EP0619597B1 - Ionization chamber with high sensitivity for gamma radiation - Google Patents

Description

Technical area

The present invention relates to a high-efficiency ionization chamber for detecting γ (gamma) radiation. It finds applications in the fields of nuclear reactors and nuclear fuel reprocessing plants.

State of the art

In installations comprising nuclear reactors as well as in reprocessing plants, it is generally necessary to know the quantity of γ radiation that can pass through a particular medium in these installations. For this, it is known to use measuring stations implementing the gamma densitometry method. This method consists in placing, on one side of the medium which one seeks to characterize, a source of γ radiation and, on the other side of this medium, a detector capable of determining the quantity of γ rays having passed through the medium. . This gamma densitometry method makes it possible to evaluate physical parameters such as, for example, the vacuum rate in the hot branch of a pressurized water reactor or the mass of heavy nuclei contained in the dissolving buckets of a reprocessing.

To implement such a method, the sources of γ radiation generally used are sources of Cesium or sources of Cobalt. From this in fact, the γ radiation to be detected has an energy of approximately 661 kev for a source of Cesium and approximately 1,170 to 1,300 kev for a source of Cobalt. The energy range of the γ radiation to be detected is therefore between 500 and 1,500 kev.

In addition, background noise is very often present during these γ radiation measurements. This background noise is, in the case of reprocessing plants, the consequence of the γ rays diffused from the dissolution solution. In the case of a nuclear reactor installation, this background noise is due to the various activation products of the medium considered. The energy of such background noise is of the order of 80 to 100 kev.

To implement this method of gammadensitometry, it is known to use, as a measuring station, ionization chambers as described in the work entitled "Sensors in industrial instrumentation", pages 693 to 699, edited by DUNOD editions . The advantage of such ionization chambers is that they can be installed in hostile environments where very high temperatures prevail, a high dose rate, a high rate of contamination, corrosion, etc.

Many types of ionization chambers are known to those skilled in the art.

One of these types is an ionization chamber comprising cylindrical housings filled with air at atmospheric pressure in which the application of a high voltage to a central electrode makes it possible to obtain an electric field, thanks to which electrons created by the ionization of air are collected on the electrode.

Such an ionization chamber (which is also more generally called a γ detector) has only a low detection efficiency; its detection efficiency being of the order of 10 ^-9 A / Gy / h (amperes per absorbed dose of γ radiation per hour) when a cobalt source is used as a source of γ radiation.

Another type of ionization chamber consists of a cylindrical enclosure made of stainless steel and filled with Xenon brought to a pressure of 7 bars. This chamber has two cylindrical and coaxial electrodes arranged in the enclosure. These electrodes are electrically isolated and create an electric field between them. The passage of ionizing particles in this electric field produces ions which are collected by the electrodes.

An ionization chamber of this type is described in the catalog of PHILIPPS PHOTONIQUE detectors.

Such a chamber has a detection efficiency of γ radiation of approximately 5.10 ^-8 A / Gy / h for a Cobalt source. In addition, the energy response curve C1, represented on a logarithmic scale in the appended FIG. 1, exhibits a maximum efficiency around 100 kev, which is explained, essentially, by the large effective cross-section of ionization of Xenon in the photoelectric domain from 50 to 400 kev of gamma / matter interaction.

où l'énergie du signal utile (E signal utile) est évaluée à environ 2,5 .10^-6A/Gy/h par lecture sur la figure 1 et l'énergie du bruit de fond (E bruit de fond) est évaluée à environ 5.10^-8A/Gy/h.In addition, if we define an efficiency ratio Re as being the ratio of the useful signal corresponding to an energy of the γ rays of between 500 and 1,500 kev on the background noise which corresponds to an energy of the γ rays of l '' from 80 to 100 kev, this RE efficiency ratio is: $$\text{RE =}\frac{\text{E useful signal}}{\text{E background noise}}\text{≅}\frac{{\text{5.10}}^{\text{-8}}}{{\text{2.5.10}}^{\text{-6}}}{\text{≅ 2 10}}^{\text{-2}}\text{,}$$

where the energy of the useful signal (E useful signal) is evaluated at approximately 2.5 .10 ^-6 A / Gy / h per reading in FIG. 1 and the energy of the background noise (E background noise) is evaluated at around 5.10 ^-8 A / Gy / h.

Statement of the invention

The object of the present invention is precisely to remedy the drawbacks mentioned above and to enable the efficiency ratio to be increased considerably.

• une enceinte remplie d'un gaz, et
• deux électrodes situées à l'intérieur de l'enceinte, ces électrodes étant électriquement isolées et séparées l'une de l'autre afin de créer un champ électrique entre elles et de permettre le passage, dans ce champ électrique, de particules ionisantes produisant des ions pouvant être collectées par lesdites électrodes.

More specifically, the invention relates to an ionization chamber for the detection of γ radiation and comprising:

• an enclosure filled with a gas, and
• two electrodes located inside the enclosure, these electrodes being electrically isolated and separated from each other in order to create an electric field between them and to allow the passage, in this electric field, of ionizing particles producing ions which can be collected by said electrodes.

It is characterized by the fact that the gas filling the enclosure is Argon brought to a pressure of between substantially 5 and 10 bars.

Advantageously, the enclosure has an outer wall made of Aluminum and covered, on its inner face, with a layer of Lead capable of absorbing photons of energy less than 100 kev.

Brief description of the drawings .

• FIG. 1, already described, represents the energy response curve of an ionization chamber of the prior art;
• Figure 2 shows, schematically, a sectional view of an ionization chamber according to the invention;
• FIG. 3 represents the energy response curve of an ionization chamber comprising a stainless steel enclosure, filled with Argon;
• FIG. 4 represents the energy response curves of an ionization chamber whose enclosure, filled with Xenon, is covered, on its inner wall, with Lead; each curve corresponding to a different lead thickness;
• FIG. 5 represents the energy response curves of the ionization chamber of the prior art and of the ionization chamber of the invention.

Detailed description of embodiments

In Figure 2, there is shown a sectional view of the ionization chamber according to the invention.

The overall structure of this chamber is substantially identical to that described in the prior art. However, the materials used for its production and the gas filling said ionization chamber differ from those usually used.

This ionization chamber therefore comprises, as in the prior art, an enclosure 1. However, this enclosure 1 is made of Aluminum. The inner wall 2 of this enclosure 1 is covered with a layer of Lead, the thickness of which can vary from substantially 0.5 mm to 1 mm. This enclosure 1 is filled with Argon under a pressure which can be approximately 5 to 10 bars.

This ionization chamber further comprises two electrodes: the high voltage electrode referenced 3 and the central electrode referenced 4. These electrodes 3 and 4 are cylindrical and coaxial. The central electrode 4 is supported by an electrode support 5. The high-voltage electrode 3 (or HT electrode) is supported by three electrode supports arranged 120 degrees from one another relative to the support 5 central. The ionization chamber being represented in a sectional view, only two of these high-voltage electrode supports 3 are shown in FIG. 2. These HT electrode supports 3 are referenced 6a and 6b. As their name suggests, these electrode supports 5, 6a and 6b make it possible to maintain the respective electrodes 4 and 3 in a fixed position inside the enclosure 1.

These electrode supports 5, 6a and 6b are made of conductive materials covered with an insulating material. In this way, the support 5 and one of the supports 6 of the HT electrode 3 can be connected to an electrical source via a connection means introduced into the threaded base 15 of the enclosure 1. This connection means thus that the electrical source is not shown in this figure for the sake of simplification of FIG. 2. In FIG. 2, it is the support 6b which is connected to the electrical source. This support 6b of the high voltage electrode 3 is therefore connected to the electrical source via the connection wire 7 and the connection pin 8 as well as the connection means introduced into the threaded base 15. In addition, the support 5 of the central electrode 4 is connected, via the connection wire 9, to a pin of central connection 10 itself connected to the electrical source by the connection wire 11 and by the connection pin 12.

This ionization chamber further comprises a socket 13 allowing the enclosure 1 to be filled with gas, that is to say with Argon. This queusot 13 is connected to the central pin 10 by a ground wire, said queusot 13 itself being grounded. This set of connection pins, connection wires, sockets and electrode supports are included in the base, referenced 14, of the ionization chamber. This base 14 is mounted on the enclosure 1 and has on its surface the screw pitch constituting the threaded base 15 and making it possible to fix the whole of the ionization chamber on the medium whose γ radiation is sought to be detected, that is to say, for example, on the hot branch of a pressurized water reactor.

The electrodes 4 and 3, thus connected to an electrical source, can be brought to a voltage such that a direct potential difference is established between the central electrode 4 and the high voltage electrode 3. An electric field E s' then establishes between these electrodes in the volume of gas which is located between said electrodes, this volume of gas being called the useful volume. The incident radiation γ which passes through this detector ionizes the Argon located in the enclosure 1, and therefore releases electrical charges which are collected by the electrodes 3 and 4. More precisely, the ions released by the particles ionizers crossing the Argon go either to the high voltage electrode 3 or to the central electrode 4 depending on the sign of their charge.

In Figure 3, there is shown the curve C2 of the energy response of an ionization chamber made of a material identical to that described in the prior art, that is to say an ionization chamber comprising a stainless steel enclosure; but this enclosure is filled with Argon. The curve C2 which is represented on a logarithmic scale, like all the curves of Figures 1, 3, 4 and 5, aims to show the advantage of an enclosure filled with Argon compared to the enclosure of the prior art filled with Xenon. The energy response curve of the ionization chamber of the prior art is referenced C1 and corresponds to the curve C1 shown in FIG. 1. The curve C2 is therefore the energy response curve of an ionization chamber made of stainless steel and filled with Argon. On these curves C1 and C2, the gases used are respectively under a pressure of 7 bars (for the curve C1) and 9 bars (for the curve C2).

Unlike Xenon, the gas used in the invention to fill the enclosure 1, namely Argon, is a gas which does not have a particularly strong intrinsic efficiency; "intrinsic efficiency" means the efficiency from the point of view of creating the number of electrons. On the contrary, Argon is a gas which optimizes the RE efficiency ratio between the energy range from 500 to 1,500 kev representing the useful signal, and the average energy from 80 to 100 kev representing the parasitic signal, c is the background noise. In order to optimize this RE efficiency ratio, the Argon is brought to a pressure ranging from 5 to 10 bars. According to the embodiment shown in this figure 3, the Argon is brought to a pressure of 9 bars. As shown in Figure 3, using Argon in a room ionization provides a contribution of radiation at 80-100 kev much lower than the contribution of radiation at 80-100 kev produced by the ionization chamber of the prior art.

We see, in fact, in this figure 3, that for an energy range of about 80 to 100 kev, that is to say an energy corresponding to the background noise, the detection efficiency of the chamber d ionization relative to the curve C2 is of the order of 3.10 ^-8 A / Gy / h. In addition, for a range of energy varying from 500 to 1500 kev, the detection efficiency represented on the curve C2 is equivalent to approximately 1.5 × 10 ^-8 A / Gy / h. The RE efficiency ratio is then equivalent to approximately: $$\text{RE =}\frac{{\text{1.5.10}}^{\text{-8}}}{{\text{3.10}}^{\text{-8}}}\text{= 0.5}$$

In FIG. 4, the energy response curves of an ionization chamber are shown, the enclosure of which, filled with Xenon, is covered on its inner wall with a layer of Lead of, respectively, 0.5 and 1 mm thick.

In this FIG. 4, the curve C1 represents the energy response curve of the ionization chamber of the prior art, that is to say the ionization chamber filled with Xenon and whose enclosure is made in Stainless steel. Curve C3 represents the energy response curve of an ionization chamber filled with Xenon and whose enclosure is made of Aluminum covered on its inner face with a layer of 0.5 mm of Lead. Curve C4 represents an energy response curve of an ionization chamber, the enclosure of which is filled with Xenon and made of coated Aluminum, on its face, inside of a layer of Lead 1 mm thick.

The enclosure 1, thus made of Aluminum coated with Lead, makes it possible to significantly reduce the contribution of the average energy radiation of the order of 80 to 100 kev compared to the enclosure of the ionization chamber of the prior art, represented by the curve C1. In addition, this embodiment of the enclosure 1 does not substantially modify the contribution of the γ radiation of energy included in the range 500 to 1,500 kev, that is to say that it hardly modifies the number of electrons created, lead having the characteristic of absorbing photons whose energy is less than 100 kev.

It can therefore be read in this FIG. 4 that, for the curve C3 representing an ionization chamber comprising a lead layer of thickness 0.5 mm, the useful signal is substantially equivalent to the value of the useful signal of the curve C1, that is to say 5.10 ^-8 A / Gy / h, and that the background noise signal is substantially 3.10 ^-7 A / Gy / h. According to this curve C3, the efficiency ratio is worth: $$\text{RE =}\frac{{\text{5.10}}^{\text{-8}}}{{\text{3.10}}^{\text{-7}}}\text{= 0.167}$$

For the curve C4 which represents the energy response curve of an ionization chamber comprising a lead layer 1 mm thick, it can be read in FIG. 4 that the useful signal is substantially equivalent to the value of the useful signal of the curve C1, namely 5.10 ^-8 A / Gy / h, and that the noise signal background equals approximately 1.5.10 ^-7 A / Gy / h. The RE efficiency ratio is then equivalent to: $$\text{RE =}\frac{\text{5.10-8}}{\text{1.5.10-7}}\text{= 0.333}$$

FIG. 5 shows the energy response curves of the ionization chamber of the prior art, and of ionization chambers according to the invention, in which the lead layer of the enclosure 1 is, 0.5 mm and 1 mm thick, respectively. The curve C1 represents, as in FIGS. 1, 3 and 4, the energy response curve of the ionization chamber of the prior art. Curve C5 represents the energy response of an ionization chamber according to the invention in which the enclosure 1 is covered, on its inner face, with a lead layer 0.5 mm thick. And curve C6 represents the energy response of an ionization chamber according to the invention, the enclosure of which is covered with a layer of Lead 1 mm thick.

Et le rapport d'efficacité RE de la courbe C6 équivaut à : $$\text{RE =}\frac{{\text{1,5.10}}^{\text{-8}}}{{\text{2.10}}^{\text{-9}}}\text{= 7,5}$$

In this FIG. 5, the values of the useful signals and of the background noise signals of the curves C5 and C6 can be read. More precisely, the useful signal is substantially the same for the curve C5 and for the curve C6; it has a value of approximately 1.5.10 ^-8 A / Gy / h. The background noise signal from curve C5 is approximately 6.10 ^-9 A / Gy / h and the background noise signal from curve C6 is approximately 2.10 ^-9 A / Gy / h. Thus, the efficiency ratio RE of curve C5 is equivalent to: $$\text{RE =}\frac{{\text{1.5.10}}^{\text{-8}}}{{\text{6.10}}^{\text{-9}}}\text{= 2.5}$$

And the efficiency ratio RE of curve C6 is equivalent to: $$\text{RE =}\frac{{\text{1.5.10}}^{\text{-8}}}{{\text{2.10}}^{\text{-9}}}\text{= 7.5}$$

Reading these Figures 1, 3, 4, and especially Figure 5, allows to understand that the absolute efficiency of the ionization chamber according to the invention is significantly reduced compared to the absolute efficiency of the chamber d ionization of the prior art, but that the efficiency ratio of the useful signal to the background noise signal is considerably increased. More precisely, this efficiency ratio which was of the order of 2.10 ^-2 for the ionization chamber of the prior art is, for the ionization chamber according to the invention, of the order of 7 , 5, which allows a gain of a factor of about 375, in the case of the particular embodiment where the Argon is brought to a pressure of 9 bars and where the lead thickness of the enclosure is 1 mm.

Such an ionization chamber, with such a gain in useful signal / background noise signal ratio, makes it possible, with equivalent measurement accuracy, to use sources of γ radiation of lower intensity than that usually used; which consequently results in a lower cost of measurement stations by gammadensitometry.

Claims (3)

1. Ionization chamber for the detection of gamma radiation comprising a gas-filled enclosure (1) and two electrodes (3, 4) positioned within the enclosure, said electrodes being electrically insulated and separated from one another in order to create an electric field (E) between them and permit the passage, into said electric field, of ionizing particles producing ions which can be collected by the said electrodes, characterized in that the gas filling the enclosure is argon, which is raised to a pressure between 5 and 10 bars.
2. Ionization chamber according to claim 1, characterized in that the enclosure has a wall made from aluminium.
3. Ionization chamber according to claim 2, characterized in that the inner face (2) of the wall is covered with a lead layer.

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