EP0441853B1 - Method and device for the bimensional localization of neutral particles, particularly for low counting ratios - Google Patents

Abstract

PCT No. PCT/FR89/00553 Sec. 371 Date Apr. 26, 1991 Sec. 102(e) Date Apr. 26, 1991 PCT Filed Oct. 24, 1989 PCT Pub. No. WO90/04851 PCT Pub. Date May 3, 1990.The invention relates in particular to a detector for two-dimensional analysis of a flux of photons or neutral particles. The detector comprises a solid plane converter (5) constituted by a two-dimensional matrix of cells (such as 9c), an array of charge-multiplying wires (6) using stimulated ionization of a gas, and a cathode grid (7). The cells both convert the photons or neutral particles and collect the charge, and the ionization is preferably stimulated until self-regulating streams of electrons appear (streamer mode). The invention has application to imaging devices.

Description

• un convertisseur solide sensiblement plan, propre, sous l'impact de ces particules neutres, à produire des charges électriques, ce convertisseur comprenant des éléments de conversion électriquement autonomes les uns par rapport aux autres;
• des fils anodiques, destinés à être portés à un potentiel électrique différent de celui du convertisseur pour faire apparaître un champ électrique, et à produire une amplification des charges par ionisation d'un gaz environnant, stimulée par ce champ électrique;
• des moyens de collectage de charges, comprenant des éléments conducteurs électriquement autonomes les uns par rapport aux autres, dont certains au moins constituent des éléments de conversion; et,
• une enceinte perméable aux particules neutres, renfermant le convertisseur, les fils anodiques, les moyens de collectage des charges, et le gaz.

The present invention relates in particular to a device for detecting and locating particles in a stream of neutral particles emitted by a source, comprising:

• a solid converter substantially planar, clean, under the impact of these neutral particles, to produce electric charges, this converter comprising conversion elements electrically autonomous with respect to each other;
• anode wires, intended to be brought to an electric potential different from that of the converter to reveal an electric field, and to produce an amplification of the charges by ionization of a surrounding gas, stimulated by this electric field;
• charge collection means, comprising electrically autonomous conductive elements with respect to one another, at least some of which constitute conversion elements; and,
• an enclosure permeable to neutral particles, containing the converter, the anode wires, the charge collection means, and the gas.

The present invention further relates to a method for detecting and locating particles in a stream of neutral particles emitted by a source in such a device.

A device of this type is described in the European patent application published under the number Ep-A-0 000 271.

Although this prior device allows direct production of two-dimensional images, its design is based on the search for an optimization of the efficiency in certain particular applications to the detriment of an optimization of the resolution.

Another example of a device of the prior art appears in the document published in the journal Nuclear Instruments and Methods, Volume 156, n ° 1, October 1978 and entitled "A hybrid MWPC gamma ray detecting system for applications in nuclear medicine", pages 27-31, of JE Bateman et al. This document describes a stack of multi-wire proportional counters in which the cathodes consist of metal sheets converting the incident gamma rays into electrons and the anodes consist of layers of wires parallel to each other. Parallel grooves between them are formed on the surface of the cathodes, these grooves being alternately perpendicular to the direction of the anode wires.

The object of the present invention is, on the contrary, to allow the production of higher resolution images and to allow, on a secondary basis, the obtaining of a high contrast, even in a priori unfavorable conditions of irradiation of the object to be examined, and more particularly in the event of poor conversion rates and / or in the presence of fluxes of incident particles of low intensity.

To this end, the device of the invention is essentially characterized in that said conversion elements capable of also ensuring the collection of charges consist of cells distributed in a flat two-dimensional matrix arranged beyond the anode wires with respect to the source .

Thanks to this arrangement, the images obtained by the device of the invention have a higher resolution than that of the images obtained by the prior device described in patent EP-A-0 000 271, the two-dimensional matrix of which is formed by two parallel planes. linear elements also parallel to each other, the elements of two different planes being however crossed.

Indeed, this previous arrangement, due to the spacing of the two planes which together form the two-dimensional matrix, and the resulting parallax error, introduced for any radiation not perpendicular to the matrix, different deformations of the partial images recovered on the two planes, which results in a significant deterioration in resolution.

According to an advantageous embodiment of the invention, the converter comprises an insulating plate, one face of which carries said cells, this plate comprising, for each cell, a passage putting this cell in electrical contact with a conductor terminating on the other face of this plate.

According to a known arrangement, it is preferable for the invention that the anode wires are placed in at least one plane substantially parallel to that of the converter, and are substantially parallel to each other.

In view of the arrangement adopted in the invention, it is preferable, in order to obtain good efficiency, that the cells comprise a conversion material chosen from the group comprising gadolinium, boron, and lithium, in the case where the particles neutrals used are neutrons and the cells comprise a conversion material chosen from the group comprising iron, and silver in the case where the neutral particles used are X-rays, in particular soft X-rays.

In cases where, whatever the cause, the particle count rate is low, it is advantageous that the gas contains an extinguishing substance present in a proportion of at least 25 percent and that the ratio of the distance "S" between two neighboring anode wires, at the distance "G" between these wires and the converter, ie at least equal to 1.

These characteristics allow the device to operate according to a mode known to those skilled in the art under the Anglo-Saxon designation of "self-quenching streamer mode", characterized by the appearance of electronic avalanches stacking up to a critical size of the cloud of charges, for which they suffocate.

The particular advantages, which this otherwise known mode of operation develops in the specific application which the invention makes of it, will be better understood in the detailed description thereof.

Preferably, at least one of the charge collection elements is connected to a reference electrical potential by means of a capacity capable of accumulating the charges collected by this element.

Such an integration of the charges contributes to compensating for the negative effects associated with the flow of particles with a low counting rate.

Preferably, the device also comprises a source of voltage capable of creating an electrical voltage between the cathode and the anode wires at least equal to 2000 volts, and the anode wires have a minimum diameter greater than 20 microns.

• recevoir ces particules sur un convertisseur solide sensiblement plan, et produire ainsi des charges électriques à partir de ces particules neutres;
• amplifier ces charges par ionisation stimulée d'un gaz environnant; et
• collecter, sur le convertisseur, en différents emplacements espacés les uns des autres, les charges présentes dans au moins un plan sensiblement parallèle au convertisseur; ce procédé étant principalement caractérisé en ce que lesdits emplacements constituent une matrice bidimensionnelle plane.

The invention also relates to a method for detecting and locating particles in a stream of neutral particles emitted by a source in such a device, comprising the operations consisting in:

• receiving these particles on a substantially planar solid converter, and thus producing electric charges from these neutral particles;
• amplify these charges by stimulated ionization of a surrounding gas; and
• collecting, on the converter, at different locations spaced from each other, the charges present in at least one plane substantially parallel to the converter; this process being mainly characterized in that said locations constitute a planar two-dimensional matrix.

In the case of low counting rates, the charge amplification operation preferably comprises the application of an electric field of sufficient value to allow the appearance of electronic avalanches of self-regulated size.

The method can advantageously comprise another operation, consisting in accumulating for a certain time the collected electrical charges.

Preferably, the conversion of neutral particles into electrical charges is ensured with a conversion rate such that the number of particles detected is less than 10⁵ particles per second and per square centimeter of surface of the converter.

This process is particularly suitable, for reasons which will be detailed in the following description, for the use of thermal neutrons as neutral particles.

• la figure 1 est une vue schématique représentant, en perspective, la mise en oeuvre d'un dispositif conforme à l'invention;
• la figure 2 est une vue schématique éclatée d'un dispositif conforme à l'invention; et
• la figure 3 est une vue en coupe d'un dispositif de détection utilisable dans le système de la figure 1, faite suivant le plan III-III de la figure 2;

Other characteristics and advantages of the invention will emerge clearly from the description which is given below, for information and in no way limitative, with reference to the appended drawings, among which:

• Figure 1 is a schematic view showing, in perspective, the implementation of a device according to the invention;
• Figure 2 is an exploded schematic view of a device according to the invention; and
• Figure 3 is a sectional view of a detection device usable in the system of Figure 1, taken along the plane III-III of Figure 2;

FIG. 1 represents in 1 a source of neutral particles, for example a source of soft X-rays, but more typically a source of thermal neutrons in the main applications of the invention.

At least part of the flow of particles emitted by this source passes through an object to be examined 2 and reaches the device 3 to which the present invention relates more particularly.

As shown in more detail in FIG. 2, this device 3 firstly comprises an enclosure, intended to contain a gas and formed of a base 4a and a cover 4b made integral with each other so airtight vis-à-vis the atmosphere, the cover 4b being, however, permeable to neutral particles emitted by the source 1, to neutrons for example.

The gas contained in the enclosure is a gaseous mixture allowing the appearance of a "streamer" operating mode, in other words the appearance of avalanches of electrons whose size is self-regulated by spontaneous suffocation.

To this end, this gas comprises an effective quenching substance, consisting of carbonaceous and polyatomic molecules comprising many relaxation modes, such as isobutane or neopentane, in proportion of at least 25 percent.

For example, this gas can be a mixture of 50 percent carbon dioxide and 50 percent isobutane, subjected to a pressure of the order of one to five bars.

• un convertisseur solide sensiblement plan 5, propre, sous l'impact de ces particules neutres, à produire des charges électriques;
• un réseau plan 6 de fils conducteurs tels que 6a, 6b, disposé parallèlement au convertisseur et à distance de celui-ci;
• et, de préférence, une grille plane de fils conducteurs 7, elle-même disposée à distance du réseau 6.

Inside the enclosure, and parallel to the base 4a are arranged:

• a solid converter substantially plane 5, clean, under the impact of these neutral particles, to produce electric charges;
• a flat network 6 of conductive wires such as 6a, 6b, arranged parallel to the converter and at a distance therefrom;
• and preferably a flat grid of conductive wires 7, itself arranged at a distance from the network 6.

The converter 5 and the planar grid are connected to a reference electrical potential, and are for example brought to potentials close to the earth potential prevailing outside the enclosure, both playing the role. cathodes.

The wires of the planar network 6 are on the other hand connected to an external source of electrical potential which delivers a positive potential + V with respect to the average potentials of the converter 5 and of the grid 7, for example of the order of 2000 to 7000 volts.

The converter 5 comprises an insulating plate 8, better visible in FIG. 3, and a two-dimensional matrix of cells, such as 9a, 9b, 9c, arranged on one face of the plate 8.

Each of the cells, such as 9a, is intended to supply a signal representing a point of a two-dimensional image of the object 2.

Each of the cells therefore operates independently of its neighbors, and the image obtained consists of a matrix of points each of which corresponds to one of these cells.

The light intensity associated with a point of the image depends on the quantity of particles received by the corresponding cell, this quantity being itself dependent on the thickness and the nature of the material of which the object is made. solid angle defined by the source on the one hand and by the cell in question on the other.

The operation of the device is illustrated in Figure 3.

The wavy path T1 represents that of a neutral particle, a neutron for example, which, after having been emitted by the source 1 and passed through the object 2, the cover 4b of the detector, the grid 7 and the plane of wires 6, reaches a cell 9c of the converter 5.

Struck by this neutral particle, cell 9c, made of an adequate material, emits in a statistically observable and reproducible way a fast electron whose trajectory is represented in T2.

By crossing the enclosure, this fast electron causes the ionization of the gas on its path, and the electrons thus produced drift towards the nearest wire, for example 6c, of the network 6 under the effect of the electric field resulting from the difference between the potentials of the converter 5 and of the wire plane 6. This movement is identified by the arrows such as T3 in FIG. 3.

Having reached a few diameters of the wire 6c, these electrons are very violently accelerated by the electric field, the value of which increases considerably in the immediate vicinity of the wire.

They then acquire enough energy to ionize the gas in turn, thus causing electronic amplification.

According to a characteristic of the invention, this amplification corresponds to a mode of operation in which avalanches of electrons appear whose size is self-regulated.

This phenomenon, which results in a considerable increase in the electric charge, represented by the final number of electrons produced by each fast electron, continues until spontaneous suffocation of the avalanches.

The corresponding positive ions, representing a number of charges equal to that of the charges of all the electrons created, move away from the wire 6c from which they are repelled because of their charge, and drift towards the nearest cathodes, constituted by the grid 7 on the one hand and by the converter 5 on the other hand.

The positive ions created on the side of the grid are collected by the latter while those created on the side of the converter 5 are collected by a cell thereof, cell 9c in this case. Their movement is identified in Figure 3 by the dotted arrows T4.

As shown in FIG. 3, the converter 5 has a layered structure supported by an insulating plate 8, the latter being for example formed by a printed circuit board made of epoxy resin, with a thickness of 3.2 millimeters.

The upper surface of this plate is covered with a layer of copper 10, a few microns thick.

On the copper layer 10 is deposited a layer of conductive adhesive 11, with which the assembly can be covered with a layer of a conversion material 12, for example a sheet of gadolinium, with a thickness of tenth of a micron, previously gilded to avoid oxidation.

This stack of layers 10, 11, and 12, deposited on at least the major part of the surface of the plate 8, is then cut, by saw cutter lines such as 13 attacking the upper face of this plate, into elements. electrically isolated from each other, which constitute cells 9a, 9b, 9c, etc.

In addition, the insulating plate 8 comprises, for each cell such as 9c, a passage such as 14c putting this cell in electrical contact with a conductor such as 15c ending on the other face of the plate 8.

Preferably, each wire such as 6c is stretched exactly over a row of cells such as 9c, these advantageously having a rectangular or square shape.

By way of example, the cathode grid 7 may consist of stainless steel wires with a diameter of 50 microns each, intersecting at right angles, in a pitch of 500 microns, the role of this grid being to allow a symmetrization of the electric field on the wires such as 6c.

The plane of wires 6 is produced in the form of a weaving on an insulating frame of golden tungsten wires with a minimum diameter of at least 20 microns, and preferably from 50 to 100 microns each, arranged parallel to one another. others following a pitch S of 2.54 millimeters for example. All of the wires are connected to a source of electrical potential outside the enclosure 3, delivering a voltage of 5000 volts for example.

The distance G between the plane of wires 6 and the converter 5 on the one hand and the distance between the plane of wires 6 and the grid 7 on the other hand are preferably equal and of the order of 3 to 5 millimeters.

The cells such as 9c for example have the shape of squares of 2 millimeters on a side, produced at the same pace as the wires, 2.54 millimeters in this case.

The conversion material 12 used in cells such as 9c advantageously consists of gadolinium in the case where the neutral particles emitted by the source 1 are thermal neutrons, and of iron or silver in the case where these particles are rays X, in particular soft X.

The conductors such as 15c are on the one hand connected to the earth potential via respective capacitors such as 16c, on the other hand connected, each at least for a given time interval, to an electronic device 17, of the type known per se, the function of which is to convert the signal present on each of these conductors to a point in a video image and / or to information capable of being stored in an optical, electronic or other memory.

Under the conditions set out, the wires such as 6c play the role of means for amplification and collection of negative charges, while the converter and its cells play both the role of conversion means, of cathode, and of means of collection of positive charges.

In fact, insofar as the useful signal, for each point of the image of the object, is constituted by the electrical signal present on the conductors such as 15c, the cells such as 9c constitute more precisely the useful elements of the means of charge collection.

The invention develops all its advantages when the number of neutral particles detected is less than 10⁵ particles per second and per square centimeter of surface of the converter, and it is of particular interest when the particles detected are thermal neutrons.

As mentioned above, the charge amplification operation includes the application of an electric field of sufficient value to allow the appearance of electron avalanches of self-regulated size (streamer mode), and it is advantageous , for this purpose, that the ratio of the distance S (FIG. 3) between two neighboring anode wires 6b, 6c, to the distance G between these wires and the cathode 12, is at least equal to 1.

The interest of the "streamer" operating mode for a detector applied to imagery according to the invention has its origin in the following two reasons.

On the one hand, this mode allows the creation, for each fast electron emitted by the converter, of an extremely high number of charges, typically of the order of 10⁷ to 10⁹, so that it is possible, even from a small number of particles received by the converter, or a small number of particles converted by it, to obtain an image of an irradiated object such as 2 (Figure 1). This property is also best exploited according to the embodiment of the invention which comprises an accumulation, for a certain time, of the collected electrical charges, in a capacity such as 16c.

On the other hand and above all, this operating mode makes it possible to overcome an intrinsic defect which the solid converters exhibit under certain conditions of use, in particular for the detection of thermal neutrons.

Indeed, if the number of fast electrons created from a predetermined number of incident neutrons is constant, statistically at least, on the other hand the fast electrons from the solid converter have a very high energy dispersion.

However, the number of first ionization charges directly created per unit of distance by a fast electron passing through the gas is a rapidly variable function of the energy of this fast electron, so that the charge collecting elements, the cells such as 9c in this case, risk providing respective signals representative no longer of the number of neutral particles that these elements have received, but of the energy of the fast electrons to which these particles have led by conversion.

The "streamer" mode, which has the property of amplifying charges in a highly non-linear manner, makes it possible to re-establish this defect by giving rise, for each fast electron, to a number of charges collected which is substantially independent of the number of charges of first ionization directly created by fast electrons. The use of this operating mode thus makes it possible to reduce the fluctuations of the useful signal to a level close to the fish fluctuations of the source.

Claims (14)

1. A device for detecting and locating particles in a flux of neutral particles emitted by a source (1), the device comprising:
      a substantially plane solid converter (5) suitable for producing electrical charges under the impact of said neutral charges, the converter comprising conversion elements (9a, 9b, 9c) which are electrically independent from one another;
      anode wires (6a, 6b) for raising to an electric potential different from that of the converter to cause an electric field to appear, and for amplifying charges by ionizing a surrounding gas under stimulation from said electric field;
      charge collecting means (9c, 15c) comprising conductor elements that are electrically independent from one another, with at least some of them constituting conversion elements; and
      an enclosure (4a, 4b) permeable to the neutral particles and containing the converter, the anode wires, the charge collecting means, and the gas;
      the device being characterized in that said conversion elements also suitable for collecting charge are constituted by cells distributed over a plane two-dimensional matrix disposed on the opposite side of the anode wires to the source.
2. A device according to claim 1, characterized in that the converter comprises an insulating plate (8) having one face carrying said cells, said plate including, for each cell, a through hole (14c) putting said cell into electrical contact with a conductor (15c) leading to the other face of said plate.
3. A device according to any preceding claim, characterized in that said anode wires are disposed in at least one plane (6) substantially parallel to the plane of the converter, and are substantially parallel to one another.
4. A device according to any preceding claim, characterized in that said cells comprise a conversion material selected from the group comprising: gadolinium, boron, and lithium.
5. A device according to any preceding claim, characterized in that said cells comprise a conversion meterial selected from the group comprising: iron and silver.
6. A device according to any preceding claim in combination with claim 3, characterized in that the gas contains a quencher constituting not less than 25% thereof, and in that the ratio of the distance (S) between two adjacent anode wires (6b 6c) to the distance (G) between these wires and the converter (5) is not less than 1.
7. A device according to claim 6, characterized in that at least one of said charge collecting elements is connected to a reference electrical potential via a capacitor (16c) suitable for accumulating the charge collected by said element.
8. A device according to any preceding claim, characterized in that it includes a voltage source suitable for establishing an electrical potential of not less than 2,000 volts between the cathode and the anode wires.
9. A device according to any preceding claim, characterized in that the anode wires (6a, 6b, 6c) have a minimum diameter of not less than 20 microns.
10. A method of detecting and locating particles in a flux of neutral particles emitted by a source (1) in a device according to any preceding claim, comprising the operations consisting in:
       receiving said particles on a substantially plane solid converter (5), thereby producing electrical charges from said neutral particles;
       amplifying said charges by stimulated icnization of a surrounding gas; and
       collecting the charges present in at least one plane substantially parallel to the converter on the converter at different locations that are spaced apart from one another;
       the method being characterized in that said locations constitute a plane two-dimensional matrix.
11. A method according to claim 10, characterized in that the operation of amplifying the charges comprises applying an electric field of sufficient value to enable electron avalanches of self-regulating size to appear.
12. A method according to claim 10 or 11, characterized in that it further includes an operation consisting in accumulating the collected electrical charge over a certain length of time.
13. A method according to any one of claims 10 to 12, characterized in that the particles are converted into electrical charges with a conversion rate such that the number of detected particles is less than 10⁵ particles per second and per square centimeter of converter area.
14. A method according to any one of claims 10 to 13, characterized in that the neutral particles essentially comprise thermal neutrons.

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