EP0742954B1 - Ionising radiation detector having proportional microcounters - Google Patents

Description

Technical area

The subject of the present invention is a gas detector for detecting radiation ionizing such as α, β, γ, or even radiation x or ultra violet radiation from a multitude of proportional microcounters assembled to form a proportional counter.

Such a detector finds many applications in the fields of medical imaging, biology, particle physics, or still crystallography, and in many areas requiring non-destructive testing.

State of the art

The detector of the invention is of the type of those in which primary electrons from ionization of radiation by gas are multiplied under the effect of a strong electric field local, in a gas.

Several types of these gas detectors are currently known and used by humans in the job.

The best known of these detectors is the parallel plate detector. It has a counter realized by means of two distant parallel grids each other by a few millimeters and between which is the multiplication of electrons. This area between the two parallel grids is called "multiplication area". The area of multiplication of such a detector is therefore presented under the shape of a single volume delimited by the two grids. By the very fact that it constitutes a single volume relatively large, such a counter has the disadvantage of being very sensitive to breakdown. In addition, the counters of these detectors parallel plates can only have one resolution limited space and, due to the thickness plate / grid, they cannot be arranged so that constitute detectors of various forms.

Another type of gas detector is the wire detector. This includes a plurality of equidistant wires, stretched in a plane. By hand and on the other side of this plane, two taut grids are placed forming cathodes. The multiplication of electrons is done near the wires since it reigns, at this place, a high electric field. However, the area multiplication of such a detector cannot be isotropic; furthermore it does not allow the detector to have various forms.

Another type of gas detector, more recent, is the microstrip detector. In this microstrip detector, the counter consists of coplanar electrodes etched on an insulating support. Such a microstrip detector is described in the French patent FR-A-2 602 058. The major drawback of this detector is its relatively small gain which is significantly limited to 5,000 since it does not allow to superimpose several counters. Furthermore, just like the counters of the parallel plate detectors described above, the counters of these detectors microbands have multiplication zones anistropes, located on very fine tracks (approximately 10 µm), which makes them very sensitive to breakdown. These detectors also have the disadvantage of being relatively fragile.

Statement of the invention

The object of the present invention is precisely remedy the drawbacks of different detectors described previously. To this end, it offers a gas detector comprising a counter consisting of a plurality of proportional microcounters independent.

More specifically, the invention relates an ionizing radiation detector comprising a gas filled enclosure, inside which there is a proportional counter and an area absorption in which the gas is ionized by radiation, characterized in that the counter proportional has at least one electrode lower and at least one upper electrode, parallel to each other and separated by a layer of insulating material, the upper electrode as well as the layer of insulating material comprising at least one breakthrough in which an electric field reigns substantially uniform and which constitutes an area of multiplication of electrons from ionization of radiation.

Each portion of the counter including a upper electrode part and layer part insulating holes and part of an electrode lower constitutes an independent microcomputer, also called elementary cell.

Advantageously, the lower electrode is a anode and the upper electrode is a cathode.

According to the invention, the insulating material is a rigid material which can either be photosensitive, which makes it possible to facilitate the manufacture of the detector, or highly resistive (with a resistivity of the order of 10 ⁹ to 10 ¹³ Ω.cm) , or fluorescent, which transforms UV radiation from the multiplication into visible radiation.

According to a first embodiment of the invention, the proportional counter includes a plurality of upper electrodes arranged one above the others, in a plane parallel to the lower electrode and separated from each other by a layer of insulating material, the holes in each upper electrode being aligned with the holes in the layers of insulating material.

• une pluralité d'électrodes supérieures disposées dans un même premier plan, avec une même première direction et connectées entre elles ; et
• une pluralité d'électrodes inférieures disposées dans un même second plan, parallèle au premier plan, selon une même seconde direction et connectées entre elles.

According to another embodiment of the invention, the proportional counter comprises:

• a plurality of upper electrodes arranged in the same first plane, with the same first direction and connected to each other; and
• a plurality of lower electrodes arranged in the same second plane, parallel to the first plane, in the same second direction and connected to each other.

• une électrode supérieure et une électrode inférieure ayant toutes deux une forme de cylindre ouvert, l'électrode inférieure entourant l'électrode supérieure ;
• une couche de matériau isolant séparant les deux électrodes ;
• un fil électrique d'amenée de potentiel traversant longitudinalement le cylindre formé par les électrodes et la couche isolante pour créer entre lui-même et l'électrode supérieure une zone d'absorption ;
• au moins une percée réalisée dans l'électrode supérieure et la couche isolante dans laquelle règne un champ électrique sensiblement uniforme et qui constitue une zone de multiplication des électrons issus de l'ionisation des rayonnements. Quel que soit le mode de réalisation de l'invention, l'électrode supérieure et l'électrode inférieure peuvent être indépendantes et reliées chacune à une entrée d'un circuit électronique de traitements, pour former un détecteur pixels.

The invention also relates to an ionizing radiation detector comprising an enclosure filled with a gas inside which is arranged a proportional counter, characterized in that the proportional counter has the shape of an open cylinder in the center of which the gas is ionized by radiation, said counter comprising:

• an upper electrode and a lower electrode both having the shape of an open cylinder, the lower electrode surrounding the upper electrode;
• a layer of insulating material separating the two electrodes;
• an electric potential supply wire passing longitudinally through the cylinder formed by the electrodes and the insulating layer to create between itself and the upper electrode an absorption zone;
• at least one breakthrough made in the upper electrode and the insulating layer in which a substantially uniform electric field prevails and which constitutes a zone of multiplication of the electrons resulting from the ionization of the radiations. Whatever the embodiment of the invention, the upper electrode and the lower electrode can be independent and each connected to an input of an electronic processing circuit, to form a pixel detector.

Brief description of the drawings

• FIG. 1A represents a view in perspective of a detector of the invention comprising a proportional counter produced according to a first mode of production ;
• Figure 1B shows a front view a strip of microcounters in accordance with the embodiment of FIG. 1A;
• FIG. 2A represents a front view of a strip of microcounters according to a second mode of realization of the invention;
• FIG. 2B represents a view in perspective of a counter made with several bands microcounters of Figure 2A;
• FIGS. 3A and 3B representing, in section, two microcounters whose recesses are, respectively, conical and concave;
• Figure 4 shows a front view a set of microcounters in which several cathodes are superimposed;
• Figure 5 shows a front view a counter in which several bands of microcounters are superimposed;
• FIG. 6 represents a plate of microcounters on which each microswitch is connected by its anode to an external circuit;
• Figure 7 shows an example for arranging several strips of microcounters proportional;
• Figure 8 shows an example of cylindrical proportional counter; and
• FIG. 9 shows a spectrum representative of the resolution for measuring an energy of 6 Kev coming from a source of Fe ⁵⁵ , by a gas detector according to the invention.

Detailed description of embodiments

In Figure 1A, there is shown schematically a gas detector conforming to the invention. This detector comprises an enclosure 1 shown in phantom in the figure. This enclosure 1 is filled with a gas mixture which comprises usually a rare gas (such as argon, krypton, xenon, etc.) and which is under pressure chosen. This gas mixture ensures the absorption of radiation. received by the detector. This gas mixture is therefore ionized by radiation in an area called "absorption zone", in which a field prevails weak and uniform electric. This ionization creates electrical charges that we seeks to multiply, thanks to the proportional counter 2.

This proportional counter 2 includes a multitude of microcounters (also called "cells elementary ") referenced 4. Each of these microcounters 4 is achieved by means of two electrodes located in different planes and carried to different potentials so as to create a field electric which attracts electric charges from ionization of radiation in the gas.

As can be seen in this FIG. 1A, the microcounters are arranged in bands 3 of microcompteurs.

In this FIG. 1A, as well as in the figures which will be described later, we have represented the microcounters arranged in bands (or rows). However, it will be understood that these microcounters can be arranged according to all kinds of geometry (for example, in squares), but that they can also be independent. This choice of representation in "band" is simply intended to facilitate understanding of the appended figures.

Referring to Figure 1A, each band 3 of the microcomputer comprises an upper electrode 5, namely a cathode, a lower electrode 6, to know an anode, and a layer of insulating material 7 located between the two electrodes 5 and 6. The cathode 5 and the insulating layer 7 are perforated by holes 8 leading to the anode 6. Each breakthrough 8 constitutes a multiplication area. So each microcomputer has a cathode portion 5, a portion of insulating layer 7, an anode portion 6 and a zone of multiplication 8.

Although each band 3 may have several breakthroughs 8, each microcomputer 4 is independent since it has its own multiplication.

It will therefore be understood that a counter 2 of the invention may include a multitude of areas of multiplication, which greatly limits the risks of breakdown.

In this FIG. 1A, "a flayed "of counter 2 which shows two breakthroughs 8 belonging to bands 3 of microcounters and leading to the respective anodes 6.

Figure 1B shows more precisely a strip 3 of microcounters. As explained previously, this strip 3 comprises an electrode upper 5 and a lower electrode 6. The electrode upper 5 is a cathode and the lower electrode 6 is an anode. Cathode 5 and anode 6 are separated from each other by a layer 7 of a insulating material.

According to one embodiment, this material insulation is also photosensitive, which allows facilitate the fabrication of the detector.

According to another embodiment, the insulating material is also highly resistive. According to yet another embodiment, the material insulator is fluorescent so as to transform the UV radiation due to the multiplication in radiation visible which can, for example, be counted.

Cathode 5, as well as the insulating layer 7 are drilled with holes 8 inside which reigns an electric field, which creates areas of multiplication. In these multiplication zones 8, the electric field is intense and almost uniform. So it's naturally towards these areas of multiplication directed by the electric charges created by ionization of radiation in the area absorption.

From an electrical point of view, if the potential of the input window of the detector (i.e. enclosure) is zero volts, the cathode can be brought to a few hundred volts, so that attract the primary charges and the anode brought to a even higher voltage, so as to ensure the multiplication of these primary charges.

In addition, for some applications, it is possible to use as insulating material, in each strip of microcounters a substrate, such as ceramic to ensure better solidity of the counter.

In FIG. 2A, there is shown, in section, microcounters 4 produced according to a different embodiment from that shown in FIG. 1B.

In this embodiment, the cathodes and the anodes are arranged in two directions perpendicular: the cathodes 5 are arranged in lines and anodes 6 are arranged in rows. Each breakthrough 8 leads to an anode 6, as in the previous embodiment.

In FIG. 2B, a proportional counter 2 produced by means of a multitude of bands 3 of microcounters of the type of that shown in Figure 2A. In other words, the proportional counter 2, shown on this FIG. 2B, comprises a plurality of cathodes 5 arranged in lines and a plurality of anodes 6 arranged in columns. Just like in the figures previous, the cathodes 5 are separated from the anodes 6 by a layer 7 of insulating material, rigid and photosensitive. The cathodes 5 as well as the layer of insulating material 7 are perforated by holes 8, which lead to the anodes 6, as shown in the figure 2B.

Such an arrangement of electrodes 5 and 6 allows coding of events in two directions. It can therefore be, for example, used in imaging.

As with all meters proportional shown in the previous figures, the breakthroughs 8 of the microcounters 4 were presented on this figure 2B as round section holes. However, it will be readily understood that all of these microcounters can have 8 breakthroughs (or obviously) of different shapes. For example, these obviously can be slots, parallel or non-parallel to each other; they can be conical, cylindrical, etc., and of variable size.

In Figures 3A and 3B, there is shown two examples of these recesses. In Figure 3A, the recess 8 has a conical shape which has the advantage of preventing the ions from the multiplication do not adhere to the wall 8 'of the recess, that is to say to the material 7. In FIG. 3B, the recess 8 of the microcomputer has a concave wall 8 ′ whose advantage is similar to that of the recess of Figure 3A.

However, whatever the form of these recesses, the ratio between the full part of a microcomputer and the hollowed out part of this microcomputer is typically chosen between 1 and 10.

According to the preferred embodiment of the invention (see FIGS. 1A to 2B), the recesses are circular holes whose ratio between the hole depth and hole width varies generally between 3 and 1/2.

With 8 holes of sizes and shapes appropriate, the light emitted during multiplication can be collected to form images or to perform counting or to obtain a signal synchronization signaling the event (i.e. the avalanche of ions).

In Figure 4, a band is shown 3 of microcounters produced according to a realization different from those described previously. According to this embodiment, the strip 3 has two cathodes 5a and 5b and two layers 7a and 7b of materials photosensitive insulators: the insulating layer 7a is disposed between cathodes 5a and 5b, and the layer insulator 7b is disposed between the cathode 5b and the anode common 6. In this example, the breakthroughs 8 are made in all the thickness made up of cathodes and insulating layers.

Such a multi-stage assembly of cathodes increases the height of the holes 8 and therefore the volume of the area of multiplication. The multiplication power of this area is thus increased and the collection of ions created during multiplication is facilitated and increased.

In Figure 5, there is shown a view of front of a multistage counter produced by of several plates 3a, 3b, of microcounters superimposed one above the other. In this example, the microcounters are arranged in the form of bands roughly of the type shown on the Figure 1B. Each plate can be either placed directly on the bottom plate is separated from its neighboring plate with gas identical to that prevailing in the activation area (as is the case in this figure) or by an insulating layer. In addition, the anode 6a, 6b of each of the plates 3a, 3b comprises a hole 8a, 8b aligned with the holes in the cathodes 5a, 5b and insulating layers 7a, 7b and opening out on an additional anode 6c.

In this embodiment, anodes additional 6c are required to ensure creation of the electric field over the entire height of drilled and placed under the clearance made by the openings 8a and 8b.

All of these plates 3a, 3b and additional anodes 6c is deposited on a substrate rigid 10.

The electric field prevailing in these breakthroughs is almost uniform across the entire height breakthroughs. So although each space cathode / anode of a plate 3 has a power of multiplication lower than an area of multiplication of the counter of FIG. 2A, the overlapping of several cathode / anode spaces allows to obtain a gain which is higher than in a zone of simple multiplication (like those shown on the Figure 2A). This sandwich configuration allows significantly reduce the electric field in the insulation; it also allows cathodes additional to collect part of the ions from multiplication. The counting rate of the detector is thus greatly increased.

It is specified that the differences e and e 'between the strips 3a and 3b and between plate 3b and the anode additional 6c can be variable depending on desired results.

As previously explained, each microcount 4 has its own multiplication 8. This means that each microcomputer is independent. However, in some applications microcounters 4 can be linked between them, either through their cathode, either through their anode.

In addition, it is possible to collect electrical signals on the electrodes from the above or below the multiplication zone 8, that is to say from cathode 5 or anode 6, which facilitates connections.

In Figure 6, there is shown a plate 3 of microcounters of which microcounters 4 are connected by their anodes 6 to an external circuit. More specifically, the plate 3 is glued to a support 13 carrying the anodes 6 of the microcounters 4. Each anode 6 is connected by means of tracks P1, P2, of contact to the external circuit, for example, up to an amplifier 15 arranged itself on a support 17. According to this example, tracks P1 and P2 pass through the support 13. Furthermore, as shown on this figure 6, a voltage source 19 is connected to plate 3 by cathode 5.

According to another example in which the anodes 6 are not connected to each other, each of them can be connected directly to a separate amplifier. Each microcomputer can then to be considered as the pixel of a linear detector or two-dimensional.

It will therefore be understood that the interconnections between the anodes of the microcounters and the circuits can be done easily by means of a multilayer circuit, for example, ceramic and according to techniques known to those skilled in the art.

The invention thus has the advantage of facilitate the connection by the fact that it can either on the cathode side, or on the anode side, or still on the back of the detector. In addition, the tracks of link between microcounters and amplifiers can be engraved or screen printed when manufacturing the proportional counter, so as to further facilitate the connection.

Thus produced, each microcomputer allows to give an electrical signal, depending on the quantity of electrons received. This electrical signal can be used for energy measurement and for measurement of impact position. More specifically, the positioning of the impact of the beam (or location spatial) can be obtained directly by identifying the microcomputer affected, in case the area absorption is low. Otherwise, the electrons from ionization are found broadcast on at least part of the counter proportional. We can then proceed to the search for the centroid, that is to say the microcomputer which received the greater share of scattered electrons. For search for such a centroid among microcounters affected we can either use a logical method known which consists in digitizing the signal derived by the microcounters affected then to calculate the centroid corresponding, either by an analog method in picking up electrical signals on lines to delay type R.C, L.C or R. Whatever the process chosen to determine the location of events, you have to process the signals from the cathode, signals from the anode and, for some embodiments using an anode additional signals from this anode additional.

In Figure 7, there is shown another embodiment of the invention in which several bands 3a, 3b, 3c, 3d, 3e of microcounters are arranged to form a sequence of U and U reversed. These bands are of the type shown in Figure 1B. This particular arrangement allows realize a delay line as we can use to locate events. According to this embodiment, the different cathodes 5a-5e are juxtaposed perpendicular to each other other. Each of these cathodes 5a-5e corresponds to a anode 6a-6e separated from its corresponding cathode 5a-5e, by a layer of insulating material 7a-7e.

In FIG. 8, a further another embodiment of a proportional counter according to the invention. Unlike linear meters described in the previous embodiments, this proportional counter 2 is cylindrical. Such a counter cylindrical can be used, for example, in crystallography.

As seen in this figure 8, this counter 2 has an open cylinder shape of which opening 12 ensures the introduction of radiation to inside the cylinder. This counter 2 therefore includes a cathode plane 5 forming the inner wall of the cylinder and an anode plane 6 forming the outer wall of the cylinder. Anode 6 and cathode 5 are separated by a layer 7 of insulating and photosensitive material. This cylinder having been shown in section, we see on the section of said cylinder of the openings 8. Such openings 8 are thus distributed over the entire length of the cylinder; they are shown in dotted lines, since covered by the anode 6.

As seen in Figure 8, a wire electric 9 crosses the cylinder longitudinally, this electric wire to bring a certain potential inside the cylinder. For example, we could bring cathode 5 to zero potential, anode 6 to potential of +1000 volts and the electric wire 9 to one potential of -200 volts.

Each set of microcounters is therefore produced by means of a sheet of insulating material covered on each of its faces of a material driver. According to the embodiments, the insulator can be glass or photosensitive glass or still any other plastic material having a rigidity sufficient dielectric.

• une des méthodes consiste à exécuter des réserves dans la cathode par photogravure, puis à la creuser, par exemple par attaque chimique. La cathode sert alors de masque autosupporté. Le percement de la feuille isolante s'effectue soit par photogravure UV, soit par lithographie profonde X ou encore par attaque chimique, usinage laser, attaque ionique, etc. selon la nature de cette feuille isolante ;
• une autre méthode consiste à percer des trous borgnes directement en utilisant un laser capable de transpercer la cathode et l'isolant sans pour autant transpercer l'anode. Pour cela, l'anode sera choisie plus épaisse que la cathode, ou bien dans un matériau de nature appropriée.

To make each of the microcounters on a plate, it is necessary to pierce the composite sheet (insulating sheet covered on either side with a conductive layer) of blind holes. For this, various known techniques can be used:

• one of the methods consists in carrying out reserves in the cathode by photogravure, then in digging it, for example by chemical attack. The cathode then serves as a self-supporting mask. The insulating sheet is pierced either by UV photogravure, or by deep X-ray lithography or by chemical attack, laser machining, ion attack, etc. according to the nature of this insulating sheet;
• another method is to drill blind holes directly using a laser capable of piercing the cathode and insulating it without piercing the anode. For this, the anode will be chosen thicker than the cathode, or else in a material of an appropriate nature.

For some embodiments, such than that shown in figure 5, it is necessary to carry out a drilling through holes, i.e. holes crossing right through the composite sheet. For you can use either mechanical drilling or laser drilling in a much simpler way than for drilling blind holes.

These technologies make it possible to carry out counters for a relatively low cost. These counters can be large in size juxtaposing several identical counters.

Yet another advantage of the invention is that since the counter is mainly formed from the multiplication zone, it can be very thin, that is to say of the order of a few tens of microns. We can thus obtain a counter proportional barely thicker than a sheet of paper. This allows, as will be easily understood, to build detectors of very varied forms, for example cylindrical, as shown in Figure 6. For such cylindrical, spherical geometries, etc. the parallax generally created in an area absorption is eliminated, which allows to have a thick absorption area (around one hundred millimeters).

In addition, the multiplier areas have a geometry such that it ensures the inexistence of breakdown between the electrodes, even at the ends of the plates, since the electrodes, cathodes and anodes do not are not in the same plane. In addition, the anodes being simple and robust, they are not subject to deterioration under the effect of possible breakdown or the electronic and ion bombardment they suffer.

All types of proportional meters described in Figures 1A to 8 can be used in gas detectors to determine different types of radiation. For example, for detectors of X-rays used in crystallography, we can use circular proportional counters, linear or spherical allowing very high rates count. In this case, the counters are placed on goniometers, in front of X sources or in front of synchrotron radiation sources.

These detectors having good energy resolution and a significant gain they provide very good spatial resolution while simplifying the connection since the plans anodes or cathodes may include, for example screen printing, all electrical routes necessary to external circuits.

In FIG. 9, a spectrum is shown showing the resolution of the energy measurements, for an energy of six thousand electron volts, in an argon / CO ₂ mixture under atmospheric pressure.

For the embodiments described previously, we get an energy resolution about 20% which shows that the meter works actually in proportional regime.

For the types of meters described previously, the multiplication gain obtained can be be around 20,000, which ensures a completely correct processing of electrical signals. For example, for a counter whose cells of independent multiplication are separated by one step about 300 µm, the spatial resolution is around 50 µm. Such a proportional counter allows advantageously to support counting rates raised by micro-counter; this rate can be around 100,000 events per second.

In addition, the meters according to the invention can have a high density of microcounters, this allows working with very high fluxes.

In addition, for meters in which each microcomputer is independent, it is possible to get an even higher signal, to allow low flux detection, by operating Geiger counters.

Some other advantages of the invention are that the detectors thus constructed are compact and light with relatively low manufacturing costs low compared to detectors produced according to other technologies, which increases considerably their field of use.

Claims (8)

1. Ionizing radiation detector having an enclosure (1) filled with a gas and within which is located a proportional counter (2) defining, between itself and an upper wall of the enclosure, an absorption zone (A) in which the gas is ionized by radiation, characterized in that the proportional counter has at least one lower electrode (6) and at least one upper electrode (5), which are parallel to one another and separated by an insulating material layer (7), the upper electrode and the insulating material layer having at least one opening (8), in which prevails a substantially uniform electric field and which constitutes a multiplication zone for the electrons resulting from the ionization of the gas by the radiation.
2. Detector according to claim 1, characterized in that the lower electrode is an anode and the upper electrode is a cathode.
3. Detector according to claim 1 or 2, characterized in that the insulating material layer is rigid.
4. Detector according to any one of the claims 1 to 3, characterized in that the insulating material layer is either photosensitive, or highly resistive, or fluorescent.
5. Detector according to any one of the claims 1 to 4, characterized in that the proportional counter has a plurality of superimposed upper electrodes parallel to the lower electrode and separated from one another by an insulating material layer, the openings of each upper electrode being aligned with the openings of material layers juxtaposed with said upper electrode.
6. Detector according to any one of the claims 1 to 5, characterized in that the proportional counter incorporates a plurality of upper electrodes arranged in the same first plane, with the same first direction and interconnected and a plurality of lower electrodes arranged in the same second plane, parallel to the first plane, with the same second direction and interconnected.
7. Detector according to claim 1, characterized in that the proportional counter is in the form of a cylinder open in the centre and from which the gas is ionized by radiation, said counter comprising:

   an upper electrode (5) and a lower electrode (6), both in the form of an open cylinder, the lower electrode surrounding the upper electrode,

   an insulating material layer (7) separating the two electrodes,

   an electric power supply wire (9) longitudinally traversing the cylinder formed by the electrodes and the insulating layer in order to create between itself and the upper electrode an absorption zone,

   at least one hole (4) made in the upper electrode and the insulating layer in which there is a substantially uniform electrical field and which constitutes a multiplication zone for the electrons resulting from the ionization of the gas by the radiation.
8. Detector according to any one of the claims 1 to 7, characterized in that the upper electrode and the lower electrode are independent and each connected to an input of an electronic processing circuit.

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