Method for mapping and analysing at least one element of interest and associated device

Abstract

The invention relates to a method for mapping and analysing at least one element of interest included in a sample (150) by means of laser-produced plasma optical emission spectrometry (160), the method comprising: - a step of producing a plasma (160) by focusing a laser beam (120) onto a surface of the sample (150); - a step of collecting an optical emission from the plasma (160) that defines an intensity signal; - a step of processing and analysing the optical emission; and - characterised in that, during the processing and analysing step, the intensity signal is studied over its entire duration in order to identify a maximum intensity value.

Description

Method for mapping and analyzing at least one element of interest and associated device

[0001] The present invention relates to the field of high-resolution mapping and analysis of elements in solids, or even the analysis of elements in liquids.

[0002] More particularly, the invention relates, in particular but not exclusively, to a high-resolution analysis device for mapping elements in metallic solids.

[0003] The invention can be applied in particular to the elemental analysis of hydrogen and oxygen by optical emission spectrometry on plasma produced by laser, in the field of the nuclear industry, or even the aeronautical or space industry.

[0004] In applications such as the characterization of devices subjected to radioactive sources, or the characterization of the aging capacity of devices used in particularly harsh environments, for example in aircraft or spacecraft, it may prove essential to carry out elemental analysis of metal samples.

[0005] More specifically, it may be necessary to be able to draw up a map of these elements within the sample analyzed. By mapping, we mean an identification of the elements making up the sample analyzed and, possibly, the distribution and the link between the different elements.

[0006] Such an analysis can be particularly useful in studies of metal embrittlement by hydrogen, or in studies of aging of fuel cladding in the presence of oxygen, or in studies of embrittlement of fuel cladding caused by the formation of hydrides, the latter promoting the propagation of cracks.

[0007] There are various known methods for mapping elements present in samples.

[0008] One of these methods is elemental analysis by optical emission spectrometry on laser-produced plasma, designated by the acronym "SEOPPL", a technique which is carried out in a natural atmosphere, also designated by the English acronym "LIBS" corresponding to the English expression "laser induced breakdown spectroscopy".

[0009] This method applies in particular to the in situ control and characterization of samples of parts to be analyzed.

[0010] A method and device for elemental analysis by optical emission spectrometry on plasma produced by laser is described by the patent document published under number WO 2012 / 032024 A1, known as document “WO024”.

[0011] Conventionally, such an analysis device comprises: - a module for generating a laser beam intended to impact a sample to be studied to generate a plasma generating an optical emission; - means for collecting the optical emission; - means for analyzing the optical emission.

[0012] The laser beam generated by the generation module, after shaping by a shaping module, is applied to a sample to be studied via optical focusing means, comprising a focusing objective whose axis is perpendicular to the surface of the sample.

[0013] A plasma is then created at the impact of the laser beam on the sample to be studied, the plasma generating an optical emission to be analyzed to map the elements making up the sample studied.

[0014] The collection of the optical emission from the plasma is carried out by the collection means. These collection means comprise an optical fiber, one free end of which is brought as close as possible to the plasma.

[0015] The solution proposed by document WO024 implements an interference filter positioned between a collimating lens and a photomultiplier.

[0016] The light captured by the optical fiber is collimated by the collimating lens and the resulting light beam passes through the interference filter and illuminates the photomultiplier. This interference filter allows only a predetermined range of wavelengths to pass through, and is notably selected to provide the most selective "bandwidth" possible around the central wavelength of a signal of interest.

[0017] Once collected, the optical emission, and more specifically the intensity of this optical emission, is analyzed by the analysis means.

[0018] One problem lies in the nature of the optical emission collected.

[0019] Schematically illustrates the evolution of the intensity of the optical emission collected by the collection means as a function of time.

[0020] A curve illustrating the evolution over time of the intensity of a collected intensity signal 11 is represented using a double continuous line.

[0021] It is also illustrated, as a function of time:– the evolution of the intensity of a signal of the continuum 12, represented using a dotted line;– the evolution of the intensity of a signal of interest 13, represented using a single continuous line;– the evolution of the intensity of a signal of the matrix 14, represented using a dashed line.

[0022] The continuum signal 12 corresponds to the optical emission of the plasma. This continuum signal 12 integrates radiation located in the bandwidth of the interference filter selected according to the central wavelength of the signal of interest 13.

[0023] The signal of interest 13 corresponds to the optical emission of the element that we are seeking to quantify, this optical emission only taking place at a discrete and known wavelength, a function of the quantified element.

[0024] The signal from matrix 14 corresponds to the more general optical emission of the material in which the element that we are seeking to quantify is located, this optical emission also integrating radiation located in the bandwidth of the interference filter selected as a function of the central wavelength of the signal of interest 13.

[0025] The collected intensity signal 11 thus corresponds to radiation captured by the collection means which integrates the signal from the continuum 12, the signal of interest 13, as well as the signal from the matrix 14.

[0026] The signal from continuum 12 and the signal from matrix 14 thus form a particularly intense background noise with respect to the signal of interest 13.

[0027] In order to compensate for this background noise, document WO024 proposes solutions implementing a detection time window of determined duration whose starting time has a delay relative to the pulses of the pulsed laser adapted to the atomic line of the atomic element, and also the insufflation of a gas having the capacity to improve the signal of interest 13 with respect to the other signals.

[0028] In practice, these solutions are not entirely satisfactory.

[0029] According to document WO024, the selection allows to confer a good spectral selectivity of the emission.

[0030] On the other hand, the spectral analysis of the collected emission offers imprecise results for mapping an element of interest.

[0031] Indeed, according to document WO024, the collected intensity signal 11 is used to obtain an overall value.

[0032] In other words, the overall value corresponds to the sum of the continuum signal 12, the signal of interest 13 and the matrix signal 14.

[0033] It is therefore not possible to specifically distinguish the signal of interest 13, that is to say the signal of the element of interest that we are seeking to analyze.

[0034] Therefore, the result of the analysis is only approximate.

[0035] Furthermore, the emission of the laser pulse can fluctuate by a few nanoseconds.

[0036] However, by predetermining a time window on a theoretical pulse, it is possible that the delay is such that the time window is out of step with the signal of intensity 11.

[0037] Therefore, the collected intensity signal 11 can either be weak, or even non-existent, or be very noisy, i.e. present too much noise compared to the signal of interest (for example too much continuum signal 12).

[0038] More specifically, spectral analysis requires a significant amount of time to obtain qualitative results.

[0039] Furthermore, the definition of the time window does not guarantee a usable transmission signal for carrying out mapping.

[0040] Indeed, various parameters can influence the generation of the plasma, and thus generate the collection of a signal of intensity 11 presenting a large portion of the signal of the continuum 12.

[0041] Such collection then makes it difficult to study the signal of intensity 11 and therefore to map the element of interest.

[0042] The invention aims in particular to overcome the drawbacks of the prior art.

[0043] More specifically, the invention aims to propose a solution allowing precise analysis of the optical emission of the plasma.

[0044] The invention also aims to provide such a solution which allows rapid analysis of the optical emission of the plasma.

[0045] The invention further aims to provide such a simple implementation solution.

[0046] These objectives, as well as others which will appear subsequently, are achieved thanks to the invention which relates to a method for mapping and analyzing at least one element of interest included in a sample by optical emission spectrometry on plasma produced by laser, comprising: - a step of producing a plasma by focusing a laser beam on a surface of the sample; - a step of collecting an optical emission from the plasma, defining an intensity signal; - a step of processing and analyzing the optical emission, and - a step of mapping at least one element of interest by sequentially renewing the production step, followed by the collection step and the processing and analysis step, in different locations on the surface of the sample, characterized in that during the processing and analysis step, the intensity signal is studied over its entire duration to identify a maximum intensity value.

[0047] Compared to the method used in document WO024, which determines a time window for analyzing the intensity signal prior to the creation of the plasma, the method according to the invention proposes an analysis of the entire intensity signal after the generation of the plasma, to identify the optimal part of the signal to be used.

[0048] Such a method thus makes it possible to ensure that the useful part of the signal of interest, corresponding to the element of interest to be mapped, is exploited.

[0049] By "useful part" is meant a duration of the signal in which the signal of interest is sufficiently far from the continuum signal.

[0050] On the other hand, in the case of the method according to document WO024, if the window is poorly predetermined or external elements disturb the generation of the plasma, then obtaining a precise mapping of the element of interest is considerably limited.

[0051] According to an advantageous aspect, during the processing step, the intensity signal is filtered to extract a signal of interest.

[0052] The mapping of the element of interest is then more precise since, after filtration, the signal of interest can be used without being disturbed by the continuum signal.

[0053] Furthermore, this makes it possible to make the exploitation of the signal of interest faster since it is freed from all the noise, that is to say the signal which does not correspond to the signal of interest, or almost.

[0054] According to another advantageous aspect, during the mapping step, only a part of the signal of interest is used.

[0055] This allows to limit the analysis time to obtain an accurate mapping of the element of interest.

[0056] Indeed, given that only part of the signal of interest is studied, the volume of data used is low and therefore transfer is rapid.

[0057] In addition, the analysis is carried out on a denoised signal, i.e. filtered, and therefore more easily exploitable.

[0058] According to another advantageous aspect, the portion of the signal of interest used corresponds to a range of the intensity signal extending over 5 to 500 nanoseconds around a point corresponding to the maximum intensity value.

[0059] Defining such a value range makes it possible to guarantee a usable signal while limiting the presence of noise, in particular formed by the continuum signal.

[0060] Furthermore, the extension of the signal around the maximum intensity value ensures that it is indeed the maximum intensity signal and not a disturbance in the total signal that could be confused with the maximum intensity signal.

[0061] According to another advantageous aspect, the portion of the signal of interest used corresponds to a range of the intensity signal from 5 nanoseconds before the point corresponding to the maximum intensity value to 500 nanoseconds after said point corresponding to the maximum intensity value.

[0062] This choice of range makes it possible to finely limit the duration of the intensity signal to be exploited, and therefore the duration of the signal of interest.

[0063] According to another advantageous aspect, the portion of the signal of interest used corresponds to a range of the intensity signal extending over 5 to 500 nanoseconds from a point corresponding to the maximum intensity value.

[0064] Other characteristics and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment of the invention, given by way of illustrative and non-limiting example, and the appended drawings described below.

[0065] This is a schematic representation of the evolution of the intensity of the optical emission of a plasma as a function of time.

[0066] This is a schematic representation of a mapping and analysis device, according to the invention.

[0067] This is a schematic representation of a detection limit as a function of time.

[0068] Diagrammatically illustrates a mapping and analysis device 100, according to the invention.

[0069] The device 100 comprises: - a generation module 110 of a laser beam 120; - means 130 for collecting an optical emission; - means 140 for analyzing the optical emission.

[0070] The laser beam 120 intended to impact a sample 150 to be studied to generate a plasma 160 generating an optical emission.

[0071] Said optical emission is then collected by the collection means 130 for the purpose of being analyzed by the analysis means 140.

[0072] More precisely, the laser beam 120 generated by the generation module is applied to the sample 150 to be studied via optical focusing means 170 (schematically represented).

[0073] The sample 150 is arranged on a mobile support 155 to move the sample 150 in order to produce plasmas in different locations of the sample 150.

[0074] The focusing means 170 comprise, for example, a focusing objective whose axis is perpendicular to the surface of the sample 150 to be studied.

[0075] According to the embodiment illustrated by the, the collection means 130 comprise at least one optical fiber.

[0076] The or each optical fiber has a free end intended to be positioned in the immediate vicinity of the plasma 160 to capture a light intensity of the plasma 160. This light intensity then forms the intensity signal 11 illustrated by 1a, which is then analyzed by the analysis means 140.

[0077] With reference to the, a curve illustrating the evolution over time of the intensity of the intensity signal 11 collected by the collection means 130 is represented using a double continuous line.

[0078] More specifically, the intensity signal 11 consists of at least one continuum signal 12, one signal of interest 13 and one matrix signal 14.

[0079] It also illustrates, as a function of time:– the evolution of the intensity of a signal of the continuum 12, represented using a dotted line;– the evolution of the intensity of a signal of interest 13, represented using a single continuous line;– the evolution of the intensity of a signal of the matrix 14, represented using a dashed line.

[0080] The continuum signal 12 corresponds to the optical emission of the plasma. This continuum signal 12 integrates radiation located in the bandwidth of the interference filter selected according to the central wavelength of the signal of interest 13.

[0081] The signal of interest 13 corresponds to the optical emission of the element that we are seeking to quantify, this optical emission only taking place at a discrete and known wavelength, a function of the quantified element.

[0082] The signal from matrix 14 corresponds to the more general optical emission of the material in which the element that we are seeking to quantify is located, this optical emission also integrating radiation located in the bandwidth of the interference filter selected as a function of the central wavelength of the signal of interest 13.

[0083] The collected signal 11 thus corresponds to radiation captured by the collection means 130, which integrates the signal from the continuum 12, the signal of interest 13, as well as the signal from the matrix 14.

[0084] The continuum signal 12 and the matrix signal 14 thus form a particularly intense background noise with respect to the signal of interest 13.

[0085] To analyze the signal 11, the analysis means 140 comprise an acquisition card 141.

[0086] The acquisition card 141 is advantageously clocked at at least 200 MHz.

[0087] Preferably, the acquisition card 141 is clocked at 500 MHz.

[0088] The 141 acquisition card could even be clocked at 2GHz to provide optimal processing of the intensity 11 signal.

[0089] The acquisition card 141 is then used to study the intensity signal 11 over its entire duration in order to identify a maximum intensity value.

[0090] Furthermore, the analysis means 140 comprise a photomultiplier 142 making it possible to electrically create the intensity signal 11 of the luminosity of the plasma 160 collected by the or each optical fiber of the collection means 130.

[0091] The signal 11 thus amplified is then transmitted to the acquisition card 141 at the output of the photomultiplier 142.

[0092] The acquisition card 141 is then used to analyze the intensity signal 11 and, via one or more dedicated transformation laws, define a limit of detection curve 15 (LDD) as illustrated by the.

[0093] The detection limit 15 is thus inverse to the intensity signal 11.

[0094] More precisely, the detection limit 15 is inverse to the signal of interest 13.

[0095] Thus by identifying the lowest point of the detection limit 15, it is possible to know the moment, or almost, at which the signal of interest 13 reaches its maximum value.

[0096] To enable mapping of the sample 150 to be carried out, the signal 11 is then filtered by the analysis means 140 in order to extract the signal of interest 13.

[0097] Despite the filtering, it is possible that the signal used still contains noise around the signal of interest.

[0098] To carry out the mapping of sample 150, in particular to know the concentration of an element of interest, the signal of interest is only used over a portion of its duration.

[0099] More specifically, by analyzing the intensity signal 11 over its entire duration, it is possible to select only a portion of the intensity signal 11. Consequently, only a corresponding portion of the signal of interest 13 can be used subsequently to map the element of interest of the sample 150.

[0100] The used part of the intensity signal 11, and therefore of the signal of interest 13, corresponds for example to a range of the intensity signal 11 extending over 5 to 500 nanoseconds around a point corresponding to the maximum intensity value.

[0101] Alternatively, the used part of the intensity signal 11, and therefore of the signal of interest 13, corresponds for example to a range of the intensity signal 11 going from 5 nanoseconds before the point corresponding to the maximum intensity value to 500 nanoseconds after said point corresponding to the maximum intensity value.

[0102] According to yet another variant, the used part of the intensity signal 11, and therefore of the signal of interest 13, corresponds to a range of the intensity signal extending over 5 to 500 nanoseconds from the point corresponding to the maximum intensity value.

[0103] This allows the signal of interest to be studied over a period allowing precise results to be obtained quickly to carry out the mapping of the element of interest.

[0104] The mapping of the element of interest is thus carried out according to a method of mapping and analyzing at least one element of interest included in a sample 150 by optical emission spectrometry on plasma 160 produced by laser 120, comprising: - a step of producing the plasma 160 by focusing the laser beam 120 on a surface of the sample 150; - a step of collecting the optical emission from the plasma 160, defining the intensity signal 11; - a step of processing and analyzing the optical emission, and - a step of mapping at least one element of interest by sequentially renewing the production step, followed by the collection step and the processing and analysis step, in different locations on the surface of the sample 150.

[0105] Compared to the method used in document WO024, which determines a time window for analyzing the intensity signal prior to the production of the plasma 160, the method according to the invention proposes an analysis of the entire intensity signal 11 after the generation of the plasma 160, to identify only a part to be exploited.

[0106] This ensures that the useful part of the signal of interest 13 can be used.

[0107] By "useful part" is meant a duration of the signal in which the signal of interest 13 is sufficiently far from the continuum signal 12 and the matrix signal 14.

[0108] The mapping of the element of interest is then more precise since, after filtration, the signal of interest can be used without being disturbed by the signal from the continuum 12 or that of the matrix.

[0109] On the other hand, in the case of the method according to document WO024, if the window is poorly predetermined or external elements disturb the generation of the plasma 160, then the intensity signal 11 presents a high risk of including part of the signal of the continuum 12, thus limiting the obtaining of a precise mapping of the element of interest.

[0110] The filtration of the signal of interest 13 can be carried out using a second signal collected simultaneously with the signal of interest 11.

[0111] More precisely, the signal of interest 11 is denoised using the second signal.

[0112] Thus, the signal of interest 13 can be obtained by subtracting the second signal from the first signal of intensity 11.

[0113] For this, the method may comprise a preliminary calibration step during which a curve of evolution of an intensity of the signal of intensity 11 as a function of a concentration of the element of interest is defined using a plurality of standards each comprising a known concentration of the element of interest.

[0114] The second signal is also obtained by the collection means 130 which then comprise a secondary optical fiber dedicated to the acquisition of the second signal.

Claims

Method for mapping and analyzing at least one element of interest included in a sample (150) by optical emission spectrometry on plasma (160) produced by laser, comprising:- a step of producing a plasma (160) by focusing a laser beam (120) on a surface of the sample (150);- a step of collecting an optical emission from the plasma (160), defining an intensity signal (11);- a step of processing and analyzing the optical emission, and- a step of mapping at least one element of interest by sequentially repeating the production step, followed by the collection step and the processing and analysis step, in different locations on the surface of the sample (150), characterized in that during the processing and analysis step, the intensity signal (11) is studied over its entire duration to identify a maximum intensity value. Method according to claim 1, characterized in that during the processing step, the intensity signal (11) is filtered to extract a signal of interest (13). Method according to the preceding claim, characterized in that during the mapping step, only part of the signal of interest (13) is used. Method according to the preceding claim, characterized in that the part of the signal of interest (13) used corresponds to a range of the intensity signal (11) extending over 5 to 500 nanoseconds around a point corresponding to the maximum intensity value. Method according to the preceding claim, characterized in that the part of the signal of interest (13) used corresponds to a range of the intensity signal (11) going from 5 nanoseconds before the point corresponding to the maximum intensity value to 500 nanoseconds after said point corresponding to the maximum intensity value. Method according to one of claims 1 to 3, characterized in that the part of the signal of interest (13) used corresponds to a range of the intensity signal (11) extending over 5 to 500 nanoseconds from a point corresponding to the maximum intensity value.

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