FR2912837A1 - Electron multiplying device for e.g. alpha ray detection system, has coating presenting low resistivity characteristics to create electric field, so that multiplication of electrons is high to prevent formation of secondary avalanches - Google Patents

Abstract

The device (1) has orifices (5) provided in a plate (2) that is made of epoxy resin or glass. Each orifice has an axis (6) perpendicular to main surfaces of the plate. A coating is provided on the surface and comprises layers (9a, 9b, 12a, 12b) made of gallium arsenide and glass, respectively. The coating presents holes (10, 13) placed in correspondence to the orifices. The coating presents low initial resistivity characteristics to create high amplitude electric field in the orifices, so that avalanche multiplication of electrons is high for preventing formation of secondary avalanches.

Description

The present invention relates to a device for multiplying

  electrons capable of operating in the air and an ionizing radiation detection system comprising such a multiplication device. The detectable radiation is as follows: a, (i, y, X-rays, UV.

  Any element emitting one of these radiations, such as radon, polonium, etc., and any other ionizing or ionizable species by external excitation may also be detected. The invention also relates to the use of such a system for the detection of a flame. Various types of ionizing radiation detection systems are already known, comprising a housing in which are housed two electrodes carried at different potentials, thus creating an electric field between them. The housing is filled with a gas that is ionized by radiation, producing electrons. These electrons are driven by the electric field to the electrode of higher potential, which counts the number of electrons that reach it. It is thus possible to detect and measure the ionizing radiation. Such a system is for example described in WO 2005/116690. This system comprises an ionization chamber with two parallel flat plates. A volume of gas to be analyzed (in particular air) is taken and injected into the chamber, which is then sealed. If at least one atom of radon is injected into the chamber, it decays by emitting a radiation, which leads to the production of electrons trapped by oxygen and converted into negative ions, driven to the collection electrode, and positive ions attracted to the other electrode. With this system, the signal obtained is very weak, and therefore strongly disturbed by the noise. It is therefore very difficult to obtain a 100% efficiency for the detection of particles a. In addition, the implementation of this system is long and difficult, because of the need to take a sample to be injected into the ionization chamber. In addition, in some cases, a purge prior to the measurement is necessary. The same system, in the presence of vapor of a low ionization energy compound such as ethylferrocene (6.08 eV), can detect ultraviolet radiation in various gases, including air, but with low sensitivity (Charpak, Peskov, Scigocki in IEEE Transactions on Electrical Insulation, Vol 26, No 4, August 1991).

  Therefore, this type of detection system, even if it can operate in the air, is not fully satisfactory. In addition, many detection systems are based on the principle of a controllable electron avalanche. Such an avalanche, called controllable Townsend avalanche, is a mechanism for the multiplication of electrons in a gas, which occurs in sufficiently large electric fields, where the free electrons have a sufficient kinetic energy (greater than 5 to 25 eV) for ionizing atoms or molecules by non-elastic collisions.

  If the number of electrons initiating the avalanche is no, then the total number of electrons created by the avalanche is A.no, where A is the amplification coefficient, also called gain. This gain A depends on several parameters, such as the electric field, the gas present in the housing of the detection system (composition, properties, pressure) and the geometry of the zone in which the avalanche occurs. For low values of gain A (typically A <10), the avalanche ends in the gaseous volume without further complication and without secondary mechanism: it is a controllable avalanche. On the other hand, for critical values of A.no, or of A according to the geometry of the device, the primary avalanche can be accompanied by several secondary avalanches, or even generate uncontrollable discharges under particular conditions. Thus, it is understood that it is desirable to have a high gain A, because this leads to a large number of electrons detectable at the electrode of higher potential, so potentially to a better detection and a better efficiency of the detection system. But a gain A too high can cause a deterioration of the detection system and associated electronics. More generally, it is desirable to avoid the formation of secondary avalanches (due to a high gain A or other phenomenon), because they cause a deterioration of the detection system. There are several types of detection systems exploiting the controllable Townsend avalanche mechanism for electron multiplication. All these systems work with very specific gas mixtures, for example mixtures of noble gases (such as argon) with other gases (especially methane).

  A first type of these detection systems is the parallel plate chamber system, where the avalanches take place in the space between two metal plates forming one a positive electrode (anode) and the other a negative electrode (cathode ).

  The Townsend avalanche can be triggered by primary electrons created inside the chamber volume by external radiation (X-rays or UV photons) or charged particles (electrons, protons, particles a). As previously indicated, in some specific gas mixtures and low A values, such a detector can function stably. On the other hand, at gain values A greater than a critical value Acr, t, secondary avalanches appear, and this in all gases. This is due to two causes. The first cause is that, while the Townsend avalanche occurs, the atoms and molecules are excited by the avalanche electrons and emit UV or photons in the visible. The number of these photons is k.A.no, where k is a coefficient (usually greater than 1). These photons reach the cathode and, by a particular phenomenon, create photoelectrons from the cathode. If the probability of occurrence of this phenomenon is yph, then the number of electrons that will be extracted from the cathode is k.A.no. YPH. In turn, these photoelectrons initiate Townsend (secondary) avalanches. If A.yph 1, then these photons do not play a significant role. But if: A.yph = 1 (1) then an undesired discharge occurs, which can damage the detection system or the associated electronics. The second cause is that positive ions remain in the space between the two plates when the primary avalanche is complete. These ions move in the direction of the cathode and can also extract electrons from the cathode if: E;> 2cp (2) where E; is the ionization potential of the atoms or molecules of the gas. The probability of such a phenomenon is y +. Again, if y + 1, these secondary electrons do not play a significant role. But if: A.y + = 1 (3) then a discharge occurs.

  Thus, controllable avalanches can only be obtained in parallel plate detection systems if: A.yph 1 (4) and A.y + 1 (5) Note that the critical values of Yph and y + depend on the composition of the absorption layer of the cathode. A discharge can also occur, especially for plane geometries, in this type of detection system if A.no> Qcrit (6) where Qcrft is typically 108 electrons. This type of discharge is due to the space charge created by the avalanche. For a number of years, attempts have been made to operate this first type of detection system in the air. But it has been possible to obtain a stable operation in the air only at low values of gain (A <50), because the coefficients Yph and y, are very large in the case of the air, and also because their values vary rapidly over time due to changes in the cathode absorption layer (for example due to ion bombardment or changes in the moisture content). The deterioration of the system in the air results from the series of secondary avalanches (equations (1) and (3)), by feedback mechanism. As a result, the maximum gain that can be achieved in air is much lower than that which can be achieved in selected gas mixtures, where yph and y + are low, and where the deterioration only occurs because of the space charge (equation (6)), which makes it possible to achieve a very high gain.

  Thus, in this first type of detection system, to obtain a high gain and a stability of the phenomenon, it is essential to choose gases having low values of Yph and y ,, and the correct operation in the air is therefore not possible. A second type of these detection systems is the wire system, in which the avalanches take place in the space between a metal cylinder (the cathode) and the wire (the anode) arranged on the axis of the cylinder. Unlike parallel plate systems, in wire systems there is a focus effect of field lines in an area around the anode. As a result, this detection system operates differently. Thus, the primary electrons created by the ionization of the gas by the radiations are first driven towards this zone located near the anode, and only in this zone trigger avalanches of Townsend. This detection system operates without secondary avalanches if conditions (4) and (5) are fulfilled, whereas, if equations (1) or (3) are satisfied, corona discharges occur. If the condition (6) is fulfilled, discharge currents are created in the multiplication zone, but these are extinguished in the detector volume before reaching the cathode because the electric field decreases rapidly when away from the anode. In this second type of detection system, to obtain a high gain and stability of the phenomenon, it is therefore also essential to choose gases having low values of yph and y +, and the correct operation in the air is therefore Not possible. In addition, the implementation of the wire-in-air detection systems has the following particularity. The primary electrons created by the ionization of the gas will undergo, when they are driven towards the anode, a significant trapping by the oxygen and the water vapor, and thus to form negative ions. These ions are carried to the wire (the anode), but when they enter the area of strong electric field around the anode, they can not trigger a Townsend avalanche due to their low kinetic energy.

  This avalanche can only be triggered either by the few electrons that have detached themselves from the negative ions in the strong electric field (to the number of b.no, where b 1), or by the few electrons that have not been trapped (at number of c.no, where c 1). In these two cases, the number of electrons capable of triggering an avalanche is much lower than the initial number of electrons (no). Therefore, to observe a signal (and thus detect the radiation), it is necessary to have a gain A much higher than in the case of non-electronegative gases. However, again, it is possible to operate this wire detection system stably with gains A less than 50, because the coefficients yph and y + are very large in the case of air, and the deterioration occurs mainly because of the feedback mechanism (equations (1) and (3)). However, a gain A of 50 is generally insufficient to detect the radiation, since the signal supplied by the detection system is proportional to A.no. (b + c), where b and c are very small.

  There are therefore two reasons why the operation of the wire-in-air detection systems is not satisfactory.

  Another type of detection system exploiting the controllable Townsend avalanche mechanism uses electron multiplication devices having a plate provided with through holes. Such detection systems are described in documents FR 2 727 525 and EP 0 948 803. The plate is covered, on its two opposite main faces, with sheets of metal carried at different potentials, and it is arranged between two electrodes. In the presence of an electric field between the two electrodes, the orifices behave like electrostatic dipoles and the field lines are focused in these orifices. Thus, in these orifices, there is an important electric field and it can therefore occur a multiplication of electrons by avalanche. Such plates form a screen for the photons resulting from the ionization of the gas, so that only a portion B of these electrons can reach the cathode, B being a geometric factor (B 1). Thus, condition (1) for the occurrence of a discharge becomes: B.A.yph = 1 (7) Conditions (3) and (6) are unchanged. However, in detection systems with orifice plates, the value of Qcrit is from 106 to 10 'electrons (depending on the geometry), therefore lower than with the parallel plate or wire detectors, so the maximum gain that the 'can be obtained is weaker. One of the reasons why the value of Qcr is lower is due to the small size of the orifices (of the order of 70 μm), which is a favorable factor for the appearance of the space charge. Another reason is the presence of sharp-edged metal edges around the holes, which also promotes deterioration. When this detection system operates in the air, as in the case of wire-based detection systems, the primary electrons created by the ionization and which are driven to the orifices are trapped by oxygen molecules and form negative ions. . Thus, only a small proportion of the primary electrons will be free and may participate in triggering an avalanche in the orifices. Moreover, as indicated above, since the value of y + is high in the air, it is possible to obtain only small gains A (of the order of 20). It should be noted that the coefficient yph does not intervene here because of the coefficient B (equation (7)). But, in practice, a gain A of 20 can only be obtained in dry air. However, in the ambient air having a certain humidity, only gains A less than about 5 to 10 can be obtained, and they are very unstable over time (due to load leakage along the walls of the orifices). Thus, for the various reasons mentioned above, the implementation in the air of such a detection system with orifice plates can lead only to obtaining a very weak signal (low gain), not exploitable in convenient. In addition, the discharges occurring in such systems are much more damaging than in the other systems presented, because they irreversibly damage the plate by creating a conductive path along the wall of the orifices. In conclusion, none of the described detection systems using electron avalanches can operate in air with sufficient gain to obtain a usable signal, and without damaging the system. It follows in particular that these detection systems must necessarily include a perfectly sealed housing, preventing the entry of air. The cost of these devices is therefore very high. An object of the invention is to provide a detection system that can give satisfactory results in the air, and not only in a chamber containing a particular gas mixture. In other words, the object of the invention is to provide a detection system which makes it possible to obtain a high gain in the air (and therefore a good radiation detection), without this high gain being likely to damage the system of detection. detection or associated electronics. For this purpose, and according to a first aspect, the invention relates to an air-multiplying device capable of operating in air, intended to be used in an ionizing radiation detection system, comprising a plate of dielectric material comprising a first and a second opposite main face and in which are formed a plurality of through holes each having an axis substantially perpendicular to said main faces of the plate, and further comprising, at least on the first main face, a coating comprising at least one layer and having holes arranged in correspondence with the openings in the plate, said coating having initial resistivity characteristics sufficiently low to allow, in operation, the creation of an electric field of large amplitude in the orifices of the plate and consequently the multiplication of electrons by avalanc hes, and sufficiently large to substantially prevent, in operation, the formation of secondary avalanches. The initial resistivity means the resistivity of the material used, before its application as a coating. Throughout the patent application, the term resistivity means the initial resistivity thus defined. Thus, thanks to the coating carried in operation at a suitable potential, it is possible to create a large electric field in the orifices of the plate, because of the sufficiently low resistivity, and therefore a multiplication of electrons by avalanches. In addition, the sufficiently high resistivity of the coating makes it possible to temporarily and locally load the surface of the cathode with the ions created and thus totally prevent or at least considerably limit the formation of secondary avalanches, in order to obtain satisfactory operating stability. It also prevents the formation of sparks or discharge currents, or at least greatly limits their intensity. In practice, the resistivity of the layer or layers of the coating is in the range of 105 to 1092 am. The device according to the invention can therefore operate in air stably, at significant gains, hence the possibility of obtaining a very good sensitivity (detection of low radiation), without risk of damage. In addition, a very good detection efficiency can be obtained. The presence of air does not interfere with the proper functioning of the device and the detection system which it is intended to be part of, it is not necessary to provide a housing having a very good seal. The cost is thus reduced, and the use facilitated. According to one embodiment, the device comprises, on the first main face, a global coating layer made of a material whose resistivity is between 10 'and 1011 μm, having holes arranged in correspondence with the orifices formed in the plate. . This average resistivity makes it possible to obtain both targeted effects with a single layer. According to another embodiment, the device comprises, on the first main face: a first coating layer, made of a first material whose resistivity is between 105 and 108 am, the first layer having holes arranged in correspondence with orifices in the plate; and - a second coating layer disposed on the first layer, made of a second material different from the first, whose resistivity is between 108 and 1012 μm and is at least 10 times greater than the resistivity of the first material, the second layer having holes arranged in correspondence of the orifices in the plate. The first layer, brought into operation at a certain potential, allows the creation of a large electric field in the orifices of the plate, and therefore an increase of electrons by avalanches, while the second layer makes it possible to ensure stability, as previously indicated. The second material can be expected to have a resistivity 100 times greater than that of the first material. Whatever the embodiment relating to the first main face, one can have, on the second main face, one of the following arrangements: a) no coating. In this case, the detection system 20 comprising the electron multiplication device will have an electrode located near the second main face of the plate, to allow the generation of an electric field in the orifices; b) a first coating layer, made of a first material whose resistivity is between 105 and 108 μm, the first layer having holes arranged in correspondence of the orifices formed in the plate; c) in the case b), one can further have a second coating layer disposed on the first layer, made of a second material different from the first, whose resistivity is between 108 and 1012 SZ.m and is at least 10 times greater than the resistivity of the first material, the second layer having holes arranged in correspondence of the orifices formed in the plate; d) an overall coating layer made of a material whose resistivity is between 0 'and 1011 Ωm, having holes arranged in correspondence of the orifices formed in the plate.

  The presence of an overall layer or a second coating layer on the side of each of the first and second major faces of the plate improves the ability of the device to prevent or limit secondary avalanches, as well as sparks as the current is decreased in this case (discharge currents). For example, the resistivity of the first material is in the range of 106 to 10 μm and / or the resistivity of the second material is in the range of 108 to 1010 am. The first material may be gallium arsenide, the second electronically conductive type material, such as some glasses and / or the plate may be made of a material of the epoxy resin or glass type. According to one possible embodiment, the orifices in the plate have a substantially cylindrical central portion extended at each of its two axial ends by a frustoconical portion of the same axis flaring towards the main face of the plate into which it opens. These frustoconical portions form a chamfer which further improves the gain. It can be provided that the holes of the first layer are centered on the axis of the orifices of the plate and have dimensions greater than those of the orifices of the plate at the level of the main face or faces of said plate, and that the holes of the second layer are centered on the axis of the orifices of the plate and have dimensions substantially equal to those of the orifices of the plate at the level or the main faces of said plate. Thus, the second layer is disposed directly against the plate in the peripheral zone to the orifices. This provides better protection against sparks and discharge currents. For example, the multiplication device has a thickness of between 0.5 and 4 mm, or even between 1 and 2 mm. The orifices formed in the plate may have a mean diameter of between 0.3 and 1 mm, or even between 0.5 and 0.8 mm, and / or be arranged substantially at the corners of a side grid of between 0.5 and 1.5 mm, or even between 0.7 and 1 mm. According to one possible embodiment, the multiplication device is substantially plane. Alternatively, it may have the shape of a cylinder or sphere portion. According to a second aspect, the invention relates to a system for detecting ionizing radiation or ionizable species, comprising: a first electrode, called a drift electrode, and a second electrode, said to be substantially parallel, and between which generating an electric field capable of driving towards the collection electrode the electrons created by said radiation, the collection electrode being able to provide data representing the number of electrons that reach it; data processing means provided by the collection electrode; at least one electron multiplication device as previously described, disposed substantially parallel to the electrodes so that the plate of the device or devices is located between the two electrodes and the first face of said plate is turned towards the first electrode; coating - or the coating layer having the lowest resistivity - disposed on the first major face of the plate being carried, in operation, to a potential between the potentials which are carried, respectively, the drift electrode and the electrode of collecting, so as to cause the multiplication of electrons by avalanche in the orifices of the plate. The electrons are for example created following the ionization, by said radiation, of the gas present between the electrodes.

  According to one possible embodiment, the collection electrode is constituted by the overall coating layer - or the first coating layer - disposed on the second main face of the plate, when such a layer exists. According to another possible embodiment, the collection electrode is distinct from the multiplication device. In this case, the overall coating layer - or the first coating layer - disposed on the second main face of the plate (when such a layer exists) is carried, in operation, to a potential between the potentials to which they are applied. , respectively, the overall coating layer - or the first coating layer - disposed on the first major face of the plate and the collection electrode. Alternatively, when the second main face of the plate is free of coating, the collection electrode must be placed near the second main face, to allow the generation of an electric field and therefore avalanches.

  In order to increase the gain, the detection system may comprise two substantially identical electron multiplication devices arranged substantially parallel to the electrodes so that the plates are located between the two electrodes and the first main face of each plate is facing the first electrode, the overall coating layers or the first successive coating layers being brought into operation at increasing potentials when approaching the collection electrode. Such a device with two multiplication systems achieves a gain of 104 in the air. The detection system may further comprise an electron multiplying device of the type comprising on the one hand a resistive gate forming a cathode and on the other hand an anode, the cathode and the anode being substantially flat, parallel and kept distant from each other. less than 500 pm using micro-pillars. Such a device, said MICROMEGAS is described in WO 97/14173, and in what follows. The collection electrode may further include separate localized collection areas. According to a first embodiment, the electrodes and the electron multiplication device (s) are arranged in a casing having a window for the passage of the UV sunshine constituting the ionizing radiation to be detected, the window being arranged facing the electrode drift, having a photosensitive material such as cesium iodide, so that the UVs can create photoelectrons from the first electrode. Either the electrode is made entirely with this photosensitive material, or its active side only is covered with such a material. A resistive layer coated with a photosensitive layer has the same effectiveness as a photosensitive layer deposited on a metal substrate. According to a second embodiment, the electrodes and the electron multiplication device or devices are arranged in a housing having a passage zone for the entry of ionizing radiation and containing a gas with a low ionization potential, such as a ethylferrocene vapor. This system may further comprise a lens placed in the path of the ionizing radiation towards the interior of the housing, the axis of the lens being substantially parallel to the or each electron multiplication device and situated at a distance from the multiplication device of the electrons. electrons closest to the drift electrode less than 3 mm. Thanks to this configuration, this system can work in the air.

  Localized collection areas may be bands converging towards the center of the lens. Such a system can be used for the detection of a flame. Indeed, a flame emits UV which will cause the ionization of the gas and thus the creation of electrons then multiplied by avalanches. Moreover, the detection system may be such that the electrodes and the electron multiplication device (s) are placed in a housing having a passage zone for the entry of UV or X rays emitted by a source, this source being arranged so that said spokes 10 penetrate the housing substantially parallel to the or each electron multiplier and at a distance from the electron multiplier closest to the drift electrode less than 3 mm. In addition, the housing has an inlet opening for a gas to be analyzed. Such a system can be used for the detection of the presence in the air of gases with a low ionization potential, such as certain alcohols or hydrocarbons, toluene, benzene. We now describe, as examples Non-limiting, several possible embodiments of the invention, with reference to the accompanying figures: FIG. 1 is a schematic perspective representation of the electron multiplication device according to a first embodiment of the invention; Fig. 2 is a cross-sectional view of the multiplier of Fig. 1; Figure 3 is a schematic plan view of a face of the device of Figure 1 without the second coating layer; Figure 4 schematically illustrates a detection system according to the invention; Figures 5 and 6 schematically illustrate a detection system according to the invention, respectively for the detection of particles a and UV; Figures 7 to 11 schematically illustrate embodiments of a detection system according to the invention for the detection of a flame; FIG. 12 illustrates a variant of the detection system of FIG. 5, implementing a device according to the invention and a device of MICROMEGAS type; Fig. 13 is a schematic sectional representation of an electron multiplication device according to a second embodiment of the invention; FIGS. 14 to 16 are schematic cross-sectional representations of alternative embodiments of an electron multiplication device according to the invention; and FIG. 17 illustrates a variant of the detection system of FIG. 5. Referring firstly to FIGS. 1 to 3 which show a device 1 for multiplying electrons according to a first embodiment. The device 1 comprises a plate 2 made of a dielectric material, such as an epoxy resin, and having a thickness e of the order of 0.4 to 2 mm, for example between 1 and 2 mm. The plate 2 comprises a first and a second opposite main face 3, 4, substantially planar. A plurality of through orifices 5 are formed in the plate 2, each of these orifices 5 having an axis 6 substantially perpendicular to the faces 3, 4. As illustrated in section in FIG. 2, the orifices 5 are identical and each have a substantially cylindrical central portion 7 extended at each of its two axial ends by a frustoconical portion 8 of the same axis 6 flaring towards the main face 3, 4 in which it opens. The orifices 5 are symmetrical with respect to the median plane P of the plate 2 parallel to the main faces 3, 4. Preferably, the central portion 7 has a height at least equal to half the thickness e of the plate 2. The average diameter of the orifices 5 is between 0.5 and 0.8 mm, and these are arranged regularly, substantially at the corners of a side grid of between 0.7 and 1 mm. On each of the main faces 3, 4 is disposed a first thin coating layer 9, 9a, 9b, made of a first material whose resistivity is relatively low, and between 106 and 107 S2.m. One of the materials that can be used is gallium arsenide, but other materials are possible if they allow to create controllable avalanches of electrons. Each of the first layers 9 has holes 10 located in correspondence of the orifices 5 formed in the plate 2. The holes 10 are substantially circular and centered on the axis 6, and have a diameter D10 greater than the diameter D5 of the orifices 5 at the main faces 3, 4 (see Figure 3). There is thus a peripheral ring 11 around each orifice 5 which is not covered by the first coating layer 9. In addition, on each of the first coating layers 9 (on each of the main faces 3, 4) is disposed a second coating layer 12, 12a, 12b thin, made of a second material, different from the first, whose resistivity is much greater than that of the first layer 9, and between 108 and 1010 û.m. Any material having this characteristic, and especially glass, can be used.

  Each of the second layers 12 has holes 13 located in correspondence of the orifices 5 formed in the plate 2. The holes 13 are substantially circular and centered on the axis 6, and have a diameter D13 substantially equal to the diameter D5 of the orifices 5 at the main faces 3, 4. Therefore, at the level of the peripheral ring 11, the second layer 12 is disposed directly against the main face 3, 4 of the plate 2. In other words, on each of the main faces 3 , 4 of the plate 2, the second layer 12 of coating locally covers and surrounds the first layer 9 of coating (see Figure 2). The placing of the two coating layers 9, 12 and the creation of the orifices 5 can be carried out by any known method. Such an electron multiplication device is intended to be used in a system 14 for detecting ionizing radiation. FIG. 4 schematically represents the general structure of such a detection system 14. The detection system 14 comprises, generally in a housing 15, a first planar electrode 16, called a drift, brought to a potential V1, as well as a second electrode 17 flat and substantially parallel to the first electrode 16, said collection, brought to a potential V4. An electron multiplication device 1 is disposed between the two electrodes 16, 17, substantially parallel to them. The first coating layer 9a facing the first electrode 16 is brought to a potential V2, while the first coating layer 9b facing the second electrode 17 is brought to a potential V3. The potentials are chosen such that VI <V2 <V3 <V4. For this purpose, a voltage divider type circuit employing resistors of appropriate values may be used. Thus, an electric field E is generated between the electrodes 16, 17. The second electrode 17 is connected to data processing means 18 (electronic circuit, oscilloscope, computer, etc.) enabling an operator to visualize the detection or the measurement of ionizing radiation by the detection system 14.

  A source S1, S2 emits ionizing radiation 19 which leads to the formation of electrons 20 in the detection system 14. The detection system 14 makes it possible to detect these ionizing radiation 19 emitted from sources positioned in several ways with respect to electrodes 16, 17, as illustrated in FIG. 4 (SI or S2). The electrons 20 thus created are driven by the electric field E towards the second electrode 17. During the passage of these electrons in the orifices 5 of the multiplication device 1, there is produced a controllable avalanche 21 leading to the formation of a greater number of electrons, the role of the first coating layers 9a, 9b being to focus the electric field lines inside the orifices 5 and thus to allow this avalanche 21. These electrons are driven towards the second electrode 17. This provides the processing means 18 with data representing the number of electrons that reach it, and thus allows an operator using the system 14 to detect the presence of ionizing radiation.

  As explained below, thanks to the presence of the second layers 12 of coating, the multiplication device 1 and the detection system 14 allow on the one hand to suppress the feedback mechanism (equation (3)), and on the other hand to protect all the sparks. These, if they appear, are very weak and do not risk damaging the multiplier device 1 or the associated electronics. In prior art orifice plates having simple metal coatings, all stored energy is released as a spark, which is therefore very violent and can cause a great deal of damage.

  On the other hand, thanks to the second coating layer 12 provided by the invention, which moreover covers and surrounds the first layer 9 in the vicinity of the orifices 5, the positive ions and the electrons formed by the avalanche of Townsend will charge this second layer 12 and temporarily reduce the local electric field to a lower value. Therefore, any electrons extracted by the ions of the second layer 12 - if they exist - can not trigger a secondary avalanche. Thus, condition (3) will not be fulfilled. As for condition (1), which becomes condition (7) in the case of orifice plates, it is not filled either. Thus, only the condition remains (6). In this regard, to obtain an even greater gain, the invention provides a particular geometry of the orifices (flared portions 8) and a greater diameter of the orifices than in the prior art. Consequently, the Qcrit value is much greater than that observed in the prior art, and typically of the order of 108 electrons, ie two orders of magnitude above Qcrit in the case of known devices. Thus, with the detection system according to the invention, deterioration can occur only at very high gains. In addition, even if a spark appears, it will be very small, because of the immediate local load of the second layers 12 of coating. Thus, the capacitance participating in the discharge will be very small, and the discharge current will be limited to the high resistivity of the second layer 12. Only low discharge currents will therefore be obtained which do not risk damaging the system. Moreover, a major advantage of the invention is the possibility of detecting radiation in the air. Indeed, since the conditions (1) and (3) are not met, the detection system can operate in the air at gains greater than 104, very stable manner.

  Due to the second layer 12 of coating, no conductive path is created along the wall of the orifices which would affect the proper functioning of the multiplication device 1. This is further improved by the fact that the device 1 has a relatively large thickness e. Finally, it is possible to place between the electrodes 16, 17 of the detection system 14 a cascade of multiplication devices, and in particular two multiplication devices 1 according to the invention, despite the presence of the second layers 12 of coating. It is thus possible to considerably increase the maximum gain that can be obtained while maintaining stable operation. The successive layers 9 must be brought to increasing potentials from the first to the second electrode.

  As illustrated in FIG. 12, it is also possible to use in the detection system 14 a multiplication device 1 according to the invention associated with a multiplication device of another type (cascade configuration), for example such that described in the document WO 97/14173, known as MICROMEGAS, using it with a resistive gate in place of its metal gate, in order to obtain an operating stability. It is a detector developed in high-energy particle physics to improve the performance of gas-filled detectors by several orders of magnitude in terms of resolution time, maximum acceptable count rate, spatial resolution and maximum gain value. tolerable. The MICROMEGAS system basically comprises an amplification space with electric fields applied over a space of approximately 50 μm, preceded by a drift space where electrons are released in collisions of the atoms of the filling gas with the ionizing particles. to detect. The faces of the electrodes that produce the amplifying electric field are maintained at short distances (of the order of 50 pm) by pillars spaced a few millimeters apart. An important aspect of its operation is that the gain variation between parallel faces has a maximum close to 30 to 50 pm produced by a compensation effect, which contributes to gain stability and immunity against flatness defects. The effect of the small gap between the electrodes is to allow fields and thus high gains over short distances. The fast collection of ions reduces the effects of space charges. The MICROMEGAS test in air has shown satisfactory operation with the use of an insulated coating grid that achieves large gains without discharge and benefits from lower fields. It is clear that use with gaseous mixtures that can contain air in any proportion is possible, this combined with single or multiple layers of resistive coatings. This applies to a variety of gaseous detectors that are under development and are based on microelectronics techniques to benefit from the very low resolution times required in high energy physics. In FIG. 12, this device 40 of the MICROMEGAS type comprises on the one hand a resistive gate 41 forming a cathode and, on the other hand, an anode 42. The cathode and the anode are substantially flat, parallel and kept at a distance of less than 500 pm, for example with a distance d1 of between 0.05 and 0.5 mm, using micro-pillars. The resistive gate suppresses the feedback generated by the ions (but not the feedback generated by the photons) and it is thus possible to obtain an operation in the air at greater gains than in the case of a MICROMEGAS type device. with metal electrodes. The resistive grid also makes it possible to protect the electronics of the MICROMEGAS-type device in the event of unintentional discharges (sparks). The detection system 14 may be used for the detection of the radiation a, as illustrated in FIG. 5. The source Sa of radiation may in particular be an object containing Polonium or radon gas. This gas, which emits a particles, can accumulate for example in poorly ventilated cellars, and cause serious health problems. The source Sa can be placed as the source S1 or the source S2 of FIG. 4. In this application, the housing 15 is placed as close as possible to the source Sa, and it has an opening 22 opposite this source, so that not to stop radiation. We therefore understand the value of having a detection system operating in the air. In the case where the particles emitted by the source must pass through the first electrode 16 (FIG. 5), this electrode 16 takes the form of a grid. For example, the distance between the first electrode 16 and the closest multiplication device 1 is of the order of 4 cm, and the distance between the two devices 1 according to the invention is of the order of 2 to 5 mm . In air, and for VI = 5, 5 kV and V4 = 3 kV, the efficiency of this detection system 14 is of the order of 95% (percentage of particles detected with respect to the actual number of particles emitted) and the gain of approximately 104. If the source Sa is positioned as the source S2 of FIG. 4, approximately halfway between the first electrode 16 and the closest multiplication device 1, it is possible to obtain an efficiency of the order of 90% in the air. Thus, the efficiency remains important even if the path of the particles is not directed directly to the multiplication device 1. Thus, the detection system according to the invention is of low cost, it can operate in the air and detect particles with an efficiency close to 100%. In addition, this detection is not significantly affected by vibratory or acoustic disturbances. These features make the detection system quite competitive with commercial radon detection systems. Moreover, as illustrated in FIG. 6, the detection system 14 constitutes a very simple and inexpensive system for the detection of UV (dose received by persons exposed to the sun). The UV 23 emitted by the sun pass through the glass pane 24 (for example glass) of the housing 15 and create photoelectrons from the electrode 16, here in Cs1 (cesium iodide) in the form of a layer which can be deposited directly on the second layer of the cathode. These photoelectrons trigger avalanches in the orifices 5 of the multiplication device 1 or devices 1. It should be noted that this system can operate for example in a mixture of air and argon or in the air. Therefore, it is not necessary that the housing is perfectly sealed, since the air inlet does not lead to a malfunction or a deterioration of the system. The cost of the detection system is greatly reduced. Another application of the detection system according to the invention is the detection of a flame, as shown in FIGS. 7 to 11. Indeed, most flames in the air emit a UV light in the range from 185 to 280 nm. Now, in sunlight, these wavelengths are completely absorbed in the upper layers of the atmosphere by ozone. Thus, at ground level, the UV light emitted by a flame is not disturbed, in these wavelengths, by the light coming from the sun. In the embodiment of FIG. 7, the detection system 14 is similar to that illustrated in FIG. 4. It comprises, in a housing 15, a first electrode 16, a second electrode 17 connected to data processing means 18, and two multiplication devices 1 according to the invention, in cascade. A window 25 permits the passage of the UV emitted by a flame 26 towards the inside of the casing 15. The casing 15 contains a mixture of argon and a photon-sensitive vapor, for example an ethylferrocene vapor, at a pressure total close to 1 atm. This vapor can be obtained by placing in the casing a small amount of ethylferrocene, which is a liquid whose saturation vapor pressure is very low. If the UV emitted by the flame 26 enters the housing 15, they cause the ionization of the ethylferrocene vapor and thus the creation of electrons. These electrons cause avalanches 21 in the orifices 5 of the multiplication devices 1, and the electrons thus generated are finally detected by the second electrode 17. The gain thus obtained is generally greater than 105. The UV light penetrating parallel to the devices 1 multiplication, and not meeting a solid photocatode on this path, the detection system is insensitive to sunlight, but retains a high sensitivity to UV emitted by a flame. Indeed, a photocathode exposed to the sun's rays can create photons. On the contrary, with the system according to the invention, since the UVs interact only with the ionizable vapor, the photons will be created because of the UV rays emitted by the flame and not by the sun, since the quantum efficiency of the vapors is practically nil. at significant wavelengths (those of the sun at ground level, not absorbed by the ozone layer). For forest fires or for other applications, it is very important that the detection system can determine the position of the flame. Thus, as illustrated in FIGS. 8 to 11, it is intended to couple the detection system 14 with an optical system. As illustrated in FIGS. 8 and 9, there is provided a lens 27, with an axis 32, and having a focal length F. The second electrode 17 is here formed of a plate of dielectric material 28 on one side of which are arranged collecting metal strips 29 converging at a point, the center of the lens 27, placed at a distance F from the middle of the device 1, with F d (where d is the dimension of the electrode 17 and the device 1). Each band 29 constitutes an individual collection area. The strips 29 are fan-shaped and each strip is connected to a separate reading device. The image of any object emitting UV radiation (and especially a flame) placed at a distance greater than the focal length will form an image near the focal plane of the lens. With F d, the UV photons emitted by the object will be projected along one or two bands 29 and will cause the ionization of the photosensitive vapors. The photoelectrons created in the drift region will trigger avalanches in the orifices 5 of the multiplication device 1, which will result in a signal in the data processing means 18. By measuring the count rate from the strips 29, it will be possible to obtain a digital image of the flame in one dimension.

  Thus, the detection system 14 according to the embodiment of FIGS. 8 and 9 provides information on the precise position of the flame. Several of these detection systems 16 can be used in combination for the effective monitoring of fire starts, avoiding false alarms. An alarm will only be triggered if two or more detection systems detect a flame from the same point in space. Again, it will be appreciated that the flame detection system 14 may operate in the presence of air. For example, the housing 15 may contain a mixture of argon, ethylferrocene vapor (which is stable in air) and 10% air. It is therefore not necessary that the housing 15 has a perfect seal. In particular, for less than 1% of air, excellent efficiency is obtained. An imperfect seal may of course lead to the escape of ethylferrocene vapor, but this product is neither toxic to humans nor dangerous for the environment. In addition, the ethylferrocene evaporating slowly, there is always a vapor in the housing 15 even if it is not perfectly sealed. In the case of use in air, it is important that the photoelectric effect takes place near the first main face 3 of the plate 2 of the multiplication device 1, in order to avoid that the electrons created are trapped by oxygen molecules. As a result, the UV radiation must enter the housing 15 substantially parallel to the device 1 and at a short distance therefrom. Thus, as illustrated in FIG. 10, a collimator 33 interposed between the lens 27 and the window 25 is provided, and arranged so that the distance d2 between the path of the UV radiation penetrating the housing 15 and the first main face 3 of the plate 2 multiplication device 1 is less than 3 mm, preferably of the order of 1 mm. The detection system 14 allows the detection of a small flame (that provided by a match for example), in broad daylight and at a distance of a few meters.

  Measurements have shown that the detection system according to the invention is approximately 100 times more sensitive than the known systems, in particular the R2868 system marketed by Hamamatsu. It is recalled that the UV does not directly reach the cathode (electrode 16), but only interact with the ethylferrocene vapor, which allows the detection system to be more efficient than the known systems and to operate in full day.

  Referring now to Figure 11 which illustrates another application of the detection system according to the invention, namely the detection in the air of low ionization potential gas such as certain alcohols or fuels (hydrocarbons).

  The air to be analyzed is introduced through an inlet opening 34 of the housing 15, which air can then be discharged through an outlet opening 35. If at least one gas with a relatively low ionization potential (toluene vapors, benzene, etc. .) is present in the introduced air, it will be ionized by the radiation of a source 36. As in the case of the detection of a flame, the radiation of the source 36 must penetrate near the first main face 3 of the multiplication device 1 (with d2 of the order of 1 mm). The source 36 may be a UV lamp emitting radiation of wavelength greater than 110 nm. If there is presence of hydrocarbon vapors in the air, there will be photoinization of these vapors and creation of photoelectrons which will be amplified by the multiplication device 1. Another possibility is to use an X-ray source: in the air there will be no signal production, but if gases (or air) containing heavy elements are present, for example molecules containing 20 copper, they will be ionized by X-rays and it will be possible to obtain a signal. The use of the detection system 14 according to the invention, which can operate in the air with a large gain, can greatly increase the sensitivity with respect to known gas detection systems with relatively low ionization potential (PID - Photo lonization Detector). Indeed, the known systems measure a current in the ionization chamber, whose detection limit is 10-15 A, so that the sensitivity threshold of the detection of these gases is of the order of 10 ppb (part per billion). On the contrary, the detection system according to the invention can record each photoelectron, which makes the system more sensitive by several orders of magnitude. Electrons can also be released into a vapor contained in the air by using a UV source whose wavelength is adjusted to have a large cross section effective for the molecules of the vapor being sought. to detect. Under these conditions we can have a considerable sensitivity gain compared to the measurement of the number of electrons because we can benefit from the multiplication in the air of each electron and the advantage of the counting of individual electrons for the elimination of noise background. FIG. 13 illustrates a second embodiment of the multiplication device 1, in which, on each main face 3, 4 of the plate 2, there is a single coating layer 30 made of a material having a resistivity of between 10 'and 1011 Qm, and having holes 31 arranged in correspondence of the orifices 5 formed in the plate 2. This overall layer 30 of intermediate resistivity alone allows to obtain the desired effect.

  Several combinations are possible with respect to the coating of the main faces 3 to 4 of the plate 2, the first face 3 always comprising either the overall layer 30 or at least the first layer 9a. Some of these combinations are illustrated in FIGS. 14 to 16: FIG. 14 shows first and second layers 9a, 12a on the first main face 3, an overall layer 30 on the second main face 4; in FIG. 15, there is an overall layer 30 on the first main face 3, first and second layers 9b, 12b on the second main face 4; - In Figure 16, there is only a first layer 9a on the first main face 3, and no coating on the second main face 4. In this case, the generation of an electric field in the orifices 5 is only possible. with the use of a collection electrode 17 placed near the second main face 4.

  In all cases, the layer 4 (anode) may be metallic. Finally, reference is made to FIG. 17, which illustrates a variant of FIG. 5, in a case where the multiplication device 1 located furthest from the first electrode 16 comprises at least a first coating layer 9b on its second main face. 4 (or at least one overall layer 30), which can be brought to a potential. The collection electrode 17 is then constituted by this layer 9b or 30, and there is no separate collection electrode of the multiplication device 1. Thus, the invention provides a decisive improvement to the prior art, by providing a detection system which has the following advantages in particular: - stable operation in rare gas and air mixtures or even in air alone, with significant gains greater than 104, hence: the possibility of using the detection system in the open, without housing; the possibility that the waterproofness of the housing is not perfect, resulting in a considerably reduced cost; the application to the detection of particles a; robustness of the multiplication device: which is more rigid than the orifice plates of the prior art because it is thicker, despite the presence of orifices of larger diameter; which provides protection against sparks and discharge currents. The coating layer or layers provided on the plate make it possible to protect the electron multiplication device and the associated electronics from deterioration by sparks, whether operating in the air or in any other gas; structural simplicity and low cost; application to the detection of several types of radiation / particles or other: a, UV, flames, radon, X-rays, etc. It goes without saying that the invention is not limited to the embodiments described above as examples but that it encompasses all the variants.

Claims (31)

  An electron multiplying device operable in air for use in an ionizing radiation detection system, comprising a plate (2) of dielectric material having first and second opposite major faces (3, 4) and in which are formed a plurality of orifices (5) each having an axis (6) substantially perpendicular to said main faces (3, 4) of the plate, characterized in that it further comprises, at least on the first main face (3), a coating comprising at least one layer (9, 9a, 9b, 12, 12a, 12b, 30) and having holes (10, 31) arranged in correspondence with the orifices (5) formed in the plate ( 2), said coating having characteristics of initial resistivity sufficiently low to allow, in operation, the creation of an electric field of large amplitude in the orifices (5) of the plate (2) and consequently e multiplication of electrons by avalanches, and sufficiently large to substantially prevent, in operation, the formation of secondary avalanches.   2. Multiplication device according to claim 1, characterized in that it comprises, on the first main face (3), an overall layer (30) of coating made of a material whose resistivity is between 10 'and 1011 Qm , having holes (31) arranged in correspondence of the orifices (5) formed in the plate (2).   3. Multiplication device according to claim 1, characterized in that it comprises, on the first main face (3): a first layer (9, 9a, 9b) of coating, made of a first material whose resistivity is between 105 and 108 μm, the first layer having holes (10) arranged in correspondence of the orifices (5) formed in the plate (2); and - a second coating layer (12, 12a, 12b) disposed on the first layer (9, 9a, 9b), made of a second material different from the first one, whose resistivity is between 108 and 1012 am and is at least 10 times greater than the resistivity of the first material, the second layer having holes (13) arranged in correspondence of the orifices (5) formed in the plate (2).   4. Multiplication device according to one of claims 1 to 3, characterized in that the second main face (4) is devoid of coating.   5. Multiplication device according to one of claims 1 to 3, characterized in that it comprises, on the second main face (4), a first layer (9, 9b) coating, made of a first material whose resistivity is between 105 and 108 am, the first layer having holes (10) arranged in correspondence of the orifices (5) formed in the plate (2).   6. Multiplication device according to claim 5, characterized in that it further comprises, on the second main face (4), a second layer (12, 12b) of coating disposed on the first layer (9, 9b), made of a second material different from the first, whose resistivity is between 108 and 1012 S2.m and is at least 10 times greater than the resistivity of the first material, the second layer having holes (13) arranged in correspondence with the orifices (5) formed in the plate (2).   7. Multiplication device according to one of claims 1 to 3, characterized in that it comprises, on the second main face (4), an overall layer (30) of coating made of a material whose resistivity is between 10 'and 1011 .mu.m, having holes (31) arranged in correspondence of the orifices (5) formed in the plate (2).   8. Multiplication device according to one of claims 3 to 7, characterized in that the resistivity of the first material is between 106 and 10 'S2.m and / or in that the resistivity of the second material is between 108 and 1010 S2.m.   9. Multiplication device according to one of claims 3 to 8, characterized in that the first material is gallium arsenide.   10. Multiplication device according to one of claims 3 to 9, characterized in that the second material is a material with electronic conductivity, such as some glasses.   11. Multiplication device according to one of claims 3 to 10, characterized in that the plate (2) is made of a material of the epoxy resin or glass type.   12. Multiplication device according to one of claims 1 to 11, characterized in that the orifices (5) formed in the plate (2) have a central portion (7) substantially cylindrical extended at each of its two axial ends by a frustoconical portion (8) of the same axis (6) flaring towards the main face (3, 4) of the plate (2) in which it opens.   13. Multiplication device according to one of claims 3 to 12, characterized in that the holes (10) of the first layer (9) are centered on the axis (6) of the orifices (5) of the plate (2). ) and have dimensions greater than those of the orifices (5) of the plate (2) at the principal face (s) (3, 4) of said plate, and in that the holes (13) of the second layer ( 12) are centered on the axis (6) of the orifices (5) of the plate (2) and have dimensions substantially equal to those of the orifices (5) of the plate (2) at the level of the main face or faces ( 3, 4) of said plate, so that the second layer (12) is disposed directly against the plate (2) in the peripheral zone (11) to the orifices (5).   14. Multiplication device according to one of claims 1 to 13, characterized in that it has a thickness between 0.5 and 4 mm.   15. Multiplication device according to one of claims 1 to 14, characterized in that the orifices (5) formed in the plate (2) have a mean diameter of between 0.3 and 1 mm.   16. Multiplication device according to one of claims 1 to 15, characterized in that the orifices (5) formed in the plate (2) are arranged substantially at the corners of a side grid between 0.5 and 1, 5 mm.   17. Multiplication device according to one of claims 1 to 5 16, characterized in that it is substantially plane.   18. A system for detecting ionizing radiation or ionizable species, comprising: a first electrode (16), called a drift electrode, and a second electrode (17), said to be substantially parallel, and between which can be generated a an electric field (E) capable of driving the electrons (20) created by said radiation (19, 23) toward the collection electrode (17), the collection electrode (17) being capable of supplying data representing the number of electrons that reach it; Data processing means (18) provided by the collection electrode (17); characterized in that it comprises at least one device (1) for multiplying electrons according to one of claims 1 to 17, disposed substantially parallel to the electrodes (16, 17) so that the plate (2) 20 of the devices (1) is located between the two electrodes (16, 17) and that the first face (3) of said plate (2) is facing the first electrode (16), the coating (30) - or the coating layer (9a) having the lowest resistivity û disposed on the first main face (3) of the plate (2) being carried, in operation, to a potential (V2) between the potentials (V ,, V3, V4) to which the drift electrode (16) and the collection electrode (17) are respectively carried in such a way as to cause the multiplication of the avalanche electrons (21) in the orifices (5) of the plate (2).   19. The detection system according to claim 18, when dependent on one of claims 5 to 7, characterized in that the collection electrode (17) is constituted by the overall coating layer (30) the first coating layer (9b) disposed on the second main face (4) of the plate (2).   20. Detection system according to claim 18, characterized in that the collection electrode (17) is distinct from the multiplication device (1).   21. Detection system according to claim 20, when dependent on one of claims 5 to 7, characterized in that the overall coating layer (30) or the first coating layer (9b) disposed on the second main face (4) of the plate (2) is carried, in operation, to a potential (V3) between the potentials (V2, V4) to which the overall coating layer (30) - or the first coating layer (9a) arranged on the first main face (3) of the plate (2), and the collecting electrode (17).   22. Detection system according to one of claims 18 to 21, characterized in that it comprises two devices (1) for substantially identical electron multiplication, arranged substantially parallel to the electrodes (16, 17) so that the plates ( 2) are located between the two electrodes (16, 17), and that the first main face (3) of each plate (2) is facing the first electrode (16), the overall coating layers (30) or the first successive coating layers (9, 9a, 9b) being carried, in operation, to increasing potentials when approaching the collection electrode (17).   23. Detection system according to one of claims 18 to 21, characterized in that it further comprises a device for multiplying electrons of the type comprising firstly a resistive gate forming a cathode and secondly an anode the cathode and the anode being substantially flat, parallel and kept at a distance of less than 500 μm by means of micro-pillars.   24. Detection system according to one of claims 18 to 23, characterized in that the collection electrode (17) has separate localized collection areas (29).   25. Detection system according to one of claims 18 to 24, characterized in that the electrodes (16, 17) and the electron multiplication device (1) are arranged in a housing (15) having a window (24). ) for the passage of the UV (23) of the sun constituting the ionizing radiation to be detected, the pane (24) being disposed facing the drift electrode (16), comprising a photosensitive material such as cesium iodide, so that the UVs can create photoelectrons from the drift electrode (16).   26. Detection system according to one of claims 18 to 25, characterized in that the electrodes (16, 17) and the electron multiplication device (1) are arranged in a housing (15) having a zone of passage (25) for the entry of ionizing radiation and containing a gas with a low ionization potential, such as an ethylferrocene vapor.   27. Detection system according to claim 26, characterized in that it further comprises a lens (27) placed in the path of the ionizing radiation towards the inside of the housing (15), the axis (32) of the lens (27) being substantially parallel to the or each electron multiplying device (1) and located at a distance (d2) from the nearest electron multiplication device (1) to the drift electrode (16) less than 3 mm.   28. Detection system according to claims 24 and 26, characterized in that it further comprises a lens (27) placed in the path of the ionizing radiation towards the interior of the housing (15) and in that the collection zones localized are bands (29) arranged in a fan and converging towards the center of the lens (27), each band being connected to a separate reading device.   29. Detection system according to one of claims 18 to 24, characterized in that the electrodes (16, 17) and the electron multiplication device (1) are placed in a housing (15) having a zone of passage (25) for the input of UV or X rays emitted by a source (36) arranged so that said rays penetrate into the housing (15) substantially parallel to the or each electron multiplication device (1) and at a distance (d2) of the electron-multiplying device (1) closest to the drift electrode (16) less than 3 mm, and in that the housing (15) has an inlet opening (34) for a gas to analyze.   30. Use of a detection system according to one of claims 26 to 28 for the detection of a flame.   31. Use of a detection system according to claim 29 for the detection of the presence in the air of gas with low ionization potential, such as certain alcohols or hydrocarbons, toluene, benzene.