Method for analyzing a specimen by neutron activation

Abstract

Method for analyzing a specimen by neutron activation. This method comprises: - activating (110) several samples of the specimen using, for each sample, a traversed thickness of moderator material different from the traversed thickness used to activate the other samples, and - for each activated sample, establishing (132) a Ni(R) concentration of a radioisotope of the component sought in that activated sample using only the ionizing radiation measured from that sample, where the exponent i is an identifier of the traversed thickness used to activate that sample and R is an identifier of the radioisotope of the component sought.then - the determination (134) of an Nc(C) concentration of the target component in the specimen from the various established Ni(R) concentrations and predetermined fluences of thermal neutrons and fast neutrons for each of the thicknesses of the moderator material used to activate the samples. Fig. 4,

Description

Title of the invention: Method for analyzing a specimen by neutron activation

[0001] The invention relates to a method and system for analyzing a specimen by neutron activation. The invention also relates to a sample activation device for implementing this analysis method.

[0002] Neutron activation analysis (NAA) is a precise and non-destructive analytical method widely used in various scientific and industrial fields to determine the elemental composition of samples. This technique is based on the interaction of neutrons with the nuclei of the elements present in the sample.

[0003] The process involves using initially energetic neutrons, called fast neutrons, from a fast neutron source. These neutrons are then slowed down by a moderator material to considerably reduce their energy; they are then called thermal neutrons. When the thermal neutrons are absorbed by the nuclei constituting the sample through neutron capture, this leads to the formation of unstable radioisotopes that can undergo radioactive decay. The ionizing radiation that accompanies the decay of these newly formed radioisotopes is measured using appropriate detectors. Measuring the intensity and energy of this ionizing radiation makes it possible to identify the different elements present in the sample and to determine their concentrations. Typically, the ionizing radiation measured is gamma radiation.

[0004] The advantages of neutron activation analysis include its high sensitivity, its ability to detect low concentrations of elements, and its non-destructiveness, which allows for successive analyses of the same samples. Furthermore, it can detect most of the elements in the periodic table.

[0005] The applications of this technique are diverse. It is widely used in the analysis of materials such as metals, alloys, ceramics, and polymers. It is also applied in archaeology, geology (particularly in mineral exploration), medical sciences for diagnosis and the study of biological tissues, and environmental monitoring for detecting radioactive contamination and analyzing environmental samples. Neutron activation analysis also plays a crucial role in criminology by enabling the analysis of elements present at crime scenes and on evidence. This technique helps to identify the composition of materials, such as Fabric fibers or glass shards can be detected, providing clues about the origin of the found elements. Furthermore, it is used to detect illicit substances such as drugs or explosives. Neutron activation analysis also helps optimize the recycling of electric vehicle batteries by characterizing their composition at the end of their life.

[0006] Neutron activation analysis, although based on reactions induced by thermal neutrons leading to the creation of a radioisotope, can be subject to interference from other nuclear reactions induced this time by fast neutrons, also usually present in the neutron fields used in neutron activation analysis, resulting in the creation of the same radioisotope. These reactions complicate the interpretation of the results and make the precise determination of the composition of the samples more difficult. Indeed, unlike thermal neutrons and the capture reactions that characterize them, fast neutrons have the particularity of being able to induce various nuclear reactions such as transmutation reactions, inelastic scattering reactions (n, n'y), and fission reactions (n,f).

[0007] To compensate for these interferences, the use of correction factors has already been proposed. However, the calibration or determination of these correction factors is complicated. An example of this prior art is represented by US patent application US7778783B2.

[0008] The invention aims to improve neutron activation analysis by simplifying and improving interference compensation.

[0009] The invention therefore relates to a method for analyzing a specimen by neutron activation, this method comprising:

[0010] - an activation phase during which:

[0011] - a fast neutron source emits fast neutrons, and

[0012] - at least one sample of the specimen is irradiated with neutrons emitted by the a source of fast neutrons that have passed through a thickness of moderator material, this moderator material slowing down some of the incident fast neutrons to transform them into thermal neutrons, the fluences of fast neutrons and thermal neutrons incident on the sample being a function of the thickness of moderator material passed through,

[0013] - a measurement phase of the ionizing radiation resulting from the radioactive decay of each sample thus activated, then

[0014] - a quantification phase of a component sought in the specimen from the measured ionizing radiation,

[0015] in which:

[0016] - during the activation phase, several samples of the specimen are activated in using, for each sample, a different traversed thickness of moderator material than the traversed thickness used to activate the other samples, and

[0017] - during the quantification phase:

[0018] - for each activated sample, a concentration N'(R) of a radioisotope of The component sought in this activated sample is established using only the ionizing radiation measured from this sample, where the exponent i is an identifier of the traversed thickness used to activate this sample and R is an identifier of the radioisotope of the component sought, then

[0019] - an NC(C) concentration of the target component in the specimen is determined from the different established N'(R) concentrations and predetermined fluences of thermal neutrons and fast neutrons for each of the thicknesses of the moderator material used to activate the samples.

[0020] Embodiments of this process may include one or more of the following features:

[0021] 1) During the quantification phase, the concentration NC(C) is determined by solving a system of equations comprising Nbe equations of the form NC(C) = [N'(R) - CJAp / C)*^1) and at least two unknowns NC(C) and Nc(Ik), where

[0022] - Nbe is a number greater than or equal to the number of unknowns and less than or equal to the number of different thicknesses of moderator material used to activate the samples,

[0023] - o / C) is the predetermined cross section of the reaction that generates the radioisotope R by capturing a thermal neutron by the component being sought,

[0024] - ¢ / is a predetermined fluence of thermal neutrons for the thickness traversed of moderating material identified by the exponent i,

[0025] - C1 is an interference correction coefficient comprising at least one term Nc(Ik )*0r(D* ¢ / ,

[0026] - Nc(L) is the concentration of an interfering component Ik, the component Ik being capable, when interacting with a fast neutron emitted by the fast neutron source, of generating a radioisotope identical to the radioisotope generated by thermal neutrons when they react with the desired component,

[0027] - or(Ik) is the predetermined cross section of the reaction that generates the radioisotope R by interaction of a fast neutron with the interfering component Ik,

[0028] - ¢ / is a predetermined fluence of fast neutrons for the thickness traversed of moderator material identified by the exponent i.

[0029] 2) The number Nbe is greater than the number of unknowns in the system of equations solve.

[0030] 3) During the activation phase, said several samples are activated by irradiating simultaneously with neutrons emitted by the same source of fast neutrons.

[0031] The invention also relates to a system for analyzing a specimen by neutron activation, this system comprising:

[0032] - a specimen sample activation device, this device comprising:

[0033] - a fast neutron source capable of emitting fast neutrons,

[0034] - a moderator material exposed to fast neutrons emitted by the neutron source fast, this moderator material being capable of slowing down some of the incident fast neutrons, transforming them into thermal neutrons, and

[0035] - at least one housing suitable for receiving a sample of the specimen to be irradiated by neutrons emitted by the fast neutron source and which have passed through a thickness of the moderator material, the fluences of fast neutrons and thermal neutrons incident on the sample received in the housing being a function of the thickness of the moderator material passed through,

[0036] - a unit of measurement of ionizing radiation resulting from the radioactive decay of each sample activated using the activation device, and

[0037] - a processing unit configured to quantify a component sought in the specimen from the ionizing radiation measured by the unit of measurement,

[0038] in which:

[0039] - the activation device is capable of enabling the activation of several samples of the specimen using, for each sample, a different thickness of the moderator material than the thickness used to activate the other samples, and

[0040] - the processing unit is configured to:

[0041] - for each activated sample, establish a concentration N'(R) of a radio isotope of the target component in this sample activated using only the ionizing radiation measured from this sample, where the exponent i is an identifier of the thickness used to activate this sample and R is an identifier of the radioisotope of the target component, then

[0042] - determine an NC(C) concentration of the component sought in the specimen at based on the different established N\R concentrations and predetermined fluences of thermal neutrons and fast neutrons for each of the thicknesses traversed of the moderator material used to activate the samples.

[0043] Embodiments of this system may include one or more of the following features:

[0044] 1) The activation device comprises several housings, each capable of receiving a respective sample of the specimen to be irradiated by neutrons emitted by the fast neutron source and which have passed through a thickness of the moderator material, these housings being arranged relative to each other so that each housing corresponds to a thickness of the moderator material passed through that is different from the thickness of the other housings passed through.

[0045] 2) The dwellings are arranged one behind the other along a straight axis which passes through the center of the neutron source.

[0046] 3) The activation device comprises:

[0047] - a niche carved inside a solid block of moderating material, this niche comprising an opening leading to an outer face of the block made of moderating material, and

[0048] - an insert suitable for being introduced into the niche by means of the opening, this insert being made in a moderating material in which each of the housings are hollowed out.

[0049] 4)

[0050] - each dwelling unit comprises:

[0051] - an opening through which the sample is introduced and, alternately, withdrawn from the housing, and

[0052] - a bottom located on the side opposite the opening of the housing, and

[0053] - for each housing, the activation device includes a plug made of material moderator that can be moved reversibly along an insertion axis of this plug between:

[0054] - a closed position in which the stopper completely obstructs the opening of the dwelling and leaves only a cavity surrounded by moderating material inside the dwelling, and

[0055] - an open position into which the sample can be inserted and removed from the accommodation,

[0056] the thickness of this plug, in a direction parallel to its insertion axis, being greater than 1 cm.

[0057] 5) The unit of measurement is suitable for measuring gamma radiation from the radioactive decay of the sample activated using the activation device.

[0058] The invention also relates to a sample activation device for implementing an activation phase of the above-mentioned method of analyzing a specimen by neutron activation, this device comprising:

[0059] - a fast neutron source capable of emitting fast neutrons,

[0060] - a moderator material directly exposed to the fast neutrons emitted by the as a source of fast neutrons, this moderator material is capable of slowing down some of the incident fast neutrons, transforming them into thermal neutrons, and

[0061] - at least one housing suitable for receiving a sample of the specimen to be irradiated by neutrons emitted by the fast neutron source and which have passed through a thickness of the moderator material, the fluences of fast neutrons and thermal neutrons incident on the sample received in the housing being a function of the thickness of the moderator material passed through,

[0062] in which the activation device comprises several housings each capable of receiving a respective sample of the specimen to be irradiated by neutrons emitted by the fast neutron source and which have passed through a thickness of the moderator material, these housings being arranged relative to each other so that each housing corresponds to a thickness of the moderator material different from the thickness of the other housings.

[0063] The invention will be better understood upon reading the following description, given solely by way of non-limiting example and made with reference to the drawings in which:

[0064] - Fig. 1 is a schematic illustration, in vertical section, of the architecture of a neutron activation analysis system for specimens,

[0065] - [Fig. 2] is a schematic, longitudinal sectional view of an insert of the system of the [Fig.l],

[0066] - [Fig.3] is a schematic, cross-sectional view of the insert of [Fig.2],

[0067] - [Fig. 4] is a flowchart of a method for analyzing a specimen by neutron activation, and

[0068] - [Fig. 5] is a schematic, perspective illustration of a device activation usable in the system of the [Fig.l].

[0069] In this description, the terminology, conventions, and definitions of the terms used in this text are introduced in Chapter I. Detailed examples of embodiments are then described in Chapter II with reference to the figures. Variants of these embodiments are presented in Chapter III. Finally, the advantages of the different embodiments are specified in Chapter IV.

[0070] Chapter I: Definitions, terminology and conventions:

[0071] In the figures, the same references are used to designate the same elements.

[0072] In the remainder of this description, the well-known characteristics and functions of a person skilled in the art are not described in detail.

[0073] The figures are oriented with respect to an orthogonal XYZ coordinate system, where the X and Y directions are horizontal and the Z direction is vertical. Terms such as "above", "below", "top", "bottom", "superior", "inferior" are defined with respect to the Z direction.

[0074] The expression "an element made of a material A" or the expression "an element made of material A" means that material A represents 90% or 95% of the mass of that element.

[0075] The symbol “*” denotes scalar multiplication.

[0076] A thermal neutron means a neutron whose energy is in the thermal domain, that is to say whose energy is between 0.012 eV and 0.1 eV.

[0077] A fast neutron is defined as a neutron with an energy greater than 0.8 MeV. Fast neutrons produce interference.

[0078] An epithermal neutron is a neutron whose energy is greater than that of a thermal neutron and less than that of a fast neutron. These epithermal neutrons do not produce interference.

[0079] A moderator material is a material capable of slowing down fast neutrons to transform them into thermal neutrons. For this purpose, a moderator material has a high hydrogen content. Thus, a moderator material is defined here as a material that contains at least 2% or 3% by mass of hydrogen and, preferably, at least 8% or 10% by mass of hydrogen.

[0080] Interference is caused by one or more nuclear reactions produced by fast neutrons that degrade the accuracy of neutron activation analysis. These nuclear reactions are typically a transmutation reaction or a fission reaction. A transmutation reaction is a reaction in which fast neutrons cause a transmutation reaction in nuclei, leading to the formation of a radioisotope different from the original element. These transmutation reactions can therefore lead to the creation of radioisotopes identical to those created by the neutron capture of thermal neutrons, yet unrelated to the neutron capture of thermal neutrons. These radioisotopes created by the transmutation reaction interfere with the interpretation of the results. The most common transmutation reactions are the (n,p) and (n,a) reactions.They occur when the incident neutron has sufficiently high energy, generally at least equal to the mass difference between the target and produced nuclei, plus the additional energy required to overcome the Coulomb barrier for charged particle emission. In geological samples, this interference produced by the principal matrix elements is common. Important transmutation reactions include Si(n,p)Al, . P(n,a)28Al, 27Al(n,p)27Mg, 30Si(n,a)27Mg, 24Mg(n,p)24Na and 27Al(n,a)24Na. In biological samples, important transmutation reactions are: 24Mg(n,p)24Na and 54Fe(n,a)51Cr. In alloys, the most common transmutation reactions include 27Al(n,p)27Mg, 28Si(n,p)28Al, 30Si(n,a)27Mg, 52Cr(n,p)52V, 54Fe(n,a)51Cr,55Mn(n,a)52V, 56Fe(n,p)56Mn, 59Co(n,a)56Mn, 60Ni(n,p)60Co, 62Ni(n,a)59Fe, 63Cu(n,a)60Co, 64Zn(n,p)64Cu, and 65Cu(n,p)65Ni [3, 4]. During an (n,f) fission reaction, fast neutrons induce nuclear fission in certain so-called "fissile" nuclei present in the analyzed samples. When a nucleus undergoes fission, it splits into two lighter nuclei and typically emits several additional neutrons. These lighter nuclei can be radioactive and emit gamma rays. This fission reaction therefore also produces unwanted radioisotopes and additional radiation.

[0081] A quasi-monoenergetic neutron source is a source that emits neutrons that all have practically the same average energy. Typically, for a quasi-monoenergetic source, the dE / Em ratio is less than or equal to 10% and, preferably, less than 5% or 2%, where:

[0082] - Em is the average energy of the neutrons emitted by the source, and

[0083] - dE is the full width at half maximum of the energy peak centered on the average energy Em.

[0084] The concentration of an element, such as a radioisotope or a target compound, here refers to the ratio between the amount of that element in a sample and the total amount in that sample. The amount can be expressed as mass. In this case, the concentration of the element is equal to the ratio of the mass of that element in the sample to the total mass of the sample. The concentration can also be expressed in other units such as atomic percentage.

[0085] Gamma radiation is composed of photons resulting from radioactive decay.

[0086] Chapter II: Examples of embodiments

[0087] Figure 1 represents a system 2 for analyzing a specimen by neutron activation. System 2 allows the concentration of a target component C in the specimen to be determined. This embodiment is subsequently described in the specific case where the target component C is aluminum 27Al and the specimen also contains silicon 28Si. The presence of silicon in the specimen generates interference because, when exposed to fast neutrons, silicon 28Si can be transmuted into the radioisotope 28Al, that is, the same radioisotope obtained by exposing aluminum 27Al to thermal neutrons.

[0088] System 2 comprises:

[0089] - a specimen sample activation device 4,

[0090] - a unit 6 for measuring ionizing radiation from radioactive decay of each activated sample, and

[0091] - a processing unit 8 configured to quantify the component sought in the specimen from the ionizing radiation measured by the unit of measurement.

[0092] Here, the ionizing radiation measured by unit 6 is the gamma radiation from the activated sample. For example, measuring unit 6 is a spectrometer.

[0093] The processing unit 8 is connected to the measuring unit 6 to acquire each measurement performed by the unit 6. Typically, the processing unit 8 includes a programmable microprocessor 81 and a memory 82 containing the data and instructions necessary for implementing the process of [Fig. 4] when these instructions are executed by the microprocessor 81. The unit 8 also includes a human-machine interface 84 for acquiring data and communicating analysis results to a human. For example, the interface 84 includes a screen and a keyboard.

[0094] Device 4 is designed to simultaneously activate several samples of the same specimen, i.e., to irradiate the samples with thermal neutrons and fast neutrons. To this end, device 4 comprises:

[0095] - a fast neutron source 10 capable of emitting fast neutrons,

[0096] - a moderator material 12 through which neutrons emitted by the source 10 pass, and

[0097] - NbL housings L; each suitable for receiving a respective sample of the specimen to irradiate, where the index i identifies a specific dwelling among all dwellings.

[0098] Source 10 is a quasi-mono-energy source. For example, it is a deuterium-deuterium generator, also called a "dD generator", which produces fast neutrons with an average energy of 2.45 MeV. Typically, the fast neutrons emitted by source 10 are almost uniformly distributed in all directions.

[0099] In this embodiment, the moderator material 12 is high-density polyethylene. However, as described in Chapter III, other materials are possible. In this embodiment, the moderator material 12 comprises a block 20 and an insert 22, both made of the same moderator material. The block 20 has an inner face 24 that defines a cavity 26 within which the source 10 is housed. The source 10 is located at the center of this cavity 26. By way of illustration, the cavity 26 is a rectangular parallelepiped with center O, and the source 10 is centered on this center O. Thus, to a first approximation, the fast neutrons emitted by the source 10 are emitted from this center O. The faces of the cavity 26 are either vertical or horizontal. For example, the cavity 26 is filled with air.

[0100] Block 20 is a rectangular parallelepiped bounded by six external faces. Each of these external faces extends parallel to a corresponding face of cavity 26. In [Fig.1], the numerical references 28 and 29 designate two opposite vertical external faces of block 20. Block 20 is centered on the center O. Therefore, the median axes that pass through the centers of two opposite external faces of block 20 intersect at this center O. In [Fig.1], the numerical reference 30 designates the one of these median axes that passes through the centers of faces 28 and 29.

[0101] The thickness of block 20 at these median axes, that is to say the distance measured along these median axes between the outer and inner faces of block 20, is chosen so that the ratio O.AD is greater than ten at the outer face 28, where <e>and <e>r are the fluences, respectively, of thermal neutrons and fast neutrons emitted by the source 10 measured at the outer face 28. For this, here, the thickness of the block 20 is, typically, at least 20 cm.

[0102] The block 20 has a niche 32 cut along the axis 30 and into which the insert 22 is received by sliding. The niche 32 is centered on the axis 30. The niche 32 has an opening 34 which opens onto the outer face 28. Here, the niche 32 also has an opening 36 which opens into the interior of the cavity 26.

[0103] The cross-section of the niche 32 is shaped to guide the insert 22 in translation between a retracted position and an extended position. In the retracted position, a proximal face 40 ([Fig. 2]) and a distal face 42 ([Fig. 2]) of the insert 22 completely obstruct the openings 34 and 36, respectively. In the extended position, the insert 22 is entirely located outside the niche 32 and the block 20. Moreover, in the retracted position shown in [Fig. 1], the faces 40 and 42 of the insert 22 are flush with the face 29 and the inner face of the block 20, respectively.

[0104] Typically, the cross-sections of the niche 32 and the insert 22 are constant along the axis 30. Here, the shape of the cross-section of the niche 32 is the same as the shape of the cross-section of the insert 22, except that it is slightly larger to allow a gap that permits the insert 22 to be inserted into the niche 32 and then pushed into the niche 22 until it reaches its retracted position. Typically, this gap is less than 1 mm or 0.5 mm.

[0105] An example of an embodiment of the insert 22 is shown in more detail in Figures 2 and 3. In this example, the cross-section of the insert 22 is a circle centered on an axis 46 of revolution. Thus, the proximal face 40 and distal face 42 are connected to each other by a cylindrical face 44 whose direction curve is a circle and whose generatrix is ​​a straight line parallel to the axis 46. The face 44 is centered on the axis 46 of revolution parallel to the X direction in [Fig. 2]. In the retracted position of the insert 22, the axis 46 coincides with the axis 30.

[0106] The insert 22 comprises the L-slots. Here, the insert 22 is shown in the particular case where the number NbL of L-slots is equal to five. The L-slots are arranged one behind the other along the axis 46. The L-slots are arranged in ascending order of index i, starting from face 42 and proceeding towards face 40. Preferably, all the L-slots are identical except that they do not occupy the same position along the axis 46. Typically, each L-slot has two vertical walls perpendicular to the axis 46, a bottom F ([Fig. 3]), and, on the side opposite the bottom F, an opening O ([Fig. 3]) which opens into face 44. The opening O allows a sample of the specimen to be analyzed to be introduced into the slot and then removed once it has been activated. Here, the bottom F; of housing L; is located in its lower part so that the sample introduced into this housing rests on this background F;.

[0107] Two immediately consecutive housings L and Li+i along axis 46 are separated from each other by a slice Ti>i+i of moderator material of thickness ei>i+i. Each slice Tiji+i is formed by the moderator material of the insert 22, which extends between the vertical walls of housings L and Li+i. Here, all the slices Ti>i+i have the same thickness ei>i+i. Housing Li is separated from face 42 by a slice Ti of the moderator material of thickness eb. For example, the thickness ei is equal to the thickness ei>i+i of the slices Ti>i+i.

[0108] With such a conformation of the insert 22, the housings L; are all separated from the source 10 by a different thickness of moderator material. More precisely, in the particular case described here, the thickness of moderator material separating housing Li from the source 10 is equal to i*eb

[0109] For each housing L, the activation device 4 includes a plug B. This plug B is reversibly movable along an insertion axis, perpendicular to axis 46, between a closed position, shown in Figures 2 and 3, and an open position. In the closed position, the plug B completely obstructs the opening O, leaving only a cavity Ca inside housing L, into which the sample is received. Typically, each cavity Ca is centered on axis 46. For example, cavity Ca is a parallelepiped.

[0110] The plug B; is made of the same moderating material as the insert 22. Thus, the cavity Ca; is surrounded by moderating material. This ensures a uniform amount of moderating material around the cavity Ca;. For this purpose, the thickness of the plug B;, in the vertical direction, is greater than 1 cm or 3 cm.

[0111] In the open position, the stopper does not obstruct the opening O; so that it is possible to introduce or remove a sample from the housing L;.

[0112] The block 20 and the insert 22 are made by molding or machining an initial block of high-density polyethylene.

[0113] A method for analyzing a specimen using system 2 is now described with reference to the method in [Fig.4].

[0114] The process begins with a step 100 initializing the various data required to perform the specimen analysis. In particular, during step 100, the thermal neutron fluence and the fast neutron fluence μ / at each housing L; are determined. These fluences μ / and μ / depend, in particular, on the thickness of the moderator material 12 that the neutrons must traverse before reaching housing L;. Here, these fluences μ / and μ / therefore depend on the position of housing L; along the axis 30 in the retracted position of the insert 22. Since each housing L; is separated from the source 10 by a different thickness of moderator material, the fluences μ / and μ / are different for each housing L;. More precisely, it is the ratio that is different for each housing L;. The fluences and ¢ / also depend on other parameters such as the average energy of the fast neutrons emitted by the source 10 and the nature of the moderator material used.However, CIV and ¢ / fluences are independent of specimen composition. For example, CIV and ¢ / fluences are determined by numerical simulations, such as Monte Carlo simulations. CIV and ¢ / fluences can also be measured experimentally.

[0115] During step 100, the cross section ot(C) of the target component C is stored in memory 82. The cross section ot(C) is the cross section of the reaction that generates the radioisotope R by the capture of a thermal neutron by the target component C. Similarly, the cross section or(Ik) of each interfering component Ik is stored in memory 82. The cross section or(Ik) is the cross section of the reaction that generates the radioisotope R by the interaction of a fast neutron with the interfering component. These cross sections can be obtained from the scientific literature or determined by simulation or experimentally. In this example, the desired component C is aluminum27Al and the interfering component Ik is silicon Si. Thus, during step 100, the cross sections ot(Al) and or(Si) are stored in memory 82. More precisely, the cross section ot(27Al) is the cross section of the reaction Al(n,y)Al.The cross section or( Si) is the cross section of the reaction 28Si(n,p)28Al. .

[0116] In step 102, samples are taken from the specimen. There are as many samples taken as there are housings L;. Preferably, the samples taken all have the same mass.

[0117] Next, in step 104, each sample taken is introduced into a respective compartment L;. For each compartment L;, once the sample is introduced, the stopper B; is moved from its open position to its closed position. The sample is then trapped inside the cavity Ca;.

[0118] When each sample is trapped inside a respective cavity Ca;, in a step 106, the insert 22 is moved from its exit position to its retracted position.

[0119] A phase 110 of simultaneous sample activation is then performed. During phase 110, the source 10 emits fast neutrons for a predetermined duration. Some of the emitted neutrons pass through the moderator material 12 and irradiate the samples contained in each compartment L. The moderator material 12 slows down some of the fast neutrons, transforming them into thermal neutrons. The proportion of fast neutrons transformed into thermal neutrons varies depending on the thickness of the moderator material 12 through which it passes. Thus, during phase 110, each sample is irradiated with fluences and <D, de neutrons thermiques et rapides qui varient en fonction du logement L; à l’intérieur duquel est reçu cet échantillon. Cette irradiation provoque l’apparition du radio-isotope 28Al du composant recherché dans chacun des échantillons. Le radio-isotope 28A1 est généré par la réaction Al(n,y) Al de capture d’un neutron thermique par l’aluminium Al.This radioisotope 28 28 28 28. Al is also generated by the Si(n,p)Al reaction when silicon Si interacts with a fast neutron.

[0120] After the predetermined time, the samples are said to be "activated". Each activated sample has a concentration N'(28A1) of the radioisotope 28A1 which depends on the housing in which the sample was located during the activation phase. Indeed, the concentration N'(28A1) depends in particular on the fluences <e>t 1 and EEC. In addition, the concentration N'(28A1) also depends on the concentration NC(27A1) of aluminium27Al in the specimen as well as the concentration Nc(28Si) of silicon28Si in the specimen.

[0121] Once the samples are activated, in step 112, the insert 22 is moved to its extended position and the plugs B; are moved to their open positions, then the samples are removed from the housings L;. Each removed sample is associated with an identifier of the housing in which it was located.

[0122] During a measurement phase 120, each activated sample is placed in the measurement unit 6 and the unit 6 measures the gamma radiation spectrum of each sample.

[0123] The measured spectra are then acquired by the processing unit 8. Each acquired spectrum is associated with the identifier i of the housing L in which the sample was located. corresponding to this spectrum. For example, the identifier i of housing L; associated with a spectrum is acquired by unit 8 via interface 84. The spectra thus acquired include, in particular, a peak characteristic of the presence of the radioisotope 28Al. The amplitude of this characteristic peak is proportional to the concentration of the radioisotope 28Al present in the sample.

[0124] During a quantification phase 130, the processing unit 8 determines the concentration of aluminium27Al in the specimen from the measurements of the measuring unit 6.

[0125] For this purpose, during step 132, for the spectrum associated with housing L;, unit 8 i 28 28 i 28 establishes the concentration N ( Al) of the radioisotope Al. This concentration N ( Al is established from the amplitude of the characteristic peak of the gamma radiation of the radioisotope 28A1 in the measured spectrum associated with housing L;.

[0126] Next, in step 134, the concentrations NC(27Al) and Nc(28Si) are determined by solving a system (1) of equations. The system (1) of equations is formed by the following five equations: NC(27Al) = [N'(28Al) - C'J / Co / ^Al)*^1), where:

[0127] - i varies from one to five, and

[0128] - C1 is an interference correction coefficient equal, in this example, to Nc(28Si)*or( 28Si)* ¢ / .

[0129] In system (1), the concentrations NC(27Al) and Nc(28Si) are the unknowns. The terms ot(27Al), <6 / , or(28Si), and ¢ / were pre-recorded during the initialization step 100. The terms N'(28Al) are those established during step 132. System (1) therefore has two unknowns and five equations. Such a system of equations is, for example, solved using the ML-EM (Maximum Likelihood Reconstruction (ML) using Expectation Maximization (EM)) algorithm.

[0130] At the end of step 134, during a step 136, the determined NC(27A1) concentration is communicated to a human being via interface 84.

[0131] Figure 5 shows an activation device 150 that can be used in place of the activation device 4. The device 150 is identical to the device 4 except that it has a recess in each face of the block 20. Here, the block 20 is a rectangular parallelepiped. Thus, in addition to the recess 32, the block 20 has five additional recesses 152 to 156 visible in Figure 5, through the block 20. The recesses 152 to 156 are identical to the recess 32 except that they are located in the center of a respective face of the block 20.

[0132] The activation device 150 also includes six inserts, namely insert 22 and five other inserts identical to insert 22.

[0133] Thus, it is possible to introduce an insert into each of the niches 32 and 152 to 156. Therefore, during the activation phase, it is possible to activate thirty samples of the specimen simultaneously.

[0134] In the case of device 150, during the activation phase, there are therefore six Li>p compartments separated from the source 10 by the same thickness of moderator material, where the index p is an identifier of the insert in which this Li>p compartment is made. Under these conditions, during step 132, the processing unit 8 obtains six concentrations of racQo_isotOpe 28Al associated with the same traversed thickness i of moderator material. For each traversed thickness i, these six concentrations N1,P(28Al1) are averaged to obtain an average concentration N'(28Al1). Then, during step 134, it is this average concentration N'(28Al1) that is used in the system (1) of equations.

[0135] Chapter III: Variants:

[0136] Activation device variants:

[0137] Other neutron sources can be used. For example, the deuterium-deuterium generator can be replaced by a deuterium-tritium (dT) generator, which produces neutrons with an average energy of 14.1 MeV. The fast neutron source can also be a nuclear reactor, a particle accelerator, or an isotopic source such as an Am-Be or 252Cf source. The fast neutron source can be a continuous or pulsed neutron source.

[0138] Other moderating materials can be used. For example, high-density polyethylene can be replaced by paraffin. High-density polyethylene (HDP) can also be replaced by medium-density polyethylene (MDPE), linear low-density polyethylene (LLDPE), cross-linked polyethylene (PEX), low-density polyethylene (LDPE), polypropylene (PP), polymethylpentene (PMP), polybutene-1 (PB-1), or other polyalphaolefins. In another embodiment, polyethylene is replaced by concrete containing 2% or 3% hydrogen by mass.

[0139] The moderator material may be a mixture, preferably homogeneous, of several different moderator materials. The moderator material may also be composed of a juxtaposition, one after the other, of several layers of different moderator materials.

[0140] Other shapes are possible for block 20. For example, instead of being a parallelepiped, block 20 can be a sphere or a cylinder with a circular cross-section.

[0141] The cavity 26 can also have other shapes. For example, alternatively, the cavity 26 is spherical or cylindrical with a circular cross-section.

[0142] Numerous variations of the niches are possible. In particular, the number of niches can be modified. For example, it is also possible to provide several niches that open onto the same external face 28 of the block 20.

[0143] In another embodiment, the niche 32 does not open into the cavity 26. In this case, the niche 32 has a base separated from the cavity 26 by a thickness of the moderator material. In this embodiment, in its retracted position, the distal face 42 of the insert 22 rests directly on this base of the niche.

[0144] In another variant, the insert 22 is made of a moderator material different from that used to make the block 20.

[0145] The cross-section of the insert 22 is not necessarily circular. For example, alternatively, the cross-section of the insert 22 is rectangular.

[0146] The dimensions of the cross-section of the insert 22 can also be much smaller than the dimensions of the cross-section of the niche 32. In this case, an adapter, made of the same moderating material as the insert 22, is used to fill the gap between the insert 22 and the niche 32. This adapter has no housing. For example, using an adapter makes it possible to use an insert with a rectangular cross-section in a niche with a circular cross-section. Adapters also allow the use of different inserts with different cross-sections in the same niche.

[0147] In a simplified embodiment, the plugs B; are omitted.

[0148] The thicknesses eiji+i and the thickness ei are not necessarily all equal.

[0149] Alternatively, in the retracted position, the L slots are aligned one behind the other along an axis that does not pass through the center O of the source 10. In fact, in the retracted position, it is also possible that the L slots are not aligned one behind the other along the same straight axis. For example, alternatively, in the retracted position, each L slot is located on a respective axis that passes through the center O of the source 10, and these respective axes are angularly spaced from each other so that the sample received in each of these L slots is irradiated by neutrons that have not previously passed through another sample received in another slot.

[0150] In another embodiment, the housings L; are directly hollowed out in the block 20 and the insert 22 is omitted. In this case, each housing L; opens directly onto an exterior face of the block 20 in order to allow the introduction and removal of the sample in that housing L;.

[0151] In a highly simplified embodiment, the activation device comprises a single housing Li, and the thickness of moderating material traversed before reaching this single housing Li is adjustable. For example, the activation device comprises:

[0152] - an insert identical to insert 22 except that it only includes the Lb housing

[0153] - niche 32 has a base made of moderating material which separates it from cavity 26, And

[0154] - a set of shims made of moderating material of different thicknesses.

[0155] In the case of the activation device described above, with a single compartment Lb for implementing the analysis method, the samples are activated sequentially. For example, a first sample is placed inside compartment Li, then the insert is introduced into niche 32 until its distal face 42 rests against the bottom of niche 32. In this position of the insert, the first sample is activated. Next, the first sample is removed from compartment Li and replaced by a second sample. Before introducing the insert into niche 32, one of the moderator material shims is inserted into niche 32 and pushed in until it rests against the bottom of the niche. Then, the insert containing the second sample is introduced into the niche until its distal face 42 rests against the shim.In this position, the second sample is irradiated by neutrons that pass through a greater thickness of moderator material than in the case of the first sample. These operations are repeated for each of the wedges in the wedge set in order to activate several samples using different thicknesses of moderator material traversed.

[0156] In a variant of this activation device with a single housing, all the wedges have the same thickness and the different thicknesses through which moderating material is passed are obtained by introducing a greater or lesser number of these wedges inside the niche.

[0157] Many different geometries are possible for the Ca; cavities. In particular, the width, length, or depth of the Ca; cavities can be modified. Thus, alternatively, the Ca; cavities are not parallelepipeds. For example, the Ca; cavities can be cylindrical with a circular cross-section, or the Ca; cavities can be spherical.

[0158] Variants of the analysis system:

[0159] Other ionizing radiations resulting from the radioactive decay of the activated sample can be used instead of gamma radiation. For example, alternatively, unit 6 measures the beta or alpha radiation of the sample.

[0160] The measuring unit 6 can be integrated into the activation device to allow measurement of the ionizing radiation of each sample while it is still located inside the block 20.

[0161] Variants of the analysis method:

[0162] Alternatively, the number Nbe of equations in the system (1) of equations is greater than two and less than the number of housings L;. For example, the number Nbe is equal to the number of unknowns in this system of equations. This is the case, for example, if some of the equations of the form NC(C) = [N'(R) - CJ / Co / C)*^1) are ignored or if housings L; are not used to activate samples.

[0163] If necessary, in addition to what has already been described, the activation and measurement phases can be repeated several times for the same thickness of moderator material traversed. These redundant measurements are then, for example, averaged to increase the measurement accuracy, as described in the specific case of the activation device 150. Instead of averaging the redundant measurements, it is also possible to introduce into the system of equations (1) an equation for each redundant measurement performed.

[0164] In one embodiment, the samples are activated one after the other, rather than simultaneously. For example, a first sample is placed in a first compartment and then activated. Next, the first activated sample is removed from the first compartment, and a second sample is placed in a second compartment and then activated. These operations are repeated as many times as necessary to activate several samples using different thicknesses of moderator material.

[0165] Alternatively, to increase the accuracy of the measurement of each concentration N'(R) for each sample, this measurement is repeated several times at different times. These times are sufficiently spaced apart so that the ionizing radiation measured is different at each of these times due to radioactive decay.

[0166] The correction coefficient C1 may also include additional terms. This is the case, for example, if the specimen contains several interfering components Ik. In this case, the coefficient C1 is written in the form of a sum of several terms Nc(Ik)*or (10*0 / , where the index k identifies the interfering component.

[0167] What has been described above also applies to the simultaneous quantification of several target components Cm in the same sample, where the subscript m identifies the target component and its radioisotope Rm. In this case, everything described in the specific case of the target component C is carried out for each of the target components Cm. In particular, the activation phase is common to all the target components Cm. If unit 6 measures the gamma spectrum of the activated samples, the measurement phase 120 can also be common to all the target components Cm. Then, during the quantification phase, a concentration N'(Rm) is established for each radioisotope Rm and for each traversed thickness i. Finally, to determine each concentration Nc(Cm), a respective system of equations similar to System of equations (1) is solved. The system of equations solved to determine the concentration Nc(Cm) is identical to system of equations (1) except that:

[0168] - the data and measurements relating to the radioisotope R are replaced by the data and corresponding measurements relating to the radioisotope Rm, and

[0169] - the data relating to each interfering component are replaced by the corresponding data of the interfering component(s) which react with fast neutrons to generate Rm radioisotopes.

[0170] Samples of different materials can also be activated simultaneously. For example, device 150 is used for this purpose. Device 150 allows the simultaneous activation of samples of six different materials A to F. To this end, five samples of material A are introduced into each of the Li>p compartments of the same insert p. The same procedure is followed for materials B to F, but using a separate insert for each. The samples of each material A to F are then simultaneously activated during step 110. Subsequently, in step 132, the processing unit 8 establishes, for each material A to F, five different radioisotope concentrations, each associated with a different thickness i of moderator material. Then, for each material A to F, the corrected concentration of the target component is determined as described in step 134.

[0171] Several of the variants described above can be combined in the same embodiment.

[0172] Chapter IV: Advantages of the embodiments described:

[0173] Using ionizing radiation measurements from several samples activated with different thicknesses of the same moderator material allows for the establishment of several concentrations N'(R) for different known fluences of thermal and fast neutrons. From these different concentrations N'(R) established for different known fluences, it is possible to compensate for interferences simply and efficiently. It is therefore possible to determine more precisely the concentration NC(C) of the desired component.

[0174] The fact that the number Nbe of equations in the system of equations (1) is greater than the number of unknowns allows the accuracy of the analysis to be improved even further.

[0175] Irradiating the different samples at the same time increases the accuracy because, in this case, the differences between the fluences of thermal and fast neutrons incident on each sample come only from the differences in thicknesses traversed by the moderator material and not from a temporal variation of the neutron source.

[0176] The fact that the activation device has several compartments for simultaneously receiving several samples of the specimen separated from the neutron source by using different thicknesses of moderator material, it is possible to activate several samples simultaneously using different thicknesses of the moderator material traversed and therefore with different O / O ratios.

[0177] Aligning the housings L one behind the other along the axis 30 which passes through the center O of the source 10 allows for a simpler and more precise determination of the thermal and fast neutron fluences for each thickness of the moderator material traversed. This therefore increases the accuracy of the analysis method.

[0178] The use of an insert simplifies the introduction and removal of samples into the housings.

[0179] The use of moderator material plugs B; to seal the Li housings allows for a uniform distribution of the moderator material around the sample during its activation. This simplifies and increases the accuracy of the thermal and fast neutron fluences determined for each L; housing. Ultimately, the accuracy of the analytical procedure is improved.

[0180] Measuring gamma radiation allows for obtaining spectra with very high resolution, making it easier to distinguish the different characteristic peaks of various radioisotopes. Thus, measuring the gamma radiation of the sample improves the accuracy of the analytical process and system.< / e> < / e> < / e>

Claims

Demands

1. A method for analyzing a specimen by neutron activation, this method comprising: - an activation phase (110) during which: - a fast neutron source emits fast neutrons, and - at least one sample of the specimen is irradiated with neutrons emitted by the fast neutron source and which have passed through a thickness of moderator material, this moderator material slowing down a portion of the incident fast neutrons to transform them into thermal neutrons, the fluences of fast neutrons and thermal neutrons incident on the sample being a function of the thickness of moderator material passed through, - a phase (120) of measuring the ionizing radiation resulting from the radioactive decay of each sample thus activated, then - a phase (130) of quantifying a component sought in the specimen from the measured ionizing radiation, characterized in that: - during the activation phase (110),Several samples of the specimen are activated using, for each sample, a traversed thickness of moderator material different from the traversed thickness used to activate the other samples, and - during the quantification phase (130): - for each activated sample, a concentration N'(R) of a radioisotope of the target component in that activated sample is established (132) using only the measured ionizing radiation from that sample, where the exponent i is an identifier of the traversed thickness used to activate that sample and R is an identifier of the radioisotope of the target component, then - a concentration NC(C) of the target component in the specimen is determined (134) from the different concentrations N1(R) established and predetermined fluences of thermal neutrons and fast neutrons for each of the thicknesses of moderator material used to activate the samples.

2. A method according to claim 1, wherein, during the quantification phase, the concentration NC(C) is determined by solving a system of equations comprising Nbe equations of the form NC(C) = [N'(R) - CJApXC)*^1) and at least two unknowns NC(C) and Nc(Ik), where - Nbe is a number greater than or equal to the number of unknowns and less than or equal to the number of different thicknesses of moderator material used to activate the samples, - o / C) is the predetermined cross section of the reaction that generates the radioisotope R by capturing a thermal neutron by the component of interest, - <f>t' is a predetermined fluence of thermal neutrons for the traversed thickness of moderator material identified by the exponent i, - C1 is an interference correction coefficient comprising at least one term Nc(Ik)*or(Ik)* ¢ / , - Nc(Ik) is the concentration of an interfering component Ik, the component Ik being capable, when it interacts with a fast neutron emitted by the fast neutron source, of generating a radioisotope identical to the radioisotope generated by thermal neutrons when they react with the desired component, - or(Ik) is the predetermined cross section of the reaction that generates the radioisotope R by interaction of a fast neutron with the interfering component Ik, - ¢ / is a predetermined fluence of fast neutrons for the traversed thickness of moderator material identified by the exponent 1

3. 1. Method according to claim 2, wherein the number Nbe is greater than the number of unknowns in the system of equations to be solved.

4. A method according to any one of the preceding claims, wherein, during the activation phase (110), said several samples are activated by irradiating them simultaneously with neutrons emitted from the same fast neutron source.

5. A system for analyzing a specimen by neutron activation, this system comprising: - a device (4) for activating samples of the specimen, this device comprising: - a source (10) of fast neutrons capable of emitting fast neutrons,

6. - a moderator material (12) exposed to the fast neutrons emitted by the fast neutron source, this moderator material being capable of slowing down a portion of the incident fast neutrons to transform them into thermal neutrons, and - at least one housing (L;) suitable for receiving a sample of the specimen to be irradiated by neutrons emitted by the fast neutron source and which have passed through a thickness of the moderator material, the fluences of fast neutrons and thermal neutrons incident on the sample received in the housing being a function of the thickness of the moderator material passed through, - a unit (6) for measuring the ionizing radiation resulting from the radioactive decay of each sample activated using the activation device, and - a processing unit (8) configured to quantify a component sought in the specimen from the ionizing radiation measured by the measuring unit, characterized in that: - the activation device (4) is capable of enabling the activation of several samples of the specimen by using, for each sample, a different thickness of the moderator material than the thickness used to activate the other samples, and - The processing unit (8) is configured to: - for each activated sample, establish a concentration N'(R) of a radioisotope of the component sought in that activated sample using only the ionizing radiation measured from that sample, where the exponent i is an identifier of the thickness used to activate that sample and R is an identifier of the radioisotope of the component sought, then - determine an NC(C) concentration of the desired component in the specimen from the different established N'(R) concentrations and predetermined fluences of thermal neutrons and fast neutrons for each of the thicknesses traversed of the moderator material used to activate the samples. A system according to claim 5, wherein the activation device comprises several compartments (L;) each capable of receiving a respective sample of the specimen to be irradiated by neutrons emitted by the fast neutron source and which have passed through a thickness of the moderator material, these housings being arranged relative to each other so that each housing corresponds to a thickness of the moderator material passed through that is different from the thickness of the other housings passed through.

7. System according to claim 6, wherein the housings (L;) are arranged one behind the other along a straight axis (30) which passes through the center of the neutron source.

8. System according to claim 7, wherein the activation device comprises: - a niche (32; 152-156) hollowed out inside a solid block (20) of moderator material, this niche having an opening (34) leading to an outer face (28) of the block of moderator material, and - an insert (22) able to be introduced into the niche through the opening, this insert being made of a moderator material in which each of the housings (U Li>p) are hollowed out.

9. A system according to any one of claims 5 to 8, wherein: - each housing comprises: - an opening (O;) through which the sample is introduced and, alternately, removed from the housing, and - a bottom (Fi) located on the side opposite the opening of the housing, and - for each housing, the activation device comprises a plug (B;) made of moderator material reversibly movable along an insertion axis of this plug between: - a closed position in which the plug completely obstructs the opening of the housing and leaves only a cavity surrounded by moderator material inside the housing, and - an open position in which the sample can be introduced and removed from the housing, the thickness of this plug, in a direction parallel to its insertion axis, being greater than 1 cm.

10. A system according to any one of claims 5 to 9, wherein the measuring unit (6) is capable of measuring gamma radiation emitted of the radioactive decay of the sample activated using the activation device.

11. Sample activation device for implementing an activation phase of a specimen analysis method by neutron activation according to claim 1, this device comprising: - a fast neutron source (10) capable of emitting fast neutrons, - a moderator material (12) directly exposed to the fast neutrons emitted by the fast neutron source, this moderator material being capable of slowing down a portion of the incident fast neutrons to transform them into thermal neutrons, and - at least one housing (L) capable of receiving a sample of the specimen to be irradiated by neutrons emitted by the fast neutron source and which have passed through a thickness of the moderator material, the fluences of fast neutrons and thermal neutrons incident on the sample received in the housing being a function of the thickness of the moderator material traversed, characterized in that the activation device comprises several housings (L;) each capable of receiving a respective sample of the specimen to be irradiated by neutrons emitted by the fast neutron source and which have passed through a thickness of the moderator material, these housings being arranged relative to each other so that each housing corresponds to a different thickness of the moderator material than the thickness of the other housings.;< / f>

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