WO2022238928A1

WO2022238928A1 - Fast and low re -absorption composite scintillator and process for detecting high-energy particles and/or electromagnetic radiation

Info

Publication number: WO2022238928A1
Application number: PCT/IB2022/054391
Authority: WO
Inventors: Angelo Maria MONGUZZI; Angiolina Comotti; Piero Ernesto SOZZANI; Anna Graziella VEDDA; Jacopo PEREGO
Original assignee: Universita' Degli Studi Di Milano - Bicocca
Priority date: 2021-05-13
Filing date: 2022-05-11
Publication date: 2022-11-17
Also published as: EP4338175A1; IT202100012347A1

Abstract

A fast and low re-absorption composite scintillator for detecting high-energy particles and/or electromagnetic radiation, comprising a matrix charged with metal-organic nanocrystals containing at least a metal and at least two fluorescent ligands, wherein the two ligands have respective absorption and emission bands defining a large Stokes shift and are arranged at an intermolecular distance from each other equal or shorter than 15 angstroms. The scintillator of the present invention thus makes it possible to minimise the scintillation light re-absorption effects resulting in increased scintillation efficiency and activate ultrafast non-radiative energy trans fer mechanisms resulting in improved response times on the detected signals.

Description

"FAST AND LOW RE-ABSORPTION COMPOSITE SCINTILLATOR AND PROCESS FOR DETECTING HIGH-ENERGY PARTICLES AND/OR ELECTROMAGNETIC RADIATION"

Cross-Reference to Related Applications

This Patent Application claims priority from Italian Patent Application No. 102021000012347 filed on May 13, 2021, the entire disclosure of which is incorporated herein by reference.

Technical Field of the Invention

The present invention relates to a fast and low re absorption composite scintillator for detecting high-energy particles, such as ionising particles and neutrons, and/or electromagnetic radiation, in particular ionised radiation, such as x-rays and gamma rays, and the high-frequency portion of ultraviolet rays.

The invention also relates to a process for detecting high-energy particles and/or electromagnetic radiation by means of the present scintillator. In particular, the process describes the generation of fast and high-efficiency scintillation pulses, emitted by the scintillator in response to the interaction with high-energy particles and/or electromagnetic radiation and detectable, in turn, by conventional photodetection systems (e.g. by a photomultiplier) .

For example, the invention has applications in the field of science (particle physics, astrophysics, chemical and biochemical analysis), medicine (X-rays, tomography, etc.) and industry (non-destructive testing, safety checks on materials and manufactured goods, etc.).

State of the Art

As regards the medical field, scintillators are known

1 to be widely used in radiodiagnostic techniques, such as in positron-emitting tomography (PET) and in the more recent Time of Flight-PET (TOF-PET), as the most accurate and effective investigation technique in the oncological field to obtain for example, imaging of tumours, in the search for metastases, to differentiate between benign and malignant lesions, etc.

However, to date, the TOF-PET technique suffers from a limited spatial resolution, resulting in image reconstructions with a poor precision, of a few centimetres, and consequent diagnostic determination that is not always accurate.

In detail, the TOF-PET technique is based on the measurement of the time difference in detecting two gamma photons emitted in opposite directions during the decay of a positron. Given the proportionality between the time difference of the pair of gamma photons and the length of the path they travel before being intercepted and detected by the scintillator, it is possible to determine the position at which the positron annihilates with an electron in the tissue, producing said pair of gamma photons, and to subsequently reconstruct the image of the tissue/organ in which the tracer radio-isotopes, contained in a radiopharmaceutical previously administered to the patient, are most concentrated. However, the localisation of the positron annihilation points is often affected by the time uncertainty in detecting the pair of gamma photons by the scintillator, resulting in an image reconstruction with limited spatial resolution having an uncertainty of the order of a centimetre.

In order to improve the spatial resolution of the TOF- PET technique, it is therefore necessary to develop fast

2 time resolution scintillators that make it possible to reach spatial resolutions of the order of a millimetre in the final reconstructed image without using complex reconstruction algorithms, and therefore with shorter acquisition times and simpler, less expensive equipment suitable for widespread use.

The time resolution of a TOF-PET system depends on many factors, several of which affect directly the scintillator and, specifically, the formation of the scintillation pulse which is, in turn, detected by a photodetecting system optically coupled to the scintillator.

In theory, the scintillators used in TOF-PET must be selected so that their chemical and physical characteristics meet the requirements imposed by the TOF-PET application, including high quantum efficiency (or scintillation efficiency or light output) and, obviously, good time performance. Scintillation efficiency is defined as the ratio between the number of photons emitted by the scintillator and the energy deposited in the scintillator, related, for example, to the number of gamma photons absorbed by the scintillator. On the other hand, in order to ensure good time performance, it is necessary for the scintillator to emit scintillation pulses by fluorescence in a short time, i.e. a good scintillator must show short decay times of the fluorescence radiation.

Among the chemical and physical characteristics of the scintillator that most affect its scintillation efficiency and time response there are its chemical composition, electronic structure, possible intra- and inter-molecular interactions, symmetry properties in crystalline systems, atomic density and its optical properties.

In practice, the scintillators currently used do not

3 have the versatility required to simultaneously control and modulate most of their physicochemical characteristics outlined above and, therefore, are not capable of ensuring the requirements of a good scintillator for TOF-PET.

For example, scintillation efficiency is often limited by the re-absorption of the scintillation pulse by the scintillator itself before it reaches the photodetection system. In this respect, they are in fact known scintillators that show good time performance, but have low quantum yields, around 20%, due to the significant phenomenon of re absorption promoted by a small Stokes shift between the absorption band of a first species, comprised in the scintillator, and the emission band of a second species, also comprised in the scintillator, i.e. the two bands tend to overlap, favouring the re-absorption by the first species of the photon emitted by the second species. In fact, considering the typical concentrations of use of first and second species and the size of the scintillators, the fraction of light re-absorbed by the scintillator is non- negligible. This phenomenon severely limits the actual scintillation photon density (h) and therefore both the sensitivity of the photodetection system and the final time resolution (At) of the TOF-PET system, because according to the relation,

1

At o —

V⁷? the lower the density of the scintillation photons, the greater the system response time to the detected signals.

At present, various approaches are used to minimise the overlap between the absorption and emission bands of the species comprised in the scintillator in order to increase the Stokes shift and, consequently, reduce the re-absorption

4 phenomenon, thereby promoting high scintillation efficiency.

For example, scintillators based on rare earth ion doped nanoparticles are known. These scintillators show a large Stokes shift that leads to high scintillation efficiencies, however suffer from long decay times, as the time evolution of the scintillation pulse emission process, ranging from tens of nanoseconds to milliseconds, thus limiting the time resolution of the TOF-PET system and, consequently, the quality, in terms of spatial resolution, of the reconstructed TOF-PET image.

Scintillators based on suitably doped semiconductor nanocrystals are also known, however low scintillation efficiencies and long decay times make them unsuitable for gamma-ray detection in TOF-PET systems.

There also exist fluorescent organic scintillators comprising various emissive species, where the re-absorption phenomenon is mitigated by the mechanism of radiative energy transfer between the emissive species. In this case, the cascade energy transfer between the emissive species allows the Stokes shift to increase, since the resulting photon, emitted by one of the emissive species, has an emission band in the near infrared, while the other emissive species, intermediaries in the transfer mechanism, have absorption bands in the near ultraviolet. However, the radiative energy transfer mechanism is efficient at high emissive species concentrations. However, high emissive species concentrations induce long decay times, thus nullifying the increase in Stokes shift achieved through the radiative energy transfer mechanism between emissive species. In addition, the number of emissive species comprised in the organic scintillator is such that the process of manufacturing the scintillator is too complex.

5 Subject and Summary of the Invention

It is therefore an aim of the present invention to provide a composite scintillator for detecting high-energy particles and/or electromagnetic radiation which overcomes the drawbacks described in the prior art.

In particular, an aim of the invention is to provide a composite scintillator that shows both a low re-absorption phenomenon, to achieve a high scintillation efficiency, and fast response times on the detected signals.

It is a further aim of the invention to provide a process for detecting high-energy particles and/or electromagnetic radiation, wherein the present composite scintillator acts as a detector. In particular, it is an aim of the invention to provide a process making it possible to achieve a fast time response of detected signals without compromising scintillation efficiency.

In accordance with such aims, in a first aspect, the present invention thus relates to a fast and low-absorption composite scintillator for detecting high-energy particles and/or electromagnetic radiation according to Claim 1.

The composite scintillator of the invention is particularly advantageous in that it allows to reduce the re-absorption phenomenon, and therefore to guarantee high scintillation efficiencies, thanks to the specific electronic structures of the at least two fluorescent ligands incorporated in the metal-organic nanocrystals, in turn charged in the matrix, which, stimulated by the detected pulses, determine specific electronic transitions associated with absorption and emission bands, defining a large Stokes shift.

At the same time, the composite scintillator of the invention provides good time resolutions for detecting high-

6 energy particles and/or electromagnetic radiation, so that, for example, if the present scintillator is used as a detector in the TOF-PET systems, the time resolution of the TOF-PET system is increased, as well as the reconstructed image achieves an improved spatial resolution. The high time resolution of the present scintillator is achieved by activating ultrafast non-radiative energy transfer mechanisms, induced by the close proximity of the two ligands incorporated in the scintillator. The efficiency of such ultrafast mechanisms is in fact closely related to the intermolecular distance between the fluorescent ligands in the scintillator: the typical distances required to activate such ultrafast mechanisms are of the order of tens of angstroms, more precisely equal to or lower than 15 angstroms.

In a second aspect, the present invention provides a process for detecting high-energy particles and/or electromagnetic radiation according to Claim 6.

The process of the invention is particularly advantageous in that the scintillation pulse, exiting the present scintillator and directed to the photodetection system, is the result of an ultrafast non-radiative energy transfer mechanism which does not have intermediate electronic transitions such as, for example, the emission of the first ligand and the transfer of the emitted energy to the second ligand, as it occurs, instead, in known organic fluorescent scintillators. Specifically, the first ligand, embedded in the metal-organic nanocrystals, which are in turn charged into the matrix comprised in the scintillator, acts as a donor and is excited by absorbing the pulses sent to the scintillator. The excited donor, instead of decaying to a fundamental electronic state by radiative processes

7 (e.g. by fluorescence), transfers its excitation in a resonant or non-radiative manner to the second ligand, which is also embedded in the metal-organic nanocrystals that are in turn charged in the matrix comprised in the scintillator, which, acting as an acceptor, receives the excitation energy and emits a scintillation pulse in the direction of the photodetection system. This resonance excitation transfer from donor to acceptor is promoted by the intermolecular proximity of the donor and acceptor and is most efficient when the emission band of the donor significantly overlaps the absorption band of the acceptor. Therefore, the scintillation pulse is emitted by the scintillator in accordance with the invention in a short time, of the order of a nanosecond, from the instant when the high-energy particles and/or electromagnetic radiation are absorbed and thereby detected by the scintillator, as intermediate processes, inevitably slowing down the emission of the scintillation pulse and also possibly adversely affecting the scintillation efficiency due to secondary radiative and non-radiative recombination processes, are eliminated.

Advantageously, the donor ligand constitutes a main ligand of the metal-organic nanocrystal, while the acceptor ligand constitutes a substituent ligand that is incorporated into the metal-organic nanocrystal as a dopant, i.e. in significantly smaller amounts than the first ligand. As an indication, the doping ligand is present in amounts smaller than 10% with respect to the main ligand.

Brief Description of the Drawings

Further features and advantages of the present invention will be apparent from the description of the following non-limiting embodiments, with reference to the figures of the accompanying drawings, wherein:

8 - Figure 1 schematically shows the structure of a composite scintillator, comprising a matrix charged with metal-organic nanocrystals, in accordance with an embodiment of the invention, and the steps of the luminescence activation process therein,

Figure 2 shows the structure formula and crude formula of an exemplary component of the composite scintillator of Figure 1, in particular the component defining the nodes of the metal-organic nanocrystals,

- Figure 3 shows the absorption and photoluminescence spectra and respective structure formulas of two examples of ligands, such as DPA and DPT, embedded in the metal-organic nanocrystals of the composite scintillator in Figure 1,

- Figure 4 shows a diffractogram from metal-organic nanocrystal powders used in the scintillator of Figure 1; the inset shows the region between 10° and 80° of the diffractogram,

- Figure 5 shows an adsorption isotherm measured at 77 K on a sample of the metal-organic nanocrystals,

- Figure 6 shows, on the left, an image acquired by Scanning Electron Microscopy (SEM) of metal-organic nanocrystals; on the right, the size distribution of the nanocrystals determined by the SEM image is shown,

- Figure 7 shows an H-NMR spectrum of metal-organic nanocrystals,

- Figure 8 shows an absorption spectrum (dotted line) and a photoluminescence spectrum (solid line) of metal- organic nanocrystals dispersed in benzene,

- Figure 9 shows two radioluminescence spectra of a scintillator in accordance with an embodiment of the present invention (solid line) and of a comparative scintillator (dashed line),

9 Figure 10 shows radioluminescence signals of a scintillator in accordance with an embodiment of the present invention (on the right) and a comparative system (on the left),

Figure 11 shows the decay over time of the scintillation pulse emitted by a scintillator in accordance with an embodiment of the invention.

Detailed Description of Preferred Embodiments of the

Invention

As schematically shown in Figure 1, a scintillator according to the invention comprises a matrix charged with porous metal-organic nanocrystals containing metals, in particular transition metals, also known by the acronym MOF (Metal-Organic Frameworks) nanocrystals.

The matrix represents the support structure of the scintillator. In particular, the matrix is made of polymeric or copolymeric material that is transparent to the visible radiations between 400 nm and 700 nm, thus excluding possible losses in scintillator efficiency due to the phenomenon of re-absorption by the matrix.

The matrix may have a crystallinity degree ranging from the amorphous to the semi-crystalline form with crystallite sizes smaller than 200 nm. In addition, the matrix may be rigid or flexible and may vary in shape, size and geometry, depending also on the specific use for which the scintillator is intended.

In a preferred embodiment, the matrix is made of polydimethylsiloxane (PDMS). However, it is understood that other polymers and copolymers may be used, such as polyacrylics, polyvinyls, polyamides, polyesters, polyurethanes and polysiloxanes.

The MOF nanocrystals, charged on the matrix, represent

10 the functional material of the scintillator which, by interacting with the high-energy particles and/or electromagnetic radiation intercepted by the scintillator, implement scintillation mechanisms through which scintillation pulses are emitted that may be detected by conventional photodetection systems coupled to the scintillator .

As known, MOF nanocrystals consist of agglomerates, serving as "nodes" in the crystal lattice, containing ions or groups of ions of metals, such as transition metals, coordinated with organic ligands, acting as "bridges" in the lattice to form structures with very high porosity.

Preferably, MOF nanocrystals contain transition metals.

As shown in Figure 2, in a preferred embodiment, the transition metal in MOF nanocrystals is zirconium and the agglomerates, defining the nodes of the lattice, are of the zirconium oxo-hydroxide type of formula Zr604(OH)4(CO2)12. It is understood that other metals and/or agglomerates may be used.

According to the invention, the MOF nanocrystals are two-component MOF nanocrystals, as they embed at least two organic ligands, defining the lattice bridges, of different types.

Among the various chemical species comprised in MOF nanocrystals, ligands are the main responsible for scintillation mechanisms. For example, in the case of two- component MOF nanocrystals, a first ligand absorbs the energy of the detected signals and the second ligand, receiving energy from the first ligand, emits scintillation pulses.

Given the relevant involvement of ligands for the detection of high-energy particles and/or radiation in the

11 scintillator of the present invention, these must meet specific requirements, including exhibiting high fluorescence quantum efficiency, having non-overlapping absorption and emission bands defining a large Stokes shift, and being arranged in MOF nanocrystals at intermolecular distances such as to allow ultrafast energy transfers.

Examples of ligands meeting these requirements are comprised in the polyacenic molecular systems consisting of polycyclic aromatic hydrocarbons made up of both linear (e.g. anthracene) and peri-fused (e.g. pyrene) conjugated aromatic rings.

As shown in Figure 3, in a preferred embodiment of the invention, the two ligands are two linear polycyclic aromatic hydrocarbons respectively containing three aromatic rings, e.g., 9,10-di(4-carboxyphenyl)anthracene (with the acronym DPA), and four aromatic rings, e.g., 5,12-di(4- carboxyphenyl)tetracene (with the acronym DPT).

Specifically, DPA in resonance with the signals intercepted by the scintillator, absorbs the energy thereof and acts as a donor ligand. Whereas DPT, not in resonance with the detected signals, receives energy from the donor ligand and emits it by fluorescence, acting as an acceptor ligand.

A large Stokes shift emitter denotes as a system showing an energy difference between the maximum of the emission spectrum and the maximum of the absorption band greater than or equal to the width at half of the emission band.

In accordance with the invention, the two ligands are selected so as to have respective absorption and emission bands defining a large Stokes shift, wherein a large Stokes shift denotes an energy difference between the maximum of

12 the emission band of the acceptor ligand and the maximum of the absorption band of the donor ligand that is equal to or greater than the width, in energy unit, of the emission band of the acceptor ligand measured at half of its maximum value.

As further shown in Figure 3, for example, the absorption band of the donor ligand, DPA, and the emission band of the acceptor ligand, DPT, define a Stokes shift with a theoretical maximum of 0.87 eV given by the energy difference between the maximum of the DPT emission band and the maximum of the DPA absorption band. Since the emission band width of DPT, measured at half of its maximum value, is equal to 0.32 eV to be compared with the relative Stokes shift (equal to 0.87 eV), the two ligands DPA and DPT show a large Stokes shift, thus allowing limited resorption phenomena.

In this preferred embodiment of the invention, wherein the present scintillator comprises a matrix charged with two-component MOF nanocrystals of the DPA and DPT type, the intramolecular distance between DPA and DPT is 12 angstroms and the overlap between the DPA emission band (solid line in the top panel of Figure 3) and the DPT absorption band (dashed line in the bottom panel of Figure 3) is significant. Considering these two characteristics, a scintillation sensitisation effect in the present scintillator is therefore possible due to ultrafast non-radiative energy transfer mechanisms between the DPA ligand and the DPT ligand, thus achieving an improved time resolution of the present scintillator with a characteristic decay time of 10 ns (see Figure 11).

Indications about the generation of scintillation pulses in two-component MOF nanocrystals of the DPA and DPT type via the ultrafast non-radiative energy transfer

13 mechanism are shown in Figure 1. In detail, the DPA ligand is excited by absorbing an impulse (hui_n), e.g. ionising or ultraviolet radiation, thus creating an electron-hole pair linked by Coulombic interaction which is called an exciton. The exciton can diffuse into the MOF nanocrystal until it reaches a nearby DPT ligand. In the vicinity of the DPT ligand, ultrafast non-radiative resonant energy transfer occurs between the exciton and the DPT ligand, resulting in non-radiative relaxation of the DPA ligand to its fundamental state and simultaneous excitation of the DPT. In a subsequent step, the excited DPT returns to the fundamental state through a process of radiative decay, spontaneously emitting a scintillation pulse (hu_0ut).

EXAMPLES

Experimental examples describing the manufacture and effectiveness of scintillators according to the invention are given below.

1. Synthesis and characterisation of two-component metal-organic nanocrystals (MOFs) of the DPA and DPT type

Two-component MOF nanocrystals were made through a modulated synthesis process under solvothermal conditions. The reagents ZrCl4 (116.5 mg), 9,10-di(4- carboxyphenyl)anthracene, DPA, (191.6 mg) and 5,12-di(4- carboxyphenyl)tetracene, DPT, (18.74 mg) were dispersed at room temperature in a mixture of anhydrous dimethylformamide (50 mL) and deionised water (25 pL). Finally, acetic acid (1.43 mL) was added. A stream of nitrogen was bubbled into the solution for 10 minutes. The mixture was sonicated for 1 minute and placed in an oven preheated to 120°C for 22 hours. Once the reaction was completed, the mixture was cooled to room temperature, filtered through a membrane filter and washed with dimethylformamide (50 mL) and

14 tetrahydrofuran (50 mL). The resulting powder was air dried and then heated to 130°C at reduced pressure.

The structure and composition of the two-component MOF nanocrystals of the DPA and DPT types were studied by powder X-ray diffraction, nitrogen adsorption isotherm at a temperature of 77 Kelvin, scanning electron microscopy (SEM) and H-NMR spectroscopy, the results of which are shown in Figures 4 to 8, respectively.

Figure 4 shows a powder diffractogram of the two- component MOF nanocrystalline powder. Whereas the inset shows a magnification of the region between 10° and 80° of the diffractogram.

Figure 5 shows an adsorption isotherm measured at 77 K on the sample of two-component MOF nanocrystals in which the adsorption curve is represented by grey circles with a black border, while the desorption curve is denoted by diamonds .

Figure 6 shows, on the left, an image of the two- component MOF nanocrystals obtained by SEM, from whose analysis it was possible to determine the size distribution of the two-component MOF nanocrystals, shown on the right.

Figure 7 shows the H-NMR spectrum of the two-component MOF nanocrystals digested in deuterated trifluoroacetic acid and dissolved in deuterated dimethyl sulfoxide. The peaks highlighted with grey dots denote signals for the DPT molecule (5,12-di(4-carboxyphenyl)tetracene in full).

Figure 8 shows the absorption spectrum (dotted line), acquired by means of a spectrophotometer, and the photoluminescence spectrum (solid line), acquired by means of a fluorometer (excitation wavelength 355 nm), of two- component MOF nanocrystals dispersed in benzene. The measured Stokes shift value of photoluminescence of 0.75 eV

15 is also shown.

2. Synthesis and characterisation of composite scintillators comprising PDMS charged with two-component MOF nanocrystals of the DPA and DPT type.

A dispersion of two-component MOF nanocrystals (16.5 mg), obtained according to the process described above, in anhydrous tetrahydrofuran was added to a polydimethylsiloxane-based prepolymer, PDMS, (typically base RTV 615, 3.0 g). The component containing the cross-linking agent (typically RTV 615 curing agent, 0.3 g) was added to the mechanically stirred dispersion. The dispersion was degassed under reduced pressure for 2 hours and poured into a special mould. The mixture was degassed again and then reacted for 16 hours at 60°C. The samples obtained were cooled to room temperature and removed from the mould.

By means of radioluminescence measurements, in which the sample is irradiated with X-rays, the spectra in Figure 9 were acquired. The dotted line shows the radioluminescence spectrum of a comparative scintillator comprising the PDMS matrix charged with single-component MOF nanocrystals of the DPA type. The net line shows the radioluminescence spectrum of a scintillator of the present invention comprising the PDMS matrix charged with two-component MOF nanocrystals of the DPA and DPT types. It is noted that the luminescence signal emitted by the scintillator of the present invention is five times higher than that from the comparative scintillator based on single-component MOF nanocrystals (DPA). This result demonstrates the improved effectiveness of the two-component MOF nanocrystal as a scintillator. The scintillation quantum efficiency (Light Yield) is 5000 photons per MeV.

The results in Figure 10 derive from further

16 temperature-dependent radioluminescence measurements, wherein the thermal stability of two-component MOF nanocrystals in powder form (left) and charged onto the PDMS matrix (right) is investigated. The invariance of the radioluminescence signal at high temperatures demonstrates the stability of the two- component MOF nanocrystals both in powder form and charged onto the PDMS matrix.

By means of time-resolved photoluminescence measurements, carried out at temperatures ranging from 10 K to 300 K, the dynamics, shown in Figure 11, of the scintillation pulses emitted by the composite scintillator comprising a PDMS matrix charged with two-component MOF nanocrystals when excited with pulsed laser light at 405 nm are captured.The intensity of the photoluminescence signal decays in a characteristic time of 10 ns, faster than the commercial reference system which shows a decay time of 13 ns.

In practice, it was ascertained that the composite scintillator of the invention achieves the intended aims.

In particular, the composite scintillator of the invention shows improved performance compared to known scintillators for detecting high energy particles and/or electromagnetic radiation both in terms of scintillation efficiency and time response. In fact, the scintillator of the invention combines negligible phenomena of scintillation pulse re-absorption with ultrafast pulse generation mechanisms, making it possible to overcome the above-mentioned limits of the prior art.

Therefore, the scintillator of the invention may be advantageously used in TOF-PET systems, making it possible to achieve a significant improvement in the quality of the reconstructed images even reducing the dose of radiopharmaceutical administered to the patient and a simplification of the equipment required to process the images.

17

Claims

1. Fast and low re-absorption composite scintillator for detecting high-energy particles and/or electromagnetic radiation, comprising a matrix charged with metal-organic nanocrystals containing at least a metal and at least two fluorescent ligands, wherein

- a first ligand absorbs photons at a given spectral range, and a second ligand, by radiative or non-radiative interactions with the first ligand, is able to emit photons falling in a lower energetic spectral range than the first ligand;

- the two ligands have respective absorption and emission bands, defining a large Stokes shift, wherein the energetic difference between the maximum of the emission band of the second ligand and the maximum of the absorption band of the first ligand is equal or higher than the width, in energy unit, of the emission band of the second ligand measured at half of its maximum value;

- the intermolecular distance between the two ligands is equal or shorter than 15 angstrom.

2 . Scintillator as claimed in claim 1, wherein the two ligands are selected from polyacenic molecular systems comprising conjugated aromatic rings, which are fused to form linear or branched structures, in particular selected from polyacenic molecular systems made up of linearly three fused aromatic rings and linearly four fused aromatic rings, respectively.

3. Scintillator as claimed in one of the previous claims, wherein the first ligand is 9,10-bis(4- carboxyphenyl)anthracene, DPA, and the second ligand is 5,12-bis (4-carboxyphenyl)tetracene, DPT.

4 . Scintillator as claimed in one of the previous

18 claims, wherein the metal-organic nanocrystals are metal- organic nanocrystals containing a transition metal, preferably zirconium.

5 . Scintillator as claimed in one of the previous claims, wherein the matrix is made of polymeric or copolymeric material, which is transparent to the visible spectrum, in particular a polymeric or copolymeric material selected from polyacrylic materials, polyvinyl materials, polyamide materials, polyester materials, polyurethane materials, polyether materials, polysiloxane material and mixtures thereof, preferably polydimethylsiloxane, PDMS.

6. Process for detecting high-energy particles and/or electromagnetic radiation comprising the steps of: i) making a fast and low re-absorption composite scintillator for detecting high-energy particles and/or electromagnetic radiation comprising a matrix charged with metal-organic nanocrystals containing at least a metal and at least two fluorescent ligands, wherein a first ligand absorbs photons at a given spectral range, and a second ligand, by radiative or non- radiative interactions with the first ligand, is able to emit photons falling in a lower energetic spectral range than the first ligand; the two ligands have respective absorption and emission bands, defining a large Stokes shift, wherein the energetic difference between the maximum of the emission band of the second ligand and the maximum of the absorption band of the first ligand is equal or higher than the width, in energy unit, of the emission band of the second ligand measured at half of its maximum value;

19 the intermolecular distance between the two ligands is equal or shorter than 15 angstrom, ii) sending high-energy particles and/or electromagnetic radiation to the scintillator of step i), iii) exciting the first ligand by means of the interactions between the first ligand and high-energy particles and/or electromagnetic radiation, obtaining a first excited ligand, iv)transferring non-radiative excitation energy from the first excited ligand to the second ligand, obtaining a second excited ligand, v)emitting a scintillation pulse by radiative relaxation of the second excited ligand.

7. Process as claimed in claim 6, wherein the two ligands are selected from polyacenic molecular systems comprising conjugated aromatic rings, which are fused to form linear or branched structures, in particular selected from polyacenic molecular systems made up of linearly three fused aromatic rings and linearly four fused aromatic rings, respectively.

8.Process as claimed in claim 6 or 7, wherein the first ligand is 9,10-bis(4-carboxyphenyl)anthracene, DPA, and the second ligand is 5,12-bis(4-carboxyphenyl)tetracene, DPT.

9. Process as claimed in one of the claims from 6 to 8, wherein the metal-organic nanocrystals are metal-organic nanocrystals containing a transition metal, preferably zirconium.

10. Process as claimed in one of the claims from 6 to 9, wherein the matrix is made of polymeric or copolymeric material, which is transparent to the visible spectrum, in particular a polymeric or copolymeric material selected from polyacrylic materials, polyvinyl materials, polyamide materials, polyester

20 materials, polyurethane materials, polyether materials, polysiloxane material and mixtures thereof, preferably polydimethylsiloxane, PDMS.

21