single arrows 3 can enter the photocathode, and a stack 4 of double layer perforated foil sheets.

The entry window 2 therefore advantageously consists of a material which is easily permeated by X-rays, such as aluminium, beryllium or a plastics material, for instance.

The window 2 should act as an electrode at the same time. If the window 2 consists of an insulating material, a thin layer of highly electrically conductive material, such as aluminium, may be applied to it, for example by sputtering, i.e. by depositing atoms which have been ejected from a cathode under the effect of a high voltage. The layer thickness of this applied material can be about 1 micron.

Only three perforated foil sheets 5 to 7 are indicated in the figure, although a practical form of construction of the stack 4 would contain a considerably higher number of such sheets, for example 20 to 50 sheets. The sheets 5 to 7, whose structure will be described in detail below, contain material having a high atomic number such as gold because of the required high absorption of the X-rays. They may also consist of nickel or copper which is subsequently gold plated.

The sheets are disposed parallel to one another and to the entry window 2 and are spaced apart from one another. Advantageously their transparency, i.e. the proportion of the total surface area of each sheet taken up by the perforations, is relatively high and is at least 30%, preferably at least 50%. This can serve to prevent too large a proportion of charge carriers produced as a result of the external X-ray photoelectric effect from impinging on the crosspieces defining the perforations and thus being neutralised and lost for use in the production of the image. The sheets may be 3 to 10 microns thick, for example, and may be spaced about 0.3 to 1 mm apart.

Using a stack 4 of sheets whose geometric measurements are adapted to the range of the emitted photoelectrons enables a relatively high quantum yield to be achieved.

The individual sheets largely absorb the Xrays on the one hand, and, on the other hand, due to their transparency they let the charges produced directly or indirectly in the filler gas through, with the result that these charges can be collected on a suitable image carrier 8 and can yield an electrostatic image of the intensity distribution of the X-rays. To this end a sufficiently high electric field gradient across the sheets of the stack is required. This gradient is expediently produced by a different high potential applied to the front and reverse side of each sheet and to the entry window 2 and the image carrier 8. The potentials denoted U to Un indicated in the figure add up to a total potential difference between the window 2 and the image carrier 8.

A method of producing double layer perforated foil sheets for use in such a photocathode is illustrated in Figures 2 and 3 which show a sheet in cross-section at different stages in its manufacture. A simple perforated gold foil 10 produced by a known galvanic technique is first taken. By a simple perforated foil is meant a foil which consists of a single stratum or layer and on which no further layers are applied. According to a particular embodiment, one such gold perforated foil is about 3 microns thick and has a weight per unit area of 3.5 mg/cm2. Its perforations 11, which are square in shape and have a side length of about 16 microns, are bounded by cross-pieces 12 having a width of about 9 microns.

This simple perforated gold foil 10 is provided on one side with an electrically insulating layer 13 of a positive photoresist lacquer. The layer can be a few microns thick. Then, as indicated in Figure 2 by single arrows denoted 14, the parts 15 of the photoresist lacquer covering the perforations of the foil are exposed to UV radiation through the foil. The UV radiation dissociates the parts 15 of the lacquer which are not covered by the crosspieces 12 of the foil 10.

After these lacquer layers parts 15 have been removed by a suitable chemical treatment a perforated insulating layer 16 is left on the underside of the perforated gold foil 10, as shown in Figure 3. An electrically conductive layer 17 of gold or another metal or a suitable semiconductor is then applied in known manner on to this insulation layer 16, for instance by vapour deposition. The double layer perforated foil sheet thus produced and represented in Figure 3 is denoted 18.

A further possible method of producing a double layer perforated foil sheet is indicated in Figures 4 and 5. As in Figure 2, the starting point is a simple perforated gold foil 10. As indicated in Figure 4, by single arrows 19, a layer 20 of insulating material is then vapour deposited or sputtered on to one side of the foil 10. Examples of suitable insulating materials are A1203, SiO2 or organic polymers. According to Figure 5, a layer 17 of gold or another metal of high atomic number is then applied on to the insulating layer 20 according to the method of Figure 3. To do this, as indicated in Figure 5 by single arrows 21, the body consisting ofthe gold perforated foil 10 and the applied insulating layer 20 may be subjected, for instance, to a gold vapour beam. The double-layer perforated foil sheet produced by this method is denoted 22 in the Figure 5.

To complete the photo cathode a number of sheets produced as described with reference to Figures 2 and 3 or 4 and 5 are arranged in a stack and suitable electrical connections are provided so that a voltage can be applied to the two layers 10 and 17 of each sheet which are made of electrically conductive material and are separated electrically from one another by an insulating layer 16 or 20, to provide the potential gradient required for removal of the charge carriers produced as a result of the external X-ray photoelectric effect. The stack arrangement of the individual double layer perforated foil sheets serves to prevent too high a total potential being set up.

Such a photocathode is particularly suitable for use in an electroradiographic or electrofluoroscopic device used in the field of medical engineering, as it comprises a plurality of double layer perforated foil sheets arranged in a stack and each sheet being provided on either side with an electrically conductive layer of a material of high atomic number.

A further method of producing double layer perforated foil sheets is indicated in Figures 6 to 15.

Figure 6 shows a cross-section through parts of a self-supporting insulating sheet 32, i.e. one not requiring a special supporting structure, whose thickness is between approximately 0.1 and a few microns. This sheet is stretched on a frame 33. Such sheets are obtainable commercially (e.g. Union Carbide:parylene). They may also be produced according to a known method on suitable substrates and then detached from these and stretched in the desired manner.

The sheet material should be at least almost free of disruptive ingredients which result in a reduction of the dielectric strength. The dielectric strength of the sheet should be at least 104 V/cm, and preferably over 105 V/cm. By way of example, sheets available commercially having 25 micron layer thickness have a dielectric strength of 2 to 3.106 V/cm. The specific resistance of these sheets is about 6.10'6 ohm.cm.

The insulating sheet 32 is provided on both sides with a thin electrically conductive layer, of a few microns thickness for example, of a material of high atomic number, as shown in Figure 7. These layers 35 and 36 may consist of gold, for instance, and may advantageously be vapour deposited or sputtered onto the sheet 32. To improve the adhesion between the sheet and the layers, short term plasma etching of the sheet surfaces undertaken beforehand in an oxygen or oxygenargon plasma is advantageous.

According to Figure 8, the two layers 35 and 36 are each coated with a layer 39 or 40 of positive photoresist lacquer, for instance.

The lacquer layers may be applied, for example, by centrifuging onto the gold layers.

According to Figure 9, parts of the two photoresist lacquer layers 39 and 40 are then subjected to UV radiation as indicated by arrows 42 or 43. The parts of the lacquer layers which are not to be exposed are screened from the UV radiation by masks 44 or 45. The structure of the masks corresponds to the structure of the perforated foil sheet to be produced. Only the parts of the lacquer layers 39 and 40, therefore, which are not shielded by the masks 44, 45 are exposed to the UV radiation.

After the exposed lacquer layer parts have been developed and detached, the remaining lacquer layer parts may act as perforation masks 47, 48, as shown in Figure 10, for the perforations to be produced in the gold layers 35 and 36 and the insulating sheet 32.

According to one embodiment, the gold layers 39 and 40 are then etched at the points which are not covered by the lacquer masks 47 and 48 by sputter etching, in an argon plasma, for example. Advantageously, a low oxygen partial pressure of preferably below 10-6 Torr is provided to avoid burning away the lacquer. At the points not covered by the lacquer, the gold of the layers can also be detached by chemical etching, it necessary.

This step thus produces the perforated gold layers 50 and 51 represented in Figure 11, which layers have a perforation structure corresponding to the structure of the masks 47, 48.

The lacquer layers 47, 48 are then chemically detached in the known manner, to give the structure shown in Figure 12. A reaction between the substance used for dissolving the photoresist lacquer and the material of the insulating sheet 32 is not generally to be feared nor is it of importance. The parts 53 of the insulating sheet 32 which are not covered by the perforated gold layers 50 and 51 are then removed, for example by etching and the result is the structure represented in Figure 13. In Figure 13 the perforated insulating sheet thus produced is denoted 55.

Because of the high resistance of the sheet material detaching the parts 53 of the sheet 32 by chemical means can involve problems.

Sputter etching in an oxygen or argonoxygen plasma is then advantageously utilized. It is preferable to employ plasma etching in which burning away and thus etching of the sheet parts takes place in an oxygen plasma of low power density by means of the active oxygen produced by the plasma. The proportion of the sheet material sputtered in this case is low. The material removal is essentially by burning. Damaging thermal stress of the perforated gold layers 50 and 51 which could lead to faults is avoided.

Gold atoms are also prevented from condensing on the material of the insulating sheet due to the substantially higher sputter rate of the gold relative to this material.

If the desired layer thicknesses of the gold layers 50 and 51 cannot be achieved, it is possible to effect subsequent reinforcement of these layers, by galvanisation for example.

In Figures 14 and 15 parts of a double layer perforated foil sheet produced as described with reference to Figures 6 to 13 are illustrated respectively in cross-section and in plan view. The parts deposited by galvanisation on the individual cross pieces 57 of the perforated gold layers 50 and 51 are indicated in these figures by thick lines denoted 58. As a result of the reinforcement of these cross-pieces, the cross-section of the perforations 59 formed between the crosspieces is made correspondingly smaller relative to the perforations 60 in the perforated insulating sheet 55.

If possible, gold layers having a fairly large thickness, for instance over 1 micron, are desirable. Such layer thicknesses may be particularly advantageous with large surface double layer perforated foil sheets since the sheets are then mechanically more stable and have less tendency to sag. To produce such sheets additional metal masks are advantageously provided between the respective gold layers and the photoresist lacquer layers. In this way it is possible to prevent the photoresist lacquer layers being completely disintegrated in the sputter operation for etching of the perforations of the gold layers before the gold layer parts are removed. Titanium is particularly suitable as the material for these additional masks. This material can be applied on to the gold layers by vapour deposition or sputtering for example.In order to produce the perforations in the titanium layers, the layers are sputter etched through the photoresist lacquer masks, in accordance with the method described for etching the gold layers. Advantageously, an argon plasma is provided with the lowest possible oxygen partial pressure, preferably below 10-6 Torr. The titanium layer thickness is selected such that the lacquer mask lasts at least until the titanium perforation pattern is completely formed, i.e. the titanium layers are completely perforated. Thereafter some oxygen is added to the argon plasma without interrupting the continuous sputter etching process, until a partial pressure of 10-4 Torr for example, is established.

This causes the titanium masks to be superficially oxidized. Since titanium oxide (TiO) has a lower sputter rate than titanium or gold, those parts of the gold layers not covered by the titanium layers may then be completely removed to produce the required perforations, even when only thin titanium layers have been applied. Any remaining parts of the lacquer layers are removed completely by burning off. No problems arise in etching the insulating sheet since an oxygen-containing plasma has already been provided.

After the complete etching of the gold layers, parts of the titanium masks may still be present. In that case the sputter etching process can be continued until the titanium layers are completely removed, since there are no disadvantages so far as the exposed parts of the insulating sheet are concerned, provided a low power density plasma is used.

Damaging thermal stress of the insulation sheet is thus avoided.

In the method described with reference to Figures 6 to 15, the masking process and the etching processes are conducted simultaneously on both sides of the insulating sheet.

However, the individual masking and etching processes can just as well be carried out first on one side and then on the other side of the sheet.

The advantages of the photocathodes described above with reference to the drawings lie, one one hand, in the fact that using a material of a high atomic number as the cathode material means that a relatively strong absorption of the X-rays and thus a correspondingly high quantum yield is achieved. The quantum yield, i.e. the num ber of electrons produced by an X-ray quantum, is essentially the product of the photoabsorption coefficients and the electron range and depends on the energy of the Xrays and the atomic number of the cathode material. On the other hand, because of the enlargement of the effective surface of the photo cathodes due to the stack arrangement of the perforated foil sheets, the quantum yield of these photo cathodes is substantially higher than the quantum yield of a comparable solid plane cathode.The electron emission capability of such photocathodes increases proportionally with the enlarged surface as long as attenuation of the X-rays in these structures is of secondary importance. The penetration depth of an X-ray quantum of the X-rays generally used in medical diagnostics in a material of a high atomic number, such as gold for example, is between approximately 40 and 200 microns depending on wavelength, whilst the range of the photoelectrons produced is generally below 2 microns. For this reason, in the case of the known solid plane cathodes, only a small percentage of the photoelectrons produced as a result of the photoelectric effect are utilized.Moreover, since the quantum flux density at the cathode is prescribed, it follows that a rise in the yield can only be achieved by also using the quanta absorbed in the case of plane cathodes in layers deeper than 2 microns and the generated photoelectrons. This is achieved in the embodiments described with reference to the drawings by providing individual layers of a prescribed thickness and appropriate structure so as to ensure that the photoelectrons can emerge from the structure. It is important that no substantial local shifting of the charges away from the site of origin occurs as otherwise lack of definition in the charge image produced would result. A stack arrangement of perforated foil sheets satisfies this requirement, it being unnecessary to adjust the individual sheets relative to one another, which is an advantage.The sheets are expediently spaced apart by from 5 microns to 5 millimetres and preferably from about 10 microns to 1 millimetre.

The thickness of each of the perforated foil sheets is advantageously selected to be less than ten times the range of the photoelectrons produced in the sheet. Preferably it is less than or equal to double the range. The majority of the photoelectrons produced can then emerge from the sheets.

If the stack arrangement contains so many perforated foil sheets that its total thickness is a multiple of the penetration depth of the Xrays, and is conveniently at least double and preferably at least five times the penetration depth in the material of high atomic number, a particularly high quantum yield is obtained due to the external photoelectric effect, since virtually all the X-ray quanta can then be utilized and since the majority of the photoelectrons generated during absorption can also emerge from the material.

The above described photocathodes according to the present invention have high sensitivity and high resolving power when used for low-pressure ionography.

WHAT WE CLAIM IS:- 1. An electroradiographic or electrofluoroscopic device having a photocathode for emitting photoelectrons when acted upon by X-rays of a suitable wavelength, the photocathode comprising a stack of sheets which are spaced apart and contain material of high atomic number, each sheet having a multiplicity of perforations and incorporating an electrically insulating layer sandwiched between first and second layers of electrically conductive foil.

2. A device according to claim 1, wherein the thickness of each sheet is less than ten times the range of the photoelectrons produced in the sheet when acted upon by Xrays of said suitable wavelength.

3. A device according to claim 2, wherein the thickness of each sheet is less than or equal to double the range of the photoelectrons produced in the sheet when acted upon by X-rays of said suitable wavelength.

4. A device according to any one of claims 1 to 3, wherein the total thickness of the stack is at least double the penetration depth of X-rays of said suitable wavelength in said material of high atomic number.

5. A device according to any one of claims 1 to 4, wherein adjacent sheets are spaced apart by a distance in the range from 5 microns to 5 millimetres.

6. A device according to claim 5, wherein adjacent sheets are spaced apart by a distance in the range from 10 microns to 1 millimetre.

7. A device according to any one of claims 1 to 6, wherein at least 30% of the surface area of each sheet is perforated.

8. A device according to claim 7, wherein at least 50% of the surface area of each sheet is perforated.

9. A device according to any one of claims 1 to 8, wherein said material of high atomic number is gold.

10. A device according to claim 9, wherein each sheet contains gold foil or goldplated nickel foil or gold-plated copper foil.

11. A method of manufacturing a photocathode comprising a stack of sheets which are spaced apart and contain material of high atomic number, each sheet having a multiplicity of perforations and incorporating an electrically insulating layer sandwiched between first and second layers of electrically conductive foil, in which method each sheet is produced by covering the unperforated portions on one side of a first layer of perforated electrically conductive foil with an electrically insulating layer and subsequently covering the unperforated portions of the side of the electrically insulating layer facing away from the first layer of perforated electrically conductive foil with an electrically conductive layer to produce a second layer of perforated electrically conductive foil.

12. A method according to claim 11, wherein the whole of said one side of the first layer of perforated electrically conductive foil is first covered with the electrically insulating layer and then the parts of the electrically insulating layer covering the perforations of the foil are removed.

13. A method according to claim 12, wherein the electrically insulating layer is a positive photoresist lacquer and the parts of the photoresist lacquer covering the perforations of the foil first layer of perforated electrically conductive foil are removed by irradiating these parts with ultraviolet radiation through the perforations of the foil and chemicqlly treating the irradiated parts.

14. A method according to claim 11, wherein the unperforated portions on one side of the first layer of perforated electrically conductive foil are covered with the electrically insulating layer by vapour deposition or by sputtering.

15. A method according to any one of claims 11 to 14, wherein the said unperforated portions of the said electrically insulating layer are covered with the said electrically conductive layer by vapour deposition.

16. A method according to any one of claims 11 to 15, wherein the said electrically conductive layer is constituted by a metal.

17. A method according to claim 16, wherein the first layer of perforated electrically conductive foil is constituted by the same material as said electrically conductive layer.

18. A method of manufacturing a photocathode comprising a stack of sheets which are spaced apart and contain material of high atomic number, each sheet having a multiplicity of perforations and incorporating an electrically insulating layer sandwiched between first and second layers of electrically conductive foil, in which method each sheet is produced by covering each side of a highly electrically insulating plastics layer, as herein defined, with a respective electrically conductive layer, providing the resultant first and second layers of electrically conductive foil with perforations such that the perforations in the first layer are in register with the perforations in the second layer, and removing those parts of the plastics layer separating the perforations of the first layer from the corresponding perforations of the second layer.

19. A method according to claim 18, wherein, prior to application of the electrically conductive layers, the plastics layer is etched in an oxygen or argon-oxygen plasma.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The above described photocathodes according to the present invention have high sensitivity and high resolving power when used for low-pressure ionography. WHAT WE CLAIM IS:-

1. An electroradiographic or electrofluoroscopic device having a photocathode for emitting photoelectrons when acted upon by X-rays of a suitable wavelength, the photocathode comprising a stack of sheets which are spaced apart and contain material of high atomic number, each sheet having a multiplicity of perforations and incorporating an electrically insulating layer sandwiched between first and second layers of electrically conductive foil.

20. A method according to claim 18 or

29, wherein the plastics layer is covered with the electrically conductive layers by vapour deposition or sputtering.

21. A method according to claim 18, 19 or 20, wherein the first and second layers of electrically conductive foil are provided with perforations by etching through masks applied thereto.

22. A method according to claim 21, wherein the perforations are provided by sputter etching in an argon plasma.

23. A method according to claim 21 or 22, wherein the masks are constituted by perforated photoresist layers.

24. A method according to claim 21 or 22, wherein the masks are constituted by perforated metal layers.

25. A method according to claim 24, wherein said metal layers are constituted by titanium.

26. A method according to claim 24 or 25, wherein said metal layers are vapour deposited sputtered on the first and second layers of electrically conductive foil.

27. A method according to claim 24, 25 or 26, wherein the said metal layers are provided with perforations by etching through further masks applied to said metal layers.

28. A method according to claim 27, wherein the perforations in said metal layers are provided by sputter etching in an argon plasma with an oxygen partial pressure below 10-6 Torr.

29. A method according to claim 27 or 28, wherein the further masks are constituted by perforated photoresist layers.

30. A method according to any one of claims 18 to 29, wherein those parts of the plastics layer separating the perforations of the first layer of electrically conductive foil from the corresponding perforations of the second layer of electrically conductive foil are removed by etching.

31. A method according to claim 30, wherein said parts of the plastics layer are removed by plasma etching in an oxygen or argon-oxygen plasma.

32. A method according to any one of claims 18 to 31, wherein the first and second layers of electrically conductive foil are subsequently galvanically reinforced.

33. A photocathode comprising a stack of sheets which are spaced apart and contain material of high atomic number, each sheet having a multiplicity of perforations and incorporating an electrically insulating layer sandwiched between first and second layers of electrically conductive foil.

34. A photocathode substantially as hereinbefore described with reference to Figures 1 to 3 or Figures 1, 4 and 5 of the accompanying drawings.

35. A photocathode substantially as hereinbefore described with reference to Figures 6 to 15 of the accompanying drawings.

36. An electroradiographic or electrofiu- oroscopic device having a photo cathode substantially as hereinbefore described with reference to Figures 1 to 3 or Figures 1, 4 and 5 of the accompanying drawings.

37. An electroradiographic or electrofluoroscopic device having a photocathode substantially as hereinbefore described with reference to Figures 6 to 15 of the accompanying drawings.

38. A method of manufacturing a photocathode, which method is substantially as hereinbefore described with reference to Figures 1 to 3 or Figures 1, 4 and 5 of the accompanying drawings.

39. A method of manufacturing a photocathode, which method is substantially as hereinbefore described with reference to Figures 6 to 15 of the accompanying drawings.