FR2530367A1 - SCINTILLATOR SCREEN RADIATION CONVERTER AND METHOD FOR MANUFACTURING SUCH SCREEN - Google Patents

Abstract

THE PRESENT INVENTION CONCERNS SCINTILLATING SCREENS RECEIVING X OR G RADIATION AND CONVERTING IT INTO LUMINOUS PHOTONS. THE SCREEN ACCORDING TO THE INVENTION CONSISTS OF A SPARKLING MATERIAL 2 PRESENTING A NEEDLE STRUCTURE. ITS CONCAVE SIDE IS PERFECTLY SMOOTH. THIS SCREEN IS OBTAINED BY EVAPORATION ON A SUPPORT PRESENTING A PERFECTLY SMOOTH CONVEX FACE AND CONSISTS OF A MATERIAL PRESENTING A THERMAL EXPANSION COEFFICIENT DIFFERENT FROM THAT OF THE SCINTILLING MATERIAL. AFTER EVAPORATION, THE FLASHING SCREEN IS SEPARATED FROM THE SUPPORT BY SIMPLE HEATING. APPLICATION TO RADIOLOGICAL IMAGE INTENSIFIER TUBES AND SCINTIGRAPHY TUBES.

Description

SCINTILLATOR SCREEN RADIATION CONVERTER

  AND METHOD FOR MANUFACTURING SUCH SCREEN

  The present invention relates to a convergent scintillator

  radiation weaver It also concerns a method of

manufacture of such a screen.

  Scintillator screens radiation converters are well known in the prior art These screens receive X-rays or and convert them into light photons to which is sensitive

  a photocathode covering the concave face of these screens.

  When the screen receives X-radiation, it is used in X-ray image intensifier tubes or when the screen receives Y-radiation, it is used in scintigraphy tubes. In the prior art, the scintillator scans radiation converters are generally obtained by evaporating on the concave face of a metal support, for example aluminum, which is transparent to radiation, sodium-doped cesium iodide or thallium Cesium iodide is grown spontaneously in the form of juxtaposed needles, which is a structure conducive to guiding the light created in cesium iodide by the incident radiation. The concave face of the scintillator screen thus obtained receives a photocathode sublayer which is intended to isolate the scintillator screen from the photocathode

  and / or to improve the surface state of the concave face of the screen.

  A photocathode is then deposited on this sublayer.

  The scintillator screens known in the prior art have a certain number of drawbacks among which may be mentioned: the fact that the concave face of the scintillator screen obtained is not perfectly smooth because of the needle structure of the scintillator material II it is difficult to perfectly check the irregularities of this face, even using an underlay of

  Photocathode The photocathode that is deposited has a resistance

  superficial electric power Beyond a certain flow of

  incidental radiation, significant local variations in the potential for

  On the surface of the photocathode appear, which leads to a defocusing of the electronic image In addition, the surface irregularities of the photocathode adversely affect its sensitivity The presence of numerous crevasses trapping pockets of gas nearby

  of the photosensitive layer is responsible for a photo-yield of

  reduced emission; the fact that it is necessary to limit the thickness of the screen because the number of cracks and surface discontinuities increases with this thickness This disadvantage is particularly troublesome in the case of screens for radiography o thick scintillator screens are required ; the presence in the finished scintillator screen of the support on which the scintillator material has been evaporated. This support is largely transparent to incident radiation but stops despite

  all a fraction of this radiation.

  The present invention relates to a convergent scintillator

  radiation weaver which does not have the disadvantages

above.

  The present invention relates to a convergent scintillator

  radiation weaver which is made of a scintillator material having a needle structure This screen has a convex face which receives the radiation and a concave face which is

  perfectly smooth, on which rests the photocathode.

  The present invention also relates to a method of manufacturing such a screen which comprises the following steps; the scintillator screen is obtained by evaporation on a support having a convex face perfectly smooth and made of a material having a coefficient of thermal expansion different from that of the scintillator material; after evaporation, the scintillator screen is separated from the support

by simple heating.

  The concave face of the scintillator screen according to the present invention is perfectly smooth because it is this face which is in contact with the convex face of the support during the evaporation of the scintillator material. The photocathode which is deposited on this face therefore has perfectly smooth surfaces The thickness of the screen can be varied from a few tens of microns to several

  millimeters while maintaining a perfectly smooth concave face.

  Finally, according to the present invention, the scintillator screen when

  completed does not include the support used to manufacture it.

  Other objects, features and results of the invention

  will emerge from the following description given as an example

  limiting and illustrated by the appended figures which represent: FIG. 1, the diagram of a scintillator screen according to the prior art; FIGS. 2 and 3, two embodiments of a screen

scintillator according to the invention.

  In the different figures, the same references designate the same elements, but for the sake of clarity, the dimensions and

  proportions of the various elements are not respected.

  FIG. 1 represents the diagram seen in section, of a screen

  scintillator according to the prior art.

  This screen is obtained by evaporating cesium iodide on the concave face of a thin metal support 1, for example aluminum, which is transparent to X-rays or to analyze the growth of cesium iodide. shaped needles 2 terminated by tetrahedral crystals which are represented in a box which shows in more detail the structure of the screen It is noted in the box of Figure 1 that the concave face of the scintillator screen thus obtained is very On this concave surface, an underlayer of photocathode 3 is deposited to chemically isolate the scintillator screen 2 of the photocathode

  and / or to improve the surface condition of the concave face of the screen.

  A photocathode 4 is then deposited on the underlayer 3.

  FIG. 2 represents the diagram, seen in section, of a mode of

  production of a screen according to the invention.

  On the two boxes which show in more detail the structure of the screen, according to the invention, it is found that this screen consists of a scintillator material 2 having a needle structure The right box shows that the concave face of this screen is perfectly smooth It is on this face that is deposited photocathode 4, with, possibly, between this concave face and

  the photocathode, a photocathode sub-layer 3,

vanadates for example.

  To obtain the screen according to the invention, it is possible to use the

The process described below.

  It is necessary to have a support having a convex face

  perfectly polished This support can have any thickness.

  It can be made of any material, glass or metal, which has a coefficient of thermal expansion different from that of the

scintillator material used.

  The scintillator screen is obtained by evaporation of the scintillator material on the convex face of the support After evaporation, the screen is separated from the support by simple heating thanks to the smooth surface of the convex face of the support and thanks to the difference between the coefficients of thermal expansion of the support and the scintillator material. A scintillator screen is thus obtained, such as that shown in FIG. 2, the concave face of which is perfectly smooth because it is this face which was in contact with the convex face of the support during the evaporation of the scintillator material. The concave face of the present screen an optical polish The diameter of the grains on this

  face varies from 1 to about 10 microns.

  As can be seen in the box on the left of Figure 2, it is the convex face of the screen that has a fairly

  irregular because of the ends of the needles of the scintillating

  but it does not matter because there is no conduction

on this face.

  The thickness of the screen may vary, depending on use, between a few tens of microns and several millimeters while

  keeping a perfectly smooth concave face.

  If the evaporation medium is kept cold during evaporation

  ration, the scintillator screen has a needle structure

  finely divided It can then be used in intensi

  high-definition radiological images. If, on the other hand, the evaporation support is heated during evaporation, at temperatures ranging from 100 to 600 ° C., for example, a structure of more agglomerated needles is obtained.

  monolithic that allows to use this screen in scintigraphy.

  The evaporation support may be made of aluminum, for example. The scintillator material used may be an alkaline halide, such as sodium or thallium doped cesium iodide, or thallium doped potassium iodide. scintillator material of tungstates, sulphides or

  1 S metal sulfates for example.

  It can be seen that by using the method described above, a scintillator screen is obtained, both faces of which are accessible for any subsequent treatment desired because the evaporation support is not part of the completed screen, contrary to what is going on.

  in the screens according to the prior art.

  To increase the mechanical strength of the screen, and particularly

  firstly in the case of a thin screen finely divided structure of needles can be disposed on the convex face of the screen a layer 5 ensuring its rigidity such a layer is shown in Figure 2 This layer providing rigidity the screen may consist for example of a glass or enamel with a low melting point, or of any organic substance supporting the baking temperatures of the tube such as for example the resin

  epoxy, parylene, polyimides, or cryolite, for example.

  The convex face of the screen may also be provided with a layer reflecting the light produced in the screen by the incident radiation This layer returns to the photocathode all the light arriving on the convex face of the screen This layer may be constituted for example, any evaporated metal such as

aluminum or nichrome.

  It is also possible to provide the convex face of the screen with a layer that increases the quantum detection efficiency of the screen. This layer consists of a high density material with a high atomic number, deposited in a thin layer, such as Barium, lead or tungsten oxide This type of material promotes photoelectric emission and stopping power against ionizing radiation. It is therefore possible to have on the convex face of the screen one, two or three successive layers each fulfilling one of the following functions: layer ensuring the rigidity of the screen, layer reflecting the light produced in the screen by the incident radiation, layer increasing, the efficiency of quantum detection of the screen. It is possible to use layers of materials fulfilling two of the stated functions. For example, a layer of indium or tin can serve both to reflect the light produced in the screen by incident radiation and also to provide rigidity of

  such a layer can be obtained by cathodic spraying

  by evaporation, projection, or any other known method. As regards the concave face of the screen, this face being perfectly polished, the photocathodes deposited thereon have a minimal surface electrical resistance which no longer depends so much on the scintillator screen but especially on the photocathode

deposited there.

  In order to further reduce the surface electrical resistance of the photocathode and / or to ensure the chemical compatibility between the scintillator screen and the photocathode, an underlayer 3 is deposited on the concave face of the screen. FIG. 2 This underlayer may consist of indium oxide or a metal in a thin layer, such as aluminum, which is

  largely transparent to the light produced by the scintillator.

  On the underlayer, a photocathode 4, consisting of

  for example, cesium and antimony.

  To mount the screen according to the invention in the tube where it will be used, it is possible to use a support grid 7 having the same concavity as the screen, as shown in FIG. 3. This grid must be transparent to the radiation flux. incident She can

  be made of nickel or iron, for example.

  In FIG. 3, the screen shown comprises a layer 6 increasing the quantum detection efficiency of the screen, a layer 2 of scintillator needle material whose concave face is perfectly polished, a photocathode underlayer 3 and a

photocathode 4.

  FIG. 3 shows that a metal ring 8 is evaporated on the concave face of the screen at the periphery of this face. Pressure tongues 9 are applied to this ring and serve as

connections with the photocathode.

  The screen according to the invention when it is sufficiently thick can

  also be mounted in a tube without the use of a support grid.

Claims (10)

  1 Scintillator screen radiation converter, which receives X-rays or fet converts them into light photons to which is sensitive a photocathode (4) which covers one of the faces of this screen, characterized in that this screen consists of a material scintillator (2) having a needle structure and having a convex face which receives radiation and a face   concave which is perfectly smooth, on which rests the photo- cathode (4).   2 screen according to claim 1, characterized in that the convex face of the screen is covered with at least one layer providing at least one of the following functions: layer ensuring the rigidity of the screen (5), reflecting layer the light produced in 'screen by   the incident radiation layer increases the detection efficiency quantum display of the screen (6).   1.5 3 Screen according to one of claims 1 or 2, characterized in   the concave face of the screen is covered with at least one layer (6) ensuring at least one of the following functions: a layer ensuring compatibility with the photocathode, a layer providing the   reduction of the surface resistivity of the photocathode.   4 Screen according to one of claims 1 to 3, characterized in that   that the scintillator material (2) used is an alkaline halide, such as cesium iodide doped with sodium or thallium or as   potassium iodide doped with thallium.   Screen according to one of Claims 1 to 4, characterized in that   that the scintillator material (2) has a needle structure   finely divided or agglomerated needle structure.   Screen according to one of claims 1 to 5, characterized in that   that the scintillator material has a thickness ranging from a few   tens of microns to several millimeters.   Screen according to one of claims 1 to 6, characterized in that   that the perfectly smooth concave face contains grains whose diameter varies from 1 to 10 Vm.   Screen according to one of claims 1 to 7, characterized in that   that the layer providing the stiffness of the screen (5) consists of a glass-or a low melting point enamel, or an organic substance such as epoxy resin, parylene, polyimides, cryolite.   9. Screen according to one of claims 1 to 8, characterized in   what the light reflecting layer produced in the screen by the   Incident radiation is aluminum or nichrome.   Screen according to one of claims 1 to 9, characterized   the layer (6) increasing the quantum detection efficiency is constituted by a high density material deposited in layer   thin, such as barium, lead or tungsten oxide.   11 -Screen according to one of claims 1 to 10, characterized in   that the layer providing the reduction of the surface resistivity of the photocathode consists of indium oxide or is a thin metal layer -   12 A method of manufacturing a screen according to claim 1, characterized in that it comprises the following steps: the scintillator screen (2) is obtained by evaporation on a support having a convex face perfectly smooth and made of a material having a coefficient of thermal expansion different from that of the scintillator material (2); after evaporation, the scintillator screen (2) is separated from the support by simple heating.   Process according to Claim 12, characterized in that the support is kept cold during the evaporation, a structure in   finely divided needle being obtained.   Process according to Claim 12, characterized in that the   substrate is heated during evaporation, a structure the agglomerates being obtained.