EP0013235A1 - Electron multiplying apparatus with axial magnetic field - Google Patents

Abstract

Electron multiplier tube, comprising along the principal axis an electron-emitting surface, several dynode stages with distributed structure, capable of reflux secondary electron emission, and an electron-receiving surface, as well as means producing an electron-accelerating electric field generally oriented along the principal axis, from the electron-emitting surface to the electron-receiving surface. With it is associated coil means producing a magnetic field generally oriented along the principal axis of the tube. Particular example is a photomultiplier with high spatial resolution.

Description

The present invention relates to electron multipliers, and more particularly to photomultiplier tubes.

It is known that of ^the electron-multiplier tubes conventionally comprise, in an evacuated tubular chamber, first one or more electrodes and cathode directed source of electrons, then a series of electrodes capable of secondary emission of electrons, or dynodes, and finally an anode forming an electron collector. These different electrodes are arranged along the main axis of the tube; in operation, they are subjected to potentials capable of creating an electric field accelerating the electrons along this same axis. For photomultipliers, the electron source, made of photosensitive material, is called photo-cathode.

Several structures of dynodes are known. In one type of electron multiplier ("box-type"), the dynodes each comprise a single element, and the dynodes together define a sort of box channeling the electrons, each dynode facing partly the previous one and partly to the next one.

In other electron multipliers, each dynode has a distributed structure comprising several active elements (ensuring secondary emission), which extend transversely to the main axis of the tube. For example, each dynode is made up of the rectangular mutually parallel mutually whose long side extends perpendicular to the main axis of the tube, while their short side is inclined on this same axis. By analogy, such dynodes are said to be "Venetian", or of the "louver" type. Often, two consecutive dynodes have an alternating and symmetrical inclination relative to the main axis of the tube.

So-called multi-channel electron multipliers are also known, in which the electrons arriving on the anode are distinguished according to the point of the cathode where they were generated. These multi-channel devices therefore have several associated anodes provided with as many electrical connections. Their dynode structures are diverse.

Although they work in an acceptable manner, these multi-channel electron multipliers generally suffer from poor spatial resolution: they only distinguish a few well defined electronic impact zones at the level of the cathode. They do not in fact make it possible to establish a true general correspondence between the impact of an electron on the cathode and the impact on the anode of the electrons multiplied consequently by the dynodes.

The invention provides an electron multiplier capable of such a correspondence, which will hereinafter be called "localization".

A priori surprisingly, whereas until now magnetic fields were considered to be very harmful, the inventors observed that by applying, to an electron multiplier equipped with dynodes with distributed structure, a magnetic field oriented along its main axis, a much better spatial resolution can be obtained, while also retaining satisfactory operating characteristics.

Further research has shown that spatial resolution, gain characteristics, and temporal resolution improve when the size of the secondary emission active elements that make up the dynodes is reduced, while increasing the fields electric and magnetic.

Thus, the proposed electron multiplier tube is of the type comprising along an main axis an electron emitting surface, several stages of dynodes with distributed structure, capable of reflex electronic secondary emission, and an electron receiving surface , as well as means producing an electron accelerating electric field generally oriented along the main axis, from the electron emitting surface to the electron receiving surface.

According to the invention, there is associated with it a means producing a magnetic field generally oriented along the main axis.

By dynode with a distributed structure, capable of secondary reflex electronic emission, is meant a dynode whose surface capable of secondary emission is discontinuous, and arranged to return the secondary electrons to the side where the primary electron arrived.

Advantageously, the dynodes comprise series or grids of elongated elements or bars, prismatic or cylindrical, and parallel, each grid being substantially perpendicular to the main axis of the tube. Although such a grid does not in itself offer any possibility of locating electrons along its large dimension, we also observe surprisingly, a roughly homogeneous spatial resolution in all directions perpendicular to the main axis of the tube. It is also very advantageous for each dynode stage to be divided into several levels or sub-stages, offset between them so that the set of different sub-stages constituting a dynode is seen by the incident electrons almost as an opaque surface.

Preferably, the small dimension of the bars in the plane. perpendicular to the main axis, is less than about 1 mm, and the distance between two adjacent bars is at least equal to their small dimension. This jointly improves the spatial resolution and the gain. It is then desirable, although not absolutely necessary, for the electric field to be greater than 200 V / cm, and for the magnetic field to be greater than 50 Gauss. In the preferred embodiments of the invention, the small dimension of the bars is approximately 0.5 mm, and the electric field and the magnetic field are chosen correlatively to each other between approximately 400 and 1000 V / cm, and between about 100 and 500 Gauss, respectively.

In a first particular embodiment, each dynode comprises two grids of bars spaced along the main axis; in each grid the bars are spaced a distance equal to their small dimension; the two grids are offset from each other by a distance equal to their small dimension; and the electric and magnetic fields are correlatively chosen so that a secondary electron emitted by a bar of the first grid always passes statistically between the bars of the second grid (the word statistically means here that a secondary electron nevertheless retains a certain probability - very low - of reaching the bars of the second grid).

In another particular embodiment, each dynode comprises n grids of bars spaced along the main axis; in each grid the bars are spaced n times their small dimension; the n grids are successively offset in the same direction by a distance equal to their small dimension each with respect to the previous one; and the electric and magnetic fields are correlatively chosen so that a secondary electron emitted by a bar of a grid on a parallel to the main axis passes statistically on the same parallel substantially at the levels of the following grid and the n-th wire rack.

This second structure works particularly well for n = 5.

It is also advantageous for the bars to have a cross section symmetrical with respect to a plane parallel to the main axis of the tube and passing through the axis of their largest dimension. This provides better homogeneity of the electric field, and improves spatial resolution.

In practice, the cross section of the bars is of the isosceles right triangle triangle of hypotenuse directed downstream of the electronic trajectories, of the circular kind, or of the flat rectangle kind of predetermined inclination on the main axis, the bars then being slats. However, other types of cross section can be envisaged.

The main application studied was that of photo-multipliers, but the invention applies generally to any type of electron multiplier tube whose dynodes, with distributed structure, are capable of secondary reflex emission. The electron emitting and receiving surfaces which surround these dynodes can take different forms.

Thus, the electron-emitting surface can be an electron-emitting electrode, cathode or photocathode, or a surface transparent to electrons of external origin. For its part, the electron receiving surface can be a divided anode with multiple connections, an electroluminescent surface or a mosaic surface which can be analyzed by electron beam, as will be seen below.

• - la figure 1 est une coupe simplifiée d'un photomultiplicateur classique à dynodes vénitiennes,
• - la figure 2 est une coupe très schématique illustrant les trajectoires électroniques dans un photomultiplicateur classique,
• - la figure 3 est une vue en coupe d'un dispositif expérimental incluant un tube photomultiplicateur soumis à un champ magnétique axial,
• - les figures 4 et 5 sont des diagrammes relatifs au dispositif expérimental de la figure 3, et
• - les figures 6 à 8 sont des schémas en perspective cavalière illustrant différentes géométries de dynodes utilisables selon la présente invention.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows, given with reference to the appended drawings, in which:

• FIG. 1 is a simplified section of a conventional photomultiplier with Venetian dynodes,
• FIG. 2 is a very schematic section illustrating the electronic trajectories in a conventional photomultiplier,
• FIG. 3 is a sectional view of an experimental device including a photomultiplier tube subjected to an axial magnetic field,
• FIGS. 4 and 5 are diagrams relating to the experimental device of FIG. 3, and
• - Figures 6 to 8 are diagrams in perspective showing different geometries of dynodes used according to the present invention.

Although the experimental device which will be described below interests a photomultiplier, its characteristics are applicable to any type of tube electron multiplier, the behavior of these being substantially the same whatever their origin.

Figure 1 is a schematic sectional view of a conventional photomultiplier tube with Venetian dynodes. In a vacuum glass bulb 1, a photocathode 2 is arranged to receive a light beam L. Made of photoelectric material, this cathode reacts to each photon by emitting a primary electron, channeled by an electrode 3. The primary electrons are directed towards a series of 10 dynodes, referenced 4 to 13, and followed by an anode 14. All of the electrodes are polarized by potentials capable of creating an electron accelerating electric field from the cathode to the anode - main axis of the tube .

Each dynode, of distributed structure, comprises a plurality of parallel and inclined lamellae which extend perpendicular to the plane of Figure 1, and extend vertically in the plane of Figure 1. For example, in one embodiment, the slats are rectangular, 30 mm long and 3 mm wide; their short side is inclined at 45 ⁰ on the main axis of the tube, the direction of inclination being alternated from one dynode to another.

The surface of the dynodes is made of material capable of secondary electronic emission, that is to say that, struck by an electron, each dynode will emit several secondary electrons on the side where the primary electron arrived. And the secondary electrons are in turn accelerated and led by the electric field to the next dynode.

Figure 2 very schematically illustrates this process, and shows how, for a photon arriving at A on the photo-cathode, the anode receives electrons on a rather large surface denoted D.

Until now, magnetic fields were considered to be very harmful to the essential qualities of a photomultiplier tube, which are the gain (electron multiplier factor), with its linearity and its homogeneity, as well as the temporal resolution, it is ie the minimum time interval required between two primary electrons for the anode of the tube to offer distinct signals. In particular, the tubes are very often carefully protected from magnetic fields using mu-metal shielding.

The inventors nevertheless had the idea of studying more precisely the effects of magnetic fields, using the experimental device of FIG. 3. A dark enclosure 30 includes an internal partition 31 housing a converging lens 32. On both sides 'other of these are housed in the enclosure a photodiode 35 (point source of 0.5 mm) and a photomultiplier tube 36. The photodiode is movable in a plane which is homologous with respect to the lens of the plane of the photocathode 37 of the tube 36. Thus the point image of the photodiode will be able to scan the entire surface of the photocathode. The tube comprises several stages of dynodes 34 and an anode 33.

The tube 36 is for example the EMI 6262 model, conventionally polarized with a high voltage of 1500 volts. Finally, a coil 39 produces a magnetic field oriented along the main axis 38 of the tube, for example downwards (the direction of the magnetic field has proven to be of little importance so far).

FIG. 4 illustrates on the ordinate the output signal of the tube on a logarithmic scale, as a function of the magnetic field applied, plotted linearly on the abscissa. For each value of the field, the photodiode has been placed in two positions corresponding to the two edges of one of the strips constituting the dynodes. The points marked "+" and "o" correspond respectively to the lower and upper edges of the coverslip.

At zero magnetic field, the two points merge, and the gain is G _O ; As the magnetic field increases, the gain decreases, and the two points "+" and "o" separate more and more. There is a rapid fall in gain above B = 30 Gauss, while below this value, the gain remains substantially constant. The inventors estimated that the value of 30 Gauss corresponds to a radius of curvature of the electronic trajectories in the magnetic field of the order of magnitude of the width of the lamellae (3 mm). When the B field exceeds 30 Gauss, the radius of curvature of the trajectory of a secondary electron emitted by a dynode is small, and the electron is likely to be recaptured by the lamella, hence the rapid decrease in the gain with the magnetic field. On the contrary, when the field B is less than 30 Gauss, the electron has the greatest chances of gaining the following dynode, from where the gain appreciably constant.

FIG. 5 illustrates, for a magnetic field of 120 Gauss, the output signal of the tube plotted on the ordinate and on a logarithmic scale, as a function of the position of the light point on the cathode, plotted linearly on the abscissa. This figure shows that the gain varies as a function of the position of the light source, and that the shape of the gain reflects the lamellar structure of the dynodes.

This confirms the role of the radius of curvature of the electronic trajectories, because a secondary electron emitted near the top edge of a coverslip is more likely to be recaptured by it than the secondary electron emitted near the bottom edge.

The inventors have also observed a channeling of the electronic trajectories around the axis of the field B. If we return to FIG. 3, the domain D becomes all the smaller as the field B is higher. Again, this is due to the radius of curvature imposed on the electronic trajectories by the magnetic field. This results in the possibility of locating at the level of the anode the origin A of the primary electron, by using for example a fractional or multi-anode anode.

The localization effect due to the magnetic field is acquired using the electron multiplier tube arranged according to FIG. 3. In fact, this tube retains real multiplying properties, despite a gain less than the value G0 at zero field.

The experimental results given in FIGS. 4 and 5 show that the gain can be improved by reducing the width of the lamellae according to the radius of curvature imposed on the electronic trajectories by the magnetic field.

Further research has been done in this direction, using in particular a simulation of the phenomena occurring in a photomultiplier tube with Venetian dynodes such as that of FIG. 1.

Performed on a computer using a Monte Carlo program, the simulation took into account the geometrical configuration of the tube, the value of the potentials between electrodes, the experimental data on the secondary emission of the dynodes, as well as the accessory effects. such as edge loss and space charge.

The electric field being thus well established, like the secondary emission, the effects of the magnetic field on the electronic trajectories could be studied.

The simulation and its experimental verifications have led to several specific structures of dynodes which provide good localization, as well as improved gain.

Figure 6 illustrates the first of these structures, currently considered preferential. Each dynode here comprises two levels. Thus the dynode D _n-1 comprises the levels 61 and 62. The level 61 comprises a series of strips or rather of elongated bars whose section is an isosceles right triangle of basic 0.5 mm. The base is perpendicular to the main axis of the tube, and exposed to the next dynode. The free space between the vertices of the bases of two adjacent bars is also 0.5 mm. The second level 62, placed 2.5 mm from the first, is constituted in the same way, but its bars are aligned on the free spaces between those of the previous floor, so that seen from above the whole of the dynode constitutes a structure without free space. The second dynode D _n is similar to the first, its first level 63 being offset by 10 mm from level 61. Finally, the surfaces active on the plane of the secondary electronic emission are at each level the two surfaces inclined at 45 °, defined by the sides of the right angle of the isosceles right triangle. A voltage of 150 volts is established between the levels 61 and 62, a voltage of 600 volts between the stages 61 and 63, and again a voltage of 150 volts between the stages 63 and 64, the polarization being thus repeated periodically for the set of dynodes. With 14 dynodes (at two levels each), an electric field of 600 Volts / cm and a magnetic field of 400 Gauss, such an electron multiplier is likely to reach a spatial resolution (at mid-height) of! 1.5 mm for a gain of the order of 10 ⁷ . Another structure, estimated to be less interesting because it is more complex, is illustrated in FIG. 7. The concept of separate dynodes is diluted in this structure, because the set of dynodes consists of a large number of equidistant levels, such as 71 to 76 which are part of it. Each level includes bars identical to those in Figure 6, but separated by a free space of 2.0 mm. The bars of a given level are offset by 0.5 mm from those of the previous level, to the left for example. Thus, the second level 76 bar is located vertically from the first level 71 bar, from the left. The set of levels 71 to 75 forms an opaque system for an electron beam parallel to the z axis. Although the structure is regular along the Oz axis, we can therefore consider that a dynode stage corresponds to 5 consecutive grids or levels, such as 71 to 75. The pitch between stages is 12.5 mm. With 14 stages, a magnetic field of 410 Gauss, and a potential increasing by 400 Volts per stage, (ie an electric field of approximately 400 _' Volts / cm), such an electron multiplier is likely to reach a spatial resolution (halfway up the impact distribution on the anode) of ^t 1.5 mm, for a gain of the order of ₁₀ ⁷ _.

Generally, the inventors have observed that the structures of dynodes whose bars have a section symmetrical with respect to the z axis are advantageous, as providing a better homogeneity of the field electric, and thereby better spatial resolution. In this respect, it is of course possible to replace the bars with a straight section in an isosceles right triangle with equivalent bars, for example with a circular section and a diameter close to the dimension of the base or hypotenuse of the isosceles triangle, made capable of secondary emission. at least on their upper part.

Another structure of dynodes is illustrated in FIG. 8. Like that of FIG. 7, it has levels 81 to 86 of active elements regularly distributed along the axis Oz, and offset successively by a value equal to the small dimension. of these active elements, projected onto the x axis (0.5 mm); here again, the free space between two active elements is 2.0 mm, so that the active elements of level 86 are vertical to those of level 81. But, this time, instead of the bars with triangular section , the active elements are Venetian lamellae, inclined at 45 ° all on the same side, and of which only the face facing upwards is capable of secondary emission. A stage is again made up of 5 adjacent levels of lamellas, and the pitch between stages is 5 mm. With 14 stages, a voltage between stages of 300 Volts (i.e. an electric field of 600 Volts / cm and a magnetic field of 230 Gauss, the spatial resolution at mid-height is t 2 mm, and the gain of the order of ₁₀ ⁸ .

The remark made as to the role of the symmetry of the active elements with respect to the z axis is therefore confirmed, since the spatial resolution is less good than in the case of FIGS. 6 and 7. On the other hand, the gain is better.

Another important and surprising observation was made. The proposed structures are distributed fumes along the x axis, but continue along the y axis. We could therefore expect to have no localization of the electrons in the direction of the y axis. In reality, we obtain in the y direction a spatial resolution practically equivalent to that of the x direction; consequently, it is the same in all directions of the plane of the photocathode. It is estimated that this remarkable property is due to the curvature of the electronic trajectories due to the applied magnetic field.

The following explanations have been developed concerning the operation of the structures of FIGS. 6 to 8:

Figure 6:

If a primary electron hits the grid 61, the secondary electrons thus produced must pass between the bars of the grid 62 to reach one or the other of the grids 63 and 64, and so on, taking into account the dimensions of the structure geometry. We will then say that the trajectories of the electrons coming from the grid 61 form a knot between the bars of the grid 62.

Figure 7:

The trajectories of the secondary electrons from the grid 71 form a first node at the level of the grid 72 (z = 2.5 mm), between its bars. They form a second node for z = 10.5 mm, slightly below the grid 75. The two nodes are substantially aligned with the emission point in the z direction, and the electrons then have the greatest chance of touching the grid 76 or another of the consecutive grids forming the next stage, avoiding grids 72 to 75.

Figure 8:

The trajectories are more complex, due to the poorer homogeneity of the electric field, due to the asymmetry of the lamellae with respect to the z axis.

It nevertheless seems that the conditions with regard to the nodes of the trajectories are comparable to those of the structure illustrated in FIG. 7.

In all cases, the achievement of these conditions of trajectory nodes, which we call here "helical focusing" is due to the relationship between the electric and magnetic fields, taking into account the geometry and the structure and its dimensions. It is this helical focusing which gives the property of localization of the electronic trajectories, that is to say the good spatial resolution in the xy plane. In this respect, it has been observed that if the electric and magnetic fields are multiplied jointly by a factor K, while dividing the dimensions of the structure and time by the same factor K, the equation of the electronic movement remains unchanged .

Of course, the spatial resolution can be all the better as the active elements of the grids are made finer, the electric and magnetic fields then being increased accordingly.

It has also been observed that the electron multipliers according to the invention have better temporal resolution, than the photomultipliers of the louver type, their rise time being able to drop to less than 2 nanoseconds (10 to 90% of the current peak) against about 10 nanoseconds for most conventional Venetian dynode photomultipliers.

It has also been observed that multiplicas Electrons are less prone to space charge problems in the upper stages than those of the prior art. Indeed, they have a better linearity of the gain as a function of the current of electrons, compared to photomultipliers with Venetian dynodes, if not those called "box-type".

These fallout from the structures described above also constitute important advantages of the present invention, which can be used independently of the location.

The invention essentially provides an electron multiplier tube capable of localization, that is to say in which there is a fine correspondence between the points of departure of the electrons on the entry surface of the tube and the points of arrival. electrons on the exit surface of the tube. The fineness of this correspondence is defined by the spatial resolution.

The currently preferred application is that of photomultipliers, the input surface then being a photocathode. The invention can however be applied with all kinds of cathodes selectively emitting electrons on their surface (divided cathode for example). It is also possible to inject electrons produced by another source through the entry surface of the tube (electron accelerator for example). The term "electron-emitting surface" here covers all of these situations.

The exit surface of the tube, or "electron receiving surface", should of course allow selective detection of the electrons according to their point of arrival. The simplest embodiment is an anode divided into fragments, provided with electrical connections. individual. The "raw" spatial resolution thus allowed (for example Δ x = ± 2 mm) is naturally limited by the dimensions of the anode fragments. This raw spatial resolution can be improved considerably if the signals from the different anode fragments are processed, the processing comprising the amplitude analysis of the signals originating from several adjacent anode fragments. From a raw resolution Δ × = ± 2 mm, and for a dimension of the anode fragments of the same order of magnitude as this raw resolution, a resolution of ± 0.1 mm is obtained after treatment.

In addition, and this is a very important characteristic of the proposed device, the resolution is independent of the statistical fluctuation due to the quantum efficiency of the photocathode, since the source of phototelectrons is common for all the adjacent anode fragments. The resolution is therefore practically independent of the light intensity of the source analyzed.

Another type of sensitive surface applicable instead of anode fragments is the electroluminescent screen, analogous to cathode ray tube screens, which allows visual and / or photographic examination. The electron receiving surface can also be produced as in television shooting tubes, and include a mosaic of small elements which are charged by the effect of the received electrons, while a beam of analyzer electrons comes to scan this surface to read the charge of each element of the mosaic. A sequential signal is thus obtained which, linked to the scanning, defines the spatial distribution of the electrons received. Due to sequential scanning, this type of receiving surface does not fully benefit from the temporal resolution of the tube according to the invention.

The electron multiplier according to the invention is capable of numerous applications: direct detection of electrons, of photons (multi-photomultiplier), high gain image amplifier; the field of application is vast and includes in particular the detection of particles in nuclear physics and high energy physics, medicine, etc.

More precisely, a photomultiplier according to the invention with a diameter of 100 mm could replace 50 to 100 small conventional photomultipliers with Venetian dynodes by offering excellent spatial resolution ( ^± 1.5 mm), a gain that is practically as good, more homogeneous and more linear, and higher time resolution.

Of course, the present invention is not limited to the embodiments described, and extends to any variant in accordance with its spirit. For example, we can use slats with the geometry of Figure 6, or cylindrical bars with the geometries of Figures 6 and 7. We can still imagine simple variants of the cross section in isosceles right triangle, for example by making their curvilinear and concave hypotenuse.

On the other hand, it seems important to keep a layout where each dynode stage is made up of several levels, offset between them so as to constitute together a practically opaque structure for the incident electrons.

Claims (12)

1 - Electron multiplier tube, comprising along an main axis an electron emitting surface, several stages of dynodes with distributed structure, capable of reflex electronic secondary emission, and an electron receiving surface, as well as means producing an electron accelerating electric field generally oriented along the main axis, from the electron emitting surface to the electron receiving surface, characterized in that it is associated with a means producing a magnetic field generally oriented along the main axis. 2 - electron multiplier tube according to claim 1, characterized in that the dynodes comprise grids of parallel bars, each grid being substantially perpendicular to the main axis. 3 - electron multiplier tube according to claim 2, characterized in that the small dimension of the bars, in the plane perpendicular to the main axis, is less than about 1 mm, and that the difference between two adjacent bars is at least equal to their small dimension. 4 - electron multiplier tube according to claim 3, characterized in that the electric field is greater than 200 V / cm, and that the magnetic field is greater than 50 Gauss. 5 - electron multiplier tube according to claim 4, characterized in that the small dimension of the bars is approximately 0.5 mm, and that the electric field and the magnetic field are chosen correlatively to one another between 400 and 1000 V / cm approximately, and approximately 100 and 500 Gauss, respectively. 6 - electron multiplier tube according to one of claims 2 to 5, characterized in that each dynode comprises two grids of bars spaced along the main axis, that in each grid the bars are spaced by a distance equal to their small dimension, that the two grids are offset from each other by a distance equal to their small dimension, and that the electric and magnetic fields are correlatively chosen so that a secondary electron emitted by a bar of the first grid passes statistically always between the bars of the second grid. 7 - electron multiplier tube according to one of claims 2 to 5, characterized in that each dynode comprises n grids of bars spaced along the main axis, that in each grid the bars are spaced n times their small dimension, that the n grids are successively offset in the same direction by a distance equal to their small dimension, each with respect to the previous one, and that the electric and magnetic fields are correlatively chosen so that a secondary electron emitted by a bar of a grid on a parallel to the main axis passes statistically on the same parallel substantially at the levels of the next grid and of the n-th grid. 8 - electron multiplier tube according to claim 7, characterized in that n = 5. 9 - electron multiplier tube according to one of claims 2 to 8, characterized in that the bars have a cross section symmetrical with respect to a plane passing through the main axis of the tube and the axis of their largest dimension. 10 - electron multiplier tube according to one of claims 2 to 9, characterized in that the cross section of the bars is of the isosceles right triangle triangle of hypotenuse directed downstream of the electronic paths of the circular kind, or like flat rectangle of predetermined inclination on the main axis, the bars then being strips. 11 - Electron multiplier tube according to one of claims 1 to 10, characterized in that the electron-emitting surface comprises an electron-emitting electrode, cathode or photocathode, or a surface transparent to original electrons external. 12 - Electron multiplier tube according to one of claims 1 to 11, characterized in that the electron receiving surface comprises a divided anode with multiple connections, an electroluminescent surface or a mosaic surface analyzable by electron beam.

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