Radioactive gas detector, particularly for radon

Abstract

A detection probe (1), comprising an inorganic scintillator material (4), configured to generate photons, in an emission spectral band, under the effect of irradiation by alpha radiation; a photodetector (3), optically coupled to a coupling face (4r) of the scintillator material, and configured to generate a detection signal dependent on a number of photons generated in the scintillator material; an opaque envelope (5), disposed around the scintillator material, on either side of the coupling face; the probe being characterized in that it comprises an opaque, porous membrane (6), disposed facing the scintillator material, so as to allow migration, towards the scintillator material, through the membrane, of a radioactive gas, the membrane extending to a thickness greater than 100 µm.

Description

Title of the invention: Radioactive gas detector, in particular radon detector. Technical field

[0001] The invention relates to the detection of a radioactive gas, in particular radon. EARLIER ART

[0002] Radon is a naturally occurring radioactive gas, resulting from the decay of uranium or thorium isotopes, which are naturally present in the Earth's crust. Significant exposure to radon can pose a health risk, causing lung diseases. Radon ingestion is considered the second leading cause of lung cancer.

[0003] 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 in the absence of ventilation, can accumulate to dangerous levels.

[0004] The European Euratom Directive 2013 / 59 establishes a maximum annual average reference value of 300 Bq-m3 for radon, radon being defined as corresponding to 222Rn.

[0005] Radon has two radioactive isotopes: 222,238 - Rn, which comes from the decay of U, whose radioactive period is 3.8 days; - 220Rn, sometimes called Thoron, which comes from the decay of 232Th, of which The radioactive half-life is 55.6 seconds

[0006] The danger posed by radon isotopes is due to the alpha particles emitted during each decay, but also to the radioactive decay products of each of these isotopes, which are solid decay products. For 222Rn, the decay products considered most problematic are alpha emitters (for example, 218Po or 214Po) or ³³γ emitters (214Bi or 214Pb). For 220Rn, the decay products considered most problematic are alpha emitters (for example, 216Po or 212Pb) or ³³γ emitters (212Bi or 212Pb or ²O₈T1).

[0007] French patent FR3113176 describes a scintillator detector made of 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 m²-g-'. Such a device is suitable for measuring gaseous radionuclides, such as 87Kr 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 diffusive powder form prevent optimal use (no detection of ³H and ³⁷Ar). 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 concentrations impossible.

[0008] French patent FR3133238 describes a plastic scintillator-type 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 [3] particles to the detected radiation by the production of delayed fluorescence induced by the creation of triplet states by alpha particles. The discrimination is performed by PSD (Pulse Shape Discrimination), that is, by classifying each detected pulse, according to its shape, as being due to an alpha particle or a [3] particle. Examples are given in which the detector material takes the form of lamellae, fins, or flakes.Spectrometric analysis of the detected radiation allows for the partial separation of the contributions of radon (Rn) and some of its decay products (Po and 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, the 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.

[0009] The devices described above allow for the measurement of the activity of all Rn isotopes by scintillation. However, they do not allow for discrimination between 222Rn and 220Rn. Description of the invention

[0010] A first object of the invention is a detection probe, comprising: - a scintillator material, preferably inorganic, configured to generate photons under the effect of irradiation by alpha radiation; - a photodetector, optically coupled to a coupling face of the scintillator material, and configured to generate a detection signal dependent on a number of photons generated in the scintillator material; - an opaque envelope, arranged around the scintillator material, on either side of the coupling face;

[0011] the probe being characterized in that it comprises an opaque, porous membrane, disposed facing the scintillator material, so as to allow migration towards the scintillator material, through the membrane, of a radioactive gas. The thickness of the membrane is preferably greater than 100 pm.

[0012] 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.

[0013] The probe may include a grid, covering the membrane, so that the membrane extends between the grid and the scintillator material.

[0014] The scintillator material is preferably porous.

[0015] The scintillator material can be segmented into different elements, so that the gas propagates between said elements.

[0016] According to one possibility: - the scintillator material is sensitive to radiation [3; - the probe includes a processing unit, configured to discriminate between detection signals resulting respectively from interactions of a and [3] particles in the detector material.

[0017] According to one possibility, the envelope, the scintillator and the porous membrane form a detection head, the detection head being removable from the photodetector.

[0018] A second object of the invention is a method for determining the activity of Rn in an analyzed medium, which may contain Rn, comprising at least the following steps: - exposure of a probe according to the first object of the invention to 222Rn likely to be contained in the analyzed medium; - application of a calibration function to a measurement signal, for example a count rate, resulting from the probe, to determine an activity of 222 Rn in the analyzed medium.

[0019] According to one possibility, the medium is gaseous, the probe being disposed in the analyzed medium.

[0020] According to one possibility, the analyzed medium is a liquid medium, disposed in an enclosure, the probe being disposed in the enclosure, at a distance from the analyzed medium.

[0021] According to one possibility, the analyzed medium is a liquid medium, the probe being immersed in the analyzed medium.

[0022] The invention will be better understood upon reading the description of the exemplary embodiments presented later in this description, in connection with the figures listed below. FIGURES

[0023] Fig. 1 and Fig. 2 schematically illustrate a probe according to the invention.

[0024] Fig. 3 schematically illustrates an assembly of a detection head against a photodetector.

[0025] Figure 4A schematically illustrates an example of a scintillator comprising channels.

[0026] Figure 4B schematically illustrates an example of a scintillator formed by an assembly of shimmering fibers.

[0027] Fig. 4C schematically represents a scintillator formed by an assembly of elementary scintillators.

[0028] Figure 5 represents an example of probe implementation. PRESENTATION OF SPECIFIC IMPLEMENTATION METHODS

[0029] Figure 1 schematically illustrates an embodiment of a probe according to the invention. The probe comprises a detection head 2, 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 gamma radiation. The light pulses are formed from scintillation photons due to the interactions of ionizing particles, in this case alpha or gamma, in the scintillator material. The light pulses are generated in an emission spectral band, generally located in the visible or near-ultraviolet range.

[0030] The photodetector 3 is configured to generate a usable detection signal from the scintillation light pulses generated in the scintillator material. The detection signal consists of electrical pulses, each pulse corresponding to an interaction of an ionizing particle, for example a or [3 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.

[0031] 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.

[0032] The memory effect corresponds to the trapping, by adsorption or absorption, of gaseous radionuclides in the pores or by chemical affinity with the scintillators, which induces prolonged exposure of the scintillator to the radiation emitted by the trapped radionuclides. It has been observed that in organic, non-porous scintillators, the memory effect is also present, 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. The use of an inorganic scintillator is therefore preferable.

[0033] In addition to the absence of memory effect, inorganic scintillators can be totally insensitive to humidity and more resistant over time than organic molecules.

[0034] 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 in the form of 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 / g.

[0035] 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.

[0036] An opaque casing 5 extends around the scintillator material 4, on either side of its rear face, so as 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. It may, for example, be 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 acts as mechanical protection. The casing may include a transparent face 5r, intended to cover the rear face 4r of the scintillator and to form an interface with the photodetector 3. By transparent, it is understood to mean transparent to scintillation photons.

[0037] The scintillator 4 is arranged against a photodetector 3, configured to generate an electrical pulse from the light pulse generated by the scintillator. The photodetector has a transparent entrance window 3a, against which the rear face 4r of the scintillator is applied, optionally with an optical coupler. The photodetector 3 may, for example, be a photomultiplier tube, for example a silicon photomultiplier (SiPM), or a black silicon-based sensor or a silicon diode.

[0038] The detection head 2 comprises 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 intended 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 dimensioned to so that the migration time of a gaseous radionuclide is greater than 1 minute, or even a few minutes, typically between 1 minute and 10 minutes, so that only radionuclides whose radioactive period is greater than the migration time, or 2 or 3 times the migration time, reach scintillator 4 in sufficient quantity to allow the formation of a detection signal usable by photodetector 3.

[0039] The probe is, for example, designed to measure the activity of 222Rn, minimizing the contribution of 220Rn to the detected signal. Given the difference in radioactive half-lives, 222Rn can diffuse over longer distances before decaying. 22Rn decays much more rapidly. The membrane is therefore sized so that the diffusion time of 220Rn is greater than its radioactive half-life, i.e., 55.6 seconds, while the diffusion time of 222Rn is less than its radioactive half-life (on the order of 3.8 days). The discrimination between OOH 000 Rn and Rn is carried out on the basis of their respective radioactive periods, the membrane acting as a time filter, so as to maximize the amount of 222Rn reaching the scintillator detector relative to 220Rn.

[0040] 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 illumination of the scintillator 4 by ambient light. When made from a polymer, the thickness of the membrane 6 can be between 15 pm and 1 mm. Beyond this, the diffusion kinetics of radionuclides through the membrane 6 are too slow, which reduces the sensitivity to 222Rn.

[0041] Another example of a porous polymer membrane is a so-called crystalline or semi-crystalline polymer membrane, in which certain regions of the polymer chain are regularly organized to form crystals, while other regions may remain in an amorphous state. Crystalline or semi-crystalline polymers exhibit opacity compatible with coupling to a scintillator. Examples of crystalline polymers are polyethylene (PE), polypropylene (PP), and nylon. Regions of the polymer chain in the amorphous state are also considered less favorable to the diffusion of gaseous radionuclides.

[0042] 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 of the membrane depends on its porosity and the materials from which it is made. It can be determined by experimental tests, 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 a temporal 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.

[0043] Fig. 2 represents the membrane 6 prior to its apposition against the front face 4a of the scintillator 4.

[0044] The detection head 2 may include a grid 7, covering the membrane 6, such that the membrane extends between the grid 7 and the scintillator 4. The grid 7 is shown schematically in [Fig. 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%, so as to facilitate the passage of gas through the mesh.

[0045] 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.

[0046] Preferably, the scintillator is structured to optimize the gas-accessible surface area. The aim is to obtain a high specific surface area. Indeed, alpha particles have a limited range within the scintillator. It is preferable to prioritize the gas-accessible scintillator surface area over the scintillator thickness. It can be a porous scintillator, with open porosity, so that gaseous radionuclides can diffuse into the pores. Examples of porous inorganic scintillators are described in Raphaël 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 more, provided that the scintillator material has sufficient mechanical strength.A large volume fraction of porosity (> 50% or more) is made possible by the fact that the radiation has interacted in a surface layer of the scintillator material, typically in a thickness of less than 1 mm or even less than 500 pm.

[0047] The scintillator material may also have been structured by machining or molding in order to increase the specific surface area. Examples of structuring of organic scintillators are described in FR3133238.

[0048] Figure 4A represents an example of scintillator material 4, in which cylindrical microchannels 4c have been formed, so as to allow passage of radioactive gas passes through these. The diameter of each microchannel can be several tenths of a mm and a few mm, for example between 0.5 mm and 2 or 3 mm.

[0049] 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, for example, be organic or inorganic scintillating fibers. The diameter of each scintillating fiber could, for example, be a few tenths of a millimeter, for example, between 0.5 mm and 1 mm. The scintillating fibers are held together by a framework 4m, schematically represented by dashed lines in Figure 4B. Examples of inorganic scintillating optical fibers were described in the publication by 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.

[0050] With regard to scintillating optical fibers, these may be fibers established from an organic material, for example polystyrene, comprising scintillating particles.

[0051] When the scintillator is formed of assembled scintillating fibers, the analyzed radioactive gas can propagate between the fibers and interact with them. Preferably, the scintillating optical fibers are not sheathed, so that the alpha particles emitted by the analyzed radioactive gas are not absorbed before reaching the scintillator material.

[0052] The dimensions of the 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 rear 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 a risk of saturation.

[0053] When the scintillator is of large volume, it can be obtained by a juxtaposition of elementary scintillators 4i, as shown schematically in [Fig.4C]. For example, the thickness of each elementary scintillator is 1 cm.

[0054] The size of the scintillator is adjusted according to its porosity, or specific surface area, and its intrinsic sensitivity, i.e. the number of scintillation photons produced per quantity of energy deposited.

[0055] It is considered that, with regard to the detection of 222Rn, with an inorganic scintillator of porosity lOOnr-g \ a diameter (or larger diagonal) of 1 inch and a thickness of 2 inches may be suitable for the quantification of 222Rn at levels corresponding to the previously mentioned Euratom directive.

[0056] As previously mentioned, the use of 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 detection head and the surrounding environment. This also allows for immediate re-equilibration in the event of a change in concentration.

[0057] The photodetector 3 is connected to a processing unit 10, the latter being configured to receive and process the detection signal from the photodetector to form a measurement signal. The processing unit may be a computer or include a microprocessor. The measurement signal may be a count rate, corresponding to the number of interactions detected per unit of time. In one possibility, the processing unit implements a PSD (Push Shape Discrimination) type algorithm to separate the pulses according to a duration or shape parameter. An example of using such an algorithm was described in FR3133238.

[0058] The use of a processing unit implementing a PSD-type algorithm is particularly well-suited to a SiO2-YaGCe (Y3Al50i2:Ce) scintillator. YAG stands for yttrium aluminum garnet. Such a scintillator consists of a SiO2 matrix in which scintillating YaG:Ce nanoparticles are embedded. With this type of scintillator, it is possible to discriminate: - a component, called Cherenkov, of the detection signal, which corresponds to the light pulses produced, by Cherenkov effect, by particles [3 interacting in the SiO2 matrix. - an a component of the detection signal, which corresponds to the light pulses produced by the interactions of the a radiation with the scintillating YaGCe nanoparticles. From this component, a count rate a can be obtained.

[0059] The discrimination is based on the duration of the light pulses, the pulses resulting from the Cherenkov effect being shorter (a few ns) than the pulses produced by the interactions of the alpha radiation with the scintillating nanoparticles (a few tens of ns).

[0060] The discrimination of particles a and [3] has several advantages: - when the processing unit 10 includes a spectrometry circuit, configured to form a spectrum, i.e., 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 222Rn and its O 1 Q 0 1 / 1 descendant emitters a (for example Po or Po). - A separation between 222Rn and the [3]-emitting decay products (e.g., 214Pb or Bi) makes direct measurement of Rn alpha radiation possible, thus accelerating processing without waiting for equilibrium. Furthermore, the stopping power of high-energy [3] particles is difficult to model with such a geometry. The detection efficiency for [3] particles requires calculations. Conversely, the detection of an alpha particle is immediate upon reaching the scintillator, allowing the analysis to consider the detection efficiency as 100% for each alpha particle.

[0061] The invention can be implemented using inorganic scintillators such as, but not limited to: CeF3, LaBr3, CeBr3, Csl, BGO (bismuth germanate), or YSO (yttrium orthosilicate) or LYSO (yttrium and lutetium orthosilicate).

[0062] The probe 1, as previously described, can be used by placing it in air to estimate the concentration of 222Rn in the ambient air, independently of the concentration of 220Rn. The relationship between the detection signal and the concentration of 222Rn can be determined by a calibration function established with calibration gases whose respective concentrations of 222Rn are known.

[0063] Figure 5 illustrates one possible implementation of the probe for determining the concentration of 222Rn in a liquid. The probe 1 is placed in a sealed chamber C containing a liquid L to be analyzed, above which lies an ambient gas G. An equilibrium is established between the liquid L and the ambient gas G, under the effect of which some 222Rn initially present in the liquid L migrates into the ambient gas G by outgassing. The detection head 2 extends into the ambient gas G, for example, air, opposite the liquid L. The probe allows the concentration of 222Rn in the ambient gas G to be estimated. Knowing the equilibrium allows the concentration of Rn in the liquid L to be estimated from the concentration of Rn determined in the ambient gas G. The concentration is estimated by applying a calibration function to the measurement signal resulting from the processing unit. This is usually a counting rate, that is, the number of particles detected per second.The calibration function takes into account a link between the concentration of 222Rn in the liquid and the concentration measured in the ambient gas G. The calibration function is stored in a memory of the processing unit.

[0064] According to one possibility, the probe 1 can be immersed in the liquid, provided that the porous membrane 6 and the envelope 5 are sealed.

Claims

Demands

1. A detection probe (1), comprising: - an inorganic scintillator material (4), configured to generate photons under the effect of irradiation by alpha radiation; - a photodetector (3), optically coupled to a coupling face (4r) of the scintillator material, and configured to generate a detection signal dependent on a number of photons generated in the scintillator material; - an opaque envelope (5), disposed around the scintillator material, on either side of the coupling face; the probe being characterized in that it comprises an opaque, porous membrane (6), disposed facing the scintillator material, so as to allow migration, towards the scintillator material, through the membrane, of a radioactive gas, the thickness of the membrane being greater than 100 pm.

2. Probe according to claim 1, 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.

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

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

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

6. Probe according to any one of the preceding claims, wherein the scintillator material (4) is porous.

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

8. Probe according to any one of the preceding claims, in which - the scintillator material (4) is sensitive to [3] radiation; - the probe includes a processing unit (10), configured to discriminate between detection signals resulting respectively from interactions of a and [3] particles in the detector material.

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

10. A method for determining the activity of 222Rn in an analyzed medium, which may contain 222Rn, comprising at least the following steps: - exposing a probe (1) according to any one of the preceding claims to the 222Rn which may be contained in the analyzed medium; - applying a calibration function to a measurement signal resulting from the probe to determine the activity of 222Rn in the analyzed medium.

11. A method according to claim 10, wherein the medium is gaseous, the probe being disposed in the analyzed medium.

12. Method according to claim 10, 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.

13. Method according to claim 10, wherein the analyzed medium is a liquid medium, the probe being immersed in the analyzed medium.

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