A neutron detector comprising an optical fission ionization chamber having a reflective optical cavity and a detection head coupled to the optical fission ionization chamber, equipped with an optical lens and an optical fiber coupler.

Abstract

To provide a neutron detector comprising an optical fission chamber with a reflective optical cavity, and a detection head coupled to the optical fission chamber comprising an optical lens and an optical fiber coupler.SOLUTION: The invention relates to a neutron detector (1) comprising at least one ionization chamber (2) sealed tight and referred to as an OFC, which is an acronym of an optical fission chamber. Each chamber executes optical transduction and comprises an optical cavity. The optical cavity operates on the basis of the optical transduction. A window and an optical lens as an optical interface are built in at one end of the optical cavity in its longitudinal direction. A mirror for reflecting photons traveling in a direction away from the window is built in at the other end of the optical cavity in its longitudinal direction.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates particularly to the field of measuring instruments for nuclear fission reactors and nuclear fusion reactors.

[0002] More specifically, the present invention relates to a fission ionization chamber neutron detector, and more specifically to a so-called optical fission ionization chamber, that is, a fission ionization chamber that converts neutron signals into optical signals.

[0003] The object of the present invention is to provide an optical fission ionization chamber in which the signal-to-noise ratio for neutron flux measurement is improved. [Background technology]

[0004] The operation of a nuclear reactor, whether for power generation or research purposes, involves stringent requirements for tracking multiple operating parameters.

[0005] Among these, the generated thermal output is one of the important parameters. This directly correlates with the neutron flux near or inside the vessel. Therefore, an increase in neutron flux results in an increase in the reactor's power level.

[0006] Various techniques exist for measuring neutron flux, and these are classified under the umbrella of neutron detectors.

[0007] Nuclear reactors may be equipped with multiple neutron detectors.

[0008] These detectors can be classified into two categories: active detectors, which require a bias voltage in the detection zone to collect and transmit information related to neutron detection, and passive detectors, which do not require a bias voltage in the detection zone.

[0009] To measure neutrons inside a nuclear reactor, commonly used active neutron detectors are fission ionization chambers or boron deposition chambers, which conventionally operate on the principle of converting neutron flux into electrical signals. This electrical conversion is performed by a pair of electrodes biased to several hundred volts.

[0010] Figure 1 shows a fission ionization chamber or boron deposition chamber 1. This chamber 1 is defined by a conductive hollow cylinder 10 with its ends sealed, which forms the cathode. A smaller diameter cylinder 11 is located on the central axis X of this conductive hollow cylinder 10, which forms the anode. The anode 11 is surrounded by a deposit 12 of a material that strongly interacts with neutrons, such as uranium-235 or boron 10. The chamber 1 is further filled with gas. The interaction of the deposit 12 with neutrons releases heavily charged particles into the gas of the chamber, thereby transferring the energy of these particles to the gas. These particles are slowed down by the ionization of atoms in the gas filling the chamber. Under the influence of an electric field generated by applying a bias to the chamber, electrons move towards the anode 11, i.e., towards the positive potential of the chamber. This charge collection generates an electrical signal, from which the neutron flux measurement can be estimated.

[0011] Therefore, in this type of fission ionization chamber 1, neutron flux measurement becomes impossible in situations where the chamber's bias is accidentally lost.

[0012] Furthermore, when measuring in a nuclear reactor, the only passive detector currently used in the industry is a type called a collectron or self-powered neutron detector (SPND). In a collectron, electron-emitting material (beta decay) generates an electric current, which is transmitted via a cable to the measuring device. Certain types of collectrons require emitter stabilization, which can take up to 30 minutes, but transmitting low currents over long distances remains a challenge. Specifically, as is the case with fission ionization chambers, the signal generally must be transmitted through zones prone to electromagnetic interference (caused by pumps, magnets, motors, etc.).

[0013] To obtain usable signals, electromagnetic shielding of the transmission line is necessary. This means using bulky, highly resistant cables, which creates spatial constraints.

[0014] Therefore, regardless of whether it is for safety or spatial reasons, the neutron detectors currently used to measure neutron flux, particularly inside nuclear reactors, are inadequate.

[0015] A new method has been proposed that represents a departure from electrical conversion. See, for example, publications [1] to [5] and patent FR3125135B1. In principle, this new method performs optical conversion by collecting photons generated in a box where ionization occurs (hereinafter referred to as an ionization box).

[0016] These fission ionization chambers are called OFCs, an acronym for optical fission chamber. Because they are passive detectors, they can eliminate the problems of dependence on power sources and the bulkiness of the electrical cables used for signal transmission mentioned earlier.

[0017] Therefore, OFC converts neutron signals into optical signals. Specifically, when a gas is ionized by heavy ions produced by the reaction between neutrons and active materials such as boron and uranium, an electron shower is generated, leading to excitation and subsequent de-excitation across a wide spectral range from ultraviolet to mid-infrared. This effect is schematically illustrated in Figure 2.

[0018] The resulting emission is then collected by an optical fiber designed to be resistant to radiation, thereby effectively limiting the spectral range that will be used. Specifically, silica optical fibers with a pure SiO2 core offer excellent radiation resistance and attenuate very little optical signals with near-infrared wavelengths, typically 800–1000 nm, typically by a few dB / km. [5] See also.

[0019] The conversion of optical signals to electrical signals is carried out outside the reactor vessel by one or more transducers, such as photodiodes, silicon-based photomultiplier tubes, or cameras.

[0020] Therefore, one or more of these transducers or detection modules convert the optical signal into an electrical pulse, and the neutron flux measurement is estimated from this electrical pulse. All components of the OFC-based measurement system are described below.

[0021] Table 1 below allows for a comparison of fission ionization chambers and conventional optical fission ionization chambers (OFCs) according to various criteria.

[0022] [Table 1]

[0023] Table 1 clearly shows that improving the accuracy of neutron flux measurement is key to unlocking the potential of OFC (Optical Fission Ionization Chamber).

[0024] An important feature of the OFC is the signal-to-noise ratio. The higher this ratio, the higher the measurement accuracy.

[0025] Regarding the measurement of neutron flux by an optical fission ionization chamber in a nuclear reactor, the mechanisms causing the intensities of signals and noise have been investigated by various studies.

[0026] Regarding the signal intensity, when a fission ionization chamber of a given shape is provided, the photon collection efficiency, that is, the value obtained by dividing the number of photons collected by the fiber by the number of de-excited photons emitted, is obtained by the following Equation 1.

[0027]

Equation

[0028] [[ID=?]] Here, - r chamber is the inner diameter of the box, - r fibre is the radius of the fiber, - θ max is the maximum angle at which light can be transmitted to the fiber, - g(θ) and f(r) represent the angular distribution and radial distribution (cm 2 ) calculated for the surface located on the opposite side of the fiber, respectively.

[0029] Since the function f(r) is substantially constant over the radius of the fiber, the collection efficiency is proportional to the square of the radius of the fiber. Based on this principle, increasing the radius of the fiber to increase the collection efficiency and thus the accuracy of the OFC has been proposed by multiple authors ([6], [7]).

[0030] In reality, since noise is also proportional to the radius of the fiber, this solution of increasing the radius of the fiber becomes insufficient due to the γ-ray flux present during the measurement in the nuclear reactor.

[0031] It should be noted that there are some "?" in the translation where the original text has some tags that seem to be incomplete or not clearly defined in the context. The translation has been done as accurately as possible based on the available information. Specifically, ionization is induced within the fiber by gamma rays from the nuclear reactor, causing electrons to be emitted at a speed faster than c / n. Here, c is the speed of light, i.e., the propagation speed of electromagnetic waves, and n is the refractive index of the fiber.

[0032] The movement of these electrons within the fiber generates visible electromagnetic radiation known as Cherenkov radiation. This parasitic light is superimposed on the collected optical signal, increasing noise and significantly reducing the signal-to-noise ratio.

[0033] According to research and measurements conducted by the inventors, this Cherenkov radiation was found to be the primary source of noise in optical fission ionization chambers (OFCs).

[0034] Specifically, the Frank-Tam formula, as a first approximation, shows that the intensity of Cherenkov radiation changes according to the following equation 2.

[0035]

number

[0036] Here, - r fibre This is the diameter of the fiber, - L irrad This is the length of the fiber being irradiated.

[0037] In reactor measurements where optical fibers are exposed to ionizing radiation and both signal and noise are proportional to the square of the fiber radius, the signal-to-noise ratio does not depend on the fiber radius.

[0038] Nevertheless, in order to improve the signal-to-noise ratio of optical fission ionization chambers, the authors of the publication [6] propose compensating for the loss of efficiency caused by the reduction in solid angle by using reflective mirrors in short optical cavities and lenses in long optical cavities.

[0039] Furthermore, Figure 9 in [6] suggests that collection efficiency can be improved by using optical fibers with a higher numerical aperture.

[0040] The inventors believe that this solution is not suitable when considering the Cherenkov effect in an approach to optimizing the signal-to-noise ratio. Specifically, following the proposal in [6] also increases the proportion of Cherenkov photons guided by the optical fiber. See Figure 3, which is quoted from publication [8].

[0041] As a result, all methods proposed to date to improve the accuracy of neutron flux measurements using optical fission ionization chambers (OFCs) are unpromising because they increase signal intensity but simultaneously increase optical noise or Cherenkov noise. [Prior art documents] [Patent Documents]

[0042] [Patent Document 1] French Patent No. 3125135B1 [Overview of the project] [Problems that the invention aims to solve]

[0043] Therefore, it is necessary to improve optical fission ionization chambers (OFCs), which are capable of measuring neutron flux, particularly inside nuclear reactors, in order to either increase the intensity of the measurement signal without increasing noise, or increase the intensity of the measurement signal while reducing noise.

[0044] The objective of this invention is to satisfy this need at least partially. [Means for solving the problem]

[0045] To achieve this objective, one subject of the present invention is a neutron detector, which includes the following: A sealed ionization chamber in which photoconversion takes place, called an optical fission ionization chamber (OFC), wherein each chamber extends along the longitudinal axis (X), and each chamber is A sealed hollow body comprising at least one inner wall that defines an optical cavity inside and is at least partially covered with at least one fissile material layer, wherein the optical cavity is filled with gas, preferably under pressure, and capable of being ionized by ions resulting from a reaction between neutrons and fissile material, A window located at one of the longitudinal ends of the hollow body, configured to seal the optical cavity, An optical lens that is fixed to a window by adhesive or formed integrally with the window and configured to focus photons received by the window, and A mirror positioned at the other longitudinal end of a hollow body on which a window is located, which is the end opposite to the longitudinal end of the hollow body on which the window is located, and configured to reflect photons toward the window. A sealed ionization chamber comprising, A detector head equipped with an optical fiber coupler, wherein the optical fiber coupler has multiple optical fibers as inputs, these multiple optical fibers are arranged in the focal plane of a lens, and the optical fiber coupler also has a single optical fiber as an output, and the optical signals received by the inputs are summed within this single optical fiber. It is a neutron detector equipped with [a specific feature / feature].

[0046] Advantageously, this OFC has an axisymmetric shape and a central axis (X). Therefore, the hollow body of the OFC is preferably cylindrical, spherical, or conical.

[0047] Advantageously, the surface density ρmax of fissile material is 2 mg / cm³. 2The following holds. The surface area of the hollow body increases as the mass of the fissile material to be deposited increases. To maximize the energy imparted to the gas, the diameter of the hollow body of the box is preferably at least equal to the path length of the fission fragments. However, this value depends on the filling gas and its pressure P. Thus, for a given mass of fissile material and a given gas pressure within the optical cavity, it is advantageous for the dimensions R and H of the hollow body to be the dimensions estimated from the following equation 3.

[0048] [Number]

[0049] where - R is the lateral dimension of the hollow body, i.e., the radius in the case of a cylindrical body, - H is the axial dimension of the hollow body, i.e., the length in the case of a cylindrical body, - Range(P) is the distance that the fission fragments can travel through a given gas at pressure P, - m fissile is the mass of the fissile deposit, - ρ max is the surface density of the fissile material.

[0050] Advantageously, the fissile material is selected from boron-10, lithium-7, any isotope of uranium-238, plutonium, and neptunium

[0051] Preferably, the material for making the window is silica-based. The silica window has high transmission efficiency and good radiation resistance. The thickness e determines the ability to withstand the internal pressure of the gas P, as shown by the following equation 4.

[0052] [Number]

[0053] where E SiO2R is the Young's modulus of silica, and R is the radius of the window.

[0054] The radius R of the window is substantially equivalent to the radius of the hollow body.

[0055] According to one advantageous variation of the embodiment, the optical lens is a Fresnel lens. The advantage of using a Fresnel lens is that the mass of the materials that need to be used to manufacture the lens, particularly silica, is reduced, and therefore the intensity of Cherenkov radiation emitted from the optical components (window and lens) is reduced. In an optical fiber with numerical aperture NA, the radius R of the lens is the radius of the window, and the thickness of the lens is a characteristic given by the manufacturer.

[0056] For the sake of advantage, the focal length f' of the lens satisfies equation 5 below.

[0057]

number

[0058] Preferably, and advantageously, the gas used to fill the optical cavity is a noble gas selected from helium, neon, argon, krypton, xenon, or a mixture thereof.

[0059] The optical cavity is preferably under pressure, typically under a pressure of several bars.

[0060] According to one advantageous embodiment, the detector comprises at least one spacer positioned between the window and the hollow body and / or between the mirror and the hollow body. Each spacer acts as a physical separator between the fissile material deposit and one of the optical components (mirror or window), thereby preventing these optical components from fogging prematurely as a result of collisions with fissile fragments.

[0061] Preferably, the axial dimension of this spacer is greater than or equal to the path length of the light fission fragments (LFFs) or the maximum range of ions in the gas passing through the optical cavity. LFFs are emitted following the interaction of neutrons with atoms in the fissile deposit. Although two fission fragments are emitted by fission, only the light fission fragment is considered when setting the dimensions. In the case of uranium-235 fission, an example of an LFF could be a particle with atomic mass A=95 and initial kinetic energy E=100 MeV.

[0062] Specifically, the path length of LFFs is always longer than the path length of other fission products. To increase collection efficiency, the inner surface of the spacer is preferably polished to allow light reflection.

[0063] The radius of the spacer is equivalent to the radius of the hollow body of the OFC.

[0064] Table 2 below shows the path lengths of LFFs in optical cavities filled with various noble gases at pressures of 1 bar and 5 bar.

[0065] [Table 2]

[0066] Table 2 clearly shows that it is preferable to use a gas with a high atomic number in order to reduce the height of the spacer and improve the collection efficiency of OFCs.

[0067] In one advantageous configuration, the detection head is fixed to the hollow body of the box, preferably by screwing it in. The neutron detector is a single, compact, and easy-to-handle object.

[0068] Accordingly, the present invention essentially consists of a sealed optical fission ionization chamber (OFC) and a neutron detector comprising an optical cavity, the optical cavity operating on a light conversion basis, with a window and optical lenses as optical interfaces incorporated at one end of its longitudinal direction, and a mirror for reflecting photons moving away from the window incorporated at the other end of its longitudinal direction.

[0069] The optical fiber coupler is appropriately positioned within the focal plane of the lens to expand the collection area of ​​the emitted signal from the lens exit, without resulting in a permanent increase in the volume of the optical fiber under irradiation, which would become a noise source as a result of Cherenkov radiation.

[0070] Preferably, the deposition of the reflective layer forming the mirror is adjusted to the measurement wavelength. Specifically, the reflection coefficient of light depends on the wavelength of the incident photon. Furthermore, NBK-7, the most common substrate material, must be avoided. Specifically, NBK-7 contains B10, which causes premature degradation of the mirror through (n,α) reactions ([9]). As for the reflective layer, materials such as silver and gold are preferred because they have high reflectivity and remain constant over a wide range of wavelengths and especially in the near-infrared region.

[0071] Since the window and optical lens are joined by adhesive or formed as a single, integrated part, these two optical components are subjected to mechanical stress together. Therefore, by adjusting the thickness of the lens, it is possible to apply some of the mechanical stress to the lens and thus make the window thinner. In this way, the mass of the irradiated silica is reduced, which can reduce the noise caused by Cherenkov radiation emitted in these optical components.

[0072] The neutron detector with minimized dimensions according to the present invention can be configured to withstand high-temperature, high-radiation environments such as those found inside an operating nuclear reactor.

[0073] Ultimately, the detector according to the present invention enables the realization of an excellent signal-to-noise ratio when measuring neutron flux.

[0074] This invention has numerous applications, among which the following can be listed. - Online measurement of neutron flux inside a nuclear reactor. - Characterization and monitoring of neutron flux in locations other than nuclear reactors (experimental reactors or power reactors). - Identifying molten fuel elements during or after a major incident (loss of cooling, and possibly temporary power loss). - Identifying the location of stoppers for sealing colloidal plutonium in chemical processing. - Neutron measurements on a neutron beam for the purpose of beam stability, or neutron measurements for the purpose of actually measuring the time of flight on these same lines.

[0075] Other advantages and features will become clearer by referring to the following diagrams and reading the detailed but non-limiting descriptions provided as examples. [Brief explanation of the drawing]

[0076] [Figure 1] This is a schematic cross-sectional view of a nuclear fission ionization chamber neutron detector based on prior art. [Figure 2] This is a schematic diagram illustrating the principle of light emission, which is caused by the ionization of gas by ionizing particles, most commonly called heavy ions, produced by the reaction of neutrons with active materials. [Figure 3] This is a longitudinal cross-sectional view of a neutron detector equipped with an optical fission ionization chamber (OFC) and an optical fiber coupler according to the present invention. [Figure 4] This figure shows, in the form of a curve, the calculated result of the angular distribution g(θ) of photons passing through the surface of the window of a neutron detector according to the present invention, which has the components and dimensions set as shown in Figure 3. [Figure 5] This is a longitudinal cross-sectional view of a neutron detector equipped with an optical fission ionization chamber (OFC) as a comparative example. [Figure 6]This figure shows, in the form of a curve, the calculated result of the angular distribution g(θ) of photons passing through the surface of the window of a comparative example neutron detector, which has the components and dimensions shown in Figure 5. [Figure 7] This is a front view of an irradiation chamber showing various advantageous positions for installing a neutron detector equipped with an optical fission ionization chamber according to the present invention, under irradiation conditions within a nuclear reactor. [Figure 8] This figure shows the collection efficiency, obtained as a function of the focal length of the optical lens and the distance between the optical lens and the fiber, as the output of the signal transmitted by the lens, in the form of a three-dimensional curve. [Figure 9] This figure shows the signal-to-noise ratio obtained using an optical fiber coupler in a neutron detector according to the present invention, as a linear function of the number of optical fibers in the coupler and the ratio of the lengths of coupled and uncoupled fibers. [Figure 10] This figure shows the distribution and energy level of the signal applied to a neutron detector with the dimensions of the device shown in Figure 5, along the longitudinal axis X. [Modes for carrying out the invention]

[0077] Figures 1 and 2 have already been explained in the introduction. Therefore, they will not be explained further below.

[0078] Figure 3 shows a neutron detector 1 according to the present invention.

[0079] First, the neutron detector 1 comprises a sealed ionization chamber 2 in which optical conversion takes place. This chamber is called an OFC, an acronym for optical fission ionization chamber, and extends around a central axis (X).

[0080] This OFC2 comprises a sealed hollow cylinder 20 having a length H and a diameter φ corresponding to 2R, with an optical cavity 21 defined inside it.

[0081] The inner wall 22 of the main body is covered with a layer of fissile material such as uranium-235, uranium-238, or boron-10.

[0082] The optical cavity 21 is filled with at least one noble gas, which is preferably placed under pressure P and can be ionized by ions produced as a result of the reaction between neutrons and fissile material. This gas may be selected from helium, neon, argon, krypton, xenon, or a mixture thereof.

[0083] The window 23 is positioned at one of the longitudinal ends of the hollow body 20 and is configured to seal the optical cavity. Furthermore, a spacer 26 is positioned between the window 23 and the hollow body 20 to prevent the window 23 from fogging prematurely as a result of impact from fission fragments.

[0084] An optical lens 24, preferably a Fresnel lens, is fixed to the window by adhesive or formed integrally with the window, and is configured to focus photons received by the window 23.

[0085] Mirror 25 is positioned at the other longitudinal end of the hollow body, which is the end opposite to the longitudinal end of the hollow body where the window is located. This mirror 25 is configured to reflect photons toward the window. Furthermore, a spacer 27 is positioned between the mirror 25 and the hollow body 20 to prevent the mirror 25 from fogging prematurely as a result of collisions with fission fragments. Spacers 26 and 27 have a diameter φ equal to the diameter of the hollow cylinder 20. These diameters may be the same as each other, and the axial dimension H1 may be the same as H2.

[0086] The neutron detector 1 is further equipped with a detection head 3.

[0087] The head 3 includes an optical fiber coupler 30 with multiple optical fibers 31 as inputs, and these optical fibers are positioned in the focal plane F of the lens 24. The head 3 also includes a single optical fiber 32 as an output, and the optical signals received at the inputs are summed within this optical fiber.

[0088] The inventors used a software package known in particular by the acronym PHITS (Particle and Heavy-Ion Transport code System) to perform various calculations with various dimensions. This software is an MCNP simulator (MCNP stands for Monte Carlo N-Particle Transport), which is a numerical simulation software platform that models nuclear physical processes using the Monte Carlo method. This general-purpose PHITS software package was developed as part of a collaborative research project between the Japan Atomic Energy Agency (JAEA) and several other research institutes around the world.

[0089] To verify the shapes of various components of the neutron detector 1 according to the present invention, particularly the optical components, as shown in Figure 3, the inventors compared these shapes with the shapes of OFC detector 1' used as a prototype in the laboratory.

[0090] Figure 5 schematically shows such a detector 1'. This detector 1' does not include a mirror, optical lens, or optical fiber coupler. In fact, the detector 1' has only a window 23, which is separated from the hollow cylindrical body 20 that defines the cavity 21 by a spacer 26.

[0091] A single optical fiber 31 is directly mounted to the window 23 along the X-axis.

[0092] To quantify the effect of the optical focusing system, the angular and radial distributions on the surface of window 23 were calculated for the shape of neutron detector 1 shown in Figure 3 and the shape of neutron detector 1' shown in Figure 5 as a comparative example.

[0093] The graphs in Figures 4 and 6 show the calculated angular distribution g(θ) of photons passing through the surface of window 23 for neutron detector 1 and neutron detector 1', respectively. These calculations were performed using the software package PHITS.

[0094] Note that in these two detectors 1 and 1', the light source is modeled as uniform, extended, and isotropic.

[0095] Table 3 below shows the dimensions of detectors 1 and 1' that are under consideration.

[0096] [Table 3]

[0097] Based on the evaluation of these results, by modifying the shape of the hollow body 20 of the OFC chamber and adding the mirror 25, the proportion of photons passing through the window surface (P window It can be seen that this increases by 25 times. This figure is an underestimate because it does not take into account the improvement due to the reflection of photons from the polished wall.

[0098] Figure 7 shows two neutron detectors 1, as shown in Figure 3, advantageously positioned at two locations P1 and P2. These neutron detectors are mounted on a reflector 100 inside the irradiation chamber of the reactor.

[0099] Since the radial and angular distributions of photons colliding with window 23 are known, it is possible to estimate the same distribution at the window's exit and thus the resulting collection efficiency by analyzing the ray matrix. This model assumes that the paraxial approximation is valid, i.e., that the angle of the incident ray is small. This applies to more than 60% of the photons in the case of the OFC detector 1 with the optimized dimensions described above.

[0100] The inventors compared three neutron detectors having different optical components. That is, - A neutron detector 1' having a single sealed window as shown in Figure 5, - A neutron detector 1 according to the present invention, equipped with a window 23 and a thick optical lens 24 as shown in Figure 3, - A neutron detector 1 according to the present invention, equipped with a window 23 and a Fresnel lens 24 as shown in Figure 3, They were compared.

[0101] Table 4 below shows the transfer matrices for each of these three detectors.

[0102] Table 4 shows, - L is the distance between the optical component (a window with or without an optical lens) and the optical fiber. - R is the radius of curvature of the thick lens, - f' is the focal length of the Fresnel lens, - t is the thickness of the window and thick lens, set to be equivalent to 2mm. - n1 is the refractive index of air, - n2 is the refractive index of silica, - θ and r are randomly plotted on a distribution calculated by the PHITS software package.

[0103] [Table 4]

[0104] Calculation simulations show that the collection efficiency for a single window is lower than that of an optical system with a combination of windows and optical lenses. This is reasonable, given that the receiving cone of this single window occupies only a small portion of the emission volume.

[0105] According to this calculation, the collection efficiency increases to the equivalent of 1.01 times in the case of an optical system combining a window and a thick optical lens, and to the equivalent of 25 times in the case of an optical system combining a window and a Fresnel lens.

[0106] By using an optical system that combines windows and optical lenses, the focal length and the distance between the lens and the fiber can be changed.

[0107] This efficiency sensitivity profile clearly shows that, in the case of an optical system combining windows and lenses, maximum efficiency is achieved when the optical fiber is positioned at the image focal point F of the lens.

[0108] In contrast, when using non-thin lenses, collection efficiency drops by as much as 40%.

[0109] Figure 8 shows the collection efficiency for an optical system combining a window and a Fresnel lens.

[0110] The inventors verified the effectiveness of the optical fiber coupler 30 and optimized it with respect to two parameters: the number of optical fibers to be coupled and the ratio x of the length of the optical fiber after coupling to the length of the optical fiber before coupling.

[0111] Assuming that the radius of curvature of the optical fiber is not excessively large, the inventors believe that x can be varied between 0.2 and 0.8.

[0112] The inventors of the present invention calculated the changes in the signal δs and noise δb described by Equation 6 as follows by comparing a neutron detector according to the present invention, i.e., a neutron detector having an optical fiber coupler, with a detector without a coupler.

[0113]

number

[0114] Here, N is the number of optical fibers through which the light entering the coupler passes.

[0115] Figure 9 shows the change in signal-to-noise ratio achieved using the optical fiber coupler according to the present invention.

[0116] Therefore, computational simulations demonstrated the advantages of having various optical components and optical fiber couplers in OFC neutron detector 1, because these components allow for maximizing the light intensity of the image from the light source.

[0117] Figure 10 shows the axial and radial energy distributions of the light source passing through the neutron detector 1 according to the present invention.

[0118] The shape of this light source depends on several parameters, such as the shape of the optical cavity, the gas filling the optical cavity, the pressure of that gas, and the properties of the fissile material selected from uranium-235, boron-10, lithium-7, etc.

[0119] The complexity of the light source combined with the optical components of detector 1 makes it impossible to calculate the image of the light source. Since the image of this light source on the detection surface F is unknown, the currently arranged multiple optical fibers ensure that the image of the light source occupies the brightest location. This additional degree of freedom allows for further optimization of light collection and, therefore, improvement of the signal-to-noise ratio.

[0120] The OFC neutron detector 1 described above has a signal acquisition efficiency and signal-to-noise ratio that are tens of times higher than conventional OFC detectors.

[0121] As can be understood from the above, this improvement is the result of appropriately added optical components (mirrors, windows, optical lenses), optical fiber couplers, optimization of their shapes, and selection of parameters such as the number of optical fibers through which light entering the coupler passes and the ratio of the lengths of coupled and uncoupled fibers.

[0122] Naturally, these results from preliminary analyses performed using the PHITS software package can also be obtained by using specialized optical calculation codes.

[0123] Other modifications and improvements can be considered without departing from the scope of the present invention.

[0124] (References) [1]: M. Lamotte, G. De Izarra, C. Jammes, "Heavy-ions induced scintillation experiments," J. Instrum., 14 (09) (2019), p. C09024, https: / / doi.org / 10.1088 / 1748-0221 / 14 / 09 / C09024 [2]: M. Lamotte, G. De Izarra, C. Jammes, "Development and first use of an experimental device for fission-induced spectrometry applied to neutron flux monitoring", Nucl. Instrum. Methods Phys. Res. A953 (2020), p. 163236, https: / / doi.org / 10.1016 / j.nima.2019.163236. [3]: M. Lamotte, G. De Izarra, C. Jammes, "Design and irradiation test of an innovative optical ionization chamber technology", Nucl. Instrum. Methods Phys. Res. A968 (2020), p.163945, https: / / doi.org / 10.1016j.nima.2020.163945. [4]: M. Lamotte, G. De Izarra, C. Jammes, SCENA: "A simulation tool for radiation-induced gas scintillation", Nucl. Instrum. Methods Phys. Res. A982 (2020), p. 164576, https: / / doi.org / 10.1016 / j.nima.2020.164576. [5]: Cheymol G., Long H., Villard J.-F., Brichard B., "High level gamma and neutron irradiation of silica optical fibers in CEA OSIRIS nuclear reactor", IEEE Trans. Nucl. Sci., 55 (4) (2008), pp. 2252-2258. [6]: Goburnov, B., "Perspectives d'utilisation de convertisseurs optiques nucleaires pour l'enregistrement de flux de photons dans les reacteurs nucleaires" [Prospects of using nuclear optical converters to record photon flux in nuclear reactors]. Russian Federal Nuclear Centre. (2015). [7]: Lamotte, M., "Etude du signal optique des chambres a fission et evaluation de son exploitation pour un systeme de mesure neutronique d'un reacteur de generation IV" [Study of the optical signals of fission chambers and evaluation of applicability to a neutron-measuring system for a Generation IV reactor]. Doctoral thesis of the University of Grenoble. (2021). [8]: Brichard, B. "Fiber-optic gamma-flux monitoring in a fission reactor by means of Cerenkov radiation". Measuring Science Technology - vol 18, 3257 - 3262. [9]: Wirtenson, RH "Radiation Induced Darkening of the Optical Element in the Startracker camera." Technical report of Lawrence Livermore National Laboratory. (2007) [Explanation of Symbols]

[0125] 1. Fission ionization chamber or boron deposition chamber, OFC neutron detector 2. Sealed ionization chamber 3 detection heads 10 Conductive hollow cylinder 11 Smaller diameter cylinder, anode 12 Sediment 20 Hollow body, hollow cylindrical body, hollow cylinder, sealed hollow cylinder 21 Optical Cavity 22 Inner wall 23 windows 24 Optical lenses, Fresnel lenses 25 Mirror 26 Spacers 27 Spacers 30 Fiber Optic Couplers 31 Optical Fiber 32 optical fibers 100 reflector

Claims

1. A neutron detector (1), At least one sealed ionization chamber (2) in which optical conversion takes place, called OFC, an acronym for optical fission ionization chamber, wherein each chamber extends along a longitudinal axis (X), and each chamber is A sealed hollow body (20) comprising at least one inner wall (22) that defines an optical cavity (21) inside and is at least partially covered with at least one layer of fissile material, wherein the optical cavity is filled with gas and is capable of being ionized by ions produced as a result of a reaction between neutrons and the fissile material, A window (23) is located at one of the longitudinal ends of the sealed hollow body and is configured to seal the optical cavity. An optical lens (24) fixed to the window by adhesive or formed integrally with the window and configured to focus photons received by the window, and A mirror (25) positioned at the other longitudinal end of the sealed hollow body on which the window is located, which is the end opposite to the longitudinal end, and configured to reflect photons toward the window. A sealed ionization chamber (2) comprising, A detection head (3) is provided with an optical fiber coupler (30), wherein the optical fiber coupler (30) has a plurality of optical fibers (31) as inputs, the plurality of optical fibers are arranged in the focal plane of the optical lens, and the optical fiber coupler (30) also has a single optical fiber (32) as an output, and the optical signals received by the inputs are summed within the single optical fiber, and A neutron detector (1) is provided.

2. The neutron detector (1) according to claim 1, wherein the OFC has an axisymmetric shape and a central axis (X) corresponding to the longitudinal extension axis of the ionization chamber.

3. The neutron detector (1) according to claim 2, wherein the sealed hollow body of the OFC is cylindrical, spherical, or conical.

4. The surface density ρmax of the fissile material is 2 mg / cm³. ２ The neutron detector (1) according to claim 1 is as follows:

5. The neutron detector (1) according to claim 1, wherein the fissile material is selected from boron-10, lithium-7, isotopes of uranium-238, plutonium, and neptunium.

6. The neutron detector (1) according to claim 1, wherein the gas is selected from helium, neon, argon, krypton, xenon, or a mixture thereof.

7. The neutron detector (1) according to claim 1, wherein the material for making the window is silica-based.

8. The neutron detector (1) according to claim 1, wherein the optical lens is a Fresnel lens.

9. The neutron detector (1) according to claim 1, comprising at least one spacer disposed between the window and the sealed hollow body, and / or at least one spacer disposed between the mirror and the sealed hollow body.

10. The neutron detector (1) according to claim 9, wherein the axial dimension of the spacer is greater than or equal to the path length of the light fission fragments (LFFs) passing through the gas in the optical cavity.

11. The neutron detector (1) according to claim 1, wherein the detection head is fixed to the sealed hollow body of the box.

12. Use of a neutron detection device according to any one of claims 1 to 11 for characterizing and tracking the neutron flux in a nuclear reactor.

13. Use of a neutron detection device according to any one of claims 1 to 11 for locating a molten fuel element during or after a serious incident.

14. Use of a device for detecting neutrons according to any one of claims 1 to 11 for locating a sealing stopper in a chemical processing process.

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