US3617791A

US3617791A - Image intensifier including polyimide support

Info

Publication number: US3617791A
Application number: US812543A
Authority: US
Inventors: Karl Franz; Gerhard Kochmann
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1968-03-14
Filing date: 1969-03-11
Publication date: 1971-11-02
Anticipated expiration: 1988-11-02
Also published as: DE1622999A1; DE1622999B2; FR2003856A1; GB1198597A

Abstract

A screen which comprises a support on which is arranged a layer of a substance capable of fluorescing has the support formed of a polyimide. The screen is suitable for use in a vacuum image intensifier.

Description

United States Patent Inventors Karl Franz;

Gerhard Kochmann, both of Erlangen,

Germany Appl. No. 812,543

Filed Mar. 11, 1969 Patented Nov. 2, 1971 Assignee Siemens Aktiengesellschaft Erlangen, Germany Priority Mar. 14, 1968 Germany IMAGE INTENSIFIER INCLUDING POLYIMIDE SUPPORT [56] References Cited UNITED STATES PATENTS 2,945,976 7/1960 Fridrich et al. 313/108 A 3,254,252 5/1966 Anderson et al.... 313/94 3,403,279 9/1968 Wanmaker et al 313/94 OTHER REFERENCES Deshotels et al; Chemical Abstracts, Vol. 65, 1966; 14581 d Primary Examiner-Robert Sega] Attorney- Richards & Geier ABSTRACT: A screen which comprises a support on which is arranged a layer of a substance capable of fluorescing has the support formed of a polyimide. The screen is suitable for use in a vacuum image intensifier.

PATENT Enuov 2 m: 3,6 17, 791

INVENTORS:

K. F/anz 4 GKOc/Ima/m I ATTOILNEBS IMAGE INTENSIFIER INCLUDING POLYIMIDE SUPPORT This invention relates to a screen capable of fluorescing, e.g., a screen capable of making visible invisible images, in which the screen comprises a layer of fluorescent substance arranged on a support.

Fluorescent screens of this type are used in vacuum image intensifiers, in which a photocathode is situated in an evacuated flask or bulb behind a ray-entry surface, which may be employed for the conversion of invisible ray images, for example X-ray images, into electron images. Behind the cathode are electron-optically active electrodes and a fluorescent screen on which visible images are formed by the electrons emitted by the photocathode and focused by the voltages applied to the electrodes.

Known fluorescent screens by which are meant screens which fluoresce when they absorb radiation, which are used in, for example, image intensifiers, usually have glass, mica or aluminum oxide foils which act as a support on which the fluorescing layer is applied. However, mica suffers from the disadvantage that it gives off water of crystallization, for example when being used under high vacuum at elevated temperatures and when being bombarded with electrons, and thus it can cause a decrease in transmission as well as a deterioration of the vacuum. The mica does, in fact, become cloudy because of the water of crystallization being given off. Furthermore, mica has only a small degree of hardness and is thus a material which can easily be damaged, together with the fluorescent coating. in addition, because of its laminated structure, flaking can easily occur, especially at the edges.

it is also known that aluminum oxide foils can only be obtained free from holes and with an optically satisfactory quality with very great difficulty. In addition, these foils have a low mechanical strength which presents difficulties during the handling thereof in the working operations.

Consequently, foils consisting of glass are used as the transparent supports in most cases. However, these foils suffer from the disadvantage that they produce a reduction in contrast when they have the necessary thickness to give sufficient strength for handling purposes. This is because the luminescent light of the fluorescent layer is reflected at the boundary surfaces of the support in such a manner that the light is spread out disturbingly in the support parallel to the image surface. The resultant brightening of the background causes a deterioration in the clear image formation, because of the reduction in the contrast on account of the brightening effect. Since this impairment of the image formation depends mainly on the thickness of the support, efforts have been made to use glass foils which are as thin as possible. However, the reduction in layer thickness is limited, because of the necessary mechanical strength and the methods by which these foils may be produced, to about 100p.

it is therefore an object of the present invention to provide a fluorescent screen in which the aforementioned disadvantages are deviated or reduced.

According to one aspect of the present invention, there is provided a screen which comprises a support on which is disposed a layer of substance capable of fluorescing, characterized in that the support is formed of a polyimide.

Foils formed of polyimide have proved very satisfactory under high vacuum as supports for fluorescent layers, because they are stable up to temperatures of 350 C. under vacuum, are not adversely affected by the loading caused by ionizing radiation, e.g., when used in X-ray image intensifiers, and are resistant to those chemicals which are generally employed in coating with fluorescent substance. Furthermore, polyimide foils still have sufficient strength, even when their thickness is below 15 1.. Accordingly, they provide the possibility of improving the contrast which can be obtained. Polyimide foils are, in addition, deformable and can be produced easily in different shapes, whereby the shapes normally employed for supports of fluorescent screens may be easily obtained, for example, hemispheres which are usually employed with vacuum image intensifiers.

Fluorescent screens having a polyimide foil as the support may be united with the glass flasks used in image intensifiers, as well as with other materials, in a manner resisting high vacuum and temperature by the use of various adhesives, e.g., polyimide lacquers and polysiloxanes. In this manner, it is possible to obtain good optical contact while substantially avoiding reflecting surfaces. As a result of the avoidance of reflections, an additional improvement in the image is obtained. It is an additional advantage of the polyimide foils that they are gas-permeable. This property is advantageous when the free surface of the fluorescent layer is to be covered in a known manner by flrstly covering the surface of the fluorescent layer with a smoothing intermediate lacquer layer and thereafter vapor-coating with aluminum. The intermediate lacquer layer which is covered with aluminum is then removed by heating. When the polyimide foil is used, the decomposition products of the lacquer layer can diffuse off through the foil. This avoids both a disturbance of the structure of the fluorescent layer, and the formation of bubbles on the aluminum skin. Polyimide foils are substantially impermeable to shortwave ultraviolet radiation and, consequently, when they are used with fluorescent layers which contain zinc sulphide, any damage to the layer of fluorescent substance by the shortwave rays is avoided. Since the polyimide foils can be obtained or produced in the form of synthetic plastics sheets having a large area, fluorescent screens of an equally large area can be made, out of which fluorescent screens of the desired size may then be cut.

An example of a polyimide foil which is suitable for use in the screen according to the invention may be a material obtained for example, by the polycondensation of a tetrabasic aromatic acid with an aromatic diamine. Because of the favorable combination of mechanical and electrical properties, there are especially suitable for the use as supporting foils for example are produced by polycondensation of at least one of the group including 2,3,6,7-naphthalene tetracarbonic acid dianhydride, 3,3',4,4'-diphenyltetracarbonic acid dianhydride, 2,6-dichloronaphthalene-1,4,5,8-tetracarbonic acid dianhydride, pyrrolidin-2,3,4,5-tetracarbonic acid dianhydride with at least one of the group including m-phenylenediamine, p-phenylenediamine, 4,4'-diamin0diphenylpropane, 4,4-diaminodiphenylmethane, 4,4'-diaminodiphenylether, l,5-diaminonaphthalene, 3,3-dichlorobenzidine.

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, to the accompanying drawing which is a cross-sectional view through a vacuum image intensifier.

In the vacuum image intensifier generally indicated by the reference numeral 1, a photocathode 3 is disposed behind a ray-entry window 4 in an evacuated glass flask 2. Behind the cathode 3 are

electrodes

5 and 6, which consist of sheet metal strips bent in the form of a ring. A fluorescent screen 8 is situated adjacent an end window 7 of the flask 2, which is opposite the entry window 4. An anode 9 is disposed in front of the fluorescent screen 8 and faces the interior of the flask. The anode 9, photocathode 3 and

electrodes

5 and 6 constitute the electron-optical system of the image intensifier.

The photocathode 3 consists of a polyimide support 10 which has a thickness of less than 10p. and which is covered on that surface facing the window 4 with a fluorescent layer 11 which contains a polysiloxane binder in addition to cadmium sulfide which acts as the fluorescent substance. The free surface of the fluorescent layer 11 is covered with a reflecting layer 12 consisting of a white pigment, in this case magnesium oxide. On that side of the support 10 remote from the entry window 4 is a photocathode layer 13, consisting of antimony activated with caesium. The photocathode 3 is fixed on the walls of the flask 2 by means of holder devices, two of which 14 and 15 are shown on two sides of the photocathode 3.

The fluorescent screen 8 comprises a support 16, a fluorescent layer 17 which contains silver-activated zinc-cadmium sulfide and silicon dioxide (as a-binder), and an aluminum layer 18. The complete screen 8 is arranged so that the free surface of the support 16, with the aid of an adhesive layer 19 consisting of polyimide lacquer, is in good optical contact on the inside of the end wall 7 of the flask 2.

Use of the intensifier enables an X-ray image to be made visible. The X-rays pass through the window 4 into the flask 2 where they penetrate the reflecting layer 12 and generate mainly light in the fluorescent layer 11. This light passes through the transparent support and produces an electron image in the photocathode layer 13, corresponding to the light distribution. Suitable voltages applied in a known manner to the

electrodes

5, 6 and the anode 9, as well as to the cathode 3, cause an electron image to be formed on the screen 8, where they produce in the layer 17 the intensified fluorescing and visible image.

We claim:

1. A vacuum image intensifier which comprises an evacuated flask in which are contained (a) a photocathode disposed behind a ray-entry window of the evacuated flask and including a support on that side of which facing the ray-entry window is disposed a layer of a substance capable of fluorescing and on that side of which remote from the ray-entry window is disposed a layer of a substance capable of photoemission; (b) an electrode capable, in use, of accelerating electrons emitted at the photocathode; (c) an electrode capable, in use, of focusing said electrons: and (d) an electron-reactive screen which comprises a support on which is disposed a layer of a substance capable of fluorescing, characterized in that the support of said electron-reactive screen is formed of a polyimide, said polyimide being formed by the condensation of a tetrabasic aromatic acid which is at least one of the group including 2,3,6,7-naphthalene tetracarbonic acid dianhydride 3,3',4,4'-diphenyltetracarbonic acid dianhydride 2,6-dichloronaphthalene-l,4,5,8-tetracarbonic acid dianhydride and pyrrolidin-2,3,4,5-tetracarbonic acid dianhydride;

and an aromatic diamine which is at least one of the group including m-phenylcnediamine, p-phenylenediamine, 4,4-diaminodiphenylpropane, 4,4'-diaminodiphenylmethane 4,4'-diaminonaphthalene, 1,5-diaminonaphthanlene,

and

3,3'-dichlorobcnzidine.

2. A vacuum image intensifier according to claim 1, wherein the support of the photocathode is formed of a polyimide.

3. A vacuum image intensifier according to claim 1, wherein the electron-reactive screen is adhered to that wall of the evacuated flask remote from the ray-entry window, by a polyimide lacquer layer.

Claims

2. A vacuum image intensifier according to claim 1, wherein the support of the photocathode is formed of a polyimide.
3. A vacuum image inteNsifier according to claim 1, wherein the electron-reactive screen is adhered to that wall of the evacuated flask remote from the ray-entry window, by a polyimide lacquer layer.