Detector for radioactive gas, in particular radon

Abstract

The invention relates to a probe for detecting ionizing radiation (1), the probe comprising - a detector material; - a photodetector (3), optically coupled to a coupling face (4r) of the detector material and configured to generate a detection signal, the detection signal comprising pulses resulting from photons generated in the detector material under the effect of interactions of the ionizing radiation in the detector material; - a processing unit (10), configured to: - determine a duration of the pulses of the detection signal; - discriminate the pulses according to their duration, so as to identify short pulses (imp s ), the duration of which is shorter than a time threshold, and long pulses (imp t ), the duration of which is longer than the time threshold; - the probe being characterized in that the detector material is formed of an inorganic transparent matrix (4m) and particles of an inorganic scintillator material (4p).

Description

[0001] Description

[0002] Title: Detector for radioactive gases, especially radon

[0003] TECHNICAL FIELD

[0004] The invention relates to the detection of substances, which may be volatile, radioactive emitting alpha and beta, in particular radon.

[0005] EARLIER ART

[0006] Radon is a naturally occurring radioactive gas produced by the decay of uranium or thorium isotopes, which are naturally present in the Earth's crust. Significant radon exposure can pose a health risk, potentially causing lung disease. Radon ingestion is considered the second leading cause of lung cancer.

[0007] Radon is particularly dangerous in confined or poorly ventilated spaces, such as some homes, basements, or older buildings. It seeps through cracks in foundations, walls, and floors, and can accumulate to dangerous levels without ventilation.

[0008] The European Euratom Directive 2013 / 59 sets a maximum annual average reference value of 300 Bq-rrr 3 for radon, radon being defined as corresponding to 222 Rn.

[0009] Radon has two radioactive isotopes:

[0010] 222 Rn, which comes from the decay of 238 U, whose radioactive period is 3.8 days;

[0011] 220 Rn, sometimes referred to as Thoron, which comes from the decay of 232T h, whose radioactive half-life is 55.6 seconds

[0012] The danger of radon isotopes stems from the alpha particles emitted during each decay, but also from the radioactive decay products of each isotope, which are solid alpha and beta emitters. Regarding 222 Rn, the descendants considered most problematic are emitters a (for example 218 Po or 214 Po) or py emitters ( 214 Bi or 214 Pb). Regarding 220 Rn, the descendants considered most problematic are emitters a (for example 216 Po or 212 Pb) or -y emitters ( 212 Bi or 212 Problem or 208TI). Patent FR3113176 describes a scintillator detector made with a porous metal-organic material (MOF). This type of scintillator is formed from inorganic units linked together by scintillating organic ligands. The porosity provides a high specific surface area, which can exceed several thousand square meters. 2 -g -1 Such a device is suitable for measuring gaseous radionuclides, such as 87 Kr or radon. The porous structure allows for the adsorption of gaseous radionuclides. However, for this type of scintillator, the low luminous efficiency and their highly diffusing powder form do not allow for optimal use (no detection of 3 H and 37 Ar). They are also very sensitive to humidity. The specific surface area of ​​these scintillators gives them a very significant memory effect, making real-time monitoring of gaseous isotope concentration impossible.

[0013] French patent FR3133238 describes a plastic scintillator detector comprising fluorescent molecules embedded in an organic polymer. The detector has a high surface area to volume ratio to increase detection sensitivity. The scintillator allows discrimination between the respective contributions of alpha and phosphor particles to the detected radiation by the production of delayed fluorescence induced by the creation of triplet states by alpha particles. This discrimination is performed by Pulse Shape Discrimination (PSD), that is, by classifying each detected pulse, based on its shape, as being due to an alpha particle or a phosphor particle. Examples are given in which the detector material takes the form of lamellae, fins, or flakes. Spectrometric analysis of the detected alpha radiation allows for the partial separation of the contributions of alpha particles. 222Rn and some of his descendants: 218 Po and 214 Po. However, this resolution is degraded by the complex shape of the scintillator and the resulting light loss. In the case of a mixture of two radon isotopes, identification of each isotope is not possible. Finally, this scintillator is organic, and therefore has a memory effect on radon, making it too limiting for long-term monitoring of radon concentration.

[0014] The devices described above allow measurement of the activity of all Rn isotopes by scintillation. However, they do not allow discrimination between 222 Rn and 220 Rn.

[0015] The document Gaware JJ et al "Indigenous development of online radon and thoron monitors for applications in uranium mining and thorium processing facilities" describes a device for measuring radon activity. 222Rn. The document Orfano Matteo et al “Efficient radioactive gas detection by scintillating porous metal organic framework” describes the use of a porous metal-organic scintillator (Metalorganic framework, based on Hafmium), citing radon detection among the possible applications.

[0016] The document Kim LS et al "A new approach to monitoring radon and radon progeny using glass scintillator in a fiber bundle structure" describes a lithium glass scintillator formed by a bundle of scintillating fibers.

[0017] The document Mitev K et al. "Design and Field tests of Scintillation Spectrometer for Continuous Radon in Soil gas monitoring" describes discrimination between alpha and phosphorus radiation by power shape discrimination (PSD).

[0018] The document Marques A. et al "Direct measurements of radon activity in water from various natural sources using nuclear track detectors" describes the use of scintillator detectors to measure radon emitted by water.

[0019] The Lucchetti C. document "Testing the radon in water probe set up for the measurement of radon in water bodies" describes a radon detector comprising a polypropylene membrane.

[0020] Radon can be associated with p emitters _ , particularly among its descendants. The objective of the invention is to be able to discriminate alpha radiation from p-radiation _ One application case being a radioactive gas such as 222 Rn, noting that the invention can also be applied to solid or liquid radioactive media.

[0021] DESCRIPTION OF THE INVENTION

[0022] A first object of the invention is a probe for detecting ionizing radiation, comprising

[0023] - a detector material;

[0024] - a photodetector, optically coupled to a coupling face (4r) of the detector material, and configured to generate a detection signal, the detection signal comprising pulses, each pulse resulting from photons generated in the detector material under the effect of interactions of ionizing radiation in the detector material;

[0025] - processing unit, configured for:

[0026] • determine the duration of the detection signal pulses; • discriminate the pulses according to their duration, so as to identify short pulses, whose duration is less than a time threshold, and long pulses, whose duration is greater than the time threshold;

[0027] - the probe being characterized in that the detector material is formed of a transparent matrix, preferably inorganic, (4m) and particles of a scintillator material, in particular inorganic (4p), distributed in the transparent matrix.

[0028] The processing unit can be configured to determine:

[0029] - a number of short pulses detected, these pulses being representative of a generation of photons by Cherenkov effect, by P" particles diffusing through the transparent inorganic matrix;

[0030] - and / or a number of long pulses detected, representative of interactions between alpha particles and particles of the inorganic scintillator.

[0031] Short pulses can have a duration less than a time threshold equal to 10 ns or 20 ns and less than 10000 ns.

[0032] The transparent matrix can be inorganic by comprising or being made up of a transparent metal oxide, a transparent halide, or a transparent nitride.

[0033] The probe may include an opaque envelope, arranged around the detector material, on either side of the coupling face.

[0034] The probe may include an opaque, porous membrane positioned opposite the detector material, allowing the migration of a radioactive gas through the membrane towards the detector material. The membrane thickness is preferably greater than 100 µm.

[0035] The membrane can extend to a thickness greater than a predetermined thickness, the predetermined thickness corresponding to a migration time of the radioactive gas through the membrane of at least 30 seconds. The membrane thickness can be less than 10 mm or 5 mm. The membrane can be made of a polymer or a sintered material. The probe can include a grid covering the membrane, such that the membrane extends between the grid and the scintillator material.

[0036] The sensor material may be porous.

[0037] The sensing material can be segmented into different elements, so that the gas propagates between said elements.

[0038] According to one possibility, the casing, the detector material, and the porous membrane form a detection head, the detection head being removable from the photodetector. A second object of the invention is a method for determining 222Rn in an analyzed medium, likely to contain 222 Rn, comprising at least the following steps:

[0039] - exposure of a probe according to the first object of the invention to 222 Rn likely to be contained in the analyzed medium;

[0040] - obtaining an a component of the detection signal, representative of a number of long pulses (impi) detected during the exposure;

[0041] - application of a calibration function to said component a, for example a counting rate, to determine an activity of 222 Rn in the analyzed environment.

[0042] In one scenario, the medium is gaseous, with the probe placed within the analyzed medium. In another scenario, the analyzed medium is a liquid, contained within a chamber, with the probe placed inside the chamber, at a distance from the analyzed medium.

[0043] According to one possibility, the medium being analyzed is a liquid medium, with the probe being immersed in the medium being analyzed.

[0044] The process may involve obtaining a p-component of the detection signal, representative of a number of short pulses detected during exposure.

[0045] The second object of the invention can be extended to a method for determining the activity of at least one isotope a of an analyzed medium, said medium being capable of comprising at least one emitting isotope.

[0046] The invention will be better understood by reading the explanation of the examples of embodiment presented, in the continuation of the description, in connection with the figures listed below.

[0047] FIGURES

[0048] Figure 1 and Figure 2 schematically illustrate a probe according to the invention.

[0049] Figure 3 schematically illustrates an assembly of a detection head against a photodetector. Figure 4A schematically illustrates an example of a scintillator with channels.

[0050] Figure 4B schematically illustrates an example of a scintillator formed by an assembly of scintillating fibers.

[0051] Figure 4C schematically represents a scintillator formed by an assembly of elementary scintillators. Figure 4D schematically represents an embodiment of a detector material.

[0052] Figure 5 shows an example of probe implementation.

[0053] Figure 6A is a histogram showing the number of pulses as a function of their respective durations.

[0054] Figure 6B is a histogram of the amplitude of the pulses shown in Figure 6A whose duration exceeds a threshold. Figure 6C is a histogram of the amplitude of the pulses shown in Figure 6A whose duration is below the threshold.

[0055] PRESENTATION OF SPECIFIC IMPLEMENTATION METHODS

[0056] Figure 1 schematically illustrates an embodiment of a probe according to the invention. The probe comprises a detection head 1, connected to a photodetector 3. The detection head 2 comprises a scintillator material 4, configured to generate light pulses upon detection of ionizing radiation, in particular alpha or β radiation. The light pulses consist of scintillation photons resulting from the interactions of ionizing particles, specifically alpha or β, within the scintillator material. The light pulses are generated in a spectral emission band, generally located in the visible or near-ultraviolet range.

[0057] The photodetector 3 is configured to generate a usable measurement signal from the scintillation light pulses generated in the scintillator material. The measurement signal consists of electrical pulses, each pulse corresponding to an interaction of an ionizing particle, for example a, P', or y, in the scintillator. The amplitude of each pulse is preferably proportional to the energy released in the scintillator material 4 during each interaction.

[0058] The scintillator material is preferably an inorganic scintillator. Compared to MOF porous scintillators and plastic scintillators described in connection with the prior art, the inventors have found that an inorganic scintillator does not exhibit a memory effect for radioactive gases.

[0059] The memory effect corresponds to the trapping, by adsorption or absorption, of gaseous radionuclides in the pores or by chemical affinity with the scintillators, resulting in prolonged exposure of the scintillator to the radiation emitted by the trapped radionuclides. It has been observed that the memory effect is also present in non-porous organic scintillators, as described in the publication Pressyanov, D. et al., "Sorption and desorption of radioactive noble gases in polycarbonates," Nuclear Instruments and Methods in Physics Research A 598 (2009) 620-627. The inventors believe that the memory effect is detrimental to monitoring the temporal evolution of radon concentration. Therefore, the use of an inorganic scintillator is preferable. In addition to the absence of the memory effect, inorganic scintillators can be completely insensitive to humidity and more durable over time than organic molecules.

[0060] Preferably, the scintillator material is structured to have a high specific surface area, the specific surface area being a surface area to volume ratio. It may be a porous material, or a material subdivided into different elements, for example, fibers, assembled together, and between which a gas can flow. The subdivision of the material into several elements may also be carried out as described in relation to FR3133238. The specific surface area is preferably greater than 1 cm². 2 / g.

[0061] The scintillator material 4 comprises a front face 4a, intended to be exposed to the analyzed radioactive gas, and a rear face 4r, preferably opposite the front face, intended to be positioned facing the photodetector 3, and preferably in contact with it. The rear face 4r is a coupling face intended to be optically coupled to the photodetector.

[0062] An opaque casing 5 extends around the scintillator material 4, on either side of its rear face, to prevent illumination of the latter by external visible radiation. In this example, it is a rigid casing whose inner face is coated with a material that reflects scintillation photons. This could be, for example, a metallic casing, such as stainless steel, or a reflective coating or film. In this example, the casing is cylindrical. When the casing is rigid, it also provides mechanical protection. The casing may have a transparent face 5r, intended to cover the rear face 4r of the scintillator and to form an interface with the photodetector 3. "Transparent" here refers to transparency to scintillation photons.The scintillator 4 is positioned against a photodetector 3, configured to generate an electrical pulse from the light pulse emitted by the scintillator. The photodetector has a transparent entrance window 3a, against which the rear face 4r of the scintillator is pressed, possibly with an optical coupling. The photodetector 3 can, for example, be a photomultiplier tube, such as a silicon photomultiplier tube (SiPM), or a black silicon-based sensor or a silicon diode.

[0063] The detection head 2 preferably includes a porous membrane 6, positioned opposite the front face 4a of the scintillator. Preferably, the porous membrane is applied against the front face 4a of the scintillator. The porous membrane 6 is designed to be exposed to the radioactive gas and to allow the migration of gaseous radionuclides to the scintillator material 4. The thickness of the porous membrane 6 is sized so that the migration time of a gaseous radionuclide is greater than 1 minute, or even several minutes, typically between 1 and 10 minutes, so that only radionuclides with a radioactive half-life greater than the migration time, or 2 or 3 times the migration time, reach the scintillator 4 in sufficient quantity to allow the formation of a detection signal usable by the photodetector 3.

[0064] The probe is, for example, designed to measure the activity of 222Rn, by limiting the contribution of 220 Rn in the detected signal. Given the difference in radioactive half-lives, 222 Rn can diffuse over longer distances before disintegrating. 220 Rn decays much more rapidly. Therefore, the membrane is sized so that the diffusion time of 220 Rn is greater than its radioactive half-life, i.e., 55.6 seconds, while the diffusion time of 222 Rn is less than its radioactive half-life (approximately 3.8 days). The discrimination between 220 Rn and 222 Rn is calculated based on their respective radioactive half-lives, with the membrane acting as a time filter, in order to maximize the amount of 222 Rn reaching the scintillator detector relative to 220 Rn.

[0065] The porous membrane 6 can be an opaque polymer membrane, for example, a BK5 type membrane (Thorlabs). It is important that the porous membrane 6 be opaque to prevent ambient light from illuminating the scintillator 4. When made from a polymer, the thickness of membrane 6 can range from 15 pm to 1 mm. Beyond this thickness, the diffusion kinetics of radionuclides through membrane 6 are too slow, which reduces sensitivity to 222 Rn.

[0066] Another example of a porous polymer membrane is a crystalline or semi-crystalline polymer membrane, in which some regions of the polymer chain are regularly arranged to form crystals, while other regions remain amorphous. Crystalline and semi-crystalline polymers exhibit opacity compatible with coupling to a scintillator. Examples of crystalline polymers include polyethylene (PE), polypropylene (PP), and nylon. Regions of the polymer chain in the amorphous state are also considered less conducive to the diffusion of gaseous radionuclides.

[0067] The porous and opaque membrane 6 can also be made of a sintered material, for example, a metallic sintered material, in which case its thickness can exceed 1 mm. More generally, the optimal thickness range for the membrane depends on its porosity and the materials from which it is made. This can be determined experimentally by exposing the probe to a standard radioactive gas of known composition. The migration time of the standard radioactive gas is then estimated by examining the time evolution of the detection signal resulting from the photodetector 3. The detection signal increases progressively, depending on the amount of gas that has reached the scintillator, until it stabilizes.

[0068] Figure 2 shows the membrane 6 prior to its apposition against the front face 4a of the scintillator 4.

[0069] The detection head 2 may include a grid 7, covering the membrane 6, so that the membrane extends between the grid 7 and the scintillator 4. The grid 7 is shown schematically in Figure 3. The grid 7 is preferably made of a rigid material, for example metallic, and provides mechanical protection for the membrane 6 and the scintillator 4. The grid defines an open mesh, the open surface fraction preferably being greater than 80% or even 90% or 95% in order to promote the passage of gas through the mesh.

[0070] Advantageously, the detection head 2 is removably positioned against the photodetector 3. It is thus easily interchangeable. The detection head is preferably connected to the photodetector 3 by a reversible means, for example a flange or a clamping ring 8. This allows the detection head 2 to be replaced after it has been exposed to a contaminating environment, or placed in a contaminating environment, for example in a liquid.

[0071] Preferably, the scintillator is structured to optimize the surface area accessible to the gas. The goal is to achieve a high specific surface area. Indeed, alpha particles have a limited range within the scintillator. It is preferable to prioritize the gas-accessible surface area of ​​the scintillator over its thickness. It can be a porous scintillator, with open porosity, so that gaseous radionuclides can diffuse through the pores. Examples of porous inorganic scintillators are described in Raphael ML et al., "Real-time detection and discrimination of radioactive gas mixtures using nanoporous inorganic scintillators," Nature Photonics, Vol. 18, October 2024, pp. 1037–1043. The porosity (pore volume fraction) can, for example, be greater than 10%. It can reach 50%, or even 80% or 90%, or even more, provided the scintillator material has sufficient mechanical strength.A high volume fraction of porosity (> 50% or more) is made possible by the fact that the radiation interacted within a surface layer of the scintillator material, typically less than 1 mm thick, or even less than 500 µm. The scintillator material may also have been structured by machining, molding, or 3D printing to increase its specific surface area. Examples of the structuring of organic scintillators are described in FR3133238.

[0072] Figure 4A shows an example of scintillator material 4, in which cylindrical microchannels 4c have been formed to allow the passage of radioactive gas through them. The diameter of each microchannel can be several tenths of a millimeter or a few millimeters, for example between 0.5 mm and 2 or 3 mm.

[0073] Figure 4B schematically illustrates scintillating fibers 4f, the assembly of which forms a scintillator material 4 with a high surface area to volume ratio. These could be, for example, organic or inorganic scintillating fibers. The diameter of each scintillating fiber could be, for example, a few tenths of a millimeter, for example, between 0.5 mm and 1 mm. The scintillating fibers are held together by a framework 4g, schematically represented by dashed lines in Figure 4B. Examples of inorganic scintillating optical fibers were described in the publication Dujardin C. et al., "Needs, trends and advances in inorganic scintillators," IEEE Transactions on Nuclear Science, Vol. 65, No. 8, August 2018; see in particular IV A.

[0074] With regard to scintillator fibers, these can be fibers made from an organic material, such as polystyrene, containing scintillator particles. When the scintillator is formed from assembled scintillator fibers, the radioactive gas being analyzed can propagate between the fibers and interact with them. Preferably, the scintillator fibers are not sheathed, so that the alpha particles emitted by the radioactive gas being analyzed are not absorbed before reaching the scintillator material.

[0075] The dimensions of scintillator 4 can be such that the diagonal, or largest diameter, is between 20 mm and 80 mm, or between 1 and 3 inches, and that the thickness, corresponding to the distance between the front and back faces, is also between 20 and 80 mm. The larger the volume of the scintillator, the greater the sensitivity. However, an excessively large volume may prove incompatible with exposure to a high quantity of radioactive gas, due to the risk of saturation.

[0076] When the scintillator is large, it can be obtained by juxtaposing elementary scintillators 4i, as shown schematically in Figure 4C. For example, the thickness of each elementary scintillator is 1 cm. The size of the scintillator is adjusted according to its porosity, or specific surface area, and its intrinsic sensitivity, that is, the number of scintillation photons produced per amount of energy deposited.

[0077] It is considered that with regard to the detection of 222 Rn, with an inorganic scintillator of 100 m porosity 2 -g -1 A diameter (or larger diagonal) of 1 inch and a thickness of 2 inches may be suitable for quantifying 222 Rn at levels corresponding to the previously mentioned Euratom directive.

[0078] As previously mentioned, using an inorganic scintillator limits the trapping of radioactive gas at the scintillator, thus facilitating the establishment of an equilibrium concentration of radioactive gas between the detector head and the surrounding environment. This also allows for immediate re-equilibration in the event of a change in concentration.

[0079] The photodetector 3 is connected to a processing unit 10, which is configured to receive and process the detection signal from the photodetector to form a measurement signal. The processing unit can be a computer or include a microprocessor. The measurement signal can be a count rate, corresponding to the number of interactions detected per unit of time. In one scenario, the processing unit implements a Pulse Shape Discrimination (PSD) algorithm to separate the pulses based on a duration or shape parameter. An example of using such an algorithm is described in FR3133238.

[0080] The use of a processing unit implementing a PSD-type algorithm is particularly well-suited to a SiC-YaGCe (Y3AI50i2:Ce) scintillator. YAG stands for yttrium aluminum garnet. Such a scintillator consists of a transparent 4m SiC matrix in which scintillating 4p YAG:Ce nanoparticles are embedded. With this type of scintillator, it is possible to discriminate:

[0081] a component, called Cherenkov, of the detection signal, which corresponds to the light pulses produced, by the Cherenkov effect, by P" particles interacting in the SiOz matrix ;

[0082] A component α of the detection signal corresponds to the light pulses produced by the interactions of α radiation with the YaGCe scintillating nanoparticles. From this component, a count rate α can be obtained. Thus, the measurement signal resulting from the processing unit can include either the α component or the Cherenkov component, the latter corresponding to the p component. _ , the Cherenkov effect due to p particles _ .

[0083] The discrimination is based on the duration of the light pulses, the pulses resulting from the Cherenkov effect being shorter (duration of a few ns) than the pulses produced by the interactions of alpha radiation with scintillating nanoparticles (duration of a few tens of ns), the latter being longer, due to the temporal de-excitation properties of scintillating particles.

[0084] The discrimination between a and P particles offers several advantages:

[0085] When the processing unit 10 includes a spectrometry circuit, configured to form a spectrum—that is, a histogram of the detected interactions as a function of their energy—a spectrum can be obtained that is representative only of the interactions generated by alpha particles. This allows discrimination between the detection signal due to 222 Rn as well as its emitting descendants a (for example 218 Po or 214 Po).

[0086] a separation between 222 Rn and the emitting descendants P" (for example 214 Problem or 214 Bi) making the direct measurement of the radiation a of 222Rn is possible, which allows for faster processing without waiting for any equilibrium. Furthermore, the stopping power of high-energy P" particles is difficult to model with such a geometry. The detection efficiency for P" particles requires calculations. Conversely, the detection of an alpha particle is immediate upon reaching the scintillator, making the analysis's detection efficiency achievable at 100% for each alpha particle.

[0087] Discrimination between interactions generated by α and P particles, based on pulse duration, is possible when the detector consists of inorganic scintillating particles 4p dispersed in a transparent inorganic matrix 4m. The transparent inorganic matrix can be formed:

[0088] of a metal oxide, as previously described, SiOz being particularly suitable, and knowing that it can also be Al2O3, MgO, ZnO or, to a lesser extent, TiOz-. Metal oxides, for example SiC or TiC, can be synthesized simply, and in a wide variety of sizes, by sol-gel method. of a halide, for example CaFi, LiF. of a nitride, for example BN (boron nitride).

[0089] The concentration of scintillating particles must be high enough to increase the probability of interaction with alpha particles, while being limited so as to limit the probability of interaction with phosphorus particles. - and to increase light emission by the Cherenkov effect using P particles - The scintillating particles can have a size (for example, 10 nm) ranging from nanometric to between 5 nm and 100 nm. The mass fraction of scintillating particles can, for example, be between 10% and 50%.

[0090] The invention can be implemented using various types of inorganic scintillators, such as, but not limited to: CeF3, LaBr3, CeBr3, Csl, BGO (bismuth germanate), or YSO (CeF3-doped yttrium orthosilicate). 3+ or Pr 3+ ) or LYSO (ce-doped yttrium lutetium orthosilicate 3+ or Pr 3+ ), as well as other materials: garnet (which includes YAG:Ce) (for example LuAG, GaGG... doped Ce), and those of the perovskite family (YAP:Ce for example).

[0091] When scintillators take the form of scintillating particles, inorganic scintillators with a decay time on the order of or greater than a few tens of nanoseconds are preferred, ideally greater than 10, 20, or 40 ns, to facilitate temporal discrimination between short pulses, corresponding to the Cherenkov effect, and long pulses, resulting from scintillation. Scintillator materials include YAG:Ce, YAP:Ce (YAP for Yttrium Aluminum Perovskite YAIO3), CeF3, Csl, YSO4, LuAGCe (LuAG for Aluminum Lutetium Garnet), and BGO.

[0092] Thus, pulses are classified according to their duration, taking into account a temporal threshold that is higher than the decay rate of light pulses emitted by the Cherenkov effect and lower than the decay rate of light pulses emitted by scintillation. The decay rate of scintillation pulses depends on the scintillator material forming the scintillating particles. The temporal threshold can be defined a priori, based on the decay characteristics of scintillation light, which are generally known. The temporal threshold can also be defined, or adjusted, experimentally.

[0093] Figure 4D schematically depicts a detector material 4 comprising a transparent inorganic matrix 4m, in which inorganic scintillating particles 4p have been distributed. The schematic representations include Cherenkov photon emission by a p- particle (dashed arrows) and scintillation photon emission (dashed arrows) by an alpha particle interacting with a scintillating particle 4p. Short electrical pulses are also schematically represented. s and long impressions t resulting from a photodetector 3, corresponding respectively to the emission of Cherenkov photons and the emission of scintillation photons. In Figure 4D, each schematic pulse corresponds to the amplitude (ordinate axis) as a function of time (abscissa axis).

[0094] Photon generation by the Cherenkov effect occurs when the energy of the particles P -exceeds c / n, where c is the speed of light in a vacuum and n is the refractive index of the transparent matrix. In the SiO2 matrix as previously described, with a YAG particle mass fraction of 30%, the refractive index is on the order of 1.12. Therefore, it is the P" particles whose energy is greater than 1.13 MeV. Knowing that the maximum energy of the p particles of 222 Rn is equal to 3 MeV.

[0095] The higher the refractive index of the matrix, the lower the energy threshold for generating photons via the Cherenkov effect. The matrix's refractive index can be increased by arranging transparent, non-scintillating, high-index particles within the transparent matrix. Adding high-index particles allows control over the Cherenkov effect threshold. Lowering the energy threshold for the Cherenkov effect improves the detector's sensitivity to P particles.

[0096] Adjusting the Cherenkov energy threshold can allow for the selection of detected isotopes. In the case of a gas containing a mixture of 222 Rn and of 85 Kr, which is common in some online environmental monitoring, a Cherenkov energy threshold above 600 keV allows the detection of only the descendants of 222 Rn emitters P - because the P particles - issued by 85 Kr have an energy below 600 keV. Conversely, a Cherenkov energy threshold below 600 keV also allows detection of P" particles emitted by 85 Kr. By implementing two different detectors, one with a Cherenkov energy threshold below 600 keV, the other with a Cherenkov energy threshold above 600 keV, it is possible to measure the respective contributions of the progeny, emitters P - , of 222 Rn as well as P particles - issued by 85Kr or only the contributions of descendants, issuers P - , of 222 Rn. The transparent inorganic matrix can be porous, with open porosity, which can be adapted for gas detection.

[0097] Probe 1, as previously described, can be used by placing it in air, so as to estimate a concentration of 222 Rn in ambient air, regardless of the concentration of 220 Rn. The relationship between the measurement signal and the concentration of 222 Rn can be determined by a calibration function established with calibration gases whose respective concentrations in 222 Rn are known. Figure 5 shows one possible implementation of the probe, in order to determine a concentration of 222Rn in a liquid. Probe 1 is placed in a sealed chamber C containing a liquid L to be analyzed, overlain by an ambient gas G. An equilibrium is established between the liquid L and the ambient gas G, under the effect of which 222 Rn, initially present in liquid L, migrates into the ambient gas G through outgassing. The detection head 2 extends into the ambient gas G, for example air, facing the liquid L. The probe allows for the estimation of a concentration of 222 Rn in ambient gas G. Knowledge of the equilibrium allows us to estimate the concentration of 222 Rn in liquid L from the concentration 222 Rn is determined in ambient gas G. The concentration is estimated by applying a calibration function to the measurement signal resulting from the processing unit. This is generally a count rate, i.e., the number of particles detected per second. The calibration function takes into account a relationship between the concentration of222 Rn in the liquid and the measured concentration in the ambient gas G. The calibration function is stored in the processing unit's memory. Depending on the scenario, probe 1 can be immersed in the liquid, provided that the porous membrane 6 and the casing 5 are sealed.

[0098] The detector can be protected by a non-porous, airtight membrane, for example, a Mylar membrane (trademarked polyethylene terephthalate), thin enough to allow a high proportion of alpha particles to pass through, while still exhibiting good mechanical resistance. The membrane thickness is preferably less than 10 µm. A detector consisting of a porous matrix of SiO₂ and cylindrical YaGCe particles was implemented. The detector was interposed between two pipes through which air flowed. The air flowed between the two pipes, passing through the porous matrix. The air contained 222Rn and his descendants.

[0099] The dimensions of the transparent, porous inorganic matrix were 25 mm in diameter by 41 mm in length. The porosity was approximately 120 µm 2 / g. The porosity was obtained by drying with supercritical CO2. The pores had a maximum distribution around 30 nm, the porosity being essentially open, so as to allow gas to flow through the pores.

[0100] The detector was optically coupled to three photomultiplier tubes arranged at 120° around the periphery of the cylinder. The pulses resulting from each of the three photomultiplier tubes were collected. Figure 6A shows a histogram of the pulse durations from the photomultiplier tubes. A clear separation is observed between short pulses (duration less than 10 ns) and long pulses. The acquisition time was 1 h. Such a histogram allows the definition or adjustment of the temporal threshold at which long or short pulses can be discriminated.

[0101] Figure 6B shows a histogram of the energy of long pulses imp t This forms an energy spectrum of the alpha particles, which can be used. Figure 6C shows a histogram of the energy of the short pulses. sIn Figures 6A, 6B, and 6C, the x-axis corresponds to the channels. This is a spectrum of Cherenkov photons resulting from the passage of P particles. This histogram is not usable as such. However, Figures 6A and 6B show that the detector according to the invention allows discrimination between alpha activity and P activity. Applied to 222 Rn, this allows an estimation of the activity a by minimizing interferences with the activity P of the offspring.

[0102] The implementation of a detector according to the invention is not limited to the analysis of the activity of 222Rn, and more generally, an analysis of the activity of an alpha-emitting gas. It can be applied to measuring an alpha activity, particularly in the presence of potential P emitters, on a solid medium, or facing or within a liquid medium, for example, water, as previously described. For this purpose, the detector is exposed to the analyzed medium, positioned facing or in contact with it. The processing unit can then extract an alpha (and / or P) component from the detection signal. A calibration function can be applied to the alpha and / or P component to determine an alpha and / or p activity.

Claims

DEMANDS 1. Probe for detecting ionizing radiation (1), comprising - a detector material; - a photodetector (3), optically coupled to a coupling face (4r) of the detector material, and configured to generate a detection signal, the detection signal comprising pulses, each pulse resulting from photons generated in the detector material under the effect of interactions of ionizing radiation in the detector material; - an opaque envelope (5), arranged around the detector material, on either side of the coupling face; - processing unit (10), configured for: • determine the duration of the detection signal pulses; • discriminate between impulses based on their duration, in order to identify short impulses (imp s), whose duration is less than a time threshold, and long pulses (impi) whose duration is greater than the time threshold; - the probe being characterized in that the detector material is formed of an inorganic transparent matrix (4m) and particles of an inorganic scintillator material (4p), distributed in the transparent matrix.

2. Probe according to claim 1, wherein the processing unit is configured to determine: - a number of short pulses detected, these pulses being representative of a generation of photons by Cherenkov effect, by P" particles diffusing through the transparent inorganic matrix; - and / or a number of long pulses detected, representative of interactions between alpha particles and particles of the inorganic scintillator.

3. Probe according to any one of the preceding claims, wherein the short pulses have a duration less than a time threshold equal to 10 ns or 20 ns and less than 10000 ns.

4. Probe according to any one of the preceding claims, wherein the transparent inorganic matrix comprises or is made up of a transparent metal oxide or a transparent halide or a transparent nitride.

5. Probe according to any one of the preceding claims, comprising an opaque, porous membrane (6) disposed facing the detector material, so as to allow migration, towards the detector material, through the membrane, of a radioactive gas, the thickness of the membrane is greater than 100 pm.

6. Probe according to claim 5, wherein the membrane extends to a thickness greater than a predetermined thickness, the predetermined thickness corresponding to a migration time of the radioactive gas, through the membrane, of at least 30 seconds.

7. Probe according to any one of claims 5 or 6, wherein the thickness of the membrane is less than 10 mm or 5 mm.

8. Probe according to any one of the preceding claims, wherein the membrane is a polymer membrane or a sintered material membrane.

9. Probe according to any one of the preceding claims, comprising a grid (7) covering the membrane, so that the membrane (6) extends between the grid (7) and the sensing material (4).

10. Probe according to any one of the preceding claims, wherein the transparent inorganic matrix is ​​porous.

11. Probe according to any one of the preceding claims, wherein the sensing material is segmented into different elements (4f), so that the gas propagates between said elements.

12. Probe according to any one of the preceding claims, wherein the casing (5), the sensing material (4) and the porous membrane (6) form a sensing head (2), the sensing head being removable from the photodetector.

13. Method for determining an activity of 222 Rn in an analyzed medium, likely to contain 222 Rn, comprising the following steps: - exposure of a probe (1) according to any one of the preceding claims to 222 Rn likely to be contained in the analyzed medium; - obtaining an a component of the detection signal, representative of a number of long pulses (impi) detected during the exposure; - application of a calibration function to the a component to determine an activity of 222 Rn in the analyzed environment.

14. A method according to claim 13, comprising obtaining a component of the detection signal, representative of a number of short pulses (imp s ) detected during exposure.

15. Method according to claim 13 or claim 14, wherein the medium is gaseous, the probe being disposed in the analyzed medium.

16. Method according to claim 13 or claim 14, wherein the analyzed medium is a liquid medium (L), disposed in a chamber, the probe being disposed in the chamber, at a distance from the analyzed medium.

17. Method according to claim 13 or claim 14, wherein the analyzed medium is a liquid medium, the probe being immersed in the analyzed medium.

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