Method for characterizing isotopes by a plurality of neutron emissions of different spectra

Abstract

Method for characterizing isotopes by a plurality of neutron emissions of different spectra. The invention relates to a method (100) for characterizing at least one isotope of a target (4) by neutron irradiation. The method comprises irradiating (110) the target with neutrons generated by a first neutron generator (2); measuring (120) a first spectral response of the target during and after irradiation; identifying (130) a first line produced by a reaction of at least one isotope with neutrons; when the measurement of the first spectral response is complete, irradiating (140) the target with neutrons generated by a second neutron generator (3); measuring (150) a second spectral response of the target during and after irradiation; The identification (160) in the second spectral response of a second line produced by a reaction of the isotope with neutrons. Figure for the abstract: Fig. 2

Description

Title of the invention: Method for characterizing isotopes by a plurality of neutron emissions of different spectra. Technical field

[0001] The present invention relates to the field of non-destructive characterization of at least one chemical element present in a solid object or in a liquid.

[0002] The invention relates more particularly to a method of characterization by neutron activation and / or by neutron interrogation.

[0003] The elements that can be characterized by neutron activation are varied. They include, for example, aluminum, copper, gold, silver, manganese, and vanadium. These elements are of particular interest in fields of application including metallurgy, archaeology, and materials science.

[0004] The elements characterized by neutron interrogation are typically major actinides, such as isotopes of uranium and plutonium, but are not limited to these elements. Applications for the characterization of these elements include, in particular, civil security, the characterization of packages of radioactive waste from the nuclear industry, and the detection of nuclear material, for example, for the inspection of luggage or freight containers. Previous technique

[0005] The non-intrusive and / or non-destructive detection of chemical elements can be implemented by various methods, some of which are based on neutron irradiation. In particular, some methods rely on the identification of chemical elements by detecting capture gamma rays produced by thermal neutrons, by detecting inelastic scattering gamma rays, or by detecting gamma rays induced by other reactions produced by fast neutrons.

[0006] Among the existing solutions for detection by neutron irradiation, it is known to use neutron generators by deuterium - deuterium (DD) or deuterium - tritium (DT) reactions, or even radioactive sources emitting neutrons such as AmBe or Cf 252.

[0007] For example, the company Sodern markets models of neutron generators under the references GENIE 16 and GENIE 35. Analysis devices based on such neutron generators are marketed by the company Malvem Panalytical in the CNA range.

[0008] The article “Schemes for producing multi-energy fast neutrons using a low-energy ion accelerator,” G. Mishra et al., Journal of Instrumentation, Volume 16, September 2021 (DOI: 10.1088 / 1748-0221 / 16 / 09 / T09003) describes the use of an ion accelerator for generating neutrons at different energies. The ions can be protons, deuterium, or light ions such as lithium-7. The device, shown in [Fig. 1], is approximately 20 meters long, which makes industrial deployment impractical.

[0009] Some existing neutron irradiation analysis devices allow for real-time measurements. However, the existing detection and analysis methods implemented with these analysis devices only allow the detection of a limited number of reactions of interest and elements.

[0010] French patent application FR 2939895 A1 describes a method for detecting chemical elements by neutron emission onto an object. The neutron emission consists, on the one hand, of a continuous emission of neutrons from a particle-associated neutron generator and, on the other hand, of the emission of neutron pulses superimposed on the continuous emission. The neutron pulses originate from a pulsed neutron generator.

[0011] There is a need to improve existing characterization processes, in particular to allow more reactions by neutron activation or neutron interrogation and to better target the chemical elements sought.

[0012] The aim of the invention is to meet at least part of this need. Summary of the invention

[0013] To this end, one aspect of the invention relates to a method for characterizing at least one isotope of a target by neutron irradiation, the method comprising the steps of:

[0014] a / irradiate the target with neutrons generated by a first neutron generator and having a first neutron spectrum;

[0015] b / measure a first spectral response of the target during and after irradiation a / by means of a gamma radiation detector;

[0016] c / identify in the first spectral response at least one first line produced by a reaction of at least one isotope with neutrons;

[0017] d / when the measurement according to step b / of the first spectral response is completed, irradiate the target with neutrons generated by a second neutron generator and having a second neutron spectrum different from the first neutron spectrum;

[0018] e / measure a second spectral response of the target during and after irradiation d / using the gamma radiation detector;

[0019] f / identify in the second spectral response at least a second line produced by a reaction of at least one isotope with neutrons, which may or may not be identical to at least one first line.

[0020] According to the invention, the target is irradiated by at least two neutron generators that produce neutrons with different spectra, i.e., different energy distributions. The irradiation phases by the first and second generators are not simultaneous. Since the neutron spectra are different, the spectral response of the target for each irradiation has its own characteristics and provides additional information about the isotope of interest.

[0021] Preferably, the second neutron spectrum has lower energy than the first neutron spectrum.

[0022] Preferably, the neutron generators are configured to generate neutrons having an energy distribution that favors reactions with at least one isotope of interest. One or more isotopes of interest can be characterized simultaneously during the process according to the invention.

[0023] During the irradiation phases, at least one isotope of the target reacts with neutrons and generates reaction products. Consequently, gamma lines are emitted by the target during the irradiation phases, through the generation of reaction products and their decay, and for a certain period after the end of irradiation, when the reaction products are decaying. The measurement of the spectral response is advantageously carried out during the irradiation phase but also during the decay phase of the reaction products. The duration of the irradiation and measurement phases is preferably chosen according to the isotopes of interest. For example, an irradiation phase may last approximately 2 hours and the measurement may continue for approximately 2 hours after the end of irradiation.

[0024] The gamma radiation detector is advantageously configured to be sensitive to energy ranges including at least one gamma line produced by a reaction of at least one isotope with a neutron. In particular, the gamma radiation detector may be sensitive to photons with energies greater than or equal to 100 eV, 1 keV or 10 keV and / or less than or equal to 50 MeV, 20 MeV or 10 MeV, especially with energies between 1 keV and 20 MeV.

[0025] The gamma radiation detector is therefore suitable for measuring the spectral response of the target to irradiation, that is, the distribution of gamma photons emitted by the target as a function of their energy. Analysis of the spectral response makes it possible to detect emission lines that indicate the presence of isotopes. Analysis of these lines makes it possible to quantify the identified isotopes.

[0026] The invention enables the real-time, active, and non-destructive characterization of a target. The method advantageously allows the combined use of different neutron spectrum profiles to irradiate the target. In particular, the method can use at least one high-energy neutron spectrum and one lower-energy neutron spectrum. The method thus makes it possible to identify gamma lines produced by reactions with a high-energy threshold reaction and then to target the gamma lines of the identified isotope by performing a more precise and less noisy count thanks to the lower-energy irradiation.

[0027] Compared to known prior art measurement devices and methods, the invention allows the use of neutron spectra covering a wide range of energies, enabling the detection of more isotopes with greater confidence in the measurements performed.

[0028] According to an advantageous feature, the method includes a step of calculating the mass of at least one isotope from at least one first line and / or at least one second line.

[0029] Identifying one or more characteristic emission lines of an isotope allows the mass of the latter to be calculated from the net emission lines under the lines, as described below. The characterization process thus makes it possible not only to identify the presence of one or more isotopes of interest but also to quantify their mass.

[0030] According to another advantageous feature, the first neutron generator generates fast neutrons, in particular with an energy greater than 1 MeV.

[0031] The use of high-energy neutrons makes it possible to overcome the reaction thresholds of many reactions and thus generate a greater number of emission lines. This facilitates and makes the identification of an isotope of interest more reliable.

[0032] The first neutron generator may in particular be a deuterium-deuterium generator, generating neutrons of 2.45 MeV, or a deuterium-tritium generator, generating neutrons of 14.1 MeV. Such neutron generators are known as such.

[0033] Preferably, the first neutron generator is coupled with a thermalization cell configured to thermalize neutrons generated by the first neutron generator. This makes it possible to obtain a neutron spectrum comprising a high-energy component and another thermalized component of lower energy.

[0034] According to a preferred feature, the second neutron generator generates thermal or epithermal neutrons, in particular with an energy of less than 1 MeV.

[0035] By "thermal neutron" is meant a neutron with an energy less than 1 eV. By "epithermal neutron" is meant a neutron with an energy between 1 eV and 1 MeV.

[0036] It is particularly advantageous to combine a first fast neutron generator with a second thermal or epithermal neutron generator. The different hardness of the generated neutron spectra makes it possible to obtain spectral responses with different characteristics and to exploit these differences. In particular, the first neutron generator makes it possible to overcome many reaction thresholds and to produce a greater number of emission lines, facilitating the identification of isotopes of interest. The second neutron generator makes it possible to obtain emission lines with a more favorable signal-to-noise ratio, notably allowing for more reliable isotope mass calculations.

[0037] Preferably, the second neutron generator comprises an ion or electron accelerator coupled to a secondary target that produces neutrons when irradiated with ions or electrons generated by the accelerator. The secondary target may, in particular, be a volume of heavy water, beryllium, or deuterium gas. An ion accelerator may, in particular, be a proton or deuteron accelerator such as the AMANDE accelerator of the IRSN.

[0038] Advantageously, such a generator can generate a plurality of different neutron spectra by simply changing the volume of the secondary target.

[0039] Alternatively, the first neutron generator can be a deuterium-tritium generator and the second neutron generator can be a deuterium-deuterium generator.

[0040] According to a preferred feature, the gamma radiation detector is collimated in the direction of the target.

[0041] This advantageously reduces measurement noise.

[0042] Preferably, the method comprises, after irradiation d / and measurement d, one or more additional phases of irradiation of the target with neutrons having a spectrum different from the first and second neutron spectra and one or more additional phases of measurement of an additional spectral response of the target during and after each additional phase of irradiation by means of the gamma radiation detector.

[0043] In other words, the method may comprise three or more phases of irradiating the target and measuring the resulting spectral response. For each phase, the neutron spectrum irradiating the target is different in order to obtain complementary information for each irradiation. It is possible to use a different neutron generator for each irradiation phase or to use one or more neutron generators for two or more irradiation phases, these generators then being configured to generate different neutron spectra.

[0044] Preferably, at least one of the first or second neutron generators is configured to generate a plurality of different neutron spectra. Brief description of the drawings

[0045] [Fig. 1] Fig. 1 represents a device adapted to implement the process according to the invention.

[0046] [Fig.2] Fig.2 illustrates a characterization method according to the invention.

[0047] [Fig.3] Fig.3 represents a simulated device adapted to implement the method according to the invention.

[0048] [Fig.4] The [Fig.4] is a graph representing the evolution of the number of nuclei produced by activation during an irradiation phase of the target of the device of the [Fig.3],

[0049] [Fig.5] The [Fig.5] is a graph representing the gamma radiation spectra produced by the target of the device in [Fig.3] after the first and second irradiations of the target.

[0050] [Fig.6] The [Fig.6] is a detail view of the graph in [Fig.5].

[0051] [Fig.7] The [Fig.7] is a detailed view of the graph in [Fig.5].

[0052] [Fig.8] The [Fig.8] is a detail view of the graph in [Fig.5].

[0053] [Fig.9] Fig.9 is a graph representing the neutron spectra produced by twelve different neutron generator configurations. Detailed description

[0054] Figure 1 shows a device 1 adapted to implement a method for characterizing at least one isotope by neutron emission.

[0055] The device 1 comprises a first neutron generator 2 and a second neutron generator 3. These generators are configured to irradiate a target 4 containing at least one isotope of one or more chemical elements of interest that are to be identified and quantified. The first and second neutron generators produce different neutron spectra in order to irradiate the target 4 with neutrons of different characteristics, in particular with different energies.

[0056] Advantageously, the first neutron generator 2 can be a fast neutron generator. In particular, it can be a deuterium-deuterium generator producing 2.45 MeV neutrons or a deuterium-tritium generator producing 14.1 MeV neutrons. It may or may not be combined with a thermalization cell which, when irradiated by neutrons from the generator, thermalizes them. The resulting neutron spectrum can thus include a monoenergetic line at 2.45 MeV or 14.1 MeV and / or a lower-energy component extending over a certain energy range.

[0057] Advantageously, the second neutron generator 3 can be a thermal and / or epithermal neutron generator, in particular with energies below 1 MeV. It can, in particular, be an ion and / or electron accelerator, for example linear accelerator such as a Linatron® M9 type accelerator, coupled to a secondary neutron-producing target, in particular a photoneutron-producing target, for example heavy water D2O. Irradiation of the secondary target by the accelerated ions or electrons produces a neutron spectrum, at least part of which can irradiate the target 4.

[0058] Target 4 may be a solid object or a fluid, in particular a liquid. By way of example, target 4 may be a package of radioactive waste, a shipping or road container, a suitcase, an elemental material such as a rock or mineral, or even an archaeological object.

[0059] In the illustrated example, device 1 further includes a gamma 5 radiation detector configured to detect gamma radiation resulting from the interaction of neutrons emitted by the neutron generators with the target. The gamma 5 radiation detector can be a detector of the HPGe (high-purity germanium), Nal, NaI(Tl+Li), LaBR, CeBr, or BGO (bismuth germanate) type.

[0060] The device 1 may more generally comprise one or more gamma radiation detectors and / or one or more neutron detectors.

[0061] The device 1 also includes an electronic measurement system 6 connected to the gamma radiation detector 5. The electronic measurement system 6 may include a data acquisition computer configured to acquire measurement data from the detector. It may also include processing means, which may be the data acquisition computer, that process the signals from the detector, in particular to identify lines in the spectral response of the detector and to estimate the mass of an isotope from the identified lines, as described below.

[0062] [Fig.2] represents the steps of a characterization process 100 according to the invention, for example implemented by the device 1 shown in [Fig.1].

[0063] The process 100 allows the identification and characterization of a number N greater than or equal to 1 of chemical elements in the form of isotopes of interest Xb X2, ..., XN.

[0064] The reaction products of each isotope of interest X; with a neutron generate gamma radiation comprising emission lines R; i, Ri2, ..., Rij at energies 0(¾).

[0065] The gamma radiation detector 5 is adapted to detect gamma emission lines of interest among the spectral response of the target to irradiation.

[0066] Preferably, for each isotope of interest X, gamma emission lines of interest are selected as a function of the radioactive decay half-life T of the isotope Xi and the intensity hj of the emission lines, so that the counting of the lines can be statistically significant for a determined irradiation time Tiirad.

[0067] The analysis of these lines of interest makes it possible to identify and characterize the isotopes of interest.

[0068] Preferably, the gamma 5 radiation detector is collimated in the direction of the target. This improves the signal-to-noise ratio of the measurement.

[0069] Preferably, the measurement environment comprising the gamma radiation detector 4 and the electronic measurement system 6 has a reduced number of components and / or is far from the target, so as to limit the contribution of active background noise to the measured signal.

[0070] In a first step 110, the target 4 is irradiated for a duration Tiirad by A neutrons generated by the first neutron generator 2. The A neutrons have an energy distribution forming a first neutron spectrum. The neutron emission from the first neutron generator 2 can be continuous throughout the duration Tin-ad-

[0071] The generated neutrons may include fast neutrons, for example with energies greater than 1 MeV. High-energy neutrons make it possible to overcome the reaction thresholds of a large number of reactions producing gamma radiation.

[0072] During the irradiation phase and for a duration Tpost_inad after the irradiation phase, the spectral response produced by the reaction of neutrons with the target is measured by the gamma radiation detector 5 (step 120).

[0073] During the irradiation phase, the spectral response includes the radiation produced by the generation of reaction products and their decay. After the irradiation phase, reaction products are no longer generated. The spectral response still includes the radiation produced by the decay of the reaction products.

[0074] Denoting D(E) as the energy response of the gamma radiation detector, the spectral response EA(E) corresponds to the convolution product of the energy response and the gamma radiation flux. <bA(E) induit par l’irradiation de la cible et entrant à la surface du détecteur, soit

[0075] [Math.l] rA(E)=jD(£-F)x04(F)dF

[0076] The gamma radiation flux <bA(E) comprend les flux provenant de l’échantillon induits par l’irradiation de la cible <1> *A(E), the parasitic flux <hA(E) provenant de l’activation de l’environnement, et le bruit de fond passif <DA(E) provenant des émissions naturelles des éléments de l’environnement, tels que :

[0077] [Math.2] 0a(E) = ^a(E)+^a(E)+^(E)

[0078] The useful flux from the sample is calculated based on Bateman's equations. We obtain

[0079] [Math.3] Naf (E) <p.(E )rfE x ( xp-xl x3(E.\x S^x £: y '—l'—rl >L„aii T a ij .. i,j 'J i ••J >

[0080] N; is the number of atoms of isotope X; present in the target, oA is the fast neutron-induced reaction cross section beyond the reaction threshold Eseuii, <pA est le flux de neutrons rapides incidents, Ej est l’efficacité géométrique de la raie d’énergie 0(¾) émise avec un rapport d’embranchement [3^ et son intensité hj, Sjj est un facteur prenant en compte des effets propres à la géométrie de la cible tels que l’auto-atténuation des rayonnements gamma émis, X, est la constante de désintégration partielle.

[0081] Similarly, the flux of gamma radiation emitted by the Xk isotopes due to the activation of the environment and perceived by the detector is calculated according to the equation:

[0082] [Math.4] ^'^E) = EtE / E")(p^Ë)dE x ( xfik .xI^ x3(Ek^xSkj x Ej

[0083] The gamma radiation flux due to the natural radioactivity of the environment, emitted by the radioactive isotopes -¾ and perceived by the detector, is calculated according to the equation:

[0084] [Math.5] L ( E ) = EJE jN h xe^' xT ^x. xi x ÜE^ x S xEj Ëj hj

[0085] In a third step 130, gamma radiation R;j emitted by the isotope of interest X, is identified in the spectral response rA(E).

[0086] Step 130 may include a step 131 of searching for peaks in the spectral response and a step 132 of extracting net areas under the identified peaks. These steps are known per se and can be implemented by computer.

[0087] Preferably, in the event of the presence of spurious radiation R'kj produced by the activation of surrounding elements or R*hjj produced by natural radioactivity of the environment and likely to interfere with the useful radiation R^, one relies on one or more emission lines R; u with u different from j to identify and quantify the isotope X,.

[0088] The mass mAi>u of the isotope X can be estimated in a step 133 with the RA ijU lines identified from the net area under the peak of the lines:

[0089] [Math.6] cc4-b4) = VxT...... ■ •iu1 fncs

[0090] yijU is the emissivity, i.e., the number of gamma rays Riu emitted per gram of isotope Xu per unit time, Tmes is the total measurement time corresponding to the

[0091]

[0092]

[0093]

[0094]

[0095]

[0096]

[0097]

[0098]

[0099]

[0100]

[0101]

[0102] The sum of the irradiation time Tinad and the post-irradiation measurement time Tpost iirad, CA ijU is the raw count in the region of interest Rdl and BAijU is the parasitic count of active and natural background noise in the region of interest Rdl. Thus: [Math.7] ct,=\ Rdl r A {E)dE [Math. 8] B Read = \ Rdi r' A (E')dE+ ^f^E^dE The uncertainty in the mass, associated with the standard deviation in the net count statistic of the RA u line above the Compton background following a Poisson distribution, is: [Math.9] ^+2^, m in ) ~ --- X IJ* 7 ' iti* mes If a plurality of RAijU lines are measurable, the total mass of X isotopes, estimated from the useful RAi>u lines, is the average of the masses weighted by their counting statistics, i.e.: [Math. 10] The associated uncertainty can be calculated by the quadratic sum [Math. 11] In a subsequent step 140, the target is irradiated by B neutrons generated by the second neutron generator 3. The neutron emission from the second neutron generator 3 can be continuous throughout the irradiation time Tiirad, which may be equal to or different from the duration of irradiation by the first neutron generator 2. This second irradiation is not simultaneous with the first irradiation and takes place after the end of step 120 of measuring the spectral response of the first irradiation. Preferably, the second neutron generator 3 is configured to generate a neutron spectrum containing a significant proportion of thermal neutrons with cross sections much larger than the crd cross sections of the neutrons generated by the first neutron generator 2. For equal neutron emission, the interrogator neutron flux <pB est plus important que le flux de neutrons interrogateurs <pA issus du premier générateur de neutrons 2 car le taux de réactions sur un isotope X; est plus important en l’absence d’un seuil de réaction à haute énergie.

[0103] The TilTad irradiation time in this step can be shortened if the half-life of the isotope of interest allows it.

[0104] During the irradiation phase and once the irradiation is complete, the spectral response produced by the reaction of the neutrons with the target is measured by the gamma radiation detector 5 (step 150).

[0105] Similar to the first irradiation, the measured spectral response is:

[0106] [Math. 12] F b(E) =\d(EF) x$B(F)dF

[0107] where = ^B(E) + &b(E) +

[0108] The useful gamma radiation flux from the sample is

[0109] [Math. 13] 0*b(£) )dE \ xfl^I Ep

[0110] E' ' is the energy of the questioning neutrons, af is the neutron-induced reaction cross section, slower than during the first irradiation, is the interrogating neutron flux in this second phase of irradiation.

[0111] The flux of the active background noise induced by the second irradiation is

[0112] [Math. 14] ^b^E) =£^4F;N^{e'}(Pb{e}dE"xx £p

[0113] The flux çp^jg^ of the background noise due to natural emissions of the isotopes Xkest is identical to that of the first irradiation.

[0114] In a subsequent step 160, the gamma Rip radiations emitted by the isotope of interest X, are identified in the spectral response rB(E).

[0115] Preferably, in the event of the presence of parasitic radiation R1^ produced by the activation of surrounding elements or R*h>p produced by natural radioactivity of the environment and likely to interfere with the useful Rip radiation, one relies on one or more emission lines R; w with w different from p.

[0116] The mass mBijU of the isotope X; can be estimated in a step 163 with the Rb;jw lines identified from Faire net below the peak of the p? — c? - R? lines:

[0117] [Math. 15] (c8 -sfy nv ï.i<' fRi = — Ms.

[0118] with^ =Jmrg(£)rfEet

[0119] The uncertainty in the mass can be calculated in a similar way to the case of the first irradiation:

[0120] [Math. 16] F;

[0121] Typically, R? lines of intensity and branching ratio equivalent to those of the lines obtained during the first irradiation phase will exhibit a more pronounced signal than / fd lines due to higher reaction rates.

[0122] If a plurality of RBi>w lines are measurable, the total mass of X isotopes, estimated from the useful RBi>w lines, is the average of the masses weighted by their counting statistic, i.e.:

[0123] [Math. 17]

[0124] The associated uncertainty can be calculated by the quadratic sum:

[0125] [Math. 18]

[0126] Optionally, further irradiation and measurement phases can be carried out after the first and second phases described, by repeating the same steps. The energy spectra of the generated neutrons are advantageously different between each irradiation phase. This provides more precise information on the isotopes of interest X;.

[0127] It is possible to use the first or second neutron generator to generate the interrogating neutrons of these later phases if the generators are configured to generate different neutron spectra. It is also possible to use additional neutron generators.

[0128] When the irradiation and measurement phases are completed, the mass of each isotope of interest X; can be estimated more accurately from the useful masses U measured for each irradiation phase M (step 170):

[0129] [Math. 19]

[0130] The uncertainty in the mass m can be calculated according to the equation

[0131] [Math.20] Examples

[0132] The characterization of an aluminium sample using a characterization process according to the invention was simulated by the inventors.

[0133] As shown in [Fig.3], a 670.72 gram aluminum sample, corresponding to a disc 2 cm thick and 6 cm in radius, constitutes target 4.

[0134] The first neutron generator generates mono-energetic neutrons of 14.1 MeV and the second neutron generator generates mono-energetic neutrons of 2.45 MeV. In this example, the neutrons are emitted isotropically from a point source 7.

[0135] Part of the neutrons generated by the generators are thermalized by a thermalization cell 8 located near the point source 7. The thermalization cell 8 is for example made of high-density polyethylene (HDPE).

[0136] With each of the generators, the target is continuously irradiated for two hours, followed by a two-hour post-irradiation counting phase. The counting is performed by the gamma 5 radiation detector, which in this example is an HPGe detector with a relative efficiency of 33%.

[0137] Figure 4 represents the number NP of nuclei produced by the activation of the target by a neutron generator (in units of shots per source neutron) as a function of time. The irradiation phase of duration Tiirad takes place between times t0 and tb. The post-irradiation measurement phase of duration Tacquisition takes place between times ti and t2. The products of the target activation are decreasing during this phase.

[0138] The first phase of irradiation with 14.1 MeV neutrons causes the threshold of the 27Al(nr, a)24Na reaction to be exceeded, which is 6 MeV. The activation product 24Na undergoes [3] decay into one of the metastable states of the daughter nucleus 24m xMn. The latter de-excites to its stable ground state by emitting two main gamma lines with energies of 1368 keV and 2754 keV. The neutrons thermalized by cell 8 induce 27Al(nth, y)28Al radiative capture reactions which generate a gamma line at 1778 keV.

[0139] Thus, irradiation of the target by the first neutron generator at 14.1 MeV highlights the presence of aluminium 27 through the three characteristic lines at 1368 keV, 1778 keV and 2754 keV.

[0140] Lower-energy neutron irradiation (below the reaction threshold) generates only a single characteristic line at 1778 keV. This line alone is insufficient to conclude the presence of aluminium-27 because unstable elements that may be present in the target or the environment emit radiation at energies close to 1778 keV. For example, 91Kr, 138Cs and 132I isotopes are frequently encountered during nuclear material inspections and emit gamma radiation at 1778.85 keV, 1778.25 keV, and 1778.5 keV, respectively. In an environment containing these isotopes, only the simultaneous presence of all three characteristic lines confirms the presence of aluminum 27.

[0141] Analysis of the signal net of background noise of these three lines makes it possible to estimate the mass of aluminium 27 present in the target, as described previously.

[0142] Next, the target is irradiated by 2.45 MeV neutrons generated by the second generator, thermalized or not by cell 8. The neutrons from this second irradiation are less energetic than those from the first irradiation. At equal neutron emission, this second irradiation makes it possible to limit the impact of active background noise caused by high-energy activation products, to increase the proportion of thermal interrogating neutrons and therefore the 27 Al(nth, y)28Al radiative capture reactions, and to avoid exceeding the activation thresholds above 2.45 MeV of elements present in the environment, including in particular oxygen present in the air, which has a 16O(n,p)16N reaction with a threshold at 10 MeV.

[0143] Consequently, the second irradiation phase improves the signal-to-noise ratio for the 1778 keV line, with equal irradiation and measurement times compared to the first irradiation.

[0144] Fig. 5 represents the count Np of gamma photons detected by detector 5 as a function of energy E, for the first irradiation phase (14.1 MeV curve) and for the second irradiation phase (2.45 MeV curve).

[0145] Fig. 6 is a detailed view of the area of ​​interest ZI of Fig. 5. It can be seen that the 1368 keV line is present only in the 14.1 MeV curve.

[0146] Fig. 7 is a detailed view of the area of ​​interest Z2 of Fig. 5. The 1778 keV line can be clearly identified in the curves of the two irradiation phases.

[0147] Fig. 8 is a detailed view of the area of ​​interest Z3 of Fig. 5. It can be seen that the 2754 keV line is present only in the 14.1 MeV curve.

[0148] The inventors have calculated that the signal-to-noise ratio of the 1778 keV line is improved by a factor of 3.456 between the first and second irradiation phases.

[0149] Thus, the process as implemented in this example makes it possible to confirm the presence of aluminium 27 during the first phase of irradiation at 14.1 MeV thanks to the presence of three characteristic lines and to make a first estimate of the mass of this isotope, then to improve the mass estimate thanks to the second irradiation which has a significantly more favorable signal-to-noise ratio on the 1778 keV line. Tunable neutron spectra

[0150] In an advantageous embodiment, one or more of the neutron generators can generate neutrons with a tunable energy spectrum. This makes it possible, in particular, to select a preferred neutron spectrum in order to facilitate the characterization of certain isotopes of interest.

[0151] Such a neutron generator may include an accelerator, in particular linear, of ions and / or electrons coupled to a secondary target which interacts with the ions or electrons to produce neutrons.

[0152] The secondary target is preferably adapted to allow photonuclear reactions. The secondary target may include heavy water D2O, beryllium, or gaseous deuterium.

[0153] Fig. 9 represents the different neutron spectra obtained by twelve different configurations of neutron generators.

[0154] Ten of the generator configurations comprise a 9 MeV monoenergetic photon source coupled to a spherical reservoir of pure heavy water with volumes ranging from 1 mL to 20 L. Two configurations correspond to DD and DT monoenergetic neutron generators. The spectra of these two configurations are monoenergetic lines at 2.45 MeV and 14.1 MeV, respectively, and represent the neutron emission ND of the generators, normalized to the emission maximum of the configuration at a volume of 20 L of heavy water as a function of energy. The spectra of the ten heavy water configurations represent the number Nn of neutrons per source photon as a function of energy.

[0155] It is observed that the neutron spectrum of heavy water configurations varies with the volume of the heavy water target. The neutron spectrum hardens with increasing heavy water volume, thus increasing the proportion of thermal interrogating neutrons. This can improve the signal produced by (nth,y) radiative capture reactions. Indeed, unlike threshold reactions of the (nr,p), (nr,a), or (nr,3He) type, which have better cross sections for fast neutrons, radiative capture reactions have better cross sections for thermal interrogating neutrons.

[0156] The characteristics of certain neutron-induced reactions for certain metallic isotopes of interest in the field of steelmaking are summarized in Table 1. These isotopes exhibit reactions with a reaction threshold and other reactions without a threshold, and therefore allow the use of several irradiation phases at different energies to improve their identification and characterization. Isotope Reaction Threshold Reaction Half-life Associated gamma radiation (keV) Associated IY intensities (%) Aluminum -27 27Al(nr, a)24Na 6 MeV 14.96 h 1368.63 2754.05 99.99 99.86 27Al(nth, y)28Al No threshold 5.120 min 1778.97 100 Copper-63 63Cu(nr, 3He)61Co 12 MeV 1.65 h 67.42 908.63 85 3.6 63Cu(nth, Y)64Cu No threshold 12.7 h 1345 100 Manganese -55 55Mn(nr, a)52V 3 MeV 3.74 min 1530.67 1434.07 11.6 100 55Mn(nth, Y)56Mn No threshold 2.57 h 735.58 1037.94 2848.72 14.6 27.9 56.3 Vanadium- 51 51V(nr, a)48Sc 2.1 MeV 43.67 h 1037.59 1312.09 97.6 100 51V(nr, p)51Ti 1.7 MeV 5.76 min 320.08 928.6 93 6.9 51V(nth, y)52V No threshold 3.74 min 1530.67 1434.07 11.6 100

[0157] Other variants and improvements may be envisaged without departing from the scope of the invention.

Claims

Demands

1. A method (100) for characterizing at least one isotope of a target (4) by neutron irradiation, the method comprising the steps of: a / irradiating (110) the target with neutrons generated by a first neutron generator (2) and having a first neutron spectrum; b / measuring (120) a first spectral response of the target during and after irradiation a / by means of a gamma radiation detector (5); c / identifying (130) in the first spectral response at least one first line produced by a reaction of at least one isotope with neutrons; d / when the measurement according to step b / of the first spectral response is completed, irradiating (140) the target with neutrons generated by a second neutron generator (3) and having a second neutron spectrum different from the first neutron spectrum; e / measure (150) a second spectral response of the target during and after irradiation d / using the gamma radiation detector;f / identify (160) in the second spectral response at least a second line produced by a reaction of at least one isotope with neutrons, which may or may not be identical to at least one first line.;

2. A method according to claim 1, comprising a step of calculating the mass of at least one isotope from at least one first line and / or at least one second line.

3. A method according to any one of the preceding claims, the first neutron generator generating fast neutrons, in particular with an energy greater than 1 MeV.

4. Method according to the preceding claim, the first neutron generator being a deuterium-deuterium or deuterium-tritium generator.

5. A method according to any one of claims 3 or 4, the first neutron generator being coupled with a thermalization cell (8) configured to thermalize neutrons generated by the first neutron generator.

6. A method according to any one of the preceding claims, the second neutron generator generating thermal or epithermal neutrons, in particular with an energy of less than 1 MeV.

7. A method according to the preceding claim, the second neutron generator comprising an ion or electron accelerator coupled to a secondary target that produces neutrons when subjected to irradiation with ions or electrons generated by the accelerator, the secondary target preferably being a volume of heavy water, beryllium or deuterium gas.

8. Method according to any one of the preceding claims, the gamma radiation detector being collimated in the direction of the target.

9. A method according to any one of the preceding claims, comprising, after irradiation d / and measurement e / , one or more additional phases of irradiation of the target with neutrons having a spectrum different from the first and second neutron spectra and one or more additional phases of measurement of an additional spectral response of the target during and after each additional phase of irradiation by means of the gamma radiation detector.

10. A method according to any one of the preceding claims, wherein at least one of the first or second neutron generator is configured to generate a plurality of different neutron spectra.

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