EP1952420B1 - Ionization device, measurement chain for measuring the activity of a gas emitting beta-radiation and method of employing this device - Google Patents

Description

Technical area

The present invention relates to an ionization device, a chain for measuring the activity of a gas emitting β radiation, which may for example be a tritium measurement chamber, and a method for implementing this device.

State of the art

A tritium chain with an ionization chamber, as described in the document referenced [1] at the end of the description, serves to measure the activity of a gas emitting β-radiation in a given gaseous environment, for example that of a glove box, that of a ventilation network of a laboratory, or that of a chimney inspection of a nuclear building. The ionization chamber, which is dipped directly into the medium to be controlled, provides a current proportional to the activity to be quantified. A processing electronics can measure intensities between 10 ^-14 and 10 ^-8 A.

The document referenced [2] describes an ionization chamber. This ionization chamber 10, which is illustrated on the figures 1 and 2 is cylindrical in shape. It comprises a central anode 11 formed by a bar of electrically conductive material, by stainless steel example, two cylindrical lower flanges 12 and upper 13 of non-magnetic and insulating material, centered on the anode 11 and arranged perpendicularly thereto at its two ends, and a cathode 14 formed of a wire wound conductive material on the peripheral portion of these two flanges 12 and 13, thereby to surround the anode 11.

The anode 11, at its lower end, is connected to a first plug 16. The two ends of the wire forming the cathode 14 are connected to two other plugs 16 arranged on the lower flange 12.

The figure 1 also illustrates a mechanical base 20, provided at its two upper and lower ends respectively with a contact holder assembly 21 and a connector 22. The assembly 21 comprises four female sockets 23, able to receive four male plugs 16 arranged on the lower flange 12. These female sockets 23 are connected by conductive wires 24 to lugs of the connector 22. The ionization chamber 10 is thus removable.

On the upper part of the flange 20 is fixed a cylindrical protective body 25, which is disassembled in the measurement phases. Also shown are a ring 26 and a nut 27 for fixing the flanges 12 and 13 to the anode, an insulating barrel 28, a stainless steel coated gasket 29.

As illustrated on the figure 2 these two flanges 12 and 13 are respectively provided with three openings 15 of circular shape, which allows to lighten the structure and reduce the contaminating surface.

This referenced document [2] also describes a β radiation emitter gas activity measurement chain which comprises an ionization chamber as described above, a preamplification assembly, a signal processing electronics and a signal cable. connection between the preamplifier assembly and the processing electronics. The electronics can be deported several meters away from the preamplifier assembly.

• un préamplificateur 70 relié d'une part à une chambre d'ionisation de l'art connu 10 (tension d'ionisation Ui et courant d'ionisation Ii) et d'autre part à :
  • un tiroir de mesure 71,

reliés entre eux par un câble 83 d'une longueur maximale de 25 mètres. Ce câble, composé de plusieurs fils, véhicule différentes tensions analogiques et numériques.The figure 3 illustrates such a measurement chain of the known art comprising:

• a preamplifier 70 connected on the one hand to an ionization chamber of the known art 10 (ionization voltage Ui and ionization current Ii) and on the other hand to:
  • a measuring slide 71,

interconnected by a cable 83 with a maximum length of 25 meters. This cable, consisting of several wires, carries different analog and digital voltages.

• un étage analogique 72 comprenant un électromètre 69,
• un étage de numérisation 73.

The preamplifier 70 comprises:

• an analog stage 72 comprising an electrometer 69,
• a digitization stage 73.

• un module de stockage de paramètre 75,
• un module de surveillance des états 76,
• un module de calcul de courant 77,
• un module de changement de gamme 78,
• un module de calcul d'activité 79,
• un interface utilisateur 80,
• un module de gestion des entrées sorties 81,
• un module d'élaboration des alarmes 82.

The measuring slide 71 comprises an analog stage 74 and a digital stage 68 comprising:

• a parameter storage module 75,
• a state monitoring module 76,
• a current calculation module 77,
• a range change module 78,
• an activity calculation module 79,
• a user interface 80,
• an input-output management module 81,
• a module for generating alarms 82.

In the device of the known art illustrated on the figures 1 and 2 the sealed passage, which is the intermediate piece between the ionization chamber and its measurement electronics, has several defects that influence the measurement quality.

This sealed passage, in effect, provides the electrical connection between the ionization chamber placed in a potentially contaminated medium and its electronics placed in uncontaminated medium, and acts as a support for the ionization chamber. It must be subjected to a helium leak test to ensure that no radioactive material is released to the outside of the glove box.

The sealing of the contact flange is made using resin. This poorly polymerized resin can generate leakage currents that interfere with the measurement.

• ils contribuent à dériver une partie du courant d'ionisation à la masse introduisant par là même une erreur de gain dans la mesure,
• ils introduisent un décalage constant de la meure par superposition d'un courant issu du potentiel d'ionisation provoquant de ce fait une erreur de zéro.

The insulation defects of the sealed passage are related to the quality of the insulation of the electrical connections. These defects have two direct consequences on the measurement:

• they contribute to divert a part of the ionization current to the mass, thereby introducing a gain error in the measurement,
• they introduce a constant offset of the moment by superposition of a current resulting from the ionization potential, thereby causing an error of zero.

Moreover, in the measurement system of the known art, the measurement is parasitized by leakage currents flowing between the anode and the cathode of the ionization chamber. These currents originate from the high voltage of the ionization chamber and the presence of ambient electromagnetic fields. This results in a limitation of the measurements to minimum values of 10 ^-14 A. The parasitic currents are superimposed on the measurement and must therefore be eliminated.

The object of the present invention is to overcome these disadvantages by making it possible to reduce the volumes and the masses of the elements of the measurement chain and thus, ultimately, to generate a smaller volume of potentially contaminated waste.

Presentation of the invention

The present invention relates to a cylindrical ionization device comprising a removable ionization chamber adapted to be inserted into a base, this chamber comprising an anode formed of a central bar of electrically conductive material and a cathode of conductive material. electrical connection surrounding said anode, both connected to two cylindrical flanges, centered on the anode and arranged perpendicular thereto at both ends, the cathode being formed of son connected to the peripheral portion of these two flanges, characterized in that each of these two flasks integrates a parasitic current collecting circuit, and is isolated from the anode, in that the two flanges are interconnected by a bundle of wires interconnected by conductive tracks internal to these two flanges to form the cathode, and a last wire parallel to the other connects parasitic current collection circuits internal to these two flanges.

In an advantageous embodiment, the base is provided at its two upper and lower ends respectively with a contact holder assembly and a connector. Said assembly comprises sockets adapted to receive male plugs arranged on the lower flange. These sockets are connected by wires to lugs of the connector. A cylindrical body of protection of the chamber can be fixed on the upper part of the base.

• une fiche centrale du circuit de mesure,
• une/des fiche(s) radiale(s) pour alimenter la haute tension,
• une autre fiche radiale pour la collecte des courants parasites.

In an advantageous embodiment, the lower flange comprises male plugs:

• a central plug of the measuring circuit,
• radial plug (s) for supplying high voltage,
• another radial plug for the collection of parasitic currents.

The present invention also relates to a chain for measuring the activity of a P-radiation emitting gas comprising such an ionization device, a preamplification assembly, which can be mounted just behind the ionization chamber, an electronics for processing the radiation. deported signal and a CAN communication link between the preamplifier assembly and the processing electronics.

Advantageously, the preamplifier assembly comprises a 24-bit analog / digital converter.

This measurement chain can be, for example, a tritium measurement chain.

• de manière à créer des mouvements de convection du mélange gazeux à mesurer ;
• de manière à stabiliser la température et à influer sur l'hygrométrie du mélange gazeux ;

et lors d'un étuvage sous vide de décontamination, la température étant supérieure à 400°C.The present invention also relates to a method for operating the ionization device, in which a heating current is circulated in the cathode during the measurement:

• so as to create convection movements of the gas mixture to be measured;
• in order to stabilize the temperature and to influence the hygrometry of the gaseous mixture;

and during vacuum decontamination curing, the temperature being greater than 400 ° C.

• contrôle des rejets des cheminées d'un bâtiment ;
• mesures environnementales ;
• réalisation d'un système de contrôle portable autonome ;
• réalisation d'un système de contrôle direct en sortie de zone réglementée.

The measurement chain of the invention can be used in many applications, and in particular in the following areas:

• control of the discharges of the chimneys of a building;
• environmental measures;
• realization of an autonomous portable control system;
• implementation of a direct control system leaving the restricted area.

Brief description of the drawings

• The figures 1 and 2 illustrate an ionization device of the prior art.
• The figure 3 illustrates a measurement chain of the known art.
• The figure 4 illustrates the ionization device of the invention.
• The Figures 5A to 5D and 6A to 6D illustrate two elements of the ionization device according to the invention.
• The figure 7 illustrates the measurement chain of the invention.
• The figure 8 illustrates comparative measurement curves obtained for the ionization chamber of the known art and for the ionization chamber of the invention.

Detailed description of the embodiments

As illustrated on the figure 4 an ionization chamber 30 according to the invention, of cylindrical shape, comprises a central anode 31 formed by a bar of electrically conductive material, for example stainless steel, two lower cylindrical flanges 32 and upper 33, centered on the anode 31 and disposed perpendicularly thereto at both ends, and a cathode 34 formed of son of conductive material, for example platinum, connected to the peripheral portion of these two flanges 32 and 33, thereby to surround the anode 31. This ionization chamber is adapted to be inserted into a base 39.

This base 39 is provided at its upper part with a contact holder assembly 50 and, in its lower part, a connector 51. The contact holder assembly 50 comprises five female sockets 52 adapted to receive five male plugs 53 disposed on the lower flange 32. These sockets 52 are connected by leads 54 to lugs of the connector 51.

A cylindrical body of protection of the chamber can be fixed on the upper part of the base.

Each of the two flanges 32 and 33, made for example of ceramic, incorporates a parasitic current collection circuit. These circuits make it possible to collect the parasitic currents and to derive them from the measurement circuit via guard rings 47 and 62 which are metallized tracks, for example of diameter 10 mm surrounding the anode 31. Each of the flanges is isolated from this anode. central 31 by a PTFE ring. The guard rings 47 and 62 are connected to the electrical ground.

• une couche conductrice 42 comprenant une piste annulaire extérieure 45 percée de trous métallisés 55, reliée à trois trous radiaux métallisés 46 ; cette couche centrale 42 comprenant également un trou métallisé central 47 relié à un trou métallisé radial 48 et à un trou extérieur 49 par une piste 50,
• une ou deux couches isolantes 41 et 43 accolées à la couche 42,
• et éventuellement deux couches conductrices extérieures 40 et 44.

As illustrated on Figures 5A to 5D , the lower flange 32 thus comprises a circuit formed of a set of layers, in the form of disks namely, at least:

• a conductive layer 42 comprising an outer annular track 45 pierced with metallized holes 55, connected to three metallized radial holes 46; this central layer 42 also comprising a central metallized hole 47 connected to a radial metallized hole 48 and to an outer hole 49 by a track 50,
• one or two insulating layers 41 and 43 contiguous to the layer 42,
• and optionally two outer conductive layers 40 and 44.

• une couche conductrice 57, identique à la couche 42, comprenant une piste annulaire extérieure 60 percée de trous métallisés 61 ; cette couche centrale 57 comportant également un trou central métallisé 62 relié à un trou extérieur métallisé 63 par une piste 64,
• une ou deux couches isolantes 56 et 58 accolées à la couche 57,
• et éventuellement deux couches conductrices extérieures 55 et 59.

As illustrated on Figures 6A to 6D , the upper flange 33 also comprises a conductive circuit formed of a set of disk-shaped layers; to know at least:

• a conductive layer 57, identical to the layer 42, comprising an outer annular track 60 pierced with metallized holes 61; this central layer 57 also comprising a metallized central hole 62 connected to a metallized outer hole 63 by a track 64,
• one or two insulating layers 56 and 58 contiguous to the layer 57,
• and optionally two outer conductive layers 55 and 59.

The two flanges 32 and 33 are interconnected by parallel wires welded to the conductive tracks to form the cathode 34.

A wire, parallel to the other wires, interconnects the parasitic current collection circuits of the two flanges 32 and 33.

• au moins une fiche centrale 35 du circuit de mesure (anode 31),
• au moins une fiche radiale 36 pour alimenter la haute tension (cathode 34),
• une fiche radiale 37 pour le circuit de garde (mise à la masse des courants parasites), qui sert aussi de détrompeur au montage.

As illustrated on the figure 4 the lower flange 32, which is the contact flange, comprises at least:

• at least one central plug 35 of the measuring circuit (anode 31),
• at least one radial plug 36 for supplying the high voltage (cathode 34),
• a radial plug 37 for the guard circuit (ground parasitic currents), which also serves as a key to mounting.

These plugs allow the mounting of the ionization chamber 30 on the base 39.

• Le flasque inférieur porte-contacts 32 est amélioré. Des plots d'isolement sont vissés en force dans le flasque 32. L'insertion de contacts électriques 37 dans ces plots d'isolement garantit l'étanchéité. De plus il n'y a plus utilisation de résine, qui risquerait de générer des courants parasites collectés par le circuit de mesure.
• Le circuit de masse est amélioré en reliant la masse mécanique à la masse analogique. Une telle liaison s'effectue par l'intermédiaire de la vis 102, fixée dans la masse métallique de l'ensemble porte-contacts 50. Cette vis est reliée à l'anneau de blindage du connecteur électrique 51 par la patte 103, elle-même soudée sur la tresse de blindage du fil 54.

To overcome the disadvantages of the ionization device of the known art, as defined above, the sealed passage is modified as follows:

• The lower flange contacts 32 is improved. Isolation studs are screwed into force in the flange 32. The insertion of electrical contacts 37 in these isolation pads ensures tightness. In addition, there is no use of resin, which could generate parasitic currents collected by the measuring circuit.
• The ground circuit is improved by connecting the mechanical ground to the analog ground. Such a connection is effected by means of the screw 102, fixed in the metal mass of the contact-holder assembly 50. This screw is connected to the shielding ring of the electrical connector 51 by the lug 103, itself even soldered to the shielding braid of the wire 54.

• un préamplificateur haute-tension 85, relié d'une part à la chambre d'ionisation de l'invention 30 et d'autre part à
• une électronique de traitement 86,

via une liaison bidirectionnelle 84 de type CAN. Le dialogue numérique entre le préamplificateur et le tiroir de mesure peut ainsi transiter via une paire de fils torsadés.The figure 7 illustrates the measurement chain of the invention. She understands :

• a high-voltage preamplifier 85, connected on the one hand to the ionization chamber of the invention 30 and on the other hand to
• a processing electronics 86,

via a bidirectional link 84 of the CAN type. The digital dialogue between the preamplifier and the measuring slide can thus pass through a pair of twisted wires.

The preamplifier 85 is powered only by a single voltage valim, provided by the processing electronics 86 or an external power supply.

• un étage analogique 87, comme dans la chaîne de l'art connu, qui comprend un électromètre 67,
• un étage convertisseur analogique-numérique 88, 24 bits,
• un étage numérique 89 comprenant :
  • un module de stockage des paramètres 90,
  • un module de surveillance des états 91,
  • un module de calcul du courant-correction-filtrage 92,
  • un module de changement de gamme 93,
  • un module bus de transmission 94,
  • un voyant d'état 95.

The preamplifier 85 comprises:

• an analog stage 87, as in the known art chain, which comprises an electrometer 67,
• an analog-digital converter stage 88, 24 bits,
• a digital stage 89 comprising:
  • a module for storing parameters 90,
  • a state monitoring module 91,
  • a module for calculating the current-correction-filtering 92,
  • a range change module 93,
  • a transmission bus module 94,
  • a status light 95.

• un module de calcul de l'activité 96,
• un interface utilisateur 97,
• un interface de gestion des entrées-sorties 98,
• un module d'élaboration des alarmes 99.

The processing electronics 86 (all-digital) comprises:

• a module for calculating activity 96,
• a user interface 97,
• an input-output management interface 98,
• a module for generating alarms 99.

• Dans l'étage analogique 87, on réalise une implantation soignée des composants à l'intérieur du préamplificateur ce qui permet de compenser la principale grandeur d'influence qui est la température du préamplificateur.
• En ce qui concerne les niveaux d'isolement des cartes électroniques, ceux-ci sont définis en réalisant une implantation soignée des composants et des points d'équipotentialité.
• La conversion analogique-numérique 88 est améliorée par la mise en place d'un convertisseur de meilleure résolution pour augmenter la sensibilité de la chaîne de mesure.
• La distance de dialogue entre le préamplificateur 85 et son électronique de mesure 86 est améliorée en utilisant une liaison bidirectionnelle 84 performante, ce qui permet de passer à une distance de plusieurs centaines de mètres entre le préamplificateur et son électronique de traitement.

The improvement of the immunity of the ionisation chamber 30 to parasitic currents allows to also improve the performance of the associated measurement electronics, including preamplifier 85. This preamplifier is capable of measuring currents of up to 10 ^-16 / 10 ^-17 amperes. Such improvements include the analog floor, isolation levels, and analog-to-digital conversion:

• In the analog stage 87, the components inside the preamplifier are carefully implanted, which makes it possible to compensate for the main influence quantity which is the temperature of the preamplifier.
• With regard to the isolation levels of the electronic boards, these are defined by performing a careful implementation of the components and equipotentiality points.
• The analog-to-digital conversion 88 is improved by setting up a converter of better resolution to increase the sensitivity of the measurement chain.
• The dialogue distance between the preamplifier 85 and its measurement electronics 86 is improved by using a high-performance bidirectional link 84, which allows a distance of several hundred meters to be passed between the preamplifier and its processing electronics.

Test of the measurement chain

A prototype chain of measurements made it possible to carry out first tests. This prototype measuring chain consists of a measuring drawer, a preamplifier, a sealed passage and an ionization chamber according to the invention. This measuring drawer has a CAN communication connection with its preamplifier. The preamplifier includes temperature compensation and a 24-bit analog / digital converter.

These tests consist of injecting a known tritium activity into two measurement chains mounted in line: a measurement chain of the known art as illustrated on FIG. figure 3 and the measurement chain of the invention illustrated on the figure 7 . An activity close to 1 LDCA (or Air Concentration Derived Limit, which is 800,000 Bq / m ³ ), then another close to 10 LCDA were injected. The results obtained are visible on the figure 8 , the curve 100 corresponding to average values measured with the measuring chain of the known art and the curve 101 corresponding to average values measured with the measurement chain of the invention.

The measurement chain of the invention is clearly more efficient than the known art chain in low activities. Indeed, the latter can not measure activities below 5.5 LDCA while the chain of the invention is quite capable and displays an average measurement of 0.85 LDCA for 1.01 LDCA injected.

This improvement is even more convincing for an activity of 10.4 LDCA injected. The measurement chain of the known art measures 0.60 LDCA and the measurement chain of the invention 9.63 LDCA average.

REFERENCES

1. [1] "Tritium And Other Radiogas Monitors" (Overhoff Technology Corporation)
2. [2] US 6,734,433

Claims (10)

1. An ionization device with a cylindrical shape including a removable ionization chamber (30) able to be inserted into a base (39), this chamber comprising an anode (31) formed by a central bar made from an electrically conducting material and a cathode (34) made from an electrically conducting material surrounding said anode, both connected in pairs to cylindrical flanges (32, 33), centered on the anode (31) and positioned perpendicular thereto at both of its ends, the cathode (34) being made up of wires connected on the peripheral part of these two flanges (32, 33), characterized in that each of the two flanges (32, 33) incorporates a circuit collecting stray currents and is isolated from the anode (31), in that the two flanges (32, 33) are connected to one another by a bundle of wires connected to one another by conducting tracks inside these two flanges to form the cathode (34), and in that a final wire parallel to the others connects the circuits collecting stray currents, inside these two flanges.
2. The device according to claim 1, wherein the base (39) is provided, at its upper and lower ends, respectively, with a set of contact holders (50) and a connector (51).
3. The device according to claim 2, wherein said set (50) comprises female sockets (52) able to receive male plugs (53) positioned on the lower flange (32), and by which these female sockets are connected by conducting wires (54) to lugs of the connector (51).
4. The device according to claim 1, wherein a cylindrical protecting body of the chamber is fastened on the upper part of the piece.
5. The device according to claim 1, wherein the lower flange (32) includes male plugs:

   - a central plug (35) of the measuring circuit,

   - one or several radial plug(s) (36) for supplying a high voltage,

   - another radial plug (37) for collecting stray currents.
6. A chain for measuring activity of a gas emitting beta-radiation comprising an ionization device according to any one of the preceding claims, a pre-amplification assembly (85), processing electronics of the signal (86) and a connection of the CAN type (84) between the pre-amplification assembly and the processing electronics.
7. The measurement chain according to claim 6, wherein the pre-amplification assembly (85) comprises a 24-bit analog/digital converter (88).
8. The measurement chain according to claim 6, which is a tritium measurement chain.
9. A method for implementing the ionization device according to any one of claims 1 to 5, wherein a heating current is circulated in the cathode (34) during the measurement.
10. A method for implementing the ionization device according to claim 1 to 5, wherein a heating current is circulated in the cathode (34) during vacuum decontamination baking, the temperature being above 400°C.

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