Ionizing radiation detector and method for manufacturing such a detector

An ionizing radiation detector comprising a plurality of conductive tubes arranged in parallel fashion containing a gas mixture under pressure, a conductive wire being tensed at the center of each tube and adapted to being polarized with respect thereto, and comprising first and second tight enclosures each having a wall provided with openings in which are tightly inserted the first and second ends of each tube, the ends of each tube being open.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of particle or ionizing radiation detectors, and in particular detectors of neutrons, γ- or X-rays.

2. Discussion of the Related Art

FIG. 1schematically shows the conventional structure of a cell2sensitive to an ionizing radiation, using the same detection principle as the present invention. This cell comprises a conductive tube4filled with a gas mixture, sealed at its ends by isolating plugs6. A conductive wire8, the ends of which tightly cross plugs6, is maintained tensed at the center of tube4by a spring10located within the tube. A positive electric voltage applied to wire8by means of a measurement circuit12enables defining within the tube an electric field which is favorable to the drifting and to the amplification of electrons generated at the passing of the ionizing radiation, which enters the tube in a direction approximately orthogonal to the axis of the tube. A resistive wire is used in a case where a position measurement along the tube is desired to be performed by charge division. The measurement circuit then comprises read electronics enabling measurement of the charge signal amplitude at each end of the wire. Another so-called “counting” operating mode uses electronics based on the comparison, with respect to a reference voltage, of the signal measured at a single end of the wire. The gas mixture contained in the tube is provided to be ionized by the particles which are desired to be detected, either directly, or after conversion into ionizing particles. For example, a mixture of CF4and He3in which He3plays the role of a converter, and CF4that of a stopping gas of the two ionizing particles (proton and triton) emitted after capture of a neutron by an He3atom, is used in the case of neutron detection

The dimensions of tube4and the pressure at which the gas mixture is confined are very variable. As an example, tube4may have a width of approximately one meter, a diameter of approximately 8 mm and a thickness of approximately 0.2 mm, and the gas mixture may be confined in the tube at a pressure of approximately 15 bars. The forming of such a cell, which implies a perfectly tight welding of plugs6under a high pressure, after positioning of the wire, is particularly expensive. It is possible to provide individual filling means for each cell, but this creates an undesirable additional mechanical bulk.

Distance δ existing between the internal wall of tube4and spring10conditions the maximum electric voltage or breakdown voltage that can be applied between the electrodes and the tube. The larger the diameter of spring10with respect to the diameter of tube4, the lower the breakdown voltage, at which electric arcs form between the spring and the tube wall. Further, the uniformity of the cell response is affected by the inaccuracy of the wire centering inside of the tube, and such a wire centering is difficult to perform by means of spring10. In practice, the presence of spring10in the tube and the difficulty of the centering of wire8by means of spring10limit the maximum amplification gain with which the detector can operate, which has direct consequences upon the detector performances (energy and position resolution).

An ionizing radiation detector is conventionally formed of several cells2, the tubes of which are juxtaposed and form a sensitive surface. The operation of a cell depends on the quality and on the pressure of the gas mixture that it contains. Now, it is difficult to form several sensitive cells comprising a same gas mixture with a long-term stability and identical for all cells. As a result, no sensitive cell really has an operation identical to the others.

The assembly of several cells requires an accurate mechanism. Further, when several sensitive cells must be used together with a minimum space between the tubes, it is difficult to ensure the continuity of the electromagnetic shielding between the tube envelope and measurement circuit12without extending beyond the external diameter of the tube, which results in creating dead spaces between cells, whereby a loss of sensitivity of the assembly. This constraint, and those imposed by inner spring10, limit the minimum diameter of the tubes to approximately 7-8 mm. Further still, a sensitive cell may wear out and need changing, for example, if the gas mixture that it contains has been altered under the influence of the received radiation. Especially, it is known that a gas mixture of butane and argon contained in the sensitive cells used for the X-ray detection may form polymers around the wires under the effect of the radiation and alter the operation of the sensitive cell. The replacing of a cell is expensive.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an assembly which is simple and inexpensive to form of cells sensitive to ionizing radiation.

Another object of the present invention is to provide such an assembly which has a low maintenance cost.

Another object of the present invention is to provide such an assembly formed of sensitive cells having a homogenous operation.

Another object of the present invention is to provide such an assembly comprising tubular sensitive cells of small diameter standing a high amplification gain.

To achieve this object, the present invention provides an ionizing radiation detector comprising a plurality of conductive tubes arranged in parallel fashion containing a gas mixture under pressure, a conductive wire being tensed at the center of each tube and adapted to being polarized with respect thereto, and comprising first and second tight enclosures each having a wall provided with openings in which are tightly inserted the first and second ends of each tube, the ends of each tube being open.

According to an embodiment of the present invention, a leaky conductive wire centering means is assembled at each end of each tube.

According to an embodiment of the present invention, the wire is maintained tensed at least one end of each tube by means of a tension means arranged outside of the tube.

According to an embodiment of the present invention, at said at least one end of each tube, the centering means comprises a cap in an isolating material attached to the tube and provided with an axial bore capable of guiding the wire.

According to an embodiment of the present invention, the cap of isolating material is crossed along the revolution axis of the tube by a first cylindrical opening in which is slidably mounted a socket imprisoning the end of the wire, the tension means bearing on the cap of isolating material and urging the socket towards the outside of the tube, a second opening crossing the cap in isolating material between the inside of the tube and of the tight enclosure to which the tube is attached.

According to an embodiment of the present invention, the tube ends have a predetermined diameter lower than the diameter of the tube bulk, the openings of the walls in which are inserted the ends of two adjacent tubes being distant by a space equal to the difference existing between the diameter of the end of the tubes and the diameter of the tube bulk.

The present invention also aims at a method for manufacturing an ionizing radiation detector comprising the steps of: inserting the first and second ends of a plurality of conductive tubes into openings made in a wall of a first and of a second tight enclosures so that the tubes are arranged in parallel fashion; attaching simultaneously or one after the other by welding each end of each tube in the opening of which said end is inserted, so that the inside of the tubes and the inside of the tight enclosures are tightly connected; and filling the tight enclosures and the tubes with a predetermined gas mixture at a predetermined pressure.

DETAILED DESCRIPTION

FIG. 2schematically shows a detector14according to the present invention, comprising a sensitive surface formed of a juxtaposition of tubular sensitive cells16. Each sensitive cell16comprises a conductive tube18, a first end of which crosses a metallic wall19of a tight enclosure20and the second end of which crosses a wall21of a tight enclosure22. The ends of tubes18are welded to walls19and21of enclosures20and22so that the tubes18and the enclosures20and22can be filled together with a single gas mixture under pressure. The ends of tubes18have a diameter smaller than the diameter of the tube bulk. The openings of walls19and21in which are inserted the ends of two adjacent tubes are distant by an interval equal to the difference between the diameter of the ends of the tubes and the tube bulk diameter. This interval between two adjacent openings enables easy welding of the tube ends to walls19and21. Enclosures20and22, formed in a conductive material, are joined together by bracings24which ensure the rigidity of the assembly while forming no screen between the radiations to be detected and the tubes. Each sensitive cell16comprises a conductive wire26, which is resistive in the case of a longitudinal localization version, maintained tensed at the center of tube18by caps28and29respectively arranged at the ends of tube18in enclosures20and22. Caps28and29are further provided to ensure the communication between enclosures20and22and tubes18. One at least of enclosures20and22is connected to means not shown enabling creating vacuum and bringing the gas mixture to the desired pressure. The ends of conductive wires26are connected to tight electric seal wires30arranged in the walls of enclosures20and22. These seal wires are connected to a measurement circuit12via appropriate connectors.

According to the present invention, the manufacturing of the detector is particularly simple. In a first step, tubes18may be assembled with no welding to walls19and21, for example, by mere insertion into openings made for this purpose in the walls. In a second step, the tubes may all be welded to walls19and21one after the other or at once in a furnace. An alternative of the present invention also provides welding together the adjacent tubes, to rigidify the tube assembly. The simultaneous welding of all the tubes of a detector according to the present invention represents a particularly advantageous time gain and saving. In a third step, walls19and21are assembled to other elements to define enclosures20and22. The inside of the assembly is degassed, after which the desired gas mixture is introduced into enclosures20and22and into tubes18.

Advantageously, the gas mixture contained in a detector according to the present invention may easily be changed. A same detector filled with different gas mixtures may thus be used for the detection of several types of ionizing radiation.

Also advantageously, a wall of each enclosure is removable to enable easy access to the wires of the sensitive cells, and thereby easy and inexpensive replacement of a defective or damaged wire.

Advantageously, a tube assembly according to the present invention forms a single mechanical block, which suppresses assembly problems which used to be posed with individual tubes according to prior art.

FIG. 3shows an end of a tube18attached to an opening of wall19. Wire26is maintained tensed at the center of tube18by a cap of isolating material28attached to the end of tube18. Cap28is crossed along the revolution axis of the tube by a cylindrical opening34in which is slidably assembled a crimp socket36. The end of wire26is crimped in socket36. A spring38bears on cap28and urges socket36to the outside of the tube to maintain wire26tensed at the center of the tube. An opening40crosses cap28to have the gas mixture contained in the tube and in enclosure20or22communicate. Cap29attached to the end of tube18attached to wall21has a structure identical to that ofFIG. 3, but comprises no spring38. Socket36directly bears against cap29.

The centering and tension holding structure of wire26, comprising caps28and29, sockets36and spring38, does not aim at ensuring any tightness of tube18. As a result, the forming of such a structure is particularly simple and enables maintaining each wire26tensed precisely at the center of the ends of tube18of each sensitive cell. It is thus possible to form sensitive cells formed of tubes18of small diameter and having a high amplification gain. The structure comprising caps28and29, sockets36and spring38enabling formation of sensitive cells all having the same geometry, and the sensitive cells all containing a same gas mixture at a same pressure, the sensitive cells exhibit a high and perfectly uniform amplification gain.

FIG. 4very schematically shows a top view of tubes18of detector14of FIG.2. Tubes18, which join, are arranged in a plane so that the sensitive surface of the detector is planar. In practice, a detector according to the present invention may comprise a large number of tubes.

Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the present invention has been described in relation with a detector, the sensitive surface of which is formed of sensitive cells arranged in a plane, but those skilled in the art will easily adapt the present invention to a detector, the sensitive cells of which are arranged differently.

FIG. 5shows as an example a cross-sectional top view of the tubes18of a detector according to an alternative embodiment of the present invention. Tubes18are arranged in parallel fashion, without joining, in quincunx along two parallel planes. Such a tube arrangement especially enables improving the detection efficiency. Since tubes18do not join, the diameter of tubes18can be constant along their entire length.

FIG. 6shows a cross-section view of tubes18of a detector according to another alternative embodiment of the present invention. Tubes18join and are arranged to form a substantially curved surface, for example, in an arc of a circle.

The present invention has been described in relation with a detector comprising a group of tubes, the first and second ends of which are connected to first and second tight enclosures, the tight enclosures each comprising at least one tight electric seal wire30.

FIG. 7is a cross-section view of a tight enclosure50of a detector according to an alternative embodiment of the present invention. The detector comprises a group of tubes18, first ends of which are connected to a wall48of enclosure50. The second ends of tubes18, not shown, are attached to the wall of a tight enclosure such as enclosure20or22of FIG.2. In enclosure50, the ends of wires26located in adjacent tubes18are connected two by two, whereby enclosure50comprises no tight connector30. Such an alternative embodiment enables dividing by two the number of read paths of measurement circuit12, and decreasing the dead area generated by one of the two enclosures.

FIG. 8is a simplified cross-section view of a tube of a sensitive cell of an ionizing radiation detector according to an alternative embodiment of the present invention. A number of cathode conductive wires42are maintained tensed in parallel fashion around the central anode conductive wire26, closer to the anode wire than to the walls of the tube18. For example, for a tube with a diameter of about 2-3 cm, the cathode wires may be tensed at a distance of 2-3 mm from the anode wire.FIG. 8is not drawn to scale for clarity sake. Six cathode wires42are drawn inFIG. 8, but any appropriate number of cathode wires may be used. The caps of isolating material attached to the ends of each tube would then be crossed by cylindrical openings arranged along a circle around the central cylindrical opening to each receive slidably one of said cathode conductive wires, the end of which might be imprisoned by a socket, which would provide for an easy to build and easy to maintain structure.

In an embodiment, the cathode wires would be biased to a voltage intermediate between the voltage of the anode and the voltage of the tube. This would provide for a first electrical field called drift field between the walls of the tube and the cathode wires and for a second field called amplification field between the cathode wires and the anode wire. The drift and amplification fields may be optimized separately so as to reduce the collection time of the electrons generated in the tube by the radiations.

Moreover, the cathode wires may be connected independently or in sub-groups so as to give an angular information about where the electrons are generated.