Method and apparatus for characterizing mixed radionuclides

Abstract

The invention relates to a method (10) for characterizing at least two radionuclides mixed in a radioactive fluid, the radionuclides being substantially pure beta emitters, the method (10) being carried out by a device (1) calibrated by an average number of photons detected per decay and a decay detection yield value for each radionuclide, the method comprising: measuring (14) an average number of photons produced per decay for the radioactive fluid, determining (16) a number of decays for each radionuclide from the average number of photons detected for each radionuclide and the average number of photons detected for the radioactive fluid, and determining (18) the activity of each radionuclide from the number of decays and the calibration value for each radionuclide. [Fig. 1]

Description

Title of the invention: Method and device for characterizing radionuclides in mixture technical field

[0001] The present invention relates to a method for characterizing at least two radionuclides mixed in a radioactive fluid. The present invention also relates to a characterization device implementing such a method.

[0002] The field of the invention is, without limitation, that of the monitoring of nuclear activities. State of the art

[0003] Unstable radionuclides, whether naturally occurring or induced by human activities, naturally lead to the emission of so-called ionizing radiation. The highly efficient detection of this radiation is of paramount importance in many sectors of society, particularly health, safety, and nuclear waste management. In the case of radionuclides that decay via beta decay (emission of electrons) directly to the ground state, it is necessary to directly detect the beta radiation, which is very difficult due to the very short and complex mean free path of these particles in air or water.

[0004] Among common pure beta-emitting radionuclides, tritium (3H) is one of the most difficult to analyze, primarily because it is a low-energy beta emitter. However, tritium is involved in many nuclear activities and must be monitored, for example, during mandatory inspections by nuclear safety authorities or during the decommissioning of nuclear power plants.

[0005] Another example is krypton-85 (85Kr). This is a fission product in nuclear reactors. Since 85Kr is a noble gas, its movement within the reactor, as well as the presence of even the smallest crack, can be monitored, allowing for the verification of the integrity of the barriers between the fuel and the environment. Aside from concerns about atmospheric releases during nuclear waste processing, it is also a good indicator of a reactor's condition.

[0006] Since krypton-85 is a near-pure beta emitter with higher energies than tritium, it is somewhat easier to detect. However, its emission spectrum also contains low-energy electrons, which makes krypton-85 a significant interference in tritium measurements, especially since the two radionuclides are often present together in a reactor.

[0007] In order to detect tritium or krypton-85, it is necessary to mix the radioactive gases or liquids to be measured with a detector element. In the case of a radioactive gas, this can be achieved with an ionization chamber (constituting a gas-gas mixture) or with a liquid scintillator (gas-liquid mixture).

[0008] These current detection systems are very specific. Liquid scintillation is generally used for measuring tritium-3H in water samples. However, liquid scintillation cannot be performed on other radioactive gases. In particular, the solubility of Kr-85 is very low in the scintillation liquid.

[0009] Proportional counters for 3H and 85Kr, however, do not offer the possibility of gas discrimination. Currently, a measurement using chromatography is necessary. To discriminate between different radionuclides in a mixture, different types of detectors are required, as well as laboratory manipulation and separation of the gases. These techniques are very expensive and have a preparation and measurement time that can last up to a week, depending on laboratory practices. Therefore, it is not easy to deploy them widely across the entire territory, as this involves laboratory measurement with sample preparation. Description of the invention

[0010] An object of the invention is to provide a method and a device for characterizing a radioactive fluid containing a mixture of radionuclides and for extracting their respective activities.

[0011] Another object of the present invention is to provide a method and device that can be easily deployed for online and over-the-air measurements.

[0012] It is another object of the present invention to propose a method and a device for characterization allowing measurements requiring only compact and transportable equipment.

[0013] At least one of these objectives is achieved with a method for characterizing at least two radionuclides mixed in a radioactive fluid, the radionuclides being substantially pure beta emitters, the method being implemented by a characterization device comprising a scintillator configured to contain the radioactive fluid and measuring means configured to detect photons emitted by the scintillator due to decays of the radionuclides, the device being calibrated by an average number of photons detected per decay and a detection yield value for each radionuclide, the method comprising the steps of: - measurement of the average number of photons produced by decay for the radioactive fluid, - determination of a number of disintegrations for each radionuclide in the radioactive fluid from the average number of photons for each radionuclide, a, b, and the average number of photons for the radioactive fluid, and - determination of the activity of each radionuclide from the number of disintegrations and the photon detection yield value for each radionuclide in the radioactive fluid.

[0014] The method according to the present invention is a method for identifying and quantifying radionuclides in a radioactive fluid.

[0015] The technique for measuring a radionuclide in a fluid (radioactive gas or liquid) can be carried out using a solid or liquid scintillator exposed to this gas, in a closed chamber. The radiation from the radioactive decays interacts with the scintillator, which in turn emits light (photons). These photons can be detected by one or more photodetectors. In general, to overcome the intrinsic noise of the photodetectors, an event is considered valid if at least two photons are detected within a predefined time window.

[0016] The method according to the present invention makes it possible on the one hand to identify the nature of the radionuclide and to determine whether it is alone or mixed.

[0017] The method according to the present invention also makes it possible to identify the respective activities in the case of a mixture of radionuclides in a single measurement. The method according to the invention allows for online measurement or characterization requiring less equipment compared to state-of-the-art systems.

[0018] The method according to the invention can be implemented with fluids containing substantially pure mixtures of beta radionuclides, whether gases or liquids. The scintillator used is adapted to the nature of the radioactive fluid being measured.

[0019] The method according to the invention makes it possible to perform measurements on a radioactive fluid online and in operation, that is, on-site during the operation of the nuclear facility, for example, for monitoring purposes, and without the need to take a sample and perform laboratory analysis. The method does not produce consumable waste, such as an organic scintillator mixed with an aqueous phase containing radioactivity for liquid scintillation. The measurements can be carried out practically in real time (measurement time on the order of one minute). Furthermore, the implementation of the method is relatively simple, particularly compared to an ionization chamber.

[0020] The term "substantially beta emitters" refers to both pure beta emitters and quasi-pure beta emitters, that is, emitters having also a gamma emission with a probability of less than 10% of all decay pathways.

[0021] The term "disintegration" refers to the transformation of a radioactive nucleus, or radionuclide, into another nucleus by producing ionizing radiation (alpha, beta, and / or gamma). Beta radiation corresponds to the emission of electrons by the radioactive nucleus. These electrons interact with the scintillator, which emits light (photons) following the absorption of the electrons. The photons can then be detected by measuring instruments.

[0022] In this document, the average number of photons produced by decay corresponds to the number of photons produced and actually detected by an emitted electron. The number of photons accessible by measurement, or counting, corresponds to the number of photons produced by decay multiplied by the light-collecting efficiency of the detection or measurement means implemented and the photon detection efficiency of these detection means.

[0023] The activity of a radionuclide indicates the number of disintegrations per second in a certain quantity of radioactive material. It is expressed in becquerels (Bq).

[0024] The calibration of the device, and thus the determination of a decay detection yield and the average number of photons detected per decay for each radionuclide separately, can be carried out using at least two predetermined types of substantially beta-emitting radionuclides. These values ​​are characteristic of the radionuclides. The average number of photons detected per decay can be considered as the average energy of the emitted electrons.

[0025] The decay detection yield value can correspond to the number of double coincidences as a function of the radioactive activity of a radionuclide for at least one coincidence window.

[0026] The average number of photons produced by decay for each radionuclide separately can be measured, for the same window of coincidence, as follows: - the measurement of the number of decays resulting in a double coincidence and the number of decays resulting in a triple coincidence for each radionuclide separately, - the calculation of the relationship between the number of disintegrations resulting in a triple coincidence and the number of disintegrations resulting in a double coincidence for each radionuclide separately.

[0027] For this example, the characterization device includes, as measuring means, three photomultipliers.

[0028] Alternatively, it is possible to use only two detectors, or even a single detector provided that digitization electronics are used and with the implementation of suitable signal processing.

[0029] The average number of photons produced by decay for each radionuclide separately can also be measured, for a first and a second window of coincidence, as follows: - the measurement of the number of decays resulting in a double coincidence for the first window of coincidence and the number of decays resulting in a double coincidence for the second window of coincidence for each radionuclide separately, - the calculation of the relationship between the numbers of disintegrations giving rise to a double coincidence for two different windows of coincidence for each radionuclide separately.

[0030] For this example, the characterization device includes, as measuring means, two photomultipliers.

[0031] Alternatively, it is possible to use only one detector provided that digitization electronics are used and with the implementation of suitable signal processing.

[0032] According to one embodiment of the process, the step of measuring the average number of photons produced by decay for the radioactive fluid can be carried out, for the same coincidence window, by: - the measurement of the number of disintegrations resulting in a double coincidence and the number of disintegrations resulting in a triple coincidence for the radioactive fluid, - the calculation of the relationship between the number of disintegrations resulting in a triple coincidence and the number of disintegrations resulting in a double coincidence for the radioactive fluid.

[0033] For this embodiment, the characterization device includes, as measurement means, three photomultipliers.

[0034] Alternatively, it is possible to use only two detectors, or even a single detector provided that digitization electronics are used and with the implementation of suitable signal processing.

[0035] According to one embodiment, the step of measuring the average number of photons produced by decay for the radioactive fluid can be carried out, for a first and a second window of coincidence, by: - the measurement of the number of decays resulting in a double coincidence for the first window of coincidence and the number of decays resulting in a double coincidence for the second coincidence window for the radioactive fluid, - the calculation of the relationship between the numbers of disintegrations giving rise to a double coincidence for two different coincidence windows for the radioactive fluid.

[0036] For this embodiment, the characterization device includes, as measurement means, two photomultipliers.

[0037] Alternatively, it is possible to use only one detector provided that digitization electronics are used and with the implementation of suitable signal processing.

[0038] The implementations, or variants, of the method according to the invention presented above are equivalent in the sense that there is a correlation between, firstly, the ratio between the number of decays resulting in a double coincidence for two different coincidence windows and, secondly, the ratio between the number of decays resulting in a triple coincidence and the number of decays resulting in a double coincidence. This relationship is specific to the Poisson statistics of photon emission and depends on the radionuclide and the light emission parameters of the scintillator and the detection parameters of the device.

[0039] According to the invention, the second coincidence window can be about five times longer than the first coincidence window.

[0040] Advantageously, the second window of coincidence can be about ten times longer than the first window of coincidence.

[0041] In particular, the second window of coincidence can be approximately twenty times longer than the first window of coincidence

[0042] Advantageously, the first window of coincidence is less than or equal to the scintillator decay time. The scintillator decay time is defined as the effective and statistical time of emission of scintillation photons following interaction with the ionizing radiation. In simple cases, the statistics follow a law of the type exp(-t / r), where r is the scintillator decay time.

[0043] By way of example, and with a porous YAG:Ce scintillator, the first coincidence window can be between 5 and 70 ns, depending on the scintillator decay time. The second coincidence window can be between 100 and 1000 ns.

[0044] The method according to the invention can be implemented in particular to detect or identify the radionuclides krypton 85 (85Kr) and tritium (3H) in a fluid made radioactive by the presence of these elements and in which these radionuclides are mixed.

[0045] These fluids may include ambient air or water.

[0046] Radionuclides 85Kr and 3H are in particular subject to the obligation to be monitored in nuclear facilities and the environment.

[0047] According to another aspect of the same invention, a device is proposed for characterizing at least two radionuclides, a, b, mixed in a radioactive fluid, the device being configured to be calibrated by an average number of photons detected by decay and a decay detection yield value for each radionuclide a, b, the device comprising: - a scintillator configured to contain the radioactive fluid, - measuring devices configured to detect photons emitted by the scintillator due to the decay of radionuclides,

[0048] the device being configured to implement the steps of the process according to the invention.

[0049] The device according to the invention, implementing the method according to the invention, makes it possible to carry out measurements or characterizations online requiring a simplified implementation compared to state-of-the-art systems.

[0050] According to an advantageous embodiment, the scintillator may comprise a porous material that is transparent to the photons emitted by the scintillator.

[0051] Highly porous and transparent or translucent scintillator materials are advantageous for the measurement and characterization of beta-emitting radionuclides. Among the possible porous materials are assemblies of nanoparticles exhibiting scintillation properties, such as scintillating aerogels.

[0052] According to one example, the scintillator can be a cerium-doped yttrium aluminum garnet aerogel (YAG:Ce).

[0053] H may, in particular, be a Ce3+ and / or Ce4+ doped Y3Al50i2 aerogel. This aerogel exhibits high scintillation efficiency and a short scintillation decay time (less than 200 ns). A scintillator of this type thus makes it possible to use the narrowest possible coincidence time window.

[0054] This aerogel is particularly suitable for near real-time measurements.

[0055] This type of scintillator is particularly suitable for measurements with a radioactive gas.

[0056] According to another example, the scintillator is a liquid inorganic scintillator. This type of scintillator is particularly suitable for characterizing a liquid mixture containing substantially pure beta radionuclides, such as, for example, 3H with strontium 90 (90Sr) or carbon 14 (14C), or any other liquid mixture of radionuclides.

[0057] The method and device for characterization according to the present invention can be implemented in particular for monitoring the operation of nuclear reactors as well as the territory for nuclear activities, in particular for activities related to energy production and the recycling and storage of nuclear waste.

[0058] The method and device according to the invention make it possible to meet the regulatory monitoring obligation imposed by the nuclear safety authorities, for the control of releases and environmental monitoring. Description of the figures and methods of realization

[0059] Other advantages and features will become apparent upon examination of the detailed description of non-limiting examples and the accompanying drawings, in which: - [Fig.1] [Fig.1] schematically represents a device for characterizing a radioactive fluid according to an embodiment of the invention; - [Fig.2] [Fig.2] presents calibration curves for a scintillator cerium-doped Y3Al50i5 aerogel, separately exposed to two radionuclide gases 85Kr (a) and 3H (b); - [Fig. 3] [Fig. 3] shows measurements of the average number of emitted photons by disintegration for 85Kr and 3H according to two measurement methods; - [Fig.4] [Fig.4] is a schematic representation of embodiments of a characterization method according to the invention; - [Fig. 5] [Fig. 5] shows measurements of the evolution of the number of decays for two different windows of coincidence and for two different gas mixtures; and - [Fig. 6] [Fig. 6] shows the activities of radionuclides in mixtures presented in [Fig. 5], determined using the method according to the invention.

[0060] It is understood that the embodiments described below are in no way limiting. In particular, all the variants and embodiments described are combinable with each other provided there are no technical obstacles to such combination.

[0061] In the figures, elements common to several figures may retain the same reference.

[0062] The characterization device according to the invention includes a scintillator configured to contain the radioactive fluid, as well as measurement means for detecting the photons emitted by the scintillator due to radionuclide decays.

[0063] The measuring means comprise one or more photodetectors. For example, the measuring means may comprise one, two, or three photodetectors, such as photomultipliers, depending on the measurement method applied, as indicated below for the different embodiments of the process.

[0064] The scintillator, which can be solid or liquid, is exposed to the radioactive fluid, which can be a gas or a liquid, in a closed enclosure. The radiation from the radioactive decays interacts with the scintillator, which emits light (photons) as a result of these interactions. The photons can be detected by one or more photodetectors. Generally, to eliminate the intrinsic noise of the photodetectors, an event is considered valid if at least two photons are detected within a temporal coincidence window. An "event" corresponds to a decay.

[0065] The photodetector(s) may, in particular, be photomultiplier tubes. It is possible to use either three photodetectors for measurements with one coincidence window, or two photodetectors for measurements with two different coincidence windows. These measurements make it possible to determine the average number of photons emitted and detected by decay, this average number being a characteristic of the radionuclide to be detected and of the detection device.

[0066] It is also possible to use only one photodetector when the data processing is adapted accordingly.

[0067] Fig. 1 schematically represents a device for characterizing a radioactive fluid according to an embodiment of the present invention.

[0068] In the embodiment shown, the radioactive fluid is a gas.

[0069] The device 1 comprises a solid scintillator 2 and two photodetectors 3, 4 arranged in the immediate vicinity of the scintillator 2. The scintillator 2 is arranged in a transparent bowl or enclosure 5. The dashed arrows denote scintillation photons generated in the scintillator 2. The inlet and outlet of the radioactive gas are indicated by solid arrows 6, 7.

[0070] Preferably, the scintillator 2 comprises a porous material that is transparent to the photons emitted by the scintillator. In particular, it may comprise a Ce3+ and / or Ce4+ doped Y3A15O12 aerogel, for example as described in M. Odziomek et al., Design and Application of High Optical Quality YAG.Ce Nanocrystal-Loaded Silica Aerogels, ACS Applied Materials & Interfaces 2018 10 (38), 32304-32312.

[0071] Device 1 also includes a control module 8, called a coincidence module, which digitizes measured pulses and counts coincidences. In the example shown, module 8 performs coincidences between the signals detected by the two detectors 3 and 4, for two different coincidence windows Atb and At2.

[0072] Figure 1 also illustrates the two coincidence windows, Ati and At2, on a measurement time axis. Detected and digitized photons are indicated by vertical lines 10. The number of measured coincidences Db D2 is indicated for each coincidence window Atb At2 (Di2 = 0 or 1 in the example shown).

[0073] The device according to the invention, for example as represented in [Fig.1], can be used to implement the steps of a process for characterizing a radioactive fluid according to the invention which will be described later.

[0074] In order to carry out the process according to the invention using a characterization device such as, for example, described above, it is necessary that the device has been previously calibrated, which is typically the case for all measuring devices.

[0075] Calibrating the device allows obtaining calibration curves and values ​​for each radionuclide separately.

[0076] The calibration of the characterization device is carried out using at least two predetermined types of substantially pure beta-emitting radionuclides. These predetermined radionuclides correspond to those to be characterized in a radioactive fluid with the characterization device.

[0077] The calibration of the device is carried out firstly by measuring its decay detection efficiency. The efficiency can be expressed by the number of double coincidences as a function of the radioactive activity (act) of a radionuclide for at least one coincidence window, At.

[0078] This gives us a curve D = f(act) linking the number of disintegrations detected to the activity for each radionuclide a, b for which the device is calibrated.

[0079] The number of photons per disintegration is measured in double coincidence D, Di or D2, with the same coincidence windows as for characterization measurements.

[0080] Fig. 2 presents calibration curves D = f(act) for a cerium-doped Y3Al50i5 aerogel as a scintillator, exposed to two separate radionuclide gases ([Fig. 2 a): 85Kr; [Fig. 2] b): 3H), for two coincidence windows, Ati = 40 ns and At2 = 400 ns.

[0081] The calibration of the characterization device secondly comprises a measurement of the average number of photons produced per decay for at least two radionuclides, a, b, separately. The average number of photons per decay can be obtained by measuring the number of coincidences under different measurement configurations.

[0082] By way of example, two variants of this part of the calibration will be described in a non-limiting manner, a first variant and a second variant. First variant

[0083] According to the first variant, the number of triple coincidences T and the number of double coincidences with the same window of coincidence At are measured for each radionuclide a, b separately. The average number of photons per decay is then obtained according to the relation T / D. This measurement is carried out for each of the radionuclides a, b separately, to obtain ra = Ta / Da and rb = Tt / Db. Second variant

[0084] According to a second variant, the number of double coincidences Di with a first coincidence window Ati and the number of double coincidences D2 with a second coincidence window At2 are measured. The average number of photons per disintegration is then obtained, for each radionuclide a, b, by calculating the relation ra = Dia / D2a and rb = Dib / D2b, respectively.

[0085] Both variants, that is, determining the relationships Ta / Da and Tt / Db on the one hand and Dia / D2a and Dib / D2b on the other, yield equivalent results. Figure 3 shows measurements of the ratios T / D and Di / D2, where T / D is plotted as a function of Di / D2. In this example, the first window of coincidence is Ati = 40 ns, and the second window of coincidence is At2 = 400 ns. Di and D2 are therefore designated D4o and D4oo, respectively. The square measurement points 31 correspond to 85Kr and the star-shaped measurement points 32 correspond to 3H. This diagram demonstrates that the two estimators of the average number of photons (T / D and Di / D2) are completely correlated, justifying that the two variants for performing the calibration concerning the average number of photons produced and detected by decay can be used interchangeably.

[0086] When the device is calibrated by carrying out the aforementioned measurements for separate radionuclides, i.e., unmixed, a presumption is made of the expected presence of one and / or the other radionuclide in the mixture that one wishes to characterize with this device.

[0087] Fig. 4 is a schematic representation of a characterization process according to an embodiment of the present invention.

[0088] The process according to the invention will be described with a radioactive fluid comprising a mixture of two types of radionuclides a, b. The process according to the invention can also be carried out with a mixture of more than two radionuclides provided that the emission energy of the electrons of each is different.

[0089] By way of example, in the case of a YAG:Ce aerogel scintillator, the gap between the maximum energies of the electronic emission spectrum must be at least on the order of 50 keV.

[0090] The method according to the invention can in particular be implemented to characterize a radioactive fluid, and more particularly a gas containing a mixture of 85Kr and 3H, radionuclides subject to the obligation of monitoring in nuclear activities.

[0091] By way of example, two variants of the process according to the invention will be described later in a non-limiting manner, a first variant and a second variant.

[0092] The process 10, according to the embodiment shown in [Fig.4], includes a step 14 of measuring an average number of photons produced by decay for the radioactive fluid, i.e., the mixture of radionuclides.

[0093] It is preferably carried out on site or operand. First variant

[0094] According to the first variant, the number of triple coincidences T and the number of double coincidences D are measured for the fluid, with the same coincidence window At respectively. The average number of photons detected by decay is then obtained using the T / D relationship.

[0095] The coincidence window At is preferably less than or equal to the decay time of the scintillator used. A short coincidence window helps to reduce detection noise. Second variant

[0096] According to the second variant, for the fluid, the number of double coincidences Di with a first coincidence window Ati and the number of double coincidences D2 with a second coincidence window At2 are measured. The average number of photons detected by decay is then obtained by calculating the relation Di / D2.

[0097] Preferably, the first coincidence window Ati is less than or equal to the scintillator decay time. The second coincidence window At2 is approximately ten times longer than the first coincidence window.

[0098] For example, for a Ce-doped Y3A150i2 aerogel scintillator as shown above, the first coincidence window Ati can be between 10 and 40 ns, depending on the scintillator decay time. The second coincidence window At2 can be between 100 and 1000 ns.

[0099] Both variants, that is, determining the T / D ratio on the one hand and the Di / D2 ratio on the other, yield equivalent results, just as is the case for unmixed radionuclides, as demonstrated above with reference to [Fig. 3]. Since the two estimators (T / D and Di / D2) of the average number of detected photons are correlated, the two variants of the method according to the invention can be used interchangeably.

[0100] Once the average number of photons per decay for the radioactive fluid has been obtained, the process 10 as shown in [Fig. 4] continues with a step 16 of determination of a number of disintegrations for each radionuclide a, b from the average number of photons detected for each radionuclide, a, b (provided by the calibration of the characterization device, reference 11) and the average number of photons detected from the radioactive fluid. First variant

[0101] Each radionuclide α, β present in the mixture contributes separately, according to its activity (acta and actb), to the average values ​​of D and T measured during a defined acquisition time (corresponding to a measurement point). These average measured values ​​allow the T / D relationship to be estimated. The average values ​​of T and D of the mixture are related to the Da, Ta, Db, and Tb values ​​of the radionuclides α, β present in the mixture as follows:

[0102] T / D = (Ta+Tb) / (Da+Db) (equation 1),

[0103] D = Da + Db (equation 2).

[0104] With the average number of photons per decay for each radionuclide a, b, i.e., ra = Ta / Da and rb = Tb / Db determined during calibration, it is possible to reduce the number of variables in equation 1:

[0105] T / D = (ra.Da + rb.Db) / (Da+Db) (equation 3).

[0106] With equation 2, we obtain a system of two equations with two unknowns, Da and Db, which can thus be easily calculated. Second variant

[0107] Each radionuclide a, b present in the mixture contributes according to its activity (acta and actb) separately to the average values ​​Di and D2 measured during a defined acquisition time (corresponding to a measurement point).

[0108] Figure 5 shows measurements of the evolution of Di (At2 = 40 ns) and D2 (At2 = 400 ns) for two different gas mixtures: mixl (59 Bq of 85Kr and 756 Bq of 3H), - mix2 (271 Bq of 85Kr and 756 Bq of 3H).

[0109] The two mixtures and the two windows of coincidence are clearly distinguished in the diagram. The round points 41, 43 correspond to mixture mixl and the square points 42, 44 to mixture mix2. The empty points 41, 42 correspond to measurements with Ati = 40 ns, and the filled points 43, 44 to measurements with At2 = 400 ns.

[0110] The average measured values ​​of Di / D2 and D2, respectively, can be expressed as follows: [YES] D i / D2 = (D ia+D ib) / (D2a+D2 b) (equation 4),

[0112] D2 = D2a + D2 b (equation 5).

[0113] With the average number of photons per decay for each radionuclide a, b, i.e., ra = Dia / D2a and rb = Dn / D2b determined during calibration, it is possible to reduce the number of variables in equation 4:

[0114] Di / D2 = (ra.D2a + rb.D2 b) / (D2a+ D2 b) (equation 6).

[0115] With equation 5, we obtain a system of two equations with two unknowns, D2a and D2b, which can be easily calculated.

[0116] It should be noted that other formulas relating Di and D2 can be used to carry out the determination of a number of disintegrations for each radionuclide a, b, such as for example Di / (Di+D2) or other relationships.

[0117] Still with reference to [Fig.4], in a subsequent step 18, the radioactive activity of each radionuclide a, b (acta and actb), present in mixture in the fluid, is determined from the number of disintegrations for each radionuclide, a, b, obtained in the previous step 16 and the number of double coincidences for each radionuclide, a, b (D(acta), D(actb) provided by the calibration of the characterization device, reference 12).

[0118] Figure 6 illustrates the solution of the system of equations (equations 5 and 6) point by point. The system of equations is used to determine the values ​​of D2a and D2b, which represent an estimator of the number of decay events detected for each radioactive gas in the two mixtures (mix1 and mix2) of the example shown in Figure 5. These evolutions of D2a and D2b are extracted from the system of equations, and the activities for 85Kr and 3H present in the mixtures are deduced separately. The activities obtained correspond to the values ​​actually used in the two mixtures.

[0119] In Figures 2, 3, 5 and 6, a measurement point corresponds to an integration time of 100 s. This time thus corresponds to the time required to detect, identify and quantify the two radionuclides in the radioactive gas. The measurements can therefore be considered to be carried out in real time, or near real time.

[0120] Of course, the method and device according to the invention, illustrated above in embodiments for characterizing a radioactive gas containing 3H and 85Kr, can also be implemented with radioactive liquids. Indeed, by using a liquid scintillator, in particular an organic one, for example with its own decay time properties, it is possible to characterize a liquid mixture of pure beta emitters, such as a mixture of 3H with 90Sr, 3H with 14C, or any other common liquid mixture, by adapting the coincidence window(s) to the decay time of the liquid scintillator used.

[0121] The radionuclides to be characterized (and discriminated) according to the method must emit radiation with different energies. In the example described above, 85Kr emits electrons of higher energy than 3H.

[0122] Of course, the invention is not limited to the examples just described and many modifications can be made to these examples without departing from the scope of the invention.

Claims

Demands

1. A method (10) for characterizing at least two radionuclides, a, b, in a mixture in a radioactive fluid, the radionuclides being substantially pure beta emitters, the method (10) being carried out by a characterization device (1) comprising a scintillator (2) configured to contain the radioactive fluid and measuring means (3, 4) configured to detect photons emitted by the scintillator (2) due to decays of the radionuclides, the device (1) being calibrated by an average number of photons detected per decay and a decay detection yield value for each radionuclide a, b, the method (10) comprising the steps of: - measuring (14) an average number of photons produced per decay for the radioactive fluid, - determining (16) a number of decays for each radionuclide in the radioactive fluid from the average number of photons detected for each radionuclide, a,b, and the average number of photons detected for the radioactive fluid, and - determination (18) of the activity of each radionuclide from the number of decays and the decay detection yield value for each radionuclide, a, b, of the radioactive fluid.

2. Method (10) according to claim 1, characterized in that the step (14) of measuring the average number of photons produced by decay for the radioactive fluid is carried out, for a window of coincidence, At, by: - ​​measuring the number of decays giving rise to a double coincidence, D, and the number of decays giving rise to a triple coincidence, T, for the radioactive fluid, - calculating the relation T / D for the radioactive fluid.

3. A method (10) according to claim 1, characterized in that step (14) of measuring the average number of photons produced by decay for the radioactive fluid is carried out, for a first and a second coincidence window, Ati and At2, by:

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9. - the measurement of the number of disintegrations resulting in a double coincidence, Db for the first window of coincidence, Atb, and the number of disintegrations resulting in a double coincidence, D2, for the second window of coincidence, At2, for the radioactive fluid, - the calculation of the Di / D2 relationship for the radioactive fluid. Method (10) according to claim 3, characterized in that the second window of coincidence is approximately five times longer than the first window of coincidence. Method (10) according to claim 3 or 4, characterized in that the first coincidence window is less than or equal to the decay time of the scintillator. Method (10) according to any one of the preceding claims, characterized in that the at least two radionuclides comprise 85Kr and / or 3H. Device (1) for characterizing at least two radionuclides, a, b, in a mixture in a radioactive fluid, the radionuclides being substantially pure beta emitters, the device (1) being configured to be calibrated by an average number of photons detected by decay and a decay detection yield value for each radionuclide a, b, the device (1) comprising: - a scintillator (2) configured to contain the radioactive fluid, - measuring means (3, 4) configured to detect photons emitted by the scintillator (2) due to radionuclide decays, The device (1) being configured to carry out the steps of the process (10) according to any one of the preceding claims. Device (1) according to the preceding claim, characterized in that the scintillator (2) comprises a porous material transparent to photons emitted by the scintillator (2). Device (1) according to claim 7 or 8, characterized in that the scintillator (2) comprises a Ce-doped Y3A150i2 aerogel.

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