EP0559550B1 - Microchannel plate type intensifier tube, especially for radiological images - Google Patents

Description

The invention relates to image intensifier tubes in which, on the one hand, incident ionizing radiation is converted into visible or near visible photons, and in which, on the other hand, a wafer of microchannels is used to achieve gain. in electrons.

Image intensifier tubes are in common use in the fields of radiology and especially in radiodiagnostics, where they are called "radiological image intensifier tubes" or abbreviated as "IIR tubes".

The principle of an IIR tube is well known. It is illustrated diagrammatically in FIG. 1, by a section view of an IIR 1 tube.

The IIR tube 1 comprises a vacuum enclosure, constituted by a central body 2 of revolution arranged around a longitudinal axis 3. The body 2 is closed at one end by an inlet window 4, and at the other end by an exit window 5.

Incident X-rays enter the IIR tube through the entry window 4 which, for this purpose, must be as transparent as possible to these rays: the window 4 is generally formed by a thin sheet of aluminum, or tantalum, or glass, etc. A suitable shape and mechanical characteristics give the window 4 mechanical strength, sufficient to withstand the atmospheric pressure which is exerted from the outside towards the inside of the tube.

The X-rays then meet an assembly called primary screen 15 which converts the incident X-radiation into electrons emitted in a vacuum, from the point where this radiation is absorbed. The primary screen is generally constituted by a "sandwich" which successively comprises: a support 6 transparent to X-rays, a layer 7 of scintillator material which converts X-radiation into lower energy radiation, generally in visible light, and a photocathode 8, deposited on the scintillator 7, which emits electrons in a vacuum under the effect of the radiation emitted by the scintillator.

The scintillator support 6 must be transparent to X-rays: it generally consists of a thin sheet of metal, or glass based on silica, etc.

The scintillator 7 often consists of a layer of cesium iodide with a thickness of the order of 0.2 to 0.8 mm.

The photocathode 8 is formed by a layer of photoemissive material generally having a very small thickness (often less than 1 micrometer).

The IIR tube 1 further comprises a set or system of electrodes 10 brought to potentials (not shown) suitable for accelerating and focusing all the electrons emitted by a same point of photocathode 8, on a homologous point of a luminescent screen. 11 located on the outlet port side 5. This electrode system is designated as the electronic optics of the IIR tube 1.

The luminescent screen 11 is composed of a layer deposited on a transparent support 12, located inside the tube and behind the exit window 5. It is thus possible to observe through the exit window, the visible image converted from the X-ray image which has been projected on the primary screen 15 through the entry window 4 of the tube.

In such a radiological image intensifier, each incident photon X of primary energy between 30 to 100 kV, absorbed in the scintillator 7, typically gives rise to several thousand light photons, and thereby the emission of several hundred electrons in a vacuum, the quantum yield of photocathodes 8 generally being between 10 and 20%.

Each of these electrons, accelerated at a voltage of 10 to 30 kV, in turn causes the emission of several hundred light photons by bombarding the luminescent screen. Each photon X absorbed by the primary screen 15 is thus converted into a number of light photons close to 10,000, emitted by the luminescent screen 11.

In addition, the electronic optics of the tube generally concentrates the output image on a format much smaller than that of the input image, typically 1/10 to 1/5, which is accompanied by a gain. important luminance for this output image. Image magnification also causes the 1 mm detail at the primary screen to be reduced to approximately 1/10 mm at the luminescent screen, and the required image resolution at the screen luminescent is thus much higher than that detected at the primary screen.

The photonic gain, and the luminance gain provided by the enlargement, make it possible to obtain, with radiological doses which can be tolerated by the patients, an output image sufficiently bright to be observed and recorded by means of a cinematographic camera or a television shooting camera, constituting x-ray systems operating in real time.

In the image intensifier tubes or in abbreviation "IIL tube" (Image intensifiers in which the incident radiation is in visible light and which therefore do not comprise a scintillator) of the second and third generation, it is known to add a microchannel cake to further increase the electronic gain. But in IIR tubes such as those shown in Figure 1, the photon gain is considered sufficient in almost all applications, and generally it is not considered useful to increase it by adding a wafer of microchannels, although such arrangements have already been proposed.

However, the use of a microchannel wafer in the IIR tubes, as a replacement for the electronic optics, is considered likely to have great advantages, such as for example: strong reduction in thickness, that is, the distance between the entry window and the exit window; uniform resolution over the entire image field (even for large images); possibility of making square or rectangular formats much more easily better suited to usual image formats, or television screens.

IIR tubes using a microchannel wafer instead of electronic optics are often called "IIR tubes with double proximity focusing". Such tubes are described in particular in "Channel Electron Multiplier Plates in X-Ray Image Intensification", by I.C.P. Millar et al., In Advances in Electronics and Electron Physics, volume 33, Académic Press, 1972. In the IIR tube described in this publication, the primary screen is flat. It is stretched parallel and at a short distance from the entry face of the microchannel wafer, while the luminescent screen is placed parallel to and at a short distance from the exit face of the wafer. To prevent the dispersion of electrons between the photocathode and the entry of the wafer, on the one hand, and between the exit of the wafer and the luminescent screen, on the other hand, from degrading the resolution, we must maintain very small distances, typically less than 1 millimeter, between these electrodes.

FIG. 2 schematically shows such an IIR tube 20 of a type similar to that described in the publication mentioned above.

As in the example in FIG. 1, the IIR tube 20 comprises a tube body 2 arranged around a longitudinal axis 3. The body 2 is closed at one end by an inlet window 4 and at the other end through an exit window 5.

Incident X-rays enter the tube 20 through the inlet window 4 and then meet a primary screen 21.

Unlike the primary screen 15 of FIG. 1, the primary screen 21 of this version is planar. It has a support 22 of scintillator, a scintillator 23 and a photocathode 24 which can be of the same nature and which ensure the same functions as the support, the scintillator and the photocathode shown in FIG. 1.

The electrons (not shown) emitted by photocathode 24 are directed by an electric field, towards the entry face 26 of a wafer 25 of microchannels. To this end, a first and a second bias potential V1, V2 are applied respectively to the photocathode 24 and to the input face 26, with the second potential V2 more positive than the first potential V1.

The microchannel wafer 25 is an assembly of a multitude of small parallel channels or microchannels 27 separated by partitions 28, and assembled in the form of a rigid plate. Each primary electron (emitted by the photocathode) which enters a microchannel 27 is multiplied by a phenomenon of secondary emission in cascade on the walls of the microchannel, so that the electronic current leaving the wafer can be more than a thousand times greater informed at the entrance. The diameter d1 of the microchannels can be between 10 and 100 micrometers. The microchannels 27 are inclined relative to the normal to the plane of the wafer, so that electrons emitted by the photocathode 24 parallel to this normal cannot emerge from a microchannel without having given rise to a phenomenon of secondary emission. In order to reduce the number of electrons that strike the entry face 26 of the wafer 25 outside the microchannels, it is common to make a flaring 30 at the entry of these microchannels and therefore at this level to reduce the thickness partitions 28. The thickness E of the plate forming the wafer 25 of microchannels is typically between 1 and 5 mm. The electronic gain of the wafer can be adjusted over a wide range of values, for example between 1 and 5000, as a function of the voltage developed between the input face 26 and an output face 31 of this wafer 25, output face. 31 to which a third bias potential V3 is applied.

The inlet face 26 and the outlet face 31 are each covered with a metallization layer respectively M1, M2 (shown in FIG. 2 in thick lines, by which the potentials V2, V3 are distributed over the inlet faces Of course, these metallizations M1, M2 must not block the microchannels 27. It should be noted that it is common to deposit the metallization layers M1, M2 on the walls of the microchannels 27 at the ends of these microchannels c ′. that is to say at the entry and exit of the latter. Generally, the metallization layers M1, M2 are deposited on the entry and exit faces 26, 31 of the microchannel wafers by an evaporation method. under vacuum of a conductive material (such as for example chromium, nickel-chromium, Inconel, etc ...), by Joule effect, using, most frequently, an electron gun to sublimate the metal to be evaporated.

This technique is classic. To limit the penetration of the metal into the channels 27, the evaporation takes place in grazing incidence ("bias").

Furthermore, the microchannel wafers are supported, during evaporation, on a planetary system which allows, by continuous rotation, to expose the surface of the wafers to the metal flow in all directions, while preserving the grazing incidence. The penetration of the metal inside the channels 27 is thus uniform, for each channel, and for all of the channels.

The electrons leaving the microchannel wafer are accelerated and focused by an electric field, on a luminescent screen 35 placed opposite the wafer, parallel to the latter, and at a distance D of the order of 1 to 5 mm. . The luminescent screen 35 has dimensions substantially equal to those of the primary screen. It locally emits a quantity of light proportional to the current of incident electrons and therefore it restores a visible and intensified image of the X-ray image projected on the scintillator, through the entry window of the tube. The luminescent screen 35 is a layer of a few micrometers thick, consisting of grains of phosphor material, and which can be deposited on the exit window 5. The face of the luminescent screen 35 facing the wafer 25 of microchannels, is coated with a very thin metallic layer 36 , aluminum for example. This metallization allows the electric polarization of the screen (by the application of a fourth potential V4 more positive than the third potential V3), and serves as a reflector for the light emitted towards the rear by this screen.

The primary screen 21 and the wafer 25 of microchannels are secured to the body 2 of the tube, using for example tabs 29, sealed in this body, and to which are applied the polarization potentials V1, V2, V3. The primary screen 21 and the wafer 25 are thus fixed so as to be electrically isolated from each other, while being separated by a relatively small distance D1, of the order of a few tenths of a millimeter (It is note that for clarity of the figures, the scale of dimensions is not respected).

Such an IIR tube structure is difficult to produce, in particular for large images. It is difficult indeed to produce and maintain parallel to the microchannel plate, and at a very small and uniform distance, a perfectly flat primary screen. This is however necessary to limit the angular dispersion of the electrons (effect which reduces the spatial resolution), and to obtain a good image resolution on the whole of the field.

Another difficulty comes from the fact that the scintillator 23 and its support 22 do not have the same expansion coefficients: they both consist of thin layers which tend to deform, and cause a deformation of the photocathode and therefore a modification. the distance between the latter and the microchannel plate.

These difficulties are all the more pronounced as the size of the IIR tubes is large, whereas the applications envisaged for an IIR tube with a microchannel wafer (that is to say an IIR tube with dual proximity focusing) ask for large useful surfaces, typically greater than 15 cm in diameter, or rectangular format with equivalent surface.

The present invention relates to image intensifier tubes using both a scintillator to convert ionizing radiation into light or near visible radiation, and a wafer of microchannels to carry out the amplification in electrons. The object of the invention is to provide a solution to the above-mentioned problems linked to the use of microchannel pancakes.

The invention proposes to place the photocathode directly on the entry face of the microchannel plate. This responds both to the problems associated with the uniformity of the spacing between the photocathode and the microchannel plate, and to the problems of electrical insulation between these two elements. The power supply is simplified, since the input face of the microchannel plate and the photocathode can be at the same potential.

This arrangement also makes it possible to eliminate the effects, on the photocathode, generated by the differences in expansion coefficient between the scintillator and its support, and may even make it possible to remove this support. The scintillator is then deposited on the microchannel wafer which has previously been coated with a photocathode. This avoids providing a specific support for the scintillator, support which absorbs part of the X-rays because it is on the side of the incident X-rays. The scintillator is not rigid enough to hold without support and the microchannel plate then advantageously serves as a support. The photocathode can also be deposited on the scintillator, which is then applied against the wafer, or in part on the scintillator and in part on the wafer, the coated scintillator being applied against the coated wafer.

The invention therefore relates to an image intensifier tube, comprising a scintillator, a photocathode, a microchannel plate, an entry face of the microchannel plate being at least partially covered by a layer. electrically conductive, characterized in that the photocathode consists of at least one layer in contact with the electrically conductive layer.

• la figure 1 déjà décrite représente un tube IIR de l'art antérieur, du type à optique électronique ;
• la figure 2 déjà décrite représente schématiquement par une vue en coupe, un tube IIR de l'art antérieur, du type à galette de microcanaux ;
• la figure 3 montre schématiquement par une vue en coupe, un tube IIR du type à galette de microcanaux conforme à l'invention ;
• la figure 4 est une vue agrandie d'une partie d'une galette de microcanaux montrée à la figure 3 ;
• la figure 5 représente plus particulièrement l'entrée de microcanaux montrés aux figures 3 et 4 ;
• la figure 6 est une vue semblable à la figure 3, et illustre la présence d'une couche de photocathode réalisée sur un scintillateur.

The invention will be better understood and other advantages which it brings will appear on reading the description which follows, given by way of nonlimiting example with reference to the appended drawings among which:

• FIG. 1, already described, represents an IIR tube of the prior art, of the type with electronic optics;
• Figure 2 already described schematically shows in a sectional view, an IIR tube of the prior art, of the wafer type of microchannels;
• Figure 3 shows schematically in a sectional view, an IIR tube of the microchannel pancake type according to the invention;
• Figure 4 is an enlarged view of part of a microchannel wafer shown in Figure 3;
• Figure 5 shows more particularly the input of microchannels shown in Figures 3 and 4;
• Figure 6 is a view similar to Figure 3, and illustrates the presence of a photocathode layer formed on a scintillator.

To simplify the figures and facilitate their reading, the scale of dimensions is not respected.

FIG. 3 represents an image intensifier tube 40 arranged according to the invention, an IIR tube for example. The IIR tube 40 includes a vacuum-tight enclosure, constituted by a tube body 2 closed at one end by an inlet window 4, and at the other end by an outlet porthole 5. This enclosure contains a scintillator 41, a scintillator support 42, a photocathode 43, a wafer 44 of microchannels and a luminescent screen 35, carried by the exit port 5, all of these elements ensuring functions similar to those provided by the support 22, the scintillator 23, the photocathode 24, the wafer 25 and the luminescent screen 35 of the IIR tube shown in FIG. 2.

According to a characteristic of the invention, the photocathode 43 is directly supported on the entry face FE of the wafer 44 (face which is oriented towards the entry window 4 and towards the scintillator 41). More specifically, in the nonlimiting example shown in FIG. 3, the photocathode 3 is produced on a conductive layer, called the first metallization layer M1, formed on the input face FE.

For the rest, the wafer 44 of microchannels is constituted in a conventional manner, and it is similar to the wafer 25 of FIG. 2: a second metallization layer M2 is deposited on the exit face FS of the wafer 44 (face oriented towards the luminescent screen). This second metallization M2 cooperates with the first metallization layer M1, to establish an electric field over the length of the microchannels 27 that comprises the wafer 44, that is to say between the inlet and the outlet of these microchannels which respectively terminate in the FE input face and in the FS output face. This electric field is obtained by applying the second and third polarization potentials V2, V3 to the metallization layers respectively M1, M2, with the third potential V3 more positive than the second potential V2. It should be noted that the potential V3 applied to the second metallization layer M1 also serves, as in the prior art, to define an electric field between the output face FS of the wafer of microchannels, and the luminescent screen 35, for operation at this level similar to that of the prior art.

To promote the establishment of the electric field in the microchannels 27, the metallizations M1 and M2 are deposited not only on the input and output faces FE, FS, but also on the walls of the microchannels 27, at the input and at the exit of the latter into which they thus penetrate slightly from a depth h1. To this end, the method of depositing metallized layers M1, M2 uses a grazing incidence evaporation technique as already explained in the preamble.

This slight penetration of the first metallized layer M1 in a part of each microchannel 27, part which constitutes the input of each microchannel, is used in the invention where it constitutes the support of the photocathode 43 according to the invention. The layer forming the photocathode 43 is thus produced on the entry face FE, as well as in the entry of each of the microchannels 27 where it constitutes a microphotocathode 43a; consequently photocathode 43 comprises as many microphotocathodes 43a as there are microchannels 27.

The scintillator 41 is disposed above the photocathode 43, and in the nonlimiting example described, it is directly supported on the input face FE of the wafer 44, that is to say directly in contact with the photocathode 43.

The scintillator 41 can be conventionally secured to a support 42 as in the nonlimiting example shown in FIG. 3, and the assembly formed by the scintillator and its support can be fixed to the wafer 44 of microchannels, for example under the thrust of one or more thrust members 56. The thrust members 56 can be formed in different ways, depending in particular on the manufacturing methods specific to each IIR tube.

In the nonlimiting example of the description, the pressure members 56 bear on an inner peripheral part 57 of the entry window 4, this peripheral part being more massive than the central part which must absorb the radiation as little as possible. X incident. In the example shown in FIG. 2, these thrust members 56 include a rigid spacer 58 and a spring washer 59: the spring washer 59 is placed on the support 42 (in a peripheral zone of the latter, and the spacer 58 is disposed between the entry window 4 and the spring washer 59. The spacers 58 have a height H2 suitable for keeping applied the scintillator 41 and its support 42 against the entry face of the wafer 44, using the washers spring 59. Several such thrust members can be used, distributed around the scintillators 41.

With a view to, on the one hand, improving the fixing of the scintillator-support assembly 41, 42, and on the other hand limiting, even canceling the mechanical deformations resulting from the differences in expansion coefficients of the scintillator and of the support, it is possible (but not compulsory) to give this scintillator-support assembly 41, 42, before being fixed to the wafer 44, a slightly concave shape (not shown) (seen from the entry window). With such a shape, when the scintillator-support assembly 41, 42 is placed above the wafer 44, it is first by its central part that it is in contact with the entry face FE on which photocathode 43 is formed. Then ensuring regular pressure on the periphery of the scintillator-support assembly 41,42 when it is fixed using the pushing members 56, this assembly is uniformly pressed on the face FE input, playing on its elasticity.

Such a concave shape of the assembly formed by the scintillator 41 and its support 42, can result from an internal mechanical tension which can itself result from a concave shape given initially to the support 42 before the deposition of the scintillator 41 on it. support. The coefficient of expansion of cesium iodide is generally higher than that of the support, and this scintillator is deposited hot on this support. In this way, the tension exerted by the scintillator 41 tends to reduce the initial concavity, and it is necessary to give the support 42 a slightly greater concavity than that which is ultimately necessary. We could for example give an initial deflection close to 1 millimeter, for a support 5 made of aluminum alloy 0.5 millimeter thick, and 15 to 25 centimeters in diameter.

However, in this configuration where the scintillator 41 is applied to the input face FE of the wafer, the presence of a support 42 of the scintillator is not compulsory. Indeed, it is known that a radiation converter or scintillator for an IIR tube can be produced on a temporary support, support which can be eliminated after production of the scintillator. Such a technique is described for example in a French patent in the name of THOMSON-CSF, published under No. 2,530,367. This patent describes a process for producing a scintillator screen made of cesium iodide with a needle structure (this type of scintillator is which is most commonly used in IIR tubes), on a temporary support which is then separated from the scintillator. In such a case, the scintillator 41 (having no support) can be fixed on the entry face FE of the wafer 44 using, for example, thrust members 56, as explained above. However, in the case of a scintillator 41 freed from its support or substrate, the problems of difference in expansion coefficients no longer arise, and it is therefore less useful to impart a concave shape (before it is fixed) to the scintillator 41.

With the invention, the photocathode 43 being produced on the input face FE of the microchannel wafer, the problems posed in the prior art are overcome by the deformations of the primary screen, and in general the problem positioning of the photocathode in relation to the microchannel plate.

The invention also provides a simplification in the electrical supply of the IIR tube 40, compared with the known art, that is to say with respect to the supply of the IIR tube of FIG. 2. Indeed, with the IIR tube of the invention, the photocathode 43 being in contact with the first metallization layer M1, it is brought to the same second polarization potential V2 as the input face FE, and the electrons it emits are immediately placed under the influence of the electric field which reigns in each of the microchannels 27.

• second potentiel de polarisation V2 alimentant simultanément la face d'entrée FE et la photocathode 43 ;
• troisième potentiel de polarisation V3 (plus positif que le second potentiel V2) appliqué à la face de sortie FS ;
• et un quatrième potentiel de polarisation V4 (plus positif que le troisième potentiel V3) appliqué à l'écran luminescent 35.

Under these conditions, compared to the conventional IIR tube of FIG. 2, the potentials necessary for the operation of the IIR tube of the invention are limited to:

• second bias potential V2 simultaneously supplying the input face FE and the photocathode 43;
• third polarization potential V3 (more positive than the second potential V2) applied to the output face FS;
• and a fourth polarization potential V4 (more positive than the third potential V3) applied to the luminescent screen 35.

It can be seen that, compared with the conventional IIR tube of FIG. 2, the first polarization potential V1 is eliminated, which first potential V1 is used in the prior art to establish an electric field between the photocathode and the entry face of the wafer microchannels.

It should also be noted that this leads, with the IIR tube according to the invention, not only to reduce the number of bias potentials, but also to significantly reduce the potential difference applied to this tube.

Figure 4 is an enlarged view of the elements contained in a box 50 of Figure 3, to better illustrate the operation of the IIR tube of the invention. FIG. 4 partially shows the scintillator 41 and its support 42, the microchannel wafer 44 and the photocathode 43 situated between the latter and the scintillator 41, and the luminescent screen 35 situated opposite the scintillator 41 relative to the wafer 44.

The scintillator 41 consists, for example, of a uniform layer of cesium iodide formed into needles 41a by growth by evaporation on the support 42, according to a conventional method. However, as has already been explained above, the support 41 no longer plays the mechanical role which it fulfills in the prior art; it can therefore be deleted, if the scintillator is produced on a temporary support. The thickness E1 of scintillator is typically 0.5 millimeter.

The scintillator 41 is disposed in contact with the photocathode 43, which itself is produced on the input face FE of the wafer 44 of microchannels.

The wafer 44 of microchannels comprises the parallel microchannels 27, separated by partitions 28. The microchannels 27 are slightly inclined relative to the normal to the plane of the wafer, that is to say relative to the longitudinal axis 3 of the tube. The FE entry face includes the first metallization layer M1, to which the second bias potential V2 is applied. The output face FS comprises the second metallization layer M2 to which the third potential V3 is applied. As an indication, a wafer 44 having a thickness E of the order of 2 millimeters, and microchannels 27 whose diameter d1 is about 50 micrometers, is suitable for this application.

The luminescent screen 35 is located relative to the exit face FS of the wafer 44, at a distance D of the order of 1 millimeter. The luminescent screen 35 receives the third polarization potential V3, by which it is brought to a positive potential of a few thousand volts relative to the output face FS of the wafer.

The layer forming the photocathode 43 is deposited by vacuum evaporation on the entry face FE, that is to say on the first metallization layer M1, and very particularly at the entry of the microchannels to form the microphotocathodes 43a there. This can be achieved, as for metallizations M1, M2, by a bias evaporation technique, that is to say in grazing incidence, as already explained (the wafer 44 of microchannels being for example on a rotating support). This technique makes it possible to evaporate the microphotocathodes 43a in the microchannels 27 to a depth h2 corresponding to approximately twice the diameter d1 of the microchannels: that is to say approximately 100 micrometers for microchannels of 50 micrometers in diameter. The photocathode 43 covers the first metallization M1, and may even exceed this, towards the inside of the microchannels 27.

When an X photon is absorbed in the scintillator 41, it gives rise to the emission of several thousand visible photons. This light, channeled by the needles 41a of the scintillator, is emitted towards the entry of the microchannels 27 (as illustrated in FIG. 4 by a light photon Ph1) where it has a high probability of coming to excite the photocathode 43 (of which the effective part is mainly constituted by microphotocathodes 43a). Electrons emitted from the photocathode, therefore, are attracted towards the interior of the microchannels 27 by the electric field, where they multiply by secondary emission in cascade, following impacts on the walls of the microchannels, according to the well-known process of the microchannel pancakes. At the exit of the microchannels 27, the electrons are accelerated towards the luminescent screen 35 where they restore, by cathodoluminescence, a visible image homologous to the X-ray image absorbed by the scintillator 41.

It should be noted that the visible photons emitted in the scintillator 41 are channeled therein either in the direction of the wafer 44 (as illustrated by the photon Ph1), or in the opposite direction, that is to say towards the support. 42. If the support 42 is reflective, all the photons will be returned to the wafer 44, which improves the sensitivity at the expense of the contrast. If the support 42 chosen is absorbent, or if there is no support, the sensitivity of the IIR tube will be reduced, to the benefit of the resolution and the contrast. The choice will be made according to the applications envisaged.

Part of the visible photons emitted in the scintillator 41 in the direction of the wafer 44 is lost: on the one hand, these lost photons (not shown) are those which are directed towards the partitions 28 and which do not penetrate into the microchannels 27; the other visible photons lost are those which are emitted towards the axis of the microchannels 27 and which consequently do not meet the photocathode 43 or more precisely the microphotocathodes 43a.

In either case, it is possible to reduce the proportion of photons lost by flaring the input of the microchannels 27, as is more fully explained in a continuation of the description made with reference to FIG. 5.

In total, the fraction of useful photons can exceed 20% of the light photons emitted, which is very sufficient, given the electronic gain provided by the wafer 44 of microchannels itself. The number of electrons extracted from photocathode 43, for each X photon absorbed in the scintillator 41, remains greater than several tens, which is sufficient to provide only negligible noise in the detected image.

FIG. 5 shows in particular the inputs of two microchannels 27 contained in a box 60 of FIG. 4, in order to illustrate the flared shape capable of being imparted to the microchannels and the resulting shape of the microphotocathodes 43a.

The flaring of the entry of the microchannels 27 (near the entry face FE) can be obtained, in itself conventional, for example using an appropriate selective chemical attack method, accomplished before the deposition of the first metallization layer M1.

This chemical attack has the effect of removing material from the walls of the microchannels (near the inlet surface) and therefore reducing the thickness E3 of the partitions 28 at this level, which results in flaring. The first metallization layer M1 and then the layer forming the photocathode 43 are then deposited, as was previously indicated. The surface area of the photocathode deposited on the surface is thus reduced, in favor of the microphotocathodes 43a formed at the entrance to the microchannels, and therefore the effective part of the photocathode 43 is increased.

To obtain a widening of the entry of the microchannels 27, it is also possible to extend the end (symbolized in FIG. 5 by a limit in dotted lines) of the partitions 28, by an additional deposit 29 of decreasing thickness E3, obtained by a vapor deposition technique. This additional deposit 29 may preferably be made of a material having a coefficient of expansion close to that of the wafer 44, silica for example if the wafer is made of glass. This additional deposit or extension is then covered by the first metallization layer M1, then by the photocathode 43.

The description of the image intensifier tube of the invention has been made with reference to an IIR tube, but the invention applies to all image intensifier tubes using a screen scintillator, to convert the incident radiation into visible or near visible radiation.

The production of an intensifier tube according to the invention can be carried out using techniques all well known to specialists.

However, it can be specified, for information only, that an image intensifier tube according to the invention must, practically, be produced by a vacuum transfer method. Indeed, the photocathode 43 must be evaporated under vacuum on its substrate (on the wafer of microchannels in the case of the invention), and this requires the necessary clearance.

• le premier sous-ensemble comprend le corps du tube, la galette du microcanaux, l'écran luminescent, le hublot de sortie (l'écran luminescent étant, par exemple, déposé directement sur la face interne du hublot), tous ces éléments étant fixés de façon définitive.
• le second sous-ensemble est constitué par le scintillateur sur son support (on un support provisoire).
• le troisième sous-ensemble est constitué par la fenêtre d'entrée, munie par exemple d'une bride (non représentée) pouvant venir se fermer sur le corps du tube.

The tube of the invention can be introduced into a vacuum transfer frame (not shown) in the form of three sub-assemblies:

• the first sub-assembly includes the tube body, the microchannel pancake, the luminescent screen, the outlet porthole (the luminescent screen being, for example, deposited directly on the internal face of the porthole), all of these elements being fixed definitively.
• the second sub-assembly is formed by the scintillator on its support (we have a temporary support).
• the third sub-assembly is constituted by the entry window, provided for example with a flange (not shown) which can come to close on the body of the tube.

Inside the frame, under vacuum, the degassing of the different parts will be carried out as usual, then the photocathode will be deposited on the entry of the wafer by evaporation at an angle, for example by using sources of antimony and alkali metals (K, Cs) arranged on the sides. The control of the photocathode evaporation will be carried out according to a known process.

Once the photocathode has been produced, a system of vacuum manipulator arms allows the scintillator to be placed and fixed on the wafer, then placed and sealed in a vacuum-tight manner, the entry window on the body of the tube.

The tube will then be returned to ambient air, ready for use.

FIG. 6 illustrates an embodiment in which the photocathode 43 is constituted not only by a layer deposited on the input face FE of the wafer 44, but also by a second layer 43s deposited on a face of the scintillator 41 oriented towards the wafer 44. For the rest, FIG. 6 is similar to FIG. 3.

The scintillator 41 being applied against the entry face FE, the second layer 43s is in contact with the first photoemissive layer 43, and is thus polarized at the same potential as the latter.

It should be noted that it is also possible, in the spirit of the invention, for the photocathode to consist of a single layer 43s deposited on the scintillator 41; in such a case, the layer 43s deposited on the scintillator 41 would be directly in contact with the first metallization M1.

The second photoemissive layer 43s makes it possible to improve the electronic efficiency, at the cost of a complication in the production, this complication being however perfectly overcome.

Indeed, the production of the photocathode 43 on the input face FE of the microchannel plate, before transferring the scintillator 41 to this input face and maintaining it in position as described above, as well as the watertight closing of the inlet window 4, requires complex equipment (although in itself well known) allowing the vacuum handling of the various parts of the tube (body of the tube equipped with the outlet screen and the wafer , primary screen or scintillator, entry window). In this same vacuum equipment, it is necessary to have sources of evaporation of the materials constituting the photocathode (antimony and alkali metals), and possibilities of relative movement (planetary system), or of multiple sources, allowing the uniform evaporation of the photocathode on the entry face of the wafer.

In this relatively complex system, the scintillator 41 may be placed, during the production of the photocathode 43, in a position symmetrical to that of the wafer 44, with respect to the sources of evaporation, so that a photocathode will be produced simultaneously on the entry face of the wafer, and on the chosen face of the scintillator 41.

Claims (9)

1. An image intensifying tube comprising a scintillation screen (41), a photocathode (43), an electronic amplification pancake (44), said pancake (44) comprising a plurality of micro-channels (27), an entry face (FE) of the pancake (44), directed toward the scintillation screen (41), being at least partially covered by a metallized layer (M1), characterized in that the photocathode (43) comprises at least one photoemitting layer (43a, 43s) in contact with the metallized layer (M1).
2. The intensifying tube as claimed in claim 1, characterized in that the photoemitting layer (43a, 43s) of the photocathode (43) is arranged on the entry face (FE) of the pancake (44).
3. The intensifying tube as claimed in claim 2, characterized in that the photocathode layer (43) extends into the inlets of the micro-channels (27) on at least part of the walls of the micro-channels.
4. The intensifying tube as claimed in any one of the preceding claims, characterized in that the scintillation screen (41) is supported on the entry face (FE) of the pancake (44).
5. The intensifying tube as claimed in claim 2, characterized in that the scintillation screen (41) is supported on the entry face (FE) of the pancake (44) and in that the photocathode (43) furthermore comprises at least one photoemitting layer (43s) arranged on the scintillation screen (41).
6. The intensifying tube as claimed in claim 1, characterized in that the scintillation screen (41) is supported on the entry face (FE) of the pancake (44) and in that the photoemitting layer (43s) in contact with the metallized layer (M1) is arranged on the scintillation screen (41).
7. The intensifying tube as claimed in any one of the preceding claims, characterized in that the inlets of the micro-channels (27) have a diverging form.
8. The intensifying tube as claimed in claim 7, characterized in that the partitions (28) which separate the micro-channels (27) are extended on the side of the entry face (FE) by an additional deposit (29) whose thickness (E3) varies in order to produce the diverging form of the entry of the micro-channels (27).
9. The intensifying tube as claimed in any one of the preceding claims, characterized in that the scintillation screen (41) is supported on the pancake (44) and in that the pancake (44) constitutes the only support for the scintillation screen (41).

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