Scintillator material and production method thereof

Abstract

The invention relates to a scintillator material and to a method for producing a scintillator material (3) comprising carbon quantum dots with a graphitic core and dispersed in a matrix, said material comprising aromatic units capable of emitting UV radiation in response to irradiation, in particular of the beta type, said UV radiation causing luminescence of the carbon dots and the emission by same of radiation in the visible domain.

Description

[0001]Description Title: Scintillation material and manufacturing method Technical field The present invention relates to scintillators capable of emitting light photons under the effect of radiation of nuclear origin, in particular β radiation. Prior art The detection of emissions in the form of waves such as gamma or X rays, or particles such as beta rays or neutrons, represents an increasingly current challenge, as the dismantling of first generation nuclear installations is carried out. It is likely that these needs will increase with the increase in the number of dismantling sites. Scintillation materials with increasingly lower detection limits will be necessary in the future in order to best ensure the radiation protection of dismantling operators.Although current gamma scintillators cover radiation protection needs relatively well, feedback from personnel in charge of dismantling operations shows that there is still progress to be made regarding alpha or beta detection. This is mainly due to the low energy involved, which makes these radiations difficult to detect. Existing beta scintillators generally use inorganic crystals (CsI, GaO, CaF2) or plastic matrices (POPOP, benzene derivatives) as luminescent agents capable of transforming incident particles (electrons or positrons for β- or β. +) into light photons. They are relatively complex to produce and require the use of exhaustible resources in the case of certain inorganic crystals such as those based on gallium, for example. US 2013 / 299742 discloses a material formed from a polymer matrix in which is dispersed a first fluorophore capable of emitting radiation when irradiated, and a second fluorophore absorbing this radiation to emit photons in the visible range. US 2017 / 096600 discloses a material comprising graphene quantum dots dispersed in a polystyrene matrix, capable of emitting radiation in the visible range when exposed to ultraviolet radiation. Finally, the article by Zhan et al. entitled "A solvent-engineered molecule fusion strategy for rational synthesis of carbon quantum dots with multicolor bandgap fluorescence" published in Carbon, volume 130, 2018, discloses a process for preparing carbon quantum dots.Disclosure of the invention There is therefore a need for a scintillator material that is relatively simple to produce, making it possible to avoid the use of rare inorganic compounds if desired, and having good detection sensitivity to beta radiation in particular. Summary of the invention The invention aims to meet this need and achieves this by means of a scintillator material comprising quantum carbon dots with a graphitic core dispersed in a matrix, said material comprising aromatic units capable of emitting UV radiation in response to irradiation, in particular of the beta type, this UV radiation causing luminescence of the carbon dots and the emission by the latter of radiation in the visible range. Such a material does not require the use of rare inorganic compounds and has good sensitivity to the detection of beta radiation in particular.The invention also relates, according to another of its aspects, to a method for detecting radiation originating from a radioactive source, in particular beta radiation, in which a scintillator material according to the invention is exposed to said radiation, and an emission of light photons by said material is detected, in response to this exposure. The invention also relates to a radiation detector, in particular beta radiation, comprising a scintillator material according to the invention, arranged so as to be able to be exposed to said radiation, and at least one sensor of light photons emitted by said scintillator material in response to this exposure. Matrix The matrix may be thermoplastic or thermosetting. It may in particular be cast or injected in the fluid state into a mold, then demolded after cooling.The matrix may have any shape, including polyhedral, for example cubic or parallelepipedal, solid or hollow cylindrical, conical, toric, among other possibilities. It may be machined, if necessary, to the desired shape. The matrix may also be in the form of a layer serving as a binder for the carbon dots. The matrix is ​​preferably transparent, but may be translucent. The matrix may be colorless and the material may have a brown color due to the presence of the carbon dots. The matrix may also include one or more colorants. It may incorporate a light-collecting element, if necessary, or form a light guide. The matrix occupies any volume appropriate to the conditions of use. Its thickness ranges, for example, from 1 mm to 100 mm. Particularly when it is polymeric, the polymer of the matrix may have the aforementioned aromatic units.The matrix may in particular be a styrenic polymer, in particular polystyrene (PS), acrylonitrile butadiene styrene (ABS) or methylmethacrylate butadiene styrene (MBS), or be chosen from aromatic polyethers (PAE), in particular poly(phenylene oxide) (PPO), polyvinyltoluene (PVT) or a mixture thereof. The matrix may in particular comprise polystyrene or be made of polystyrene, this polymer allows good results to be obtained. The matrix may be polymeric. The polymer may be a homopolymer or a copolymer. Aromatic units The aromatic units may be phenyl rings, but any aromatic unit allowing UV emission to be generated under the action of the incident radiation to be detected, then by transfer, in particular radiative, an emission of visible light by the carbon dots, may be suitable. The aromatic units may thus be rings other than phenyl rings, for example.Aromatic units may be present within the matrix. The matrix may thus be a plastic material having aromatic units, as mentioned above. The aromatic units may comprise one or more heteroatoms, if applicable, and / or be condensed in the form of aromatic polycyclic compounds. Carbon dots Carbon dots are also known by the Anglo-Saxon designation “Carbon Dots” or “CD”. They are in the form of luminescent nanoparticles with a graphitic core, the size of which is less than or equal to 50 nm, and preferably less than or equal to 10 nm. The carbon dots are preferably hydrophobic, which can facilitate their dispersion in the polymerization precursor of the matrix. In exemplary embodiments, the carbon dots have traces of nitrogen, which come from the synthesis method used.These traces of nitrogen can contribute to improving the emission efficiency of carbon dots. Indeed, the nitrogen groups act as exciton traps, which results in an increase in the quantum efficiency of the carbon dots. Carbon dots can have a non-zero nitrogen content, in particular an atomic % content of N1s of between 10 and 15%. Carbon dots can have a non-zero oxygen content, in particular an atomic % content of O1s of between 15 and 20%. Carbon dots can have an atomic % content of C1s of between 65 and 75%. The mass content of carbon dots can be between 0.01% and 0.5% relative to the total weight of the scintillator material (carbon dots plus matrix). Synthesis process Carbon dots can be produced from the degradation (for example hydrothermal or microwave) of molecules used as precursors.These precursor molecules are preferably extracted from biomass, which is an advantage given the inexhaustible nature of the resources used to produce them. The carbon dots advantageously result from a synthesis process involving the decomposition of trinitropyrene, previously solubilized in an organic solvent, which may be toluene, dimethylformamide (DMF), or better still a toluene / DMF mixture. The presence of DMF in a DMF / toluene mixture makes it possible to obtain a better mass yield of carbon dots.Manufacturing method The invention also relates to a method for manufacturing the scintillator material according to the invention, in which the carbon dots are synthesized by implementing the above synthesis method, in particular a synthesis method in which the decomposition of trinitropyrene, previously solubilized in an organic solvent, is carried out, this organic solvent comprising a mixture of toluene and dimethylformamide (DMF). As mentioned above, the matrix can be polymerized in the presence of the carbon dots. Alternatively, the matrix can be dissolved in a solution containing the carbon dots suspended in an organic solvent, then this organic solvent is evaporated, this evaporation taking place for example in a mold with the desired shape.It is thus possible to prepare a suspension of carbon dots in an organic solvent, in particular consisting of or containing toluene, dissolve polystyrene with this solution and then evaporate the solvent. Brief description of the drawings The invention may be better understood by reading the detailed description which follows, non-limiting examples of its implementation, and by examining the appended drawing, in which: [Fig 1] schematically and partially represents a test installation for a scintillator material according to the invention, [Fig 2] represents two raw images obtained from an au source. 90Sr on a PS matrix alone (left image) and on a scintillator material according to the invention (right image), with a PS matrix loaded with 0.015% by weight of carbon dots, [Fig 3] represents transmission electron microscopy (TEM) images of carbon dots (images A and B) and a diffractogram of the carbon dots (image C), [Fig 4] illustrates the effect of the scintillator thickness on the intensity of the radioluminescence, [Fig 5] illustrates the effect of the concentration of carbon dots on the intensity of the radioluminescence, [Fig 6] illustrates the effect of the presence of carbon dots in a PS matrix for a source 90 Sr (left image) and 14C (right image), [Fig 7] illustrates the XPS spectrum of the carbon dots used for scintillation (left) and the associated content of Carbon (C), Nitrogen (N) and Oxygen (O), and [Fig 8] illustrates the deconvolution of the XPS spectra associated with the carbon dots used for scintillation and the identification of the functions present in the carbon dots using fitting curves (Fits). Detailed description Figure 1 shows an installation 1 (also called a measuring bench) for testing a scintillator material 3 according to the invention, comprising a radioactive source 2 arranged on one side of the scintillator material 3 and a camera 4 arranged on the opposite side, to observe the scintillator material 3, the assembly being placed in a black enclosure 5. The camera 4 is equipped with a photosensitive detector for counting photons in a spectral range covering the UV-visible range of light (180-900 nm).The radioactive source 2 used for testing is, for example, a pure beta-emitting source, such as an au source. 90 Sr delivering an energy of 546 keV and having an activity of 12 kBq, or a source at 14C delivering an energy of 156 keV and having an activity of 33 kBq. The scintillator material 3 can be in the form of a solid block, parallelepiped or cylindrical, any shape being however possible. The manufacture of the scintillator material 3 can be done in two stages. In a first stage, the carbon dots are synthesized in the form of a powder. In a second stage, this powder is dispersed in a liquid monomer in the presence of a polymerization initiator. After a waiting time necessary for the polymerization of the monomer, the carbon dots are found fixed in a transparent plastic matrix, for example polystyrene. It is also possible to avoid the waiting time linked to the polymerization of the monomer. In this case, it is sufficient to first prepare a suspension of carbon dots in toluene. 5g of polystyrene are then dissolved in 10mL of this suspension.Finally, the mixture is placed in a mold of the desired shape and left for evaporation of the toluene. After 48 hours of waiting, a polystyrene scintillator loaded with carbon dots is obtained. The carbon dots can be synthesized from trinitropyrene of the following formula: [Chem 1]. A solution of trinitropyrene is prepared, for example, at a concentration of 10 mg / mL in dimethylformamide (DMF). This solution can then be placed either in a microwave oven at 200°C for 1 hour or in an autoclave at 200°C for 8 hours. At the end of the reaction, a blackish-purple solution is obtained. This solution is slowly evaporated, for example, at 70°C for 72 hours. Following the evaporation step, a black powder is collected. This black powder can be redispersed in 5 mL of ethanol. These 5 mL are then dialyzed, for example, for 24 hours in a 1 kDa specific tube to remove the unreacted species. The ethanol solution containing the purified carbon dots is then evaporated, for example, at 70°C overnight. After this final evaporation step, a black powder of purified carbon dots is obtained. It is this black powder that can then be used for the preparation of the scintillator material.The synthesis is preferably carried out in DMF because it is preferable to obtain hydrophobic carbon dots, which are easier to disperse in the monomer that serves as a precursor to the transparent plastic matrix. Figure 3 shows TEM images as well as the diffractogram relating to the carbon dots thus prepared. The size of the carbon dots is polydisperse, between a few nm and 50nm (images A and B). The DRX analysis (image C) gives a classic diffractogram of what is generally obtained with carbon dots, with a broad peak around 25° relative to the (002) plane of the graphitic core of the carbon dots. To produce the block of scintillator material 3, a mass of, for example, between 5mg and 15mg of carbon dots is taken and then dispersed in 20mL of styrene under stirring. Then, a mass of a polymerization initiator such as benzoyl peroxide, for example between 0 and 20mg, is added.Then the viscous liquid is placed in a mold intended to give its shape to the scintillator material, and left to mature at a temperature of, for example, between 70°C and 110°C for a time of, for example, between 5 and 25 days. The installation of Figure 1 makes it possible to test the radioluminescence properties of the scintillator materials 3 according to the invention thus obtained. The radioactive source 2 illuminates the scintillator material 3, which is in an excited energetic state. During its de-excitation, the scintillator material 3 emits a light photon which is captured by the detector of the camera 4. The signal recovered by the camera in the form of an image is then analyzed by a computer 7 running image processing software to obtain a curve making it possible to compare the efficiency of each scintillator material. Figure 2 shows the results of the radioluminescence tests carried out with the measuring bench of Figure 1.We see that the samples containing carbon dots (right image) emit much more light photons than the PS matrix alone (left image). There is therefore clearly a carbon dot effect involved in the radioluminescence mechanisms of the scintillator material according to the invention. We see in Figure 6 that the intensity of the detected signal can depend on the energy of the radiation considered, since for the same exposure time, the source at. 90 Sr (left image) causes the emission of more light photons in the visible range of light than the source at 14C (right image). Figure 5 shows the influence of the concentration of carbon dots in the matrix on the radioluminescence intensity of the scintillator material according to the invention. In this example, the scintillator material is 9 mm thick and 13 mm on each side, and the concentration is respectively 0.045% (highest intensity), 0.027% (intermediate intensity) and 0.015% by weight (lowest intensity). Figure 4 shows the effect of the thickness of the scintillator material placed on the path of the incident radiation on the intensity of the radioluminescence at 90Sr. We see that the radioluminescence effect is indeed due to the presence of the scintillator material, increasing with its thickness, and therefore to the presence of the carbon dots in this material. The highest intensity is obtained in this figure for a thickness of 9 mm, and the lowest for a thickness of 2 mm, for the same concentration of 0.027% by weight of the carbon dots. Figure 7 shows the XPS spectrum of the carbon dots and their calculated content of Carbon (69.5%), Nitrogen (12.4%) and Oxygen (18.1%). The deconvolution of the peaks of the XPS spectrum presented in Figure 8 makes it possible to identify the various groups present on the surface of the carbon dots. Of course, the invention is not limited to the exemplary embodiments which have just been described. For example, the scintillator material can be used for the detection of radiation other than beta radiation, for example alpha or gamma radiation.The matrix can be made with other polymers, and the aromatic units can be present in another form. The scintillator material can be combined with other scintillator materials aimed at detecting other radiation, for example neutron.

Claims

Claims 1. A scintillator material (3) comprising graphitic-core quantum carbon dots, having a non-zero nitrogen content, dispersed in a matrix, said material comprising aromatic motifs capable of emitting UV radiation in response to irradiation, in particular beta radiation, such UV radiation causing luminescence of the carbon dots and emission by them of radiation in the visible range.

2. A material according to claim 1, the matrix being polymeric and the matrix polymer having said aromatic motifs.

3. A material according to claim 2, the aromatic motifs being phenyl rings.

4. A material according to any one of claims 2 and 3, the matrix comprising or being made of polystyrene.

5. A material according to any one of claims 1 to 4, the carbon dots having an atomic % nitrogen content of between 10 and 15%. 6.Material according to any one of claims 1 to 5, the carbon dots having a non-zero oxygen content, in particular an O1s atomic % content of between 15 and 20%.

7. Material according to any one of claims 1 to 6, the carbon dots having a C1s atomic % content of between 65 and 75%.

8. Material according to any one of the preceding claims, the mass content of carbon dots being between 0.01% and 0.5% by weight of the material.

9. Material according to any one of the preceding claims, the carbon dots resulting from the decomposition of trinitropyrene.

10. Material according to any one of the preceding claims, the carbon dots being hydrophobic. 11.Method for detecting radiation from a radioactive source, in particular beta radiation, wherein said radiation is exposed to a scintillator material as defined in any one of the preceding claims, and an emission of light photons by said material is detected in response to this exposure.

12. A radiation detector, in particular a beta detector, comprising a scintillator material (3) according to any one of claims 1 to 10, arranged so as to be able to be exposed to said radiation, and at least one sensor (4) of light photons emitted by said scintillator material in response to such exposure.

13. A method for preparing carbon dots, in particular for their use in the manufacture of a scintillator material according to any one of claims 1 to 10, wherein trinitropyrene, previously solubilized in an organic solvent, is decomposed, said organic solvent comprising a mixture of toluene and dimethylformamide (DMF).

14. A method for manufacturing a scintillator material according to any one of claims 1 to 13, wherein the carbon dots are synthesized by carrying out the method according to claim 13.

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