EP0116806B1 - Curved electronic avalanche gaseous detector with strip-shaped electrode - Google Patents

Description

The present invention relates to the technical field of gas detectors used for the spatial localization of particles or radiation and which is defined in the preamble of claim 1.

In many applications, it is necessary to be able to detect and spatially locate a particle or a radiation. By way of examples, mention may be made of X-ray crystallography, detection of radioactivity, medical or biological research, detection of particles around the accelerators.

Generally, a gas detector, of the above type, comprises a body defining an enclosure containing a gaseous fluid under a certain pressure.

The enclosure has an entry window for radiation or a particle to be detected and comprises, internally, at least one elongated element, generally parallel to the window. This elongate element is isolated from the body and is brought to a high positive potential with respect to the body or to electrodes surrounding the elongate element, forming cathodes.

The impact of an elementary particle, having crossed the entry window with one or more atoms of the gaseous fluid, gives rise to one or more primary electrons which are attracted by the electric field produced by the positive potential applied to the elongated element forming anode. These electrons, under the influence of this field, migrate towards the anode and initiate, if the electric field is sufficient, a process of chain collisions producing an avalanche of electrons captured by the anode. The localization along the avalanche anode is carried out according to a well-known procedure, by determining the center of gravity by means of cathode strips measuring the collection of positive charges induced by the avalanche in the gaseous fluid. By means of, for example, a delay line, it is possible to locate such a center of gravity and, consequently, to know the position of the avalanche along the anode. One thus obtains a one-dimensional localization along the anode. The accuracy of the localization and the spatial resolution depend on the quality of the electronic measurement chain, on the nature and the pressure of the gas, on the nature and on the energy of the particle or of the radiation. Commonly obtained a resolution of 200 m _it to X-ray 8 keV.

In certain applications, the detector comprises several parallel anodes, which makes it possible to have a substantially increased detection area and to make a determination of the position in two dimensions.

Detectors of the above type may be qualified as rectilinear anodes, since the anode or anodes which they comprise consist of conductive wires of small diameter stretched between two anchoring and electrical connection points to extend parallel at the cathodes and the entrance window.

In certain applications, such as the study of X-ray diffraction, it would be interesting to be able to locate along an arc of a circle.

If one is satisfied with spatial resolutions of approximately 1 to 2 mm, one can use a sheet of anode son following the circumference. Reading the pulses on these wires giving the position of the radiation to the nearest wire.

With detectors with rectilinear anodes and for spatial resolutions less than a millimeter, the angular opening that can be examined does not exceed ten degrees. In fact, beyond this, account should be taken of parallax phenomena originating from the angle of incidence of the trajectory of the particles relative to the anode and also resulting from the position on this trajectory from which such a particle initiates the electron avalanche phenomenon.

- Parallax correction tests did not lead to a simple technological solution, by the simple fact that the phenomenon of electron avalanche initiation can be considered as completely random and likely to occur indifferently in upstream or downstream of the anode relative to the plane passing the latter and intersecting the direction of propagation of the particle.

In order to solve this problem, one could think of producing gas detectors of the curved type comprising a body delimiting, on a concave face, a window whose radius of curvature is centered on the emission or reflection source. The anode or anodes are each formed in the traditional way by a wire which is kept curved, being centered on the radius of curvature, by rigid insulating supports.

Such a solution is, however, not acceptable, because the supports are responsible for the existence of zones which can be considered as dead, that is to say in which the phenomenon of avalanche of electrons cannot occur as it suits.

To solve this problem, it has been proposed to produce the anode in the form of a conductive wire with a section of the order of 40 p initially bent or curved according to the radius of curvature chosen and over the convergent angular range. Such an anode is fixed at both ends on supports and is held parallel to the entry window by interaction of the field of a current passing through it with that of two permanent magnets between which the wire extends.

A variant of this construction consists in maintaining the constituent wire of the anode in the required position by electrostatic effect.

These two proposals made it possible to carry out measurements at the laboratory or experimental level. On the other hand, it was not possible to retain a satisfactory industrial application given the structural fragility of such devices and their sensitivity to vibrations applied to the support or to the device and transmitted to the anode wire, only maintained in the open angular detection by magnetic or electrostatic effect.

A third solution known by the reference Nuclear Instruments and Methods, 177 (1980), North Holland Publishing Company, 405-409, T. Izumi, Curved position-sensitive detector for X-ray crystallography, consists in producing a curved gas detector using , as anode, a conductive wire of larger cross section made of hard steel, for example 0.20 mm in diameter, replacing the wire of smaller cross section used in the previous solutions.

A wire of such a section can be bent and maintained, thanks to its mechanical qualities, by anchoring at the two ends representing points of support and conduction of an electrical operating voltage.

If such a technical solution can be considered as providing, theoretically, a solution to the problem posed, on the other hand, it has been practically observed that the radius of curvature and the length of anode were limited and therefore that the angular resolution could often be insufficient. .

This is due to the fact that the anode becomes, as its length increases, less stable and the detector more and more fragile. As before, this solution does not protect against mechanical vibrations.

The object of the invention is to propose a new curved gaseous detector providing a technological solution to the problems thus posed and capable of remedying the drawbacks observed of the solutions adopted for the constitution of curved detectors with good spatial resolution currently known.

The other object of the invention is to provide a slightly fragile curved gas detector, which can be subjected to various mechanical working conditions and which is also capable of having significant resistance to electrical breakdowns and the dimensions of which are not limited. by mechanical problems.

Another object of the invention is to propose a curved gas detector capable of being produced quickly, simply and safely, without involving a delicate operation of regular shaping of one or more anodes.

The object of the invention aims, moreover, to allow the use, as an anode, of a basic product supplied in a strip or blade commercially, according to various physical characteristics allowing a choice in relation to the particularities of a detector to be built.

To achieve the above goals, the curved gas detector with spatial localization blade is characterized in that it comprises an electron avalanche anode constituted by a structure of at least one conductive, curved blade, maintained by the detector body and having a curved longitudinal edge extending parallel to the longitudinal median axis of the window.

Various other characteristics will emerge from the description given below with reference to the appended drawings which show, by way of nonlimiting examples, embodiments of the subject of the invention.

Fig. 1 is a partial perspective illustrating the curved gas detector according to the invention.

Fig. 2 is a schematic view of a section showing the arrangement of the various electrodes.

Fig. 3 is a partial perspective illustrating another embodiment of one of the constituent elements of the object of the invention.

Fig. 4 is a perspective similar to FIG. 3 but showing another embodiment of the same constituent element.

Figs. 5 to 8 are schematic views showing different alternative embodiments of one of the constituent elements of the detector.

Fig. 9 is a schematic view showing another embodiment of the detector.

The curved gas detector, of spatial location, comprises a body 1 of generally tubular shape, delimiting an enclosure 2 intended to contain a gaseous fluid under a pressure to be chosen.

The body 1 is produced in a curved manner and therefore has a concave face 3 defined by a radius of curvature which is centered on the source of emission or reflection of a radiation to be detected. The concave face 3 defines an inlet window 4 which is, for example, closed by a cover 5 to preserve the sealed confinement of the gaseous fluid. The cover 5 is made of an appropriate material, permeable to the radiation to be detected and, for example of mylar or beryllium in the case of application to X-ray crystallography.

Another of the walls of the tubular body 1 and, preferably, the flat bottom 6, supports, directly or indirectly, an elongated element 7 electrically isolated from the body 1 and intended to constitute the electron avalanche anode. According to the invention, the element 7 is formed by a conductive strip which is held so that one of its longitudinal edges, such as 8, extends parallel to the window 4, made conductive by an internal deposit and forming with the conductive element 14 a cathode.

The conductive strip 7 is held to have a radius of curvature centered on the same center as that of the wall 3 and, for this purpose, for example, is embedded by the second longitudinal edge 9 'in an insulating support formed by or adapted on the body 1. The metal blade 7 is electrically connected to a production source, capable of applying to it a constant positive potential. Under tension, the edge 8 produces an electric field influencing the surrounding medium and the gaseous fluid confined in the enclosure 2.

Such a construction provides an absolute certainty of the position occupied by the edge 8 and of its conformation in a curved anode, exactly centered on the center of the wall 3, so that all the points of this edge are exactly equidistant from such a center. This construction makes it possible to maintain, in a rigid stable state, an elongated anode by giving it a determined radius of curvature and by developing it over an angular extent related to the possible dispersion characteristics of the emitted or reflected radiation. More generally, such a construction makes it possible to conform the edge 8 along any curve desired for detection.

In order to be able to locate the location of the edge 8 at which the avalanche of electrons resulting from the impact of an elementary particle of the radiation to be detected with the gaseous fluid occurs, the curved gaseous detector is associated with a bar 10 for measuring the collection of positive charges induced by the presence of positive ions resulting from the avalanche of electrons.

The strip 10 consists of cathode strips 11, conductive, extending parallel to each other, having a direction orthogonal to the edge 8. The cathode strips are placed parallel to the plane of the blade 7, for example, along the internal face of the wall 12 of the body 1 opposite the wall 3. The cathode strips 11 pass through the body 1 outside of which they are connected to a delay line 13 of design known in the art.

According to fig. 1, the body 1 is made of an insulating material and internally comprises a conductive coating 14 forming a cathode, in the general sense, isolated from the strips 11.

Fig. 2 shows, diagrammatically, an exemplary embodiment according to which the body 1 is made of conductive material and supports the strip 7 by an attached wall element 15, made of an insulating material. In this example, the body of conductive material is connected to earth by a connection 16. In such a case, the bar 10 is mounted without contact or electrical connection with the body 1.

In all cases, the body 1 has means making it possible to keep the enclosure 2 filled with the desired gas mixture.

According to a preferred embodiment, illustrated in FIG. 3, the blade 7 is held in the body 1 by an intermediate support 17 which is preferably made up of two complementary half-parts 18a and 18b. The half-parts 18a and 18b can be connected together by means of connecting members 19 of any suitable type. The half-parts 18a and 18b are made of an insulating material and shaped to delimit between them, once assembled, a recess 20 capable of retaining the blade 7 from its longitudinal edge 9.

The complementary half-parts 18a and 18b are shaped to present, once assembled, a curved shape centered on the center of curvature of the wall 3. Thus, by such a support 17, it becomes possible to effectively maintain the blade 7 in a stable position and, simultaneously, to impose on such a blade the desired curvature. It is thus possible to use to form the blade 7 a conductive strip of suitable thickness, elastically or plastically deformable, and which is thus maintained in a deformed state by its embedding between the complementary parts 18a and 18b. In such a case, one end of the support 17 comprises a conductive terminal 21 making it possible to establish an electrical contact between the blade 7 and a conductor 22 connecting said blade to a source of positive voltage with respect to the potential of the cathodes (which is generally to ground).

Fig. 4 illustrates an alternative embodiment in which the support 17 is produced so as to delimit itself a window 23 in which extends the edge 8 of the blade 7 maintained as said previously with reference to FIG. 3.

Good detection results are obtained using a blade 7 made of stainless steel, the thickness of which can be between 10 and 100 p.

A detector of the above type, containing in enclosure 2 a gaseous fluid constituted by a mixture of argon, methane, forane 13B1 confined under a pressure of a bar, made it possible to obtain localization results in proportional regime, by means of a 40 p thick blade to which a positive voltage of 3,700 volts was applied, for an X radiation of 8 KeV of energy.

Particularly satisfactory results have been obtained in operating mode, called self-cutting luminous wakes, by implementing the following means:

With conditions such as above, a half-height spatial resolution of 180 u was obtained, that is, in this experiment, an angular resolution of 0.05 °.

The above results were obtained by using a body 1 in stesalit with an aluminum front face to stiffen the assembly.

In these embodiments, the conductive strip had a linear length of 25 cm and was shaped according to a radius of curvature of 20 cm.

The blade 7 described above may include an active edge 8 shaped in different ways. This edge 8 can be tapered (fig. 5), with sharp edges (fig. 6) or rounded.

The edge 8 can also be constituted by a wire 8 ₁ reported in any suitable manner, in particular by gluing on a blade 7 ₁ , as illustrated in FIG. 7.

It may also be retained to constitute the edge 8 by shaping a blade 7 ₂ around a wire 8 ₂ , as illustrated in FIG. 8.

• - pour la lame 7, le support de l'arête 8 et le maintien de la courbure désirée sans perturber outre mesure le champ électrique,
• - pour l'arête 8, le lieu où le champ électrique est très intense et provoque le phénomène d'avalanche.

The above examples are only given non-limiting title because other geometries can be used to make the curved anode assume the two functions of the invention, namely:

• - for the blade 7, the support of the edge 8 and the maintenance of the desired curvature without unduly disturbing the electric field,
• - for edge 8, the place where the electric field is very intense and causes the avalanche phenomenon.

In the foregoing, it is indicated that the object of the invention allows one-dimensional position detection.

It is possible to produce a curved detector for two-dimensional detection by adopting a structure such as that shown diagrammatically in FIG. 9. According to this figure, the detector comprises, inside the sealed enclosure an insulating support 24 now n curved blades 7a, for example by embedding. The blades 7a are parallel to each other and directed so that their plane is parallel or substantially parallel to the direction of propagation of a particle or of radiation. Each blade 7a has, facing the direction of propagation, a generally concave curved edge 8a.

The determination of the blade 7a which has received the avalanche provides, by processing the negative electronic pulse which is triggered there, the location in the X dimension.

The location in dimension Y is obtained, as in the previous example, by implementing a structure 25 of cathode strips 1 la extending parallel to the edges 8a in a direction orthogonal to that of the blades 7a. The cathode strips 11a are, for example, carried by a thin insulating support 26 and are connected to a delay line 13a. The structure is arranged upstream of the edges 8a with respect to the direction of propagation along the arrow f. The invention is not limited to the examples described and shown since various modifications can be made without departing from its scope.

Claims (9)

1. Curved gas-filled detector of the type comprising a body (1) defining an enclosure (2) having the shape of an annular segment, containing a gaseous fluid, and having a concave cylindrical wall (3) in which is provided a window (4) for the admission of the radiation to be detected and the outline of which has an axis contained in a cross- sectional plane of the cylindrical wall, said enclosure containing at least one elongated element (7) which is parallel to the plane occupied by at least part of a cathode electrode (14) insulated from the body, and receiving a high positive voltage, in order to form means of picking up the avalanche of electrons created by the impact of a particle or radiation caused to traverse the gaseous fluid and of which the source of emission or the means reflecting it are situated at right angle to the window on the axis of the concave cylindrical wall (3) said enclosure containing a plurality of conducting cathode bands (11) which are all parallel and oriented orthogonally to the elongated element (7) characterised in that the electron avalanche anode (7) is constituted by a structure of at least one curved conducting strip held by the body (1) and presenting a curved longitudinal edge (8) which extends in parallel to the longitudinal middle axis of the window.

2. Gas-filled detector according to claim 1, characterised in that it comprises at least a curved strip of which the generatrix is perpendicular to the plane containing the strip curving direction.

3. Gas-filled detector according to claim 1, characterised in that it comprises at least one curved strip (7a) of which the curving direction is co-planar thereto and which is carried by the body so that its longitudinal edge (8a) extends in parallel to the inner face of the window.

4. Gas-filled detector according to claim 1, 2 or 3, characterised in that the thickness of the conducting strip (7) may vary between 10 and 100u.

5. Gas-filled detector according to any one of claims 1 to 4, characterised in that the strip (7) is supported by a body (1) in insulating material covered on the inside with a conducting layer (14) insulated from the cathode bands (11

6. Gas-filled detector according to any one of claims 1 to 4, characterised in that the strip (7) is supported by a support (15) in insulating material adapted inside a body in conducting material.

7. Gas-filled detector according to claim 1 or 2, characterised in that the conducting strip (7) extends between the wall (3) having the window (4), and a plurality of conducting cathode bands (11) connected to a delay line (13).

8. Gas-filled detector according to claim 7, characterised in that the strip (7) is supported by a support (17) composed of two complementary halves (18-18a) which immobilize the strip and give it the wanted curvature.

9. Gas-filled detector according to claim 3, characterised in that it comprises a plurality of strips (7a) parallel together and to cathode structure (25) situated upstream of the strips with respect to the direction of propagation.

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