Scintillating nanocomposites

Abstract

An improved scintillator nanocomposite comprising nanoparticles with scintillating properties and a diameter between 10 and 50 nanometer and a first matrix material comprises is obtained by introducing the nanoparticles into a dispersing medium to form a stable suspension. The dispersing medium is a precursor to the first matrix material, which is cured to form the first matrix material.

Claims

1. A method for producing a scintillator nanocomposite, the method comprising: introducing nanoparticles into a first dispersing medium to form a stable suspension, wherein the first dispersing medium is a precursor for a first matrix material; and curing the first dispersing medium to form the first matrix material, wherein the scintillator nanocomposite comprises the nanoparticles with scintillating properties and a diameter between 10 and 50 nanometer and the first matrix material.

2. The method according to claim 1, wherein the nanoparticles are garnet nanoparticles.

3. The method according to claim 1, wherein the first matrix material is a polymeric material.

4. The method according to claim 1, wherein the dispersing medium is a glycol.

5. The method according to claim 1, wherein a second dispersing medium is mixed with the first dispersing medium prior to introducing the nanoparticles or to the stable suspension, wherein the second dispersing medium is a precursor for a second matrix material.

6. The method according to claim 5, wherein the second matrix material is a polymeric material, preferably a polymeric material selected from the group of polyurethane, polyester, such as polybutylene terephthalate, unsaturated polyester resin, aromatic polyamide, aromatic polyimide, polystyrene or polysulfone.

7. The method according to claim 6, wherein the curing results in a co-polymer of the first matrix material and the second matrix material.

8. The method according to claim 1, wherein the curing is performed at elevated temperature, and/or at elevated pressure greater than 20 bar, and/or for at least 24 hours.

9. A scintillator nanocomposite comprising: nanoparticles with scintillating properties and a diameter between 10 and 50 nanometer; and a first matrix material; and wherein the nanoparticles are introduced into a first dispersing medium to form a stable suspension; and wherein the first matrix material is formed by curing the first dispersing medium.

10. The scintillator nanocomposite according to claim 9, wherein the first matrix material is polyurethane.

11. A product comprising the scintillator nanocomposite according to claim 9.

12. A method to produce a transparent scintillator comprising: melting a nanocomposite obtained with a method comprising introducing the nanoparticles into a first dispersing medium to form a stable suspension, wherein the first dispersing medium is a precursor for the first matrix material; curing the first dispersing medium to form the first matrix material; and injection molding the molten nanocomposite; and wherein the scintillator nanocomposite comprises the nanoparticles with scintillating properties and a diameter between 10 and 50 nanometer and the first matrix material.

13. A transparent scintillator obtainable by the method according to claim 12.

14. A radiation detector comprising the transparent scintillator according to claim 13.

15. An imaging system comprising the radiation detector according to claim 14.

16. The method according to claim 2, wherein the nanoparticles are Cerium-doped nanoparticles.

17. The method according to claim 2, wherein the nanoparticles are chosen from the group of Y.sub.3Al.sub.5O.sub.12:Ce nanoparticles, (Lu,Gd).sub.3Al.sub.5O.sub.12:Ce nanoparticles or (LGGAG:Ce) nanoparticles.

18. The method according to claim 3, wherein the polymeric material comprises a polymeric material selected from the group of polyurethane, polyester, polybutylene terephthalate, unsaturated polyester resin, aromatic polyamide, aromatic polyimide, polystyrene or polysulfone.

19. The method according to claim 4, wherein the glycol comprises a 1, 4 butanediol.

20. The method according to claim 8, wherein the elevated temperature is between 220 degrees Celsius and 250 degrees Celsius; and/or wherein the elevated pressure is about 40 bar.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is illustrated by drawings of which

(2) FIG. 1a, b show schematic depictions of a well-dispersed scintillator nanocomposite (1a) and a scintillator nanocomposite with several defects (1b).

(3) FIG. 2 shows a combined flowchart of a method for producing scintillator nanocomposites, a method for producing a transparent scintillator and a method to construct a radiation detector, all according to the present invention.

(4) FIG. 3a, b shows graphs of optical properties of a transparent nanocomposite according to the present invention.

(5) FIG. 4 shows a schematic depiction of a radiation detector module according to the present invention.

(6) FIG. 5 shows a schematic depiction of an exemplary imaging system according to the present invention.

(7) The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for the purpose of illustrating preferred embodiments and are not to be construed as limiting the invention. To better visualize certain features may be omitted or dimensions may be not be according to scale.

DETAILED DESCRIPTION OF EMBODIMENTS

(8) Several types of scintillator nanoparticles are known, for instance garnet-based nanoparticles. The present invention is illustrated, but not limited to, with several specific scintillating nanoparticles as examples, but a skilled person would know how to prepare and use other types of scintillator nanoparticles as well.

(9) To obtain a scintillator nanocomposite 10 the scintillator nanoparticles 12 need to be embedded in a matrix 11, here illustrated with a polymeric matrix. As mentioned earlier, known scintillator nanocomposites are prepared by dispersing the nanoparticles in a precursor material that is formed into the matrix, e.g. monomers that may be (co-)polymerized into a polymer matrix. Non-polymeric precursors are possible as well, for instance inorganic pre-cursors that may be cured in inorganic matrix materials, e.g. glass-like structures.

(10) The refractive index of the scintillator nanoparticles and the matrix must match to be suitable in optical applications, such as radiation detectors. This is however not trivial, since nanoparticles usually have a higher refractive index than most polymers. For example, the refractive index of Y.sub.3Al.sub.5O.sub.12:Ce nanoparticles particles is 1.8, and usually polymers have refractive indexes no higher than 1.7 (e.g. polyurethane has a refractive index of approximately 1.5). High refractive index polymers are rare and expensive. The refractive index of the polymer matrix might be increased by introducing inorganic particles such as TiO.sub.2 or ZrO.sub.2, but the amount of inorganic material would need to be very high and will cause even more defects in the nanocomposite, contributing even more to the problem the present invention intends to solve. A better solution is to use scintillator nanoparticles with a very small diameter. Since scattering intensity depends on the particle diameter to the 6th power (Rayleigh's equation), only the particles with a size smaller than the wavelength of visible light do not interfere with incident light and in the end composite does not appear turbid. Especially, nanoparticles smaller than 50 nm in a matrix appear transparently to the human eye.

(11) Unfortunately, such small nanoparticles have a strong tendency to form agglomerates 13 due to van der Waals forces resulting from their relatively large surface area. The agglomerates 13 usually grow to several hundred nanometers in size and act like submicron particles that scatter incident light and reduce transparency of nanocomposite, which is strongly detrimental to the transparency and quality of the scintillator nanocomposite.

(12) The present invention presents a production method resulting in a transparent scintillator nanocomposite that more homogenously distributes nanoparticles smaller than 50 nm in a matrix, while at the same time reducing defects caused by dispersants or solvents used in known methods to produce nanoparticle dispersions used in scintillator nanocomposite production.

(13) First, scintillator nanoparticles are prepared 101 using any of the many known scintillator nanoparticle synthesis methods.

(14) Next, the nanoparticles are dispersed 102 in an organic medium. It is an insight of the present invention that this organic medium is a precursor material to form (at least part of) the matrix material, for instance a monomer that may be (co-)polymerized into a polymer matrix. There are many requirements to the organic material and resulting matrix material, which makes selecting the right organic medium non-trivial. First, the organic medium should disperse the scintillator nanoparticles to form a stable suspension with a solid content of up to 50%. Secondly, the resulting matrix material (such as a polymer) needs to be radiation hard, be compatible with and have a refractive index matching any other materials present in the nanocomposite (e.g. polymers with which it may or may not form a co-polymer) and preferably also is not overly expensive or difficult to obtain. Surprisingly, it was found that glycols and particularly 1,4-butanediol [HO(CH.sub.2).sub.4OH] possess the required properties to be used as organic medium that may be (co-)polymerized into suitable polymeric matrix materials, such as polyurethanes or polyesters. 1,4-Butanediol is furthermore very attractive to use for Cerium-doped scintillator materials, since 1,4-butanediol has a stabilizing influence on Ce.sup.3+ and nanoparticles. This later results in less surface defects 15 in the nanocomposite 10 caused by Cerium migration.

(15) Examples of alternate precursor materials are precursors for aromatic polyamide or aromatic polyimide, such as dianhydride, diamine, N,N-dimethylacetamide (DMAc) or N-methylpyrrolidinone (NMP), precursors for polystyrene, such 4,4-azobis 4-cyanovaleric acid (ACVA), polyvinyl pyrrolidone (PVP) or precursors for polysulfone, such as diphenol (bisphenol-A or 1,4-dihydroxybenzene) and bis(4-chlorophenyl)sulfone. As mentioned before, also inorganic precursors are considered.

(16) The suspension of the precursor material with the nanoparticles is then cured 103 into the matrix material. To polymerize a glycol another monomer (e.g. an isocyanate or a dicarboxylic acid) needs to be added to form, for instance, a polyurethane or a polyester. This polymerization mechanism is commonly known. Other reagents, such as co-monomers to form copolymers and catalysts may be used. Using dibutyltin dilaurate (DBTDL) as a catalyst is preferable, since this catalyst is known to decrease reaction time. Using 1,6-diisocyanatohexane (HDI) as an isocyanates source is preferable since it may increase transparency and allows for an increased amount of nanoparticles in the matrix. The presence of nanoparticles does not significantly influence the reaction conditions or the resulting polymer, except that after polymerization the nanoparticles 12 are embedded in the polymer matrix 11 in a homogeneous manner with less or at least smaller agglomerations 13 and with less disruptions 14 of the matrix and less surface defects 15 (for instance as is schematically shown in FIG. 1a).

(17) The polymerization reaction is performed in a pressurized oven, such as an autoclave. The temperature must be near or above the boiling point of the organic medium. For 1,4-butanediol this provides a temperature window of 220 to 250 degrees Celsius, with a preferred working temperature of 225 degrees Celsius. High pressure must be applied, at least greater than 20 bar, preferably about 40 bar and the reaction must be kept under these conditions for at least 24 hours to obtain well-cured and well-dispersed scintillator nano composites.

(18) To be able to obtain good quality nanocomposites all starting materials must be dehydrated and degassed (preferably at a temperature between 100 and 150 degrees Celsius) to avoid inclusions or defects in the nanocomposite. Especially any presence of water makes it very difficult, or even impossible, to obtain scintillator nanocomposites, since with water present the polymerization reaction would be too fast and nanoparticles are very difficult to add controllably.

(19) Because no defects are caused by insufficient or ineffective solvent removal, nanoparticle composites are obtained with improved transparency and quality. This can be improved even further by melting 104 the obtained nanocomposite and injection molding 105 the molten composite. The fast melting and violent cooling of this process may reduce further defects. This is especially effective in case the nanocomposite comprises polymers with a block structure with harder and softer segments linked together, such as for instance is the case with polyurethanes. Transparency before injection molding is not yet optimal because the segments have a high polarity and therefore have a strong tendency to agglomerate, resulting in (pseudo)-crystallite structures that reduce transparency. Crosslinks between the segments disappear upon heating and injection, which prevents crystallites from growing, resulting in greatly enhanced transparency of the nanocomposites.

(20) The improved nanocomposites of the present invention may then be processed further to be used in various applications, such as radiation detectors. FIG. 3 shows a highly simplified, schematic depiction of a radiation detector where a scintillator 31 is optically attached to a photodiode 32, which is mounted on an integrated circuit 33. Other configurations, such as vertical arrangements, other layers, pixelated detectors, etc., are also considered in the context of the present invention.

(21) Said radiation detector 30 may then be incorporated in imaging systems 40, such as an x-ray imaging system, a CT imaging system 40 (as is schematically shown as an example in FIG. 5), a PET imaging system or SPECT imaging system. The improved nanocomposites may be further processed as the known nanocomposites and construction 106 of a radiation detector may be done using known techniques.

EXAMPLE 1

Polyurethane Filled with Y.SUB.3.Al.SUB.5.O.SUB.12.:1% Ce Nanoparticles (20 Vol. %)

(22) 11.498 g of Aluminium isopropoxide, 11.336 g of Yttrium acetate hydrate and 0.111 g of Cerium acetate hydrate were blended together with mixture of solvents: 1,4-butanediol and diethylene glycol in mass ratio 9:1. The colloidal solution was stirred on a hot plate for 3 hours with mild heating of 50 degrees Celsius. After the mixture was homogenized, it was poured into a high pressure autoclave vessel. Air present in the autoclave was flushed away with Argon before the mixture was heated to 225 degrees Celsius for 60 hours with a heating rate 1.5 degrees Celsius/min. At the end of process the mixture was cooled down to obtain a translucent yellowish suspension.

(23) Next, 9 g of the obtained dehydrated suspension was rapidly mixed with 16 g of dehydrated 1,6-diisocyanatohexane and 1 drop of dibutyltin dilaurate. The viscous material was poured out into Teflon mold and kept in an oven at 60 degrees Celsius for 8 hours to obtain nanocomposite granules.

(24) The nanocomposite granules were then introduced in an injection molder, melted down at 210 degrees Celsius and immediately cooled down in the mold.

(25) FIG. 3(a) shows transmission T, reflection R and absorption A of the resulting transparent YAG:Ce-polyurethane nanocomposite. FIG. 3(b) shows luminescence under 450 and 360 nm excitation of the obtained nanocomposite. The scintillators of this example are transparent at visible wavelengths, but absorb at x-ray wavelengths, which make them suitable for use in CT and other x-ray radiation detectors. For PET, absorption should be at 511 keV. For other purposes absorption in UV or other wavelengths may be necessary. By selecting the right scintillating nanoparticle the absorption and emission wavelengths may be set, for instance using nanoparticles based on Eu or Tb.

(26) Similar results are obtained for LGAG:Ce (checked with at least Lu.sub.2Gd.sub.1Al.sub.5O.sub.12:1% Ce) and LGGAG:CE (checked with at least Lu.sub.2Gd.sub.1Al.sub.4Ga.sub.1O.sub.12:1% Ce), accounting for stoichiometric differences in the amounts. Particularly mixed garnet scintillators, such as LGGAG:Ce are difficult to obtain in sufficient transparency and quality. The method of the present invention also results for these type of scintillators in high transparency and high quality nanocomposites.

(27) It is also possible to obtain a gel as matrix material, for instance a polyurethane gel, for the scintillator materials. Especially if the nanoparticle has luminescent properties, then several interesting applications may be produced using gel-based nanocomposites. For instance: flexible luminescent sheets (for instance for safety applications, in cloths, etc.) or health applications (e.g. to treat skin diseases with light. In this case the luminescence is excited optically). They might be applied in certain toys. Further, they can be used in the same sections as known (polyurethane or other) gels but with luminescent function: luminescent computer mouse-pad/keyboard wrist rests, luminescent bicycle parts, luminescent motorbike seating, luminescent shoe insole, luminescent padding parts for medical devices, luminescent sticky pads for holding cell phones and tablet computers, any conformal luminescent layer to cover a product, etc.

EXAMPLE 2

Polyurethane Gel (PU Gel) Filled with Lu.SUB.2.Gd.SUB.1.Al.SUB.5.O.SUB.12.:1% Ce Nanoparticles (35 Vol. %)

(28) 6.008 g of Aluminium isopropoxide, 7.529 g of Lutetium acetate hydrate, 3.350 g of Gadolinium acetate hydrate and 0.104 g of Cerium acetate hydrate were blended together with mixture of solvents: 1,4-butanediol and diethylene glycol in mass ratio 9:1. The colloidal solution was stirred on a hot plate for 3 hours with mild heating of 50 C. After the mixture was homogenized, it was poured into a high pressure autoclave vessel. Air present in autoclave was flushed away with Argon. The mixture was heated to 225 degrees Celsius for 60 hours with a heating rate 1.5 degrees Celsius/min. At the end of process the mixture was cooled down and translucent yellowish suspension was obtained.

(29) 5 g of the obtained dehydrated suspension was rapidly mixed with 6.5 g of mixture of Polymeric MDI (a mixture of oligomeric polyisocyanates) and an MDI isomer (isocyanates (1-isocyanato-4-[(4 isocyanatophenyl)methyl] benzene). The resulting viscous polyurethane gel was poured out into Teflon mold.

(30) The nanocomposite may be made into any shape or form (e.g. sheets, powder, shaped articles, foils, etc.) using the method of the present invention, as long as the precursor material disperses the nanoparticles well and (co-)polymerizes into an optically compatible matrix material for the scintillating nanoparticles. Curing is not limited to curing by heating. Other curing reactions, such as for instance UV polymerization and others known to the skilled person are also considered.

(31) Nanoparticle composites of the present invention may be applied in various manners known to a skilled person, such as depositing, coating, printing, etc. They may be shaped by injection or other molding techniques, 3D printing and other techniques known to the skilled person.

(32) While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

(33) The term about in the present application means that 10% under or over the given value is considered to be covered.

(34) Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.