ITMI20012555A1 - HIGH EFFICIENCY LUMINESCENT GLASSES, PARTICULARLY FOR USE AS SPARKLING MATERIALS FOR THE DETECTION OF IONIZING RADIATIONS - Google Patents

Description

DESCRIPTION

Patent for Industrial Invention,

The present invention relates to high efficiency luminescent glasses, particularly suitable for use as scintillator materials for the detection of ionizing radiations, and to the related manufacturing process.

It is known that they are widely used as scintillator materials for the detection of ionizing radiations to be used in numerous applications both in the medical field (radiographs, tomography, etc.), and in the industrial field (non-destructive tests, safety checks on materials and artifacts, etc.), of crystalline materials, for example NaI: Tl, CsI: Tl and the like: although these materials are characterized by a high luminescence efficiency (quantity of photons emitted per unit of energy absorbed in the form of ionizing radiation), they present a low mechanical resistance and a remarkable hygroscopicity. Other scintillator crystals based on garnets or perowskites have much better mechanical and chemical characteristics, but have lower efficiency and are difficult to produce.

As an alternative to crystals, glass-matrix scintillator materials produced by melting processes are known: these materials can be produced in a relatively simple and economical way, have good mechanical properties and allow the introduction of luminescent ions (typically rare earth ions) into even high concentrations.

However, since typically the elements belonging to the rare earth group, as well as many transition elements, have a poor solubility in silica, to allow the incorporation of the active ions into the glass matrix (and to obtain an adequate dopant concentration) it is It is necessary to include large percentages (approximately 20 ÷ 50% by weight) of network modifiers, normally oxides of alkali or alkaline earth metals, phosphorus, etc. The presence of these compounds compromises some characteristics of the materials, making them difficult to use for some applications (for example, in the optical fiber sector): in addition to this, the considerable concentration of dot defects (due to the presence of lattice modifiers in the matrix vitreous) gives rise to a high probability of non-radiative (or radiative but slow) recombinations between electrons and holes, affecting the optical performance of the material.

It is therefore an object of the present invention to provide luminescent glasses, together with a process for making such glasses, which allow the above-mentioned drawbacks of the known art to be overcome: in particular, it is an object of the invention to provide glasses with good mechanical characteristics, excellent luminescence properties (efficiency, sensitivity and response speed), high purity and which can be achieved relatively easily and economically.

It is a further object of the invention to provide new materials which can be effectively used as scintillator glasses for the detection of ionizing radiations, for example in medical diagnostics and industrial controls, and which can be used, inter alia, in the form of fibers. optics, making use of production technologies already known in the optical fiber industry for digital telecommunications.

The present invention therefore relates to a process for the production of a highly efficient luminescent glass, particularly for use as a scintillator material for the detection of ionizing radiations, characterized in that said glass is obtained by means of a sol-gel reaction. followed by sintering of the gel resulting from said sol-gel reaction.

In particular, glass is essentially composed of a glass matrix of a base component in which at least one dopant having luminescent properties is dispersed, and the process comprises the steps of: - preparing a sol, dispersing at least one precursor of the base component in a solvent of the precursor tooth and adding said dopant;

- causing the sol-gel reaction to take place, with the transformation of said sol into a gel;

- densifying, by removing the solvent, the gel and sintering to obtain said glass.

It has in fact been recognized, for the first time, that glasses obtained by sol-gel (currently destined for completely different applications) can be effectively and advantageously used as luminescent materials and, in particular, as scintillators for the detection of ionizing radiations, showing unexpected performances and in many respects superior to known materials (for example glasses obtained from melting processes).

The selection of the process of formation of the vitreous matrix by sol-gel (a process that has never before been applied to the production of materials specifically intended for use as scintillators) allows the production of high-purity glasses, adopts relatively mild physical conditions and it allows a good dispersion of doping elements in the glass matrix.

In particular, the method according to the invention allows to obtain glasses substantially free of melting substances, alkali and alkaline / earth metal oxides and recalculation modifiers in general: this reduces the presence of dot defects and allows a significant increase in optical performance. both in terms of response intensity and in terms of the time decay of the response itself.

The glasses according to the invention are therefore distinguished from known materials, among other things, for their high purity: this characteristic, together with the fact that they are fully compatible with silica (unlike classic glass obtained by fusion), makes the glasses of the invention that can be fully integrated with optical fiber production technology (using for example the production methods known as "powder in tube" or "rod in tube").

The glasses according to the invention also have good mechanical resistance, high chemical stability and potentially low production costs, aspects which make them competitive even with respect to traditional crystalline scintillators.

The invention therefore extends to a high efficiency luminescent glass, particularly for use as a scintillator material for the detection of ionizing radiations, comprising a glass matrix of a base component in which at least one dopant having luminescent properties is dispersed, the glass being characterized in that the glass matrix is essentially constituted by silica or silica / germany with purity higher than 901 molar and preferably higher than 95% molar.

The invention therefore provides, for the first time in the art, luminescent glasses essentially consisting of a vitreous matrix, of silica or silica / germany in any ratio between them, having high purity, that is to say containing a very low percentage (lower than 10% and preferably 5% molar) of other substances, of any type.

The high purity glassy matrix includes the actual dopants, having scintillating properties, and possibly co-dopants, i.e. substances introduced with auxiliary function to the dopants: as dopants, for example, active ions can be used, preferably of Rare lands; the codrogants can be selected, according to the desired function, from the rare earth group or from the group consisting of Li, B, Al, P, Cd.

Drugs and possible co-dopants can be advantageously introduced in the sol preparation phase in the form of precursors, for example nitrates, acetates, halides, alkoxides or coordination complexes more or less easily hydroisable: preferably the rare earth precursors are used in hydroalcoholic solution, previously prepared by dissolving the appropriate amount of precursor in the alcoholic solvent, if necessary operating under nitrogen and with the aid of a sonicator.

The main auxiliary functions performed by the co-authors are the following:

- "energy transfer": known phenomenon, for example, in pairs of elements Er-Yb and Ce-Gd;

- to facilitate the dispersion of rare earth (for example the co-presence of Al has been shown to work in this sense with Er already in relatively low stoichiometric ratios, for example with 5 moles of Al for each mol of Er);

- to carry out a radiation conversion (for example by introducing elements with a high neutron capture cross section, such as Li, B, Cd, into the glass matrix, allowing the detection of neutrons).

The glasses according to the invention are also characterized by the fact that they have a softening temperature ("softening point") higher than 100Q ° C, thermal expansion coefficient (evaluated between 0 ÷ 300 C) lower than 2.10-6 ° C <-1 > and preferably lower than 1-10 <-6> ° C <-1>, and a concentration of point defects such as not to induce the presence of scintillation tails in the micro-millisecond range with relevance greater than 5% of the tonal emission .

The invention envisages the use of silica (SiO2) or a silica / germany mixture (SiO2 / GeO2) as the basic component of the vitreous matrix: they can be used as precursors of silica and germany, for example, organometallic compounds of silicon, such as silicic esters (tetraethyl-orthosilicate TEOS or tetramethyl-crtosilicate TMOS), and, respectively, of germanium, such as germanium tetraethoxide. The silicic esters (TEOS or TMOS) can also be substituted or integrated with other compounds of general formula Si (R) x (A) y where: x is between 0 and 3; y is between 1 and 4; the sum of the coefficients x and y is 4; R is a generic non-hydrolyzable alkyl group (also variously substituted, for example with complexing substituents of the alkyl-ethylenediamine type); A is a hydrolyzable group of organic or inorganic nature (eg chloride, carboxyl, various alkoxides, etc.).

Depending on the precursor used, a suitable solvent is selected, for example an alcoholic solvent; as preferred solvents the following can be indicated: ethanol, methanol, dioxane, acetone, THF, methoxy-ethanol, their mixtures.

Advantageously, the solvent is removed by drying the gel and yet by extracting the solvent under hypercritical conditions, the drying being optionally preceded by a thermal pre-treatment of the gel carried out at a temperature above 100 ° C.

The glasses according to the invention are particularly suitable for the production of scintillating optical fibers: the invention therefore extends to a scintillating optical fiber characterized by the fact that it comprises a "core" which has been obtained by spinning a glass made with the process described above, or of a glass having the characteristics indicated above.

According to an important aspect of the invention, the glass obtained from the sol-gel reaction and subsequent densification and sintering is then subjected to a high temperature post-treatment, in particular at a temperature higher than the temperature at which the sintering is carried out: yes it was found that this treatment induces, compared to the same glasses not subjected to post-treatment, a significant increase in the scintillation efficiency, as confirmed by comparative experimental tests (on some of which the attached example 18 refers).

On the other hand, a high temperature heat post-treatment has never been included in sol-gel type processes, as apparently contrary to the basic idea of these processes (which consists precisely in the use of operating conditions, and in particularly at relatively mild temperatures).

Advantageously, the heat post-treatment is carried out at a temperature higher than about 1400 ° C (and preferably between 1400 ° C and 2000 ° C) for a time higher than about 1 second (preferably between about 5 seconds and about 5 minutes and more preferably between about 10 seconds and about 1 minute). Heat post-treatment can be performed, for example, by exposure to hydrogen or oxygen flame.

It was also recognized the important role played, in order to further improve the characteristics of the glass and above all the luminescence efficiency, by other operating parameters of the glass manufacturing process, such as the sintering temperature and the dopant concentration (or of dopants).

In particular, it has been observed that the luminescence efficiency increases as the sintering temperature increases: the sintering step (forming the glass from the gel) is therefore advantageously carried out at a temperature higher than about 900 ° C and preferably between 950 ° C and 1150 ° C ÷ 1200 ° C: as confirmed by Raman and IR spectroscopy analysis, in this way the complete formation of the glass is achieved and the strong reduction of the OH groups initially present is achieved.

As regards the concentration of the dopant (or dopants), the sol-gel reaction, which takes place in the liquid phase at room temperature, allows, unlike fusion processes, to obtain materials in which the dopant is incorporated in the medium regardless of the solubility of the element in the glass matrix. It is therefore also possible to obtain materials with dopant concentrations above the solubility limit, essentially supersaturated: nevertheless, these materials have proved to be sufficiently stable to allow thermal sintering processes or processing at temperatures above the glass transition. without giving rise to ceramicization phenomena.

On the other hand, it has been surprisingly observed that the glasses according to the invention provide high optical performance at relatively low dopant concentrations: in particular, it has been observed that in order to obtain a level of luminescence intensity comparable to that of a glass in accordance with According to the invention with a dopant concentration, for example, lower than 0.1 mol%, it is necessary to use traditional fusion glasses with dopant concentrations of at least one order of magnitude higher.

The invention is further described in the following non-limiting embodiments, with reference to the attached drawings in which:

Figure 1 schematically illustrates a sintering cycle of a glass according to the invention;

Figure 2 schematically illustrates a scintillating optical fiber made with the glass according to the invention;

Figure 3 reports the results of comparative radioluminescence tests carried out on a glass according to the invention which has or has not been subjected to post-heat treatment after sintering;

- figure 4 reports the results of comparative radioluminescence tests carried out on a glass according to the invention, two commercial glasses produced by fusion and a traditional scintillator crystal.

EXAMPLE -1

A sol composed of 6 ml of absolute ethanol was prepared; 2 ml of tetraethyl orthosilicate (TEOS); 0.500 ml of a solution of Ce (N03) 3.6H20 in ethanol (concentration 4 mg / ml); 1.2 ml of water.

The sol was filtered on a 0.2 micron filter, introduced into a hermetically sealed polypropylene bottle and reacted in a thermostated environment at 35 ° C. After freezing, small openings were made in the container to allow the slow drying of the gel over a couple of weeks. The product thus obtained (called xerogel) was vitrified by thermal sintering at 1050 ° C according to the scheme (purely indicative) shown in the attached figure 1.

The treatment was carried out under vacuum, but further tests confirmed that it is also possible to use He, Ar, N2 atmospheres and also mildly oxidizing atmospheres such as He: O2 at 1%; around 700 ° C it may be advantageous to operate in a Cl2: O2 atmosphere to eliminate traces of residual water.

A glass disc with a thickness of 1 mm and a diameter of 15 mm was obtained, which was then flame treated with H2: O2 at white heat for a few seconds (temperature between -1600 ° and 1800 ° C) to optimize the scintillation efficiency.

Nominal atomic composition of the boar obtained: 100 parts of Si and 0.1 parts of Ce.

EXAMPLES 2-12

The further sols were prepared, the composition of which is shown in table 1. These sols were then treated according to methods similar to those indicated in the previous example 1, varying in some cases the operating conditions of the densification / sintering phase and / or of the heat post-treatment phase.

EXAMPLE 13

A sol was prepared by refluxing 30 ml of absolute ethanol; 20 ml of tetraethyl orthosilicate (TEOS); 0.50 ml of triethyl borate B (OC2H5), 3 ml of water. After 1 hour, 3.6 ml of a solution of Ce (CH3COO) 3-xH? 0 in an ethanol / water mixture in a 5: 1 ratio (at a concentration of 0.8 mg / ml) was added to the tenth part of the sol. 0.3 ml of water.

The subsequent phases were carried out in a manner similar to the previous examples.

EXAMPLE 14

A sol was prepared consisting of: 6 ml of ethanol / water mixture in a 5: 1 ratio; 2 ml of tetraethyl orthosilicate (TEOS); 0.2 ml of Ce (N03) 3.6H2O solution in ethanol (at a concentration of 4 mg / ml); 0.500 ml of germanium tetraethoxide. The sol was placed in a glass container (test tube), made hydrophobic by silanization. Once sealed, the container was left in an incubator at 35 ° C until the sol had frozen. Once the gel was obtained, the container was opened and placed in a steel autoclave filled with absolute ethanol for the following heat treatment:

- temperature increase from room temperature to 230 ° C in 20 hours;

- temperature maintenance for 24 hours;

- cooling down to room temperature in 20 hours.

The gel obtained was then left to dry slowly and then subjected to sintering and post-heat treatment as already described.

EXAMPLE 15

With the same modalities of the previous example 14, a sol was prepared consisting of: 6 ml of absolute ethanol; 2 ml of water; 4 ml of TEOS; 0.2 ml of solution of Ce (N03) 3-6H20 in ethanol (at a concentration of 4 mg / ml); 1 ml of germanium tetraethoxide.

As in the previous case, a silanized glass container was used; after gelation, the gel was introduced into a steel autoclave containing ethanol: the solvent was tapped slowly, once the chemical-physical conditions of hypercriticity were reached (for ethanol this means temperatures higher than 245 ° C and pressure of 80 atmospheres), and completely eliminated. The material obtained from this process, called AIRGEL, was sintered without further intermediate treatments, with methods similar to those already described.

EXAMPLE 16

Further tests were carried out by varying both the compositions of the starting sols and the operating modes of the different process phases.

In particular, glasses were prepared having the nominal compositions summarized in table 2.

EXAMPLE 17

Glasses produced with the methods indicated above (in particular silica or silica / germany glass with 5 ÷ 30% of germany) were used for the realization of optical fibers, with known techniques and not reported in detail for simplicity.

An optical fiber made according to the invention is schematically illustrated in Figure 2 and indicated as a whole with the reference number 1: the optical fiber 1 comprises a glass "core" 2 surrounded by a "cladding" ) 3 of a material having a lower refractive index: the core 2 is made with the glasses according to the invention, the cladding 3 in pure silica or in polymeric materials with a lower refractive index than that of the core 2. In particular, it was made an optical fiber having a core 2 with a DC diameter of about 100 microns and a cladding 3 having a thickness of about 20 microns: other optical fibers with a DC core diameter greater than 50% of the total diameter DT of the optical fiber were also made. This type of optical fibers, having a core diameter greater than 50% of the total diameter and in particular with a ratio between total diameter and core diameter DC: DT around 140: 100, is particularly suitable for fibers with high scintillation performance, for which do not require a large signal transmission distance.

It is understood that optical fibers with traditional and standard geometries can also be produced, which clearly allow easier interfacing with "passive" fibers capable of carrying the light signal over a greater distance.

EXAMPLE 18

In order to verify the effect of the thermal post-treatment on the scintillation efficiency, measurements of radioluminescence intensity were carried out on samples of glasses produced according to the process of the invention but not subjected to post-thermal treatment and on samples of the same glasses. subjected to post-heat treatment: in all cases a significant increase in radioluminescence intensity was observed (from a minimum of 5 to about a factor of 30 depending on the concentration of the rare earth); by way of example, figure 3 shows the radioluminescence spectra at room temperature (radioluminescence intensity expressed in arbitrary units) of a glass having a silica glass matrix with 0.1% Ce and sintered at 1050 ° C but not subjected after heat treatment (curve A), and of the same glass subjected to post-treatment at 1800 ° C for a few seconds (curve B): the increase in radioluminescence intensity is about a factor of 6.

The same glass according to the invention, subjected to post-heat treatment, was then compared with two commercial glasses produced by fusion and a traditional scintillator crystal, in order to evaluate the scintillation efficiency in a comparative way. The results of the experimental tests are shown in figure 4, where the light intensity curves (in arbitrary units) are represented as a function of the wavelength for:

A, B: commercial glass produced by fusion by Applied Scintillation Technologies (AST) and named respectively GS1 and NAG;

C: Bi4Ge3O12 scintillator crystal;

D: glass produced by sol-gel according to the invention (silica matrix with 0.1% Ce).

The illustrated results confirm the high efficiency of the material according to the invention with respect to commercial glasses obtained with melting processes.

Claims (20)

CLAIMS 1. Process for the production of a highly efficient luminescent glass, particularly for use as a scintillator material for the detection of ionizing radiations, characterized in that said glass is obtained by means of a sol-gel reaction followed by sintering of the resulting gel from said sol-gel reaction. 2. Process according to claim 1, characterized in that said glass is essentially composed of a glass matrix of a base component in which at least one dopant having luminescent properties is dispersed; the procedure including the steps of: - preparing a sol, dispersing at least one precursor of the base component in a solvent of the said precursor and adding the said dopant; - causing the sol-gel reaction to take place, with the transformation of said sol into a gel; - densifying, by removing the solvent, the gel and sintering to obtain said glass. 3. Process according to claim 2, characterized in that said base component is silica and said precursor is an organometallic compound of silicon, preferably TEOS or TMOS. 4. Process according to claim 2, characterized in that said basic component is a silica / germany mixture and respective precursors of silica and germany are used, preferably consisting of organometallic compounds of silicon and, respectively, of germanium. 5. Process according to one of claims 2 to 4, characterized in that said solvent is an alcoholic solvent. 6. Process according to one of claims 2 to 5, characterized in that said dopant is selected from the rare earth group. 7. Process according to claim 6, characterized in that the dopant tooth is present in a concentration comprised between 0.017 and 17 molar. 8. Process according to one of claims 2 to 7, characterized in that in the sol preparation step a dopant and at least one co-dopant are dispersed in said sol, the dopant being selected from the rare earth group, the co-dopant being selected from the group of rare earths or in the group consisting of Li, B, Al, P, Cd. 9. Process according to one of the preceding claims, characterized in that the said solvent is removed by drying the said gel or by extracting the solvent under hypercritical conditions, the drying being optionally preceded by a heat pre-treatment of the said gel led to a temperature above 100 ° C. 10. Process according to one of the preceding claims, characterized in that the sintering is carried out at a temperature between 900 ° C and 1200 ° C. 11. Process according to one of the preceding claims, characterized in that said glass is subjected, after sintering, to a thermal post-treatment at a temperature higher than the temperature at which the sintering is carried out. 12. Process according to claim 11, characterized in that said heat post-treatment is carried out at a temperature higher than about 1400 ° C for a time higher than about 1 second. 13. Process according to claim 12, characterized in that said heat post-treatment is carried out at a temperature between 1400 ° C and 2000 ° C for a time between about 5 seconds and about 5 minutes and preferably between about 10 seconds and about 1 minute. 14. High efficiency luminescent glass particularly for use as a scintillator material for the detection of ionizing radiation, comprising a glass matrix of a base component in which at least one dopant having luminescent properties is dispersed, the glass being characterized in that the matrix vitreous consists essentially of silica or silica / germany with purity higher than 90% by mol and preferably higher than 95% by mol. 15. Glass according to claim 14, characterized in that it has a softening temperature ("softening point") higher than 1000 ° C and a coefficient of thermal expansion (evaluated between 0 ÷ 300cC) lower than 2.10-6 ° C-1 and preferably below 1.10-6 ° C-1. Glass according to claim 14 or 15, characterized in that it has a concentration of point defects such as not to induce the presence of scintillation tails in the micromillisecond range with relevance greater than 5% of the total emission. 17. A scintillating optical fiber, characterized in that it comprises a core which has been obtained by spinning a glass made with the process according to any one of claims 1 to 13, or a glass according to one of the claims 14 to 16. 18. Optical fiber according to claim 17, characterized in that it has a core having a diameter greater than about 50% of the total diameter of the optical fiber, and preferably a ratio between the total diameter and the diameter of the anima ("core") of about 140: 100. 19. Optical fiber according to claim 18 or 19, characterized in that it comprises a core, consisting of a scintillating glass whose inert matrix is silica or silica / germany 5 ÷ 30% of germany, and coated with a cladding of pure silica or a polymeric material with a lower refractive index. 20. Process for making a high-efficiency luminescent glass, particularly for use as a scintillated material, for detecting ionizing radiations, glass obtained by means of this process and optical fiber made with such glass, substantially as described.