FR2703790A1 - Ionization chamber with high radiation detection efficiency. - Google Patents

Abstract

The invention relates to an ionization chamber for detecting gamma radiation comprising: - an enclosure (1) made of aluminum and lead and filled with argon under a pressure of 5 to 10 bars, and - two electrodes (3, 4) located inside the enclosure, these electrodes being electrically isolated and separated from each other in order to create an electric field (E) between them and to allow the passage, in this electric field, of particles ionizing producing ions which can be collected by said electrodes. Application to nuclear reactors and nuclear fuel reprocessing plants.

Description

i

  IONIZATION CHAMBER HAVING HIGH EFFICIENCY DETECTION

Y RADIATION

DESCRIPTION

  TECHNICAL FIELD The present invention relates to a high efficiency ionization chamber for detecting y (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 and in reprocessing plants, it is generally necessary to know the amount of radiation that can be

  through a particular environment of these facilities.

  For this, it is known to use measuring stations

  using the gamma densitometry method.

  This method consists in placing, on one side of the medium that we are trying to characterize, a source of radiation y and, on the other side of this medium, a detector able to determine the quantity of Y rays having passed through the medium. This gammadensitometry method makes it possible to evaluate physical parameters such as, for example, the degree of vacuum in the hot branch of a pressurized water reactor or the mass of heavy nuclei contained in the

  dissolving cups of a reprocessing plant.

  To carry out such a method, the radiation sources generally used therein are sources of cesium or sources of cobalt. As a result, the radiation to be detected has an energy of about 661 kev for a cesium source and

  from about 1,170 to 1300 kev for a cobalt source.

  The energy range of the radiation y to be detected is therefore between 500 and 1500 kev. Moreover, a background noise is very

  often present during these radiation measurements y.

  This background noise is, in the case of reprocessing plants, the consequence of the scattered rays of the dissolution solution In the case of a nuclear reactor installation, this background noise is due to the different activation products of the medium. considered the energy of such a background noise is to

the order of 80 to 100 kev.

  To implement this method of gammadensitometry, it is known to use, as a measurement station, ionization chambers as described in the book entitled "Sensors in industrial instrumentation", pages 693 to 699, published by DUNOD Such ionization chambers have the advantage of being able to be implanted in hostile environments with very high temperatures, a high dose rate, a high level 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 on a central electrode makes it possible to obtain an electric field, by means of which electrons created by the ionization of

  air are collected on the electrode.

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

of radiation y.

  Another type of ionization chamber consists of a cylindrical chamber made of stainless steel

  and filled with Xenon brought to a pressure of 7 bar.

  This chamber comprises two cylindrical and coaxial electrodes arranged in the chamber. These electrodes are electrically insulated and make it possible to create between them an electric field. The passage of the ionizing particles in this electric field produces

  ions that are collected by the electrodes.

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

PHOTONICS.

  Such a chamber has a y-radiation detection efficiency of about 10-8 A / Gy / hr for a cobalt source. Furthermore, the energy response curve C1, shown in a logarithmic scale in FIG. has a maximum efficiency around 100 kev, which is mainly explained by the strong section

  effective ionization of Xenon in the field of

  50 to 400 kev electric interaction of gamma / matter interaction.

  Furthermore, if an efficiency ratio Re is defined as being the ratio of the useful signal corresponding to a ray energy y between 500 and 1500 kev on the background noise corresponding to an energy of the y-rays of the 80 to 100 kev, this efficiency ratio RE is E useful signal 5 10-8

RE = 2 10-2

  E background 2.5 10-6 o the energy of the useful signal (E useful signal) is evaluated at about 2.5 10-6 A / Gy / h by reading in Figure 1 and the energy of the noise of background (E noise of

  background) is estimated at about 10-8 A / Gy / hr.

  SUMMARY OF THE INVENTION The object of the present invention is precisely to remedy the drawbacks mentioned above and to make it possible to increase the efficiency ratio considerably. More precisely, the subject of the invention is an ionization chamber for detecting radiation y and comprising: a chamber filled with a gas, and two electrodes located inside the chamber, these electrodes being electrically isolated and separated from each other to create an electric field between them and to allow the passage, in this electric field, ionizing particles producing ions that can be collected by

said electrodes.

  It is characterized by the fact that the gas filling the enclosure is Argon brought to a

  pressure between substantially 5 and 10 bar.

  Advantageously, the enclosure comprises an outer wall made of aluminum and covered on its inner face with a layer of lead suitable for

  absorb photons with 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; FIG. 2 represents, 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 thickness of Lead; 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 FIG.

  section 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 chamber

  Ionization differs from those usually used.

  This ionization chamber thus 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 whose thickness can vary from substantially 0.5 mm to 1 mm This chamber 1 is filled with argon at a pressure which may be

about 5 to 10 bars.

  This ionization chamber further comprises two electrodes, the high-voltage electrode referenced 3 and the central electrode referenced 4.

  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 at 120 degrees from each other with respect to the central support 5 As the ionization chamber is shown in sectional view, only two of these high-voltage electrode holders 3 are shown in FIG. 2. These HT 3 electrode supports are referenced 6a and 6b, as their name indicates. these electrode supports 5, 6 a and 6 b make it possible to hold 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 electrode HT 3 can be connected to a source. electrical connection via a connection means introduced into the threaded base 15 of the enclosure 1 This connection means and the electrical source are not shown in this figure for the sake of simplification of Figure 2 In Figure 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 and the means of In addition, the support 5 of the central electrode 4 is connected, via the connection wire 9, to a central connection pin 10, itself connected to the electrical source by the connection wire 11 and by the spit

connection 12.

  This ionization chamber further comprises a tube 13 for filling the chamber 1 with gas, that is to say Argon This tube 13 is connected to the central pin 10 by a ground wire, said pipe 13 being itself grounded This set of connection pins, connection wires, test tubes and electrode holders 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 thread constituting the threaded base 15 and for fixing the assembly of the ionization chamber on the medium of which it is desired to detect the radiation y, that is, 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 continuous potential difference is established between the central electrode 4 and the high voltage electrode 3 An electric field E is established then between these electrodes in the volume of gas which is between said electrodes, this volume of gas being called the useful volume The incident radiation passing through this detector ionizes the Argon located in the chamber 1, and thus releases , electric charges which are collected by the electrodes 3 and 4 More precisely, the ions released by the ionizing particles passing through the Argon go either towards the high voltage electrode 3 or towards the

  the central electrode 4 according to the sign of their charge.

  FIG. 3 shows the curve C 2 for 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 an enclosure made of stainless steel; but this enclosure is filled with argon The curve C 2 which is represented on a logarithmic scale, like all the curves of FIGS. 1, 3, 4 and 5, aims to show the advantage of an enclosure filled with Argon relative 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. Curve C 2 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 curve C 2). Unlike Xenon, the gas used in the invention to fill the chamber 1, namely Argon, is a gas that does not have a particularly strong intrinsic efficiency; the term "intrinsic efficiency" refers to the efficiency of the creation of the number of electrons. On the contrary, Argon is a gas which optimizes the ratio RE of efficiency between the energy range of 500 to 1500 kev representing the useful signal, and the average energy of

  at 100 kev representing the parasitic signal, ie

  In order to optimize this ratio of efficiency RE, Argon is brought to a pressure ranging from 5 to 10 bars. According to the embodiment represented in this FIG. 3, the Argon is brought to a pressure of 9 bars As shown in FIG. 3, the fact of using Argon in an ionization chamber makes it possible to obtain a contribution of the radiation at 80-100 kev which is much lower than the contribution of the radiation at 80-100. kev produced by the

  ionization chamber of the prior art.

  We see, indeed, in this figure 3, that

  for an energy range of about 80 to 100 kev, that is

  ie an energy corresponding to the background noise, the detection efficiency of the ionization chamber

  relating to the curve C 2 is of the order of 3 10-8 A / Gy / h.

  Furthermore, for an energy range of 500 to 1500 kev, the detection efficiency shown on the C 2 curve is equivalent to about 1.5 10-8 A / Gy / hr. The efficiency ratio RE is then equivalent to about

1.5 10-8

RE = -0.5

3.10-8

  FIG. 4 shows the energy response curves of an ionization chamber whose enclosure, filled with Xenon, is covered on its inner wall with a lead layer of

  0.5 and 1 mm thick, respectively.

  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 chamber is made of Stainless steel The curve C 3 represents the energy response curve of an ionization chamber filled with Xenon and whose enclosure is made of aluminum covered on its inside with a layer of 0.5 mm of lead. The curve C 4 represents a power response curve of an ionization chamber whose enclosure is filled with Xenon and made of aluminum coated on its inner side with a Lead layer 1 mm thick. The chamber 1, thus made of lead-coated aluminum, makes it possible to significantly reduce the contribution of the average energy radiation of the order of 80 to 100 kev with respect to the chamber of the ionization chamber of the chamber. In addition, this embodiment of the chamber 1 substantially does not modify the contribution of the energy y radiation in the range 500 to 1500 kev, that is to say that it hardly modifies the number of electrons created, the 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 C 3 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 signal of the background noise is substantially 3 10-7 A / Gy / h According to this curve C 3, the efficiency ratio is worth

5 10-8

RE = = 0.167

3.10-7

  For the curve C 4 which represents the energy response curve of an ionization chamber comprising a Lead layer 1 mm thick, it can be seen in FIG. 4 that the useful signal substantially equals the value of the signal. the curve Ci, namely 10-8 A / Gy / h, and that the background noise signal is equivalent to about 1.5 10-7 A / Gy / hr. The efficiency ratio RE is then equivalent to

5.10-8

RE = 0.333

1.5 10-7

  FIG. 5 shows the energy response curves of the ionization chamber of the prior art, and ionization chambers according to the invention, in which the lead layer of the enclosure 1 is, The curve C1 represents, as in FIGS. 1, 3 and 4, the energy response curve of the ionization chamber of the prior art. Curve C 5 represents the energy response of an ionization chamber according to the invention in which the chamber 1 is covered on its inner face with a Lead layer 0.5 mm thick and the curve C 6 represents the energy response of an ionization chamber according to the invention whose enclosure is

  covered with a layer of Lead 1 mm thick.

  In this FIG. 5, the values of the useful signals and of the background signals of the curves C 5 and C 6 can be read. More precisely, the useful signal is substantially the same for the curve C 5 and for the curve C 6. ; it has a value of about 1.5 10-8 A / Gy / h The background signal of the curve C 5 is substantially 6 10-9 A / Gy / h and the signal of

  background noise of curve C 6 is about 2 10-

  9 A / Gy / h Thus, the efficiency ratio RE of the curve C 5 is equivalent to 1.5 × 10-8

RE -2.5

6.10-9

  And the efficiency ratio RE of the curve C 6 is equivalent to:

1.5 10-8

RE = -7.5

2 10-9

  Reading these FIGS. 1, 3, 4, and especially FIG. 5, makes it possible to understand that the absolute efficiency of the ionization chamber according to the invention is substantially reduced with respect to the absolute efficiency of the chamber. ionization of the prior art, but that the efficiency ratio of the useful signal on the

  background noise signal is greatly increased.

  More specifically, 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 bar and or the thickness of lead of

the speaker is 1 mm.

  Such an ionization chamber, with such a gain in useful signal / background signal ratio, allows, for equivalent measurement accuracy, to use radiation sources y of less intensity than that usually used; which consequently leads to a lower cost of the measuring stations

by gammadensitometry.

Claims (3)

  1 ionization chamber for detecting radiation y and comprising: a chamber (1) filled with a gas, and two electrodes (3, 4) located inside the chamber, these electrodes being electrically isolated and separated each other in order to create an electric field (E) between them and to allow the passage, in this electric field, of ionizing particles producing ions that can be collected by said electrodes, characterized in that the gas filling the enclosure is Argon brought to a pressure between substantially 5 and 10 bar.   2 ionization chamber according to claim 1, characterized in that the enclosure   has a wall made of aluminum.   3 ionization chamber according to claim 2, characterized in that the wall is covered, on an inner face (2), with a layer lead.