Method and apparatus for characterizing mixed radionuclides

Abstract

The invention relates to a method (10) for characterizing at least two radionuclides, a, b, in a mixture in a radioactive fluid, the radionuclides being substantially 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 disintegrations and the calibration value for each radionuclide, a, b, of the radioactive fluid. The invention also relates to a characterization device implementing such a method. Figure for abbreviation: [Fig. 1],

Description

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

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

[0002] The field of the invention is, in a non-limiting manner, that of the monitoring of nuclear activities. State of the art

[0003] Unstable radionuclides that exist as natural constituents or induced by human activities naturally lead to the emission of so-called ionizing radiation. The detection of this radiation with high efficiency is of paramount importance in many sectors of society, including health, safety and nuclear waste management. In the case of radionuclides decaying by beta transition (electron emission) directly to the ground state, it is necessary to directly detect 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 the common pure beta-emitting radionuclides, tritium (3H) is one of the most difficult to analyze mainly because it is a low-energy beta emitter. However, tritium is involved in many nuclear activities and must be monitored, for example, during mandatory controls by nuclear safety authorities or decommissioning of nuclear power plants.

[0005] Another example is krypton-85 (85Kr). This is a fission product in nuclear reactors. Since 85Kr is a rare gas, its movement in the reactor and the presence of even the slightest crack can be monitored, allowing for monitoring of the good condition of the barriers between the fuel and the environment. Apart from concerns about release into the atmosphere during nuclear waste processing, it is also a good indicator of the condition of a reactor.

[0006] Since krypton-85 is a pure quasi-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 disruptor for tritium measurements, especially since the two radionuclides are often present together in a reactor.

[0007] To be able to detect tritium or krypton-85, it is necessary to mixing the radioactive gases or liquids to be measured with a detector element itself. 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 the measurement of 3H tritium on water samples. However, liquid scintillation cannot be performed on other radioactive gases. In particular, the solubility of Kr-85 is very low in the scintillating liquid.

[0009] Proportional counters for 3H and 85Kr, on the other hand, do not offer the possibility of gas discrimination. Currently, a measurement using chromatography is necessary. To discriminate different radionuclides in a mixture, different types of detectors are necessary, as well as laboratory manipulations and separations of gases. These techniques are very expensive and have a preparation and measurement time of up to one week depending on laboratory practices. Also, it is not easy to deploy them widely across the entire territory as it is a laboratory measurement with sample preparation. Statement of the invention

[0010] It is an object of the invention to propose a method and a device allowing the characterization of a radioactive fluid containing a mixture of radionuclides and to extract the respective activities therefrom.

[0011] Another object of the present invention is to provide an easily deployable method and device for online and operand measurements.

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

[0013] At least one of these aims 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 disintegrations of the radionuclides, the device being calibrated by an average number of photons detected per disintegration and a detection efficiency value for each radionuclide, the method comprising the steps of: - measuring an average number of photons produced per disintegration for the radioactive fluid, - determination of a number of disintegrations for each radionuclide of the radioactive fluid from the average numbers of photons for each radio- nuclide, 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 efficiency value for each radionuclide of 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 of measuring a radionuclide in fluid (radioactive gas or liquid) can be carried out using a solid or liquid scintillator exposed to this gas, in a closed enclosure. The radiation from radioactive disintegrations interacts with the scintillator, which in return emits light (photons). These photons can be detected by one or more photo-detectors. In general, to overcome the intrinsic noise of photo-detectors, an event is considered valid if at least two photons are detected in a pre-defined temporal coincidence 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 online measurement or characterization requiring reduced equipment compared to state-of-the-art systems.

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

[0019] The method according to the invention makes it possible to carry out measurements on a radioactive fluid online and in operation, that is to say, on site during the operation of the nuclear installation for example, for monitoring reasons, and without the need to take a sample and carry out 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 of the order of one minute). Also, the implementation of the method is relatively simple, in particular compared to an ionization chamber.

[0020] By the term "substantially beta emitters" is meant both pure beta emitters and quasi-pure beta emitters, i.e., emitters also having gamma emission with a probability of less than 10% of the set of decay pathways.

[0021] The term "disintegration" means 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 means.

[0022] In this document, the average number of photons produced per disintegration 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 per disintegration multiplied by the light collection efficiency of the detection or measurement means implemented and the photon detection efficiency by 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 becquerel (Bq).

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

[0025] The decay detection efficiency value may 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 per disintegration for each radionuclide separately can be measured, for the same coincidence window, as follows: - measuring the number of disintegrations giving rise to a double coincidence and the number of disintegrations giving rise to a triple coincidence for each radionuclide separately, - calculating the relationship between the number of disintegrations giving rise to a triple coincidence and the number of disintegrations giving rise to a double coincidence for each radionuclide separately.

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

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

[0029] The average number of photons produced per decay for each radionuclide separately can also be measured, for a first and a second coincidence window, as follows:

[0030]

[0031]

[0032]

[0033]

[0034]

[0035]

[0036] - the measurement of the number of decays giving rise to a double coincidence for the first coincidence window and the number of decays giving rise to a double coincidence for the second coincidence window for each radionuclide separately, - the calculation of the relationship between the numbers of disintegrations giving rise to a double coincidence for two different coincidence windows for each radionuclide separately. For this example, the characterization device includes, as measuring means, two photomultipliers. Alternatively, it is possible to use only one detector provided that digitizing electronics are used and with the implementation of suitable signal processing. According to one embodiment of the method, the step of measuring the average number of photons produced by disintegration for the radioactive fluid can be carried out, for the same coincidence window, by: - the measurement of the number of disintegrations giving rise to a double coincidence and the number of disintegrations giving rise to a triple coincidence for the radioactive fluid, - the calculation of the relationship between the number of disintegrations giving rise to a triple coincidence and the number of disintegrations giving rise to a double coincidence for the radioactive fluid. For this embodiment, the characterization device comprises, as measuring means, three photomultipliers. Alternatively, it is possible to use only two detectors, or even a single detector provided that digitizing electronics are used and suitable signal processing is implemented. According to one embodiment, the step of measuring the average number of photons produced by disintegration for the radioactive fluid can be carried out, for a first and a second coincidence window, by: - the measurement of the number of disintegrations giving rise to a double coincidence for the first coincidence window and the number of disintegrations giving rise to 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. For this embodiment, the characterization device comprises, as measuring means, two photomultipliers.

[0037] Alternatively, it is possible to use only one detector provided that digitizing 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 numbers of disintegrations giving rise to a double coincidence for two different coincidence windows and, secondly, the ratio between the number of disintegrations giving rise to a triple coincidence and the number of disintegrations giving rise to a double coincidence. This relationship is specific to the Poisson statistics of the photon emission and depends on the radionuclide and the light emission parameters of the scintillator and detection of the device.

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

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

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

[0042] Advantageously, the first coincidence window 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] As an example and with a porous YAG:Ce scintillator, the first coincidence window can be between 5 and 70 ns, depending on the decay time of the scintillator. 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] The radionuclides 85Kr and 3H are in particular subject to the obligation to be monitored in nuclear installations and the environment.

[0047] According to another aspect of the same invention, there is provided a device for characterizing at least two radionuclides, a, b, in a mixture in a radioactive fluid, the device being configured to be calibrated by an average number of photons detected by disintegration and a disintegration detection efficiency value for each radionuclide a, b, the device comprising: - a scintillator configured to contain the radioactive fluid, - measuring means configured to detect photons emitted by the scintimeter tillator due to disintegrations of radionuclides,

[0048] the device being configured to implement the steps of the method according to any one of the preceding claims.

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

[0050] According to an advantageous embodiment, the scintillator may comprise a porous material 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 may be a cerium-doped yttrium aluminum garnet (YAG:Ce) aerogel.

[0053] This may in particular be a Ce3+ and / or Ce4+ doped Y3Al50i2 aerogel. This aerogel has a high scintillation efficiency and a short scintillation decay (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 measurements close to real time.

[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 characterization method and device 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 linked to the production of energy 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 discharges and environmental monitoring. Description of figures and embodiments

[0059] Other advantages and characteristics will appear on examining the detailed description of non-limiting examples, and the appended drawings in which: - [Fig.l] [Fig.l] schematically represents a device for characterizing a radioactive fluid according to an embodiment of the invention; - [Fig.2] [Fig.2] shows 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 average numbers of photons emitted by decay 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 disintegrations grations for two different coincidence windows and for two different gas mixtures; and - [Fig.6] [Fig.6] shows radionuclide activities in mixtures presented in [Fig.5], determined by means of the method according to the invention.

[0060] It is understood that the embodiments which will be described below are in no way limiting. In particular, all the variants and all the embodiments described can be combined with each other if nothing prevents this combination on a technical level.

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

[0062] The characterization device according to the invention comprises a scintillator configured to contain the radioactive fluid, as well as measuring means for detecting the photons emitted by the scintillator due to disintegrations of the radionuclides.

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

[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) following these interactions. The 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 time coincidence window. An “event” corresponds to a decay.

[0065] The photodetector(s) may in particular be photomultipliers. It is 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 know the average number of photons emitted and detected per disintegration, 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.l] 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 as well as two photo-detectors 3, 4 arranged in the immediate vicinity of scintillator 2. Scintillator 2 is arranged in a transparent cuvette or enclosure 5. The dotted arrows designate scintillation photons generated in 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 transparent to the photons emitted by the scintillator. It may in particular comprise a Ce3+ and / or Ce4+ doped Y3A15O 12 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] The device 1 also comprises a control module 8, called a coincidence module, for digitizing measured pulses and counting coincidences. In the example shown, the module 8 achieves coincidences between the signals detected by the two detectors 3, 4, and this for two different coincidence windows Atb At2.

[0072] [Fig.l] 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 coincidences Db D2 measured is indicated for each coincidence window Atb At2 (Di 2 = 0 or 1 in the example shown).

[0073] The device according to the invention, for example as shown in [Fig.l], can be used to implement the steps of a method for characterizing a radioactive fluid according to the invention which will be described subsequently.

[0074] In order to be able to carry out the method according to the invention by means of a characterization device such as, for example, described above, it is necessary for the device to have been previously calibrated, which is typically the case for all measuring devices.

[0075] Calibration of the device makes it possible to obtain calibration curves and values ​​for each radionuclide separately.

[0076] The calibration of the characterization device is carried out by means of at least two predetermined types of substantially pure beta-emitting radionuclides. These predetermined radionuclides correspond to those which it is desired to characterize 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] We thus obtain 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 the characterization measurements.

[0080] [Fig.2] shows calibration curves D = f(act) for a cerium-doped Y3Al50i5 aerogel as 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 an average number of photons produced per disintegration for at least two radionuclides, a, b, separately. The average number of photons per disintegration can be obtained by measuring the number of coincidences according to 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 coincidence window At are measured for each radionuclide a, b separately. The average number of photons per disintegration is then obtained according to the relationship T / D. This measurement is carried out for each of the radionuclides a, b separately, to obtain ra = Ta / Da and rb = Tb / Db. Second variant

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

[0085] Both variants, that is to say, the determination of the relations Ta / Da and Tt / Db on one side and Dia / D2a and Dib / D2b on the other side, make it possible to obtain equivalent results. [Fig. 3] shows measurements of the T / D and Di / D2 ratios, where T / D is plotted against Di / D2. In this example, the first coincidence window is Ati = 40 ns, and the second coincidence window is At2 = 400 ns. Di and D2 are therefore named D40 and D400, 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 calibration of the device is carried out by carrying out the aforementioned measurements for separate, i.e., unmixed, radionuclides, 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 method according to an embodiment of the present invention.

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

[0089] For example, in the case of a YAG:Ce aerogel scintillator, the difference between the maximum energies of the electronic emission spectrum must be at least of 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 method according to the invention will be described below in a non-limiting manner, a first variant and a second variant.

[0092] The method 10, according to the embodiment shown in [Fig.4], comprises a step 14 of measuring an average number of photons produced by disintegration for the radioactive fluid, that is to say, 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 disintegration is then obtained using the relationship T / D.

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

[0096] According to the 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 for the fluid. The average number of photons detected by disintegration is then obtained by calculating the Di / D2 relationship.

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

[0098] For example, for a Ce-doped Y3Al50i2 aerogel scintillator as presented above, the first coincidence window Ati may be between 10 and 40 ns, depending on the decay time of the scintillator. The second coincidence window At2 may be between 100 and 1000 ns.

[0099] Both variants, i.e., the determination of the T / D relationship on one side and the Di / D2 relationship on the other side, make it possible to obtain 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 photons detected are correlated, the two variants of the method according to the invention can be used interchangeably.

[0100] Once the average numbers of photons per disintegration for the radioactive fluid have been obtained, the method 10 as shown in [Fig.4] continues with a step 16 of determining a number of disintegrations for each radionuclide a, b from the average numbers 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 a, b present in the mixture contributes according to its activity (acta and actb) separately to the average values ​​of D and T measured during a defined acquisition time (corresponding to a measurement point). These average measured values ​​make it possible to estimate the T / D relationship. The average values ​​of T and D of the mixture are related to the values ​​of Da, Ta, Db, Tb of the radionuclides a, b 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 = Tt / 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 calculated easily. Second variant

[0107] Each radionuclide a, b present in the mixture contributes 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] [Fig.5] shows measurements of the evolution of Di (Ati = 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 as well as the two coincidence windows are clearly distinguished in the diagram. The round points 41, 43 correspond to the mixture mixl and the square points 42, 44 to the mixture mix2. The empty points 41, 42 correspond to the measurements with Ati = 40 ns, and the solid points 43, 44 to the measurements with At2 = 400 ns.

[0110] The measured average values ​​of Di / D2 and D2, respectively, can be expressed as follows:

[0111] Dj / D, = (Dla+Dlb) / (D2a+D2b) (equation 4),

[0112] D2 = D2a + D2b (equation 5).

[0113] With the average number of photons per decay for each radionuclide a, b, i.e., ra= D[a / 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.D2b) / (D2a+D2b) (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 linking Di and D2 can be used to determine 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], during a following step 18, the radioactive activity of each radionuclide a, b (acta and actb), present in a mixture in the fluid, is determined from the number of disintegrations for each radionuclide, a, b, obtained in the previous step 16 and from 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] [Fig.6] illustrates the resolution of the system of equations (equations 5 and 6) point by point, and reports the values ​​of D2a and D2b, i.e. an estimator of the number of disintegration events detected for each of the radioactive gases for the two mixtures (mixl and mix2) of the example presented with reference to [Fig.5]. We extract from the system equations these evolutions of D2a and D2b, and we deduce the activities separately for 85 Kr and 3H present in the mixtures. The activities found correspond to the values ​​actually used in the two mixtures.

[0119] In Figures 2, 3, 5 and 6, one 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. It can therefore be considered that the measurements are carried out in real time, or quasi-real time.

[0120] Of course, the method and the 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 organic, for example with specific decay time properties, it is possible to characterize a liquid mixture of pure beta emitters, such as for example 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, 85 Kr emits higher energy electrons than 3H.

[0122] Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without departing from the scope of the invention.

Claims

Claims

1. Method (10) for characterizing at least two radionuclides, a, b, mixed in a radioactive fluid, the radionuclides being substantially pure beta emitters, the method (10) being implemented 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 disintegrations of the radionuclides, the device (1) being calibrated by an average number of photons detected per disintegration and a disintegration detection efficiency value for each radionuclide a, b, the method (10) comprising the steps of: - measuring (14) an average number of photons produced per disintegration for the radioactive fluid, - determining (16) a number of disintegrations for each radionuclide of the radioactive fluid from the average numbers of photons detected for each radionuclide, has,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 disintegrations and the disintegration detection efficiency 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 per disintegration for the radioactive fluid is carried out, for a coincidence window, At, by: - measuring the number of disintegrations giving rise to a double coincidence, D, and the number of disintegrations giving rise to a triple coincidence, T, for the radioactive fluid, - calculating the T / D relationship for the radioactive fluid.

3. Method (10) according to claim 1, characterized in that the step (14) of measuring the average number of photons produced by disintegration for the radioactive fluid is carried out, for a first and a second coincidence window, Ati and At2, by: - measuring the number of disintegrations giving rise to a double coincidence, Db for the first coincidence window, Atb and the number of disintegrations giving rise to a double coincidence, D2, for the second coincidence window, At2, for the radioactive fluid, - calculating the Di / D2 relationship for the radioactive fluid.

4. Method (10) according to claim 3, characterized in that the second coincidence window is approximately five times longer than the first coincidence window.

5. 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.

6. Method (10) according to any one of the preceding claims, characterized in that the at least two radionuclides comprise 85 Kr and / or 3H.

7. Device (1) for characterizing at least two radionuclides, a, b, mixed 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 per disintegration and a disintegration detection efficiency 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 disintegrations of the radionuclides, the device (1) being configured to implement the steps of the method (10) according to any one of the preceding claims.

8. Device (1) according to the preceding claim, characterized in that the scintillator (2) comprises a porous material transparent to the photons emitted by the scintillator (2).

9. Device (1) according to claim 7 or 8, characterized in that the scintillator (2) comprises a Ce-doped Y3Al50i2 aerogel.

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