Neutron detector comprising optical fission chamber having reflective optical cavity and probe comprising optical lens and optical fiber coupler coupled to chamber

Abstract

The invention relates to a neutron detector comprising an optical fission chamber having a reflective optical cavity and a detection head coupled to the chamber comprising an optical lens and an optical fiber coupler. Specifically, the invention relates to a neutron detector (1) comprising at least one sealed ionization chamber (2), referred to as OFC (abbreviation of optical fission chamber), each chamber implementing an optical conversion and comprising an optical cavity, the operation of which is based on the optical conversion and which contains, at one of its longitudinal ends, a window and an optical lens as an optical interface, a mirror for reflecting photons away from the window is included at the other end.

Description

Technical Field

[0001] The present invention relates to the field of instrumentation, in particular instrumentation for nuclear fission and fusion reactors.

[0002] The invention relates more particularly to fission chamber neutron detectors and more particularly to so-called optical fission chambers, ie to fission chambers which convert neutron signals into optical signals.

[0003] It is an object of the present invention to provide such an optical fission chamber in which the signal-to-noise ratio of the sub-flux measurements is improved. Background Art

[0004] Operating a reactor, whether for power generation or research, places stringent requirements on tracking multiple operating parameters.

[0005] Among them, the thermal power generated is one of the key parameters. This is directly related to the neutron flux near or inside the container. Therefore, increasing the neutron flux will lead to an increase in the reactor power level.

[0006] There are various techniques for measuring neutron flux, which are grouped together under the category of neutron detectors.

[0007] Multiple neutron detectors can be installed in a nuclear reactor.

[0008] These detectors can be divided into two categories: active detectors, ie detectors whose detection region requires a bias voltage to collect and transmit information related to the detection of neutrons, and passive detectors, whose detection region does not require any bias voltage.

[0009] For neutron measurements inside nuclear reactors, the active neutron detectors commonly used are fission chambers or boron deposition chambers, which usually operate on the principle of converting the neutron flux into an electrical signal. This electrical conversion is performed by a pair of electrodes biased at several hundred volts.

[0010] Figure 1 A fission chamber or boron deposition chamber 1 is shown. This chamber 1 is delimited by an electrically conductive hollow cylinder 10 which is closed at its ends in a sealed manner and forms the cathode, and on its central axis X is placed a cylinder 11 of smaller diameter forming the anode. The periphery of the anode 11 is coated with a deposit 12 of a material that interacts strongly with neutrons, such as uranium-235 or boron-10. The chamber 1 is also filled with a gas. The interaction of the neutrons with the deposit 12 releases heavy charged particles into the gas of the chamber, which particles transfer energy to the gas. These particles are slowed down by the ionization of the gas atoms that fill the chamber. Under the action of the electric field generated by biasing the chamber, the electrons will move in the direction of the anode 11, i.e. the positive potential of the chamber. This collection of charges generates an electrical signal from which a measurement of the neutron flux can be derived.

[0011] Therefore, in this type of fission chamber 1, the unexpected loss of chamber bias will result in a loss of neutron flux measurements.

[0012] Furthermore, for measurements in reactors, the only passive detector currently used in the industry is a type of detector known as a collectron or self-powered neutron detector (SPND). In a collectron, an electron-emitting material (beta decay) generates an electric current that is transmitted via a cable to a measuring device. Apart from the fact that some types of collectrons require a stabilized emitter (which can take up to 30 minutes), transmitting low currents over long distances remains a problem. Specifically, as is the case with fission chambers, the signal usually has to be transmitted through areas prone to electromagnetic interference (due to pumps, magnets, motors, etc.).

[0013] In order to obtain a usable signal, electromagnetic shielding of the transmission line is necessary. This means using bulky, high-noise immunity cables, which creates space constraints.

[0014] Therefore, the neutron detectors currently used to measure the neutron flux, especially in reactors, are unsatisfactory, both for safety and space reasons.

[0015] A new approach, quite different from electrical conduction, has been proposed: see for example publications [1] to [5] and patent FR 3125135 B1. In principle, it consists in performing an optical conversion by collecting the photons generated in a chamber where ionization occurs (hereinafter called ionization chamber).

[0016] These fission chambers, called OFC, short for optical fission chamber, are passive detectors that make it possible to overcome the dependence on an electric power source and the aforementioned bulkiness of the cables that transmit the signal.

[0017] Therefore, OFC realizes the conversion of neutron signal into optical signal. Specifically, when the gas is ionized by heavy ions produced by the reaction between neutrons and active materials such as fissionable materials (boron, uranium), electron showers occur and lead to excitation and then de-excitation in a wide spectral range from ultraviolet to mid-infrared. This effect is as Figure 2 Shown schematically.

[0018] This luminescence is then collected by optical fibers designed to withstand the radiation, which effectively limits the spectral region to be used. In particular, it has been shown that silica optical fibers with a pure SiO2 core withstand radiation very well and have a low attenuation, typically a few dB / km, for optical signals with wavelengths in the near infrared, typically between 800nm ​​and 1000nm: see [5].

[0019] As for converting the optical signal into an electrical signal, this is done outside the vessel of the nuclear reactor by one or more converters (such as photodiodes, silicon-based photomultiplier tubes or cameras).

[0020] The one or more converters or detection modules thus transform the optical signals into electrical pulses from which a measurement of the neutron flux is derived. All components of an OFC-based measurement system are described below.

[0021] Table 1 below allows a comparison of the fission chamber with prior art optical fission chambers (OFC) according to various criteria.

[0022] [Table 1]

[0023] Types of detectors Fission Chamber Optical fission chamber Passive no yes Temperature resistance Up to 600°C Up to 1000°C Occupancy area (cable diameter) Tens of millimeters (mm) Hundreds of micrometers (μm) Electromagnetic noise immunity Depends on cable diameter yes Accuracy of neutron flux measurements good Poor

[0024] It is clear from Table 1 that improving the accuracy of neutron flux measurements is key to unlocking the potential of OFC (Optical Fission Chamber).

[0025] An important characteristic of OFCs is their signal-to-noise ratio: the higher the signal-to-noise ratio, the more accurate the measurement.

[0026] Various studies have investigated the mechanisms responsible for signal and noise strength for measuring neutron flux through optical fission chambers within nuclear reactors.

[0027] Regarding signal intensity, for a given fission chamber geometry, the photon collection efficiency, i.e. the number of photons collected by the fiber divided by the number of de-excitation photons emitted, is given by the following equation1:

[0028] [Equation 1]

[0029]

[0030] in:

[0031] -r chamber is the inner radius of the chamber,

[0032] -r fibre is the radius of the optical fiber,

[0033] -Θ max is the maximum angle at which light can be transmitted into the fiber, and

[0034] g(θ) and f(r) are the calculated angular and radial distributions on the surface opposite to the fiber (in cm). 2 ).

[0035] Since the function f(r) is almost constant over the radius of the fiber, the collection efficiency is proportional to the square of the fiber radius. Based on this principle, many authors have suggested increasing the radius of the fiber to improve the collection efficiency and, therefore, the accuracy of OFC: see [6], [7].

[0036] In fact, the flux of gamma radiation present during measurements in a reactor makes the solution of increasing the radius of the optical fiber unsatisfactory, since the noise is also proportional to the radius of the optical fiber.

[0037] Specifically, due to the ionization caused by the reactor's gamma radiation in the optical fiber, electrons are emitted at a speed greater than c / n, where c is the speed of light, that is, the propagation speed of electromagnetic waves, and n is the refractive index of the optical fiber.

[0038] The movement of these electrons within the optical fiber produces the emission of visible electromagnetic radiation, known as Cherenkov radiation. This parasitic light is superimposed on the collected optical signal, which adds noise and greatly reduces the signal-to-noise ratio.

[0039] Studies and measurements carried out by the inventors have shown that this Cherenkov radiation is the main source of noise in optical fission chambers (OFCs).

[0040] Specifically, the Franck-Tamm formula shows that, in a first approximation, the intensity of the Cherenkov radiation varies according to Equation 2 below.

[0041] [Equation 2]

[0042]

[0043] in:

[0044] -r fibre is the diameter of the fiber, and

[0045] -L irrad is the length of the irradiated fiber.

[0046] In reactor measurements where the optical fiber is subjected to ionizing radiation, both the signal and the noise are proportional to the square of the fiber radius, and the signal-to-noise ratio does not depend on the fiber radius.

[0047] Nevertheless, in order to improve the signal-to-noise ratio of the optical fission chamber, the authors of the publication [6] proposed using mirrors in short optical cavities and lenses in long optical cavities to compensate for the efficiency loss caused by the reduced solid angle.

[0048] In addition, [6] Fig. 9It is recommended to use optical fibers with higher numerical aperture to improve collection efficiency.

[0049] The inventors consider this solution to be insignificant, taking into account the Cherenkov effect in the method of optimizing the signal-to-noise ratio. Specifically, if the suggestion of [6] is followed, the fraction of Cherenkov photons guided by the optical fiber will also be larger: see Figure 3 , the figure is taken from the publication [8].

[0050] Therefore, the methods proposed so far to improve the accuracy of neutron flux measurements by optical fission chambers (OFCs) have led to dead ends, because although they increase the strength of the signal, they also increase the optical or Cherenkov noise.

[0051] There is therefore a need to improve optical fission chambers (OFCs) allowing neutron flux measurements, in particular in nuclear reactors, in order to increase the strength of their measurement signal without increasing the noise, or to increase the strength of the measurement signal and reduce the noise.

[0052] It is an object of the present invention to at least partially meet this need. Summary of the invention

[0053] To this end, a subject of the invention is a neutron detector comprising:

[0054] - at least one sealed ionization chamber in which the optical conversion takes place, said chamber being called an optical fission chamber (OFC), each extending along a longitudinal axis (X) and comprising:

[0055] a sealed hollow body defining an optical cavity internally and comprising at least one inner wall at least partially coated with at least one layer of a fissionable material, the optical cavity being filled with a gas, preferably under pressure, and capable of being ionized by ions produced by the reaction between neutrons and the fissionable material,

[0056] a window arranged at one of the longitudinal ends of the hollow body and configured to seal the optical cavity,

[0057] an optical lens secured to the window by bonding or formed integrally with the window, the optical lens being configured to focus photons received through the window,

[0058] a reflector arranged at the other of the longitudinal ends of the hollow body, the longitudinal end being opposite to the longitudinal end at which the window is arranged, the reflector being configured to reflect photons toward the window,

[0059] - a detection head, the detection head comprising a fiber coupler, the fiber coupler comprising: a plurality of optical fibers as input ends, the plurality of optical fibers being arranged in the focal plane of the lens; and an optical fiber as an output end, in which optical signals received at the input ends are summed.

[0060] Advantageously, the OFC is axisymmetric in shape and has a central axis (X).Thus, the hollow body of the OFC is preferably a cylinder, a sphere or a cone.

[0061] Advantageously, the surface density of the fissile material is max is less than or equal to 2 mg / cm2. The surface area of ​​the hollow body increases with the mass of the fissionable material to be deposited. In order to maximize the energy deposited in the gas, the diameter of the hollow body of the chamber is advantageously at least equal to the path length of the fission fragments. However, this value depends on the filling gas and its pressure P. Therefore, for a given mass of fissionable material and a given gas pressure in the optical cavity, the dimensions R and H of the hollow body are advantageously the dimensions derived from the following equation 3:

[0062] [Equation 3]

[0063]

[0064] in:

[0065] - R is the transverse dimension of the hollow body, i.e. the radius in the case of a cylinder,

[0066] - H is the axial dimension of the hollow body, i.e. the length in the case of a cylinder,

[0067] -Range(P) is the distance that fission fragments can travel through a given gas at pressure P,

[0068] -m fissile is the mass of the fissile sediment,

[0069] -ρ max is the surface density of the fissile material.

[0070] The fissionable material is advantageously selected from all isotopes of boron-10, lithium-7, uranium-238, plutonium and neptunium.

[0071] The material used to make the window is preferably based on silicon dioxide. Silicon dioxide windows have high transmission efficiency and good radiation resistance. The thickness e determines its ability to withstand the internal pressure P of the gas, as shown in the following equation 4:

[0072] [Equation 4]

[0073]

[0074] Among them, E SiO2 is the Young's modulus of silica and R is the radius of the window.

[0075] The radius R of the window is substantially equal to the radius of the hollow body.

[0076] According to an advantageous variant of embodiment, the optical lens is a Fresnel lens. The advantage of using a Fresnel lens is that it reduces the mass of material (especially silica) needed to manufacture it and thus reduces the intensity of the Cherenkov radiation emitted by the optical components (window and lens). In an optical fiber with numerical aperture NA, the radius R of the lens is the radius of the window, while the thickness of the lens is a characteristic given by the manufacturer.

[0077] The focal length f' of the lens advantageously satisfies the following equation 5:

[0078] [Equation 5]

[0079]

[0080] Preferably, the gas filling the optical cavity is advantageously an inert gas selected from helium, neon, argon, krypton, xenon, or a mixture thereof.

[0081] The optical cavity is preferably under pressure, typically a pressure of a few bars.

[0082] According to an advantageous embodiment, the detector comprises at least one spacer arranged between the window and the hollow body and / or between the reflector and the hollow body. Each spacer acts as a physical separator between the deposit of fissile material and one of the optical components (reflector or window) to prevent premature clouding of the optical component due to the impact of fission fragments.

[0083] The axial dimension of the spacer is preferably greater than or equal to the path length of the light fission fragments (LFF) or the longest range ions in the gas through the optical cavity. LFF are emitted after the interaction of neutrons with atoms of the fissionable deposit. Fission emits two types of fission fragments, but only the light fragments are considered when setting the size. In the case of the fission of uranium-235, the LFF can be compared to a particle with an atomic mass A=95 and an initial kinetic energy E=100 MeV.

[0084] Specifically, the path length of the LFF is always longer than the path length of other fission products. To improve the collection efficiency, the inner surface of the spacer is preferably polished to allow reflection of light.

[0085] The radius of the spacer is equal to the radius of the hollow body of the OFC.

[0086] Table 2 below gives the path lengths of the LFF in an optical cavity filled with various noble gases at pressures of 1 bar and 5 bar.

[0087] [Table 2]

[0088]

[0089] It is clear from Table 2 that it is preferred to use gases with high atomic numbers to reduce the spacer height and improve the collection efficiency of the OFC.

[0090] According to an advantageous construction variant, the detection head is fastened, preferably screwed, to the hollow body of the chamber.The neutron detector is a single object, which can be compact and easy to handle.

[0091] Therefore, the present invention mainly includes a neutron detector, which includes a sealed optical fission chamber (OFC) and an optical cavity, the operation of which is based on optical conversion and contains a window and an optical lens as an optical interface at one of its longitudinal ends, and a mirror for reflecting photons away from the window at the other end.

[0092] The fiber coupler is judiciously positioned in the focal plane of the lens in order to increase the collection area of ​​the transmitted signal from the exit of the lens without permanently increasing the volume of the irradiated fiber, which is a source of noise due to Cherenkov radiation.

[0093] The deposition of the reflective layer forming the reflector is preferably tailored to the measurement wavelength. Specifically, the reflectance of the light depends on the wavelength of the incident photons. Furthermore, the most common base material NBK-7 must be avoided. Specifically, it contains B10, which leads to premature degradation of the reflector via the (n,α)-reaction: [9]. For the reflective layer, materials such as silver and gold are preferably used, since their reflectivity is high and remains constant over a wide wavelength range, in particular in the near-infrared range.

[0094] The window and the optical lens are joined into a one-piece component by bonding or integration so that the two optical components are subjected to mechanical stress together. The thickness of the lens is therefore such that a certain proportion of any mechanical stress can be applied to it and the window can be thinned. Thus, the mass of irradiated silica can be reduced, which reduces the noise due to the Cherenkov radiation being emitted into these optical components.

[0095] Neutron detectors of miniaturized size according to the present invention may be configured to withstand high temperature, high radiation environments, such as those within an operating nuclear reactor.

[0096] Finally, the detector according to the invention makes it possible to obtain an excellent signal-to-noise ratio when measuring the neutron flux.

[0097] The invention has many applications, among which the following can be mentioned:

[0098] -Online measurement of neutron flux in nuclear reactors;

[0099] - Characterization and monitoring of neutron fluxes in places other than nuclear reactors (experimental reactors or power generation reactors);

[0100] -Location of molten fuel elements during or after severe accidents (interruption of cooling or even temporary power outage);

[0101] - Positioning of encapsulation plugs, especially colloidal plutonium, during chemical processing.

[0102] - Performing neutron measurements on the neutron beam for the purpose of ensuring beam stability or indeed for the purpose of measuring the flight time on these same paths.

[0103] Other advantages and features will become more apparent on reading the detailed but non-limiting description given by way of example with reference to the following drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0104] Figure 1 It is a schematic longitudinal cross-sectional view of a fission chamber neutron detector according to the prior art.

[0105] Figure 2 is a schematic diagram illustrating the principle of light emission produced by ionization of a gas by ionized particles (generally referred to as heavy ions) produced by a reaction between neutrons and an active material.

[0106] Figure 3 is a longitudinal cross-sectional view of a neutron detector including an optical fission chamber (OFC) and a fiber coupler according to the present invention.

[0107] Figure 4 The curve shows the neutron detector according to the present invention (whose component dimensions are as follows). Figure 3 The calculation results of the angular distribution g(θ) of photons on the surface of the window (shown).

[0108] Figure 5 is a longitudinal cross-sectional view of a neutron detector including an optical fission chamber (OFC) as a comparative example.

[0109] Figure 6 The graph shows the flow of a neutron detector (with the dimensions of the components shown in FIG. Figure 5 The calculation results of the angular distribution g(θ) of photons on the surface of the window (shown).

[0110] Figure 7is a front view of a radiation chamber showing various advantageous mounting positions of a neutron detector according to the invention comprising an optical fission chamber in the case of radiation in a nuclear reactor.

[0111] Figure 8 The collection efficiency obtained as a function of the focal length of the optical lens and the distance between the optical lens and the optical fiber is shown in the form of a three-dimensional curve, which is output as the signal transmitted by the lens.

[0112] Fig. 9 The signal-to-noise ratio obtained using a fiber coupler in a neutron detector according to the present invention is shown in linear form as a function of the number of fibers in the coupler and the ratio of the lengths of the coupled fiber to the uncoupled fiber.

[0113] Fig.10 shows the distribution and energy level of the signal deposited in the detector as it is along the Figure 5 The device dimensions are shown as a function of the position of the longitudinal axis X of the neutron detector. DETAILED DESCRIPTION

[0114] Figure 1 and Figure 2 These figures have been described in the background art. Therefore, these figures will not be discussed below.

[0115] Figure 3 A neutron detector 1 according to the invention is shown.

[0116] It comprises firstly a sealed ionization chamber 2 in which the optical conversion takes place, this chamber being called OFC, short for Optical Fission Chamber, and which extends around a central axis (X).

[0117] OFC 2 comprises a sealed hollow cylinder 20 having a length H and a diameter is equal to 2R and defines an optical cavity 21 internally.

[0118] The inner wall 22 of the body is coated with a layer of fissile material, such as uranium-235, uranium-238 or boron-10.

[0119] The optical cavity 21 is filled with at least one noble gas, preferably at a pressure P, capable of being ionized by ions produced by the reaction between neutrons and fissionable material. The gas may be selected from helium, neon, argon, krypton, xenon, or mixtures thereof.

[0120] The window 23 is arranged at one of the longitudinal ends of the hollow body 20 and is configured to seal the optical cavity. In addition, a spacer 26 is arranged between the window 23 and the hollow body 20 to prevent the window 23 from being prematurely clouded due to the impact of fission fragments.

[0121] An optical lens 24 , preferably a Fresnel lens, is secured to the window by bonding or is formed integrally with the window, the optical lens being configured to focus photons received by the window 23 .

[0122] A reflector 25 is arranged at the other longitudinal end of the hollow body, which is opposite to the longitudinal end where the window is arranged. The reflector 25 is configured to reflect photons toward the window. In addition, a spacer 27 is arranged between the reflector 25 and the hollow body 20 to prevent the reflector 25 from becoming cloudy prematurely due to the impact of fission fragments. The diameters of the spacers 26 and 27 are Equal to the diameter of the hollow cylinder 20. They can be identical to each other, with the axial dimension H1 being equal to H2.

[0123] The neutron detector 1 further comprises a detection head 3 .

[0124] The detection head 3 includes a fiber coupler 30, which includes: a plurality of optical fibers 31 as input ends, which are arranged in the focal plane F of the lens 24; and an optical fiber 32 as an output end, in which optical signals received at the input ends are summed.

[0125] The inventors set various dimensions and performed various calculations, in particular using a software package abbreviated as PHITS (standing for Particle and Heavy-Ion Transport code System). The software is a MCNP simulator (MCNP stands for Monte Carlo N-Particle Transport), a numerical simulation software platform that uses Monte Carlo methods to model nuclear physics processes. The general PHITS software package was developed in the context of cooperation between the Japan Atomic Energy Agency (JAEA) and many other research institutes around the world.

[0126] To verify Figure 3 The inventors have compared the geometry of the various components of the neutron detector 1 according to the invention, in particular the optical components, with the geometry of an OFC detector 1 ′ used as a prototype in the laboratory.

[0127] Such a detector 1' is Figure 5 Schematically shown: it does not comprise mirrors, optical lenses or fibre couplers. In fact, the probe 1 ′ comprises only a window 23 spaced apart from the hollow cylindrical body 20 delimiting the chamber 21 by a spacer 26 .

[0128] The single optical fiber 31 lies directly against the window 23 along the X-axis.

[0129] In order to quantify the impact of the optical focusing system, Figure 3 The geometric shape of the neutron detector 1 is shown as a comparative example according to Figure 5 The geometry of the neutron detector 1 ′ is taken into account and the angular and radial distributions on the surface of the window 23 are calculated.

[0130] Figure 4 and Figure 6 The graphs of φ and φ show the calculated results of the angular distribution g(θ) of photons passing through the surface of the window 23 of the neutron detector 1 and the neutron detector 1 ′, respectively. These calculations were performed using the PHITS software package.

[0131] It should be noted that in both detectors 1 , 1 ′ the light source is modeled as homogeneous, extended and isotropic.

[0132] Table 3 below gives the dimensions of the considered detectors 1 , 1 ′.

[0133] [Table 3]

[0134]

[0135] In evaluating the results, it can be seen that by modifying the geometry of the OFC chamber's hollow body 20 and adding the reflector 25, the proportion of photons that pass through the surface of the window (Pwindow) is increased by a factor of 25. This value is an underestimate as it does not take into account the improvement due to the polished walls reflecting the photons.

[0136] Figure 7 Shown as Figure 3 Two neutron detectors 1 are shown advantageously mounted in two positions P1 , P2 , these neutron detectors being attached to a reflector 100 in a radiation chamber in a nuclear reactor.

[0137] Since the radial and angular distribution of the photons striking the window 23 is known, the same distribution at the window exit and hence the resulting collection efficiency can be derived analytically by means of the ray transfer matrix. The model assumes that the paraxial approximation is valid, i.e. the angle of the incident ray is small. In the case of an OFC detector 1 with the above-mentioned optimized dimensions, this is the case for more than 60% of the photons.

[0138] The inventors compared three neutron detectors with different optical components, namely:

[0139] -like Figure 5 The neutron detector 1' shown has a single sealed window.

[0140] -like Figure 3 The neutron detector 1 according to the invention is shown with a window 23 and an attached thick optical lens 24, and

[0141] -like Figure 3 A neutron detector 1 according to the invention is shown with a window 23 and an attached Fresnel lens 24 .

[0142] The transmission matrix for each of these three detectors is given in Table 4 below.

[0143] In Table 4:

[0144] - L is the distance between the optical component (window, with or without optical lens) and the fiber,

[0145] -R is the radius of curvature of the thick lens,

[0146] -f' is the focal length of the Fresnel lens,

[0147] -t is the thickness of the window, the thickness of the thick lens is set to be equal to 2mm,

[0148] -n 1 is the refractive index of air,

[0149] -n 2 is the refractive index of silicon dioxide,

[0150] -θ and r are randomly drawn from distributions calculated by the PHITS software package.

[0151] [Table 4]

[0152]

[0153] Computational simulations show that a single window has lower collection efficiency than the combined window-lens optics, which makes sense because its acceptance cone occupies only a small fraction of the emission volume.

[0154] Calculations show that the collection efficiency is increased by a factor of 1.01 for an optical system combining a window and a thick optical lens, and by a factor of 25 for an optical system combining a window and a Fresnel lens.

[0155] By combining an optical system with windows and optical lenses, the focal length and lens-fiber distance can be varied.

[0156] The sensitivity contour of the efficiency clearly shows that for the optical system combining a window and a lens, the maximum efficiency can be achieved when the fiber is placed at the image focus F of the lens.

[0157] In contrast, when a non-thinned lens was used, the collection efficiency dropped by 40 percent.

[0158] Figure 8 The collection efficiency of an optical system combining a window and a Fresnel lens is shown.

[0159] The inventors have verified the effect of the fiber coupler 30 and optimized it with respect to two parameters, namely the number of optical fibers to be coupled and the ratio x of the fiber length after coupling to the fiber length before coupling.

[0160] Considering that the radius of curvature of the optical fiber cannot be too large, the inventors believe that x can vary between 0.2 and 0.8.

[0161] By comparing the neutron detector according to the present invention (i.e., the detector with the fiber coupler) and the detector without the coupler, the inventors calculated the changes of the signal δs and the noise δb described in Equation 6 as shown below:

[0162] [Equation 6]

[0163]

[0164] Where N is the number of optical fibers through which light enters the coupler.

[0165] Fig. 9 The variation of the signal-to-noise ratio obtained by the fiber coupler according to the present invention is shown.

[0166] Computational simulations therefore allow demonstrating the advantages of the presence of various optical components and of the fiber coupler in the OFC neutron detector 1 , since they make it possible to maximize the light intensity of the light source image.

[0167] Fig.10 The axial and radial distributions of the light source energy through the neutron detector 1 according to the present invention are shown.

[0168] The shape of such a light source depends on many parameters, such as the geometry of the optical cavity, its filling gas, the pressure of the filling gas and the nature of the fissionable material, which is selected from uranium-235, boron-10, lithium-7, etc.

[0169] The complexity of the optical components of the combination of the light source and the detector 1 does not allow the calculation of the image of the light source. Since the image of the light source in the detection plane F is unknown, the multiple optical fibers positioned as they are make it possible to ensure that they occupy the position where the image of the light source is brightest. This additional degree of freedom makes it possible to further optimize the light collection and thus improve the signal-to-noise ratio.

[0170] The OFC neutron detector 1 just described has a signal collection efficiency and a signal-to-noise ratio several dozen times higher than those of OFC detectors in the prior art.

[0171] As described above, this improvement is the result of judicious choice of the optical components (mirrors, windows, and optical lenses) added to the fiber coupler, optimizing its geometry, and parameters such as the number of optical fibers through which light enters the coupler and the ratio of the length of the coupled fiber to the length of the uncoupled fiber.

[0172] Needless to say, the results of the preliminary analysis performed using the PHITS software package can be found using specialized optical calculation codes.

[0173] However, other variations and modifications may be envisaged without departing from the scope of the invention.

[0174] List of cited references

[0175] [1]: M.Lamotte, G.De Izarra, C.Jammes, "Heavy-ions induced scintillationexperiments", J.Instrum, 14(09)(2019), page C09024, https: / / doi.org / 10.1088 / 1748-0221 / 14 / 09 / C09024.

[0176] [2] M.Lamotte, G.De Izarra, C.Jammes, "Development and first use of anexperimental device for fission-induced spectrometry applied to neutron fluxmonitoring", Nucl.Instrum.Methods Phys.Res,A953(2020), page 163236, https: / / doi.org / 10.1016 / j.nima.2019.163236.

[0177] [3]: M.Lamotte, G.De Izarra, C.Jammes, "Design and irradiation test of aninnovative optical ionization chamber technology", Nucl.Instrum.MethodsPhys.Res, A968(2020), page 163945, https: / / doi.org / 10.1016j.nima.2020.163945.

[0178] [4] M. Lamotte, G. De Izarra, C. Jammes, SCENA: "simulation tool for radiation-induced gas scintillation", Nucl.Instrum.Methods Phys.Res, A982 (2020), page 164576, https: / / doi.org / 10.1016 / j.nima.2020.164576.

[0179] [5]: Cheymol G., Long H., Villard J.-F., Brichard B, "High level gamma andneutron irradiation of silica optical fibers in CEA OSIRIS nuclear reactor", IEEE Trans.Nucl.Sci, 55(4)(2008), pp. 2252-2258.

[0180] [6] Goburnov, B., “Perspectives d’utilisation de optiques nuclear converters for the enregistrement of the flux of photons in the nuclear reactors” [Prospects of using nuclear optical converters to record photon flux in nuclear reactors], Nuclear Center of the Russian Federation, (2015).

[0181] [7] Lamotte, M., “Study of the optical signals of fission chambers and evaluation of applicability to a neutron-measuring system for a Generation IV reactor” [Study of the optical signals of fission chambers and evaluation of applicability to a neutron-measuring system for a Generation IV reactor], PhD thesis, University of Grenoble, (2021).

[0182] [8] Brichard, B., “Fiber-optic gamma-flux monitoring in a fission reactor by means of Cerenkov radiation”, Measurement Science and Technology, vol. 18, 3257-3262.

[0183] [9]: Wirtenson, RH, “Radiation Induced Darkening of the Optical Element in the Startracker camera”, Technical Report from Lawrence Livermore National Laboratory, (2007).

Claims

1. A neutron detector (1), comprising: - at least one sealed ionization chamber (2) in which the optical conversion takes place, called optical fission chamber OFC, each extending along a longitudinal axis (X) and comprising: a sealed hollow body (20) defining an optical cavity (21) inside the body and comprising at least one inner wall (22) at least partially coated with at least one layer of a fissionable material, the optical cavity being filled with a gas, preferably under pressure, and capable of being ionized by ions produced by the reaction between neutrons and the fissionable material, a window (23) arranged at one of the longitudinal ends of the hollow body and configured to seal the optical cavity, an optical lens (24) secured to the window by bonding or formed integrally with the window, the optical lens being configured to focus photons received through the window, a reflector (25) arranged at the other of the longitudinal ends of the hollow body, the other longitudinal end being opposite to the longitudinal end at which the window is arranged, the reflector being configured to reflect photons toward the window, - a detection head (3), comprising a fiber coupler (30), comprising: a plurality of optical fibers (31) as input ends, the plurality of optical fibers being arranged in the focal plane of the lens; and an optical fiber (32) as an output end, in which optical signals received by the input ends are summed.

2. The neutron detector (1) according to claim 1, wherein: The OFC is axisymmetric in shape and has 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 hollow body of the OFC is a cylinder, a sphere or a cone.

4. The neutron detector (1) according to any one of claims 1 to 3, wherein: The surface density of the fissionable material is max Less than or equal to 2mg / cm 2 .

5. The neutron detector (1) according to any one of claims 1 to 4, wherein: The fissionable material is selected from isotopes of boron-10, lithium-7, uranium-238, plutonium and neptunium.

6. The neutron detector (1) according to any one of claims 1 to 5, wherein: The gas is selected from helium, neon, argon, krypton, xenon, or a mixture thereof.

7. The neutron detector (1) according to any one of claims 1 to 6, wherein: The material from which the window is made is based on silicon dioxide.

8. The neutron detector (1) according to any one of claims 1 to 7, wherein: The optical lens is a Fresnel lens.

9. The neutron detector (1) according to any one of claims 1 to 8, comprising at least one spacer arranged between the window and the hollow body and / or at least one spacer arranged between the reflector and the 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 optical fission fragments LFF through the gas of the optical cavity.

11. The neutron detector (1) according to any one of claims 1 to 10, wherein: The detection head is fastened, preferably screwed, to the hollow body of the ionization chamber.

12. Use of a neutron detector 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 detector according to any one of claims 1 to 11 for locating molten fuel elements during or after a severe accident, such as a cooling interruption or a temporary power outage.

14. Use of a neutron detector according to any one of claims 1 to 11 for positioning packaging plugs, in particular colloidal plutonium, during chemical processing.

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