Patent Application
20240094416
The present disclosure relates to devices and methods for detecting radiation such as ionizing radiation. In particular, the present disclosure relates to devices and methods for detecting gamma ray radiation. The devices and methods of the present disclosure may be used in a number of applications, including medical imaging applications, such as, for example, positron-emission tomography (PET).
One of the main research directions in the development of devices for the detection of ionizing radiation (radiation detectors) is related to the improvement of their timing resolution. Generally, in this context, the timing resolution is the uncertainty on the exact time of interaction between an incoming radiation (for example gamma rays) and the detector. Radiation detectors with improved timing resolution are faster and can therefore be used to detect shorter radiation pulses (for example below 1 ns) or a higher rate of radiation (for example higher than 100 MHz). The ability to detect radiation with high rates or short pulses is a useful, for example, for high energy physics and radiation protection applications.
In the context of nuclear medicine, particularly in positron emission tomography (PET), Coincidence Time Resolution (CTR) of the order of tens to hundreds of picoseconds (ps) allows retrieval of time-of-flight (TOF) information. This means that the reconstruction of a coincidence event in PET will not only provide the line-of-response, along which the initial positron-electron annihilation event took place, but also an estimation on the location where the event occurred. Therefore, a radiation detector with a better timing resolution improves the effective sensitivity of PET as such a radiation detector allows sufficient information to be available with less coincidence pairs, hence shortening the examination duration and reducing radiation dose absorbed by a patient (see P. Lecoq, Pushing the limits in Time-Of-Flight PET imaging, IEEE Trans. Radiat. Plasma Med. Sci. 1(6): 473-485). A common detector used in PET applications is based on the interaction between an incoming radiation and an assembly of scintillation materials, aka scintillators. Scintillation materials are materials able to absorb the energy of higher energy photons comprised in the incoming radiation and re-emit the energy in a shower of lower energy photons, in or around the visible spectrum, for example between 150 nm and 800 nm. The re-emitted photons travel within the relatively transparent volume of the scintillating material until the re-emitted photons exit at one or more of surfaces of the scintillating material. These photons are then detected upon reaching a detection surface of photon-detectors optically coupled to one or more of the scintillating material surfaces. The surfaces coupled with photon-detectors are called extraction surfaces. The photon-detectors may comprise for example, silicon photomultipliers (SiPM), avalanche photon-diodes (APD) or photo-multiplying tubes (PMT).
The assembly may also comprise light guiding materials, to facilitate light transport and extraction through simple optics. Light guiding materials may be simple optical grease or a lightguide, for example in the form of glass block (called lightguide) with a predetermined refractive index to enhance optical coupling between the scintillators and photon-detection devices, or opaque and reflective surfaces to limit the optical space within which the photons can propagate and direct them towards a particular area of the detection surface of the photon-detection device.
In scintillation materials, photons are produced through a cascade process that gradually transforms radiation energy into visible photons. The photons, hence, have random directions (which add up to an isotropic field) and timing characteristics that are depending on the intrinsic features of the scintillating materials.
For this reason, the assembly can also comprise light-guiding materials, to facilitate light transport from the location of the radiation interaction to the photon-detector and registration through simple optics. The light guiding materials may be optical grease or a glass block with particular refractive index to enhance optical coupling between scintillators and photon-detectors. The light guiding materials can alternatively comprise materials that reflect or absorb the photons in order to limit the optical space within which the photons can propagate and direct them towards a particular area of the detection surface of the photon-detector.
Nevertheless, light absorption and propagation uncertainties and difficulties in the extraction of light from the crystal to the photo-detection device further deteriorate timing resolution. These physical phenomena both limit the number of photons that reach the photon-detection device while also adding uncertainty on the time elapsed between the onset of an event (an incoming radiation interacting with the scintillator) and the detection of the reemitted photons. Timing resolution is a standard deviation that describes the distribution of time difference between each event and the photodetection of the re-emitted photons resulting from this event.
A general goal in this context relates to the choice of the design of the assembly, in particular the type and geometrical arrangement of the scintillation materials and light-guiding materials in the assembly in order to improve the timing characteristics of the photons reaching the photon-detector.
Materials such as Lutetium-Yttrium Orthosilicate (LYSO) and organic scintillators based on polyvinyltoluene (PVT) generally constitute the standard regarding timing resolution, providing a shower of more than 1000 optical photons within the first ns of scintillation.
Scintillators with large volume (for example above 25 mm per mm2 of detector surface) generally improve detector efficiency, while deteriorating timing resolution. It results that the geometry of the scintillator is chosen due to a trade-off between those two quantities. For this reason, different approaches exist to optimize light collection and time resolution for gamma detectors. A known design of devices for detecting radiation comprises monolithic scintillators (with dimensions generally exceeding 10×10×10 mm3), read out by a plurality of photon-detectors. Reconstruction and recombination of the numerous signals from the plurality of photon-detectors allows for the characterization of timing, energy, and location of the radiation interaction within the scintillator (see LAMPROU, E. et al. (2020). Exploring TOF capabilities of PET detector blocks based on large monolithic crystals and analog SiPMs, Physica Medica, 70, 10-18).
Another known design, referred to as a pixelated detector comprises elongated scintillator rods of high aspect ratio that are wrapped in reflecting material and allow light to be extracted on only one or two sides. Those, given the reduction of optically connected volume, provide an easier characterization of the gamma interaction timing. A recent trend on scintillator design favors a combination of different scintillating materials with sub-millimeter size on at least one dimension to benefit from shared interactions between the different materials and from a synergistic combination of their favorable characteristics in timing and detection efficiency. All of these approaches are limited by the intrinsic characteristics of the scintillators as well as the geometrical characteristics of the detector, which directly affect photon propagation, extraction, absorption, and statistical nature of their detection (see KONSTANTINOU, G. et al. (2020), A novel metascintillator approach for ultra-fast timing in Positron Emission Tomography, In 2020 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), pp. 1-4, IEEE).
Another method for improving timing resolution of radiation detectors relates to the use of photonic crystals (PhC). PhCs are structures with dimensions in the order of the wavelength of the scintillation photons (for example tens to hundreds of nanometers—nm) that interact with light in a non-classical manner, similarly to metamaterials. In general, this non-classical interaction corresponds to the bending of light towards desired directions in a way that would not be possible with conventional materials. Difficulty in constructing 3-dimensional PhC has led to the development of PhC slabs, where one of the three dimensions is orders of magnitude smaller than the other two (for example in the present context a PhC slab features dimensions of 3 mm×25 mm×100 nm). PhC slabs are practically two-dimensional coatings, where the lateral dimensions can extend to the level of centimeters, while the height remains within the order of nanometers. Radiation detectors comprising PhC have been successful on improving extraction of scintillation light, see e.g., M. SALOMONI et al., Enhancing Light Extraction of Inorganic Scintillators Using Photonic Crystals, Crystals 2018, 8(2), 78 and U.S. Pat. No. 8,481,948 B2 (FRACH, T et al.) Jul. 9, 2013. For an extended review on this matter, see A. KNAPITSCH et al., Review on photonic crystal coatings for scintillators, International Journal of Modern Physics A (2014), vol. 9 num. 30.
Specific examples of the application of the techniques and methods described above aimed to improve light extraction from scintillators can be found in the patent application EP2919037A1 (LERONDEL et al.) Sep. 16, 2015, and in the patent U.S. Ser. No. 10/620,323B1 (NAGARKAR et al.) Apr. 14, 2020. The former discloses an optimization procedure for the management of photons in scintillating crystals based on the use of a nano-structured material (preferably comprising a conical pattern) with a similar refraction index to that of the scintillating crystal in order to improve luminous performance and time resolution. The latter discloses a device comprising a photonic band gap structure composed by periodic posts or holes in order to achieve the same goal as the former.
In the cited detector, the PhC slabs parallel to the photon-detection surface are used to favor some light extraction modes and collimate the light in certain preferred directions. While this approach does improve timing resolution, the isotropic character of the photons in the scintillation light remains a limiting factor that is not addressed. Similarly, this solution does not affect their travel path and optical absorption. There are cases where timing should be retrieved by a mere handful of optical photons, for which only extraction improvement is not enough.
Moreover, the statistical distribution of light travel time within the scintillator is not addressed, which means that apart from a larger population of available, extracted photons, statistical information on photon-detection is not improved. There is interest in new designs that address all of the aforementioned uncertainties on timing resolution.
In what follows, the term “comprise” is synonym of (means the same as) “include” and “contains”, is inclusive and open, and does not exclude other non-recited elements. Moreover, in the present disclosure, when referring to a numerical value, the terms “about” and “substantially” are synonyms of (mean the same as) a range comprised between 80% and 120%, preferably between 90% and 110%, of the numerical value.
According to a first aspect, the present disclosure relates to a device for detecting gamma radiation comprising an assembly of scintillating and light-guiding materials, producing scintillation light upon incidence of radiation, and at least one photon-detector device comprising a detection surface, optically coupled to the assembly. The assembly may comprise at least one photonic crystal slab configured to direct said scintillation light towards the detection surface along an extraction direction (also called detection direction), being this direction substantially perpendicular to the detection surface. The at least one photonic crystal slab may be arranged substantially perpendicular to the detection surface as well.
According to one or more embodiments, the at least one photonic crystal slab may be configured to direct said scintillation light towards the detection surface along an extraction direction, the extraction direction being substantially perpendicular to the detection surface.
The applicant has demonstrated that, by virtue of the device herein described, the scintillation light produced from interaction of the incoming radiation with the detector is directed towards the extraction surface, thus having to traverse a smaller distance in the scintillator than in devices of the prior art, thereby reducing absorption. In the device according to the present disclosure, scintillation light isotropy may be controlled by the change on the travel path of photons, to create a more uniform energy flux towards extraction surface than otherwise, thus improving, inter alia, transport within the scintillator and the statistical characteristics of detection, while photons may meet the extraction surface of the scintillator at a substantially right angle, further improving extraction probability. All these factors contribute synergistically to a significant improvement of radiation detector timing resolution.
In a preferred embodiment of the invention, the photonic crystal slab of the device comprises a pattern of periodically arranged features with size and period comprised between 10 nm and 1000 nm, and it may also comprise a plurality of these patterns of periodically arranged features. Furthermore, the periodically arranged features might be different from one another, varying on size or dimensions depending on the distance between each of them and the detection surface. According to one or more embodiments, the at least one photonic crystal slab comprising a periodic pattern may be configured to direct scintillation light through a leaky mode.
In another preferred embodiment of the invention, the periodically arranged features correspond to protrusions, depressions or holes arranged in lines. More preferably, the pattern that they form comprises a line defect.
In another preferred embodiment of the invention, the photonic crystal slab comprises a scintillating material adapted to produce scintillation light within itself. Furthermore, the photonic crystal slab might be directly imprinted on a nano manufactured scintillation surface.
In another preferred embodiment of the invention, the photonic crystal slab is homogenous, and its thickness is adapted to direct scintillation light through Fabry-Perot interference.
In another preferred embodiment of the invention, the scintillating material comprises a soft material (i.e., a polymer-based material such as PVT (polyvinyltoluene), PS (polystyrene) or others) such that the photonic crystal slab is deposited, inserted, or fitted directly on its surface. This soft scintillating material, which can or cannot be loaded with other materials (such as dyes, CdSe, MAPbBrs, n-terphenyl (PPP), or others) may be etched with the application of a nanofabricated master mold, and the thickness of the photonic crystal slab deposited directly on its surface may have a thickness between 10 nm and 1000 nm.
In another preferred embodiment of the invention, the photonic crystal slab comprises an asymmetric pattern of periodically arranged features, periodically arranged features of different shape, periodically arranged features in different orientations or a combination of two or more of these characteristics such that it presents a different pattern which changes depending on the distance from the extraction surface.
In another preferred embodiment of the invention, the device comprises two or more photonic crystal slabs arranged substantially parallel and substantially separated from one another, a plurality of photonic crystal slabs arranged in a sequence along the detection direction and/or a plurality of photonic crystal slabs arranged substantially parallel to each other forming at least one stack.
Regarding the materials of which the scintillating material might be composed of, in a preferred embodiment of the invention we might have BGO, LSO, LYSO, GSO, NaI, CsI, BaF2, LaBr3, LuAP, LuAG, GGAG, CdSe, PVT-PPP, MAPbBr3 or a combination thereof.
In another preferred embodiment of the invention, the photonic crystal slab might me composed of Si, SiO2, Si3N4, SiC, GaP, GaAs, Nb2O5, Au, Ag, or a combination thereof.
According to one or more embodiments, the scintillating materials may further comprise organic scintillators, materials with a perovskite structure, or a combination thereof.
According to one or more embodiments, the at least one photonic crystal slab may have a periodic pattern configured to direct scintillation light predominantly towards a first direction.
The first direction may be the extraction direction and may be substantially perpendicular to the detection surface.
According to one or more embodiments, the periodic pattern of the at least one photonic crystal slab may be further configured to direct scintillation light towards a second direction. The second direction may be perpendicular to the first direction.
According to one or more embodiments, the at least one photonic crystal slab may comprise features that are geometrically arranged to change depending on a distance from the detection surface.
Such features may provide different light distribution impinging on the detection surface depending on the depth-of-interaction of the incoming radiation with the detector.
According to one or more embodiments, the plurality of photonic crystal slabs may have different designs, which may for example lead to different effects acting synergistically, for instance better focusing light towards a given direction or point of extraction.
According to one or more embodiments, the assembly may comprise rods, for example elongated rods, which may be separated by a plurality of photonic crystal slabs.
According to a second aspect, the present disclosure also relates to a method for detecting gamma radiation comprising performing the following steps with a device according to any of the embodiments described above:
Embodiments of the method may comprise steps corresponding to the features of any of the embodiments of the device described herein.
Other advantages and features of the disclosure will become apparent on reading the description, illustrated by the following figures which represent:
A detailed description of the disclosure related to embodiments thereof, based on
In the example shown in
Additionally, the device comprises at least one photon-detector 207 comprising a detection surface optically coupled to the scintillator assembly, and at least one photonic crystal slab 202 configured to direct the scintillation light 205 towards the detection surface along an extraction direction 209. The extraction direction 209 is substantially perpendicular to the detection surface.
The at least one photonic crystal slab can comprise a plurality of photonic crystal slabs. In the embodiment shown in
In the device according to the disclosure, an incoming gamma photon interacts with the scintillator at an initial location 203. When the interaction energy is more than the energy keeping core electrons in a stack of some of the atoms of the scintillator, the electron is excited, undergoes transition out of position, is emitted and travels 204 in the material as a recoil electron, losing energy through minor interactions with the scintillating material and production of excitons, until its excess energy is reduced, and it is reabsorbed by the material. Due to the band structure of the scintillator, these interactions lead to the production of photons 205 in the UV and optical spectrums, which will be referred to as scintillation light in the present description. Each PhC slab 202 interacts with the scintillation light 205 with a stochastic possibility of changing the general orientation of a portion 206 of the photons towards desired directions along the two largest surfaces of the PhC slab 202. As the PhC slabs 202 are arranged perpendicularly to the extraction surfaces, a portion of the photons is directed towards the extraction surface of the scintillator 208 which is optically coupled to the photon-detector 207 (extraction direction 209).
This general mechanism will be described, in the present description, as “light being directed”, and can be performed through various physical effects termed propagation modes, some of which will be described in the following paragraphs. In detail, scintillation light of particular wavelength resonates with the PhC slab depending on its dimensions, material, characteristics of the nanofabricated features in the PhC slab (said features will be introduced when describing
Scintillation light can interact with the PhC slab in different ways. If scintillation photon production takes place outside of the PhC slab, these are directed through interaction referred to as quasi-guided modes or “leaky modes”. The leaky modes are anomalous effects that became known as Wood's anomalies, as they could not be explained by ordinary theory of diffraction. When the light radiated through this leaky mode arrives on the PhC slab, part of its energy is coupled in the structure and then propagates along the PhC slab. Then the light that is decoupled interferes with both the transmitted and reflected light, and at a certain resonance wavelength, the de-coupled light interferes destructively with the transmitted light but constructively with the reflected light, resulting in highly efficient resonant reflection for a particular wavelength interval. This phenomenon named Guided Mode Resonance (GMR) occurs only when certain conditions on wave vector, frequency and polarization are met. Thanks to their high degree of tunability in terms of optical properties and the variety of possible fabrication processes and materials developed recently, GMRs have been implemented in extremely diverse applications: refractive index and fluorescence biosensors, solar cells and photon-detectors, optical communication, and signal processing, among others. In simpler terms, leaky modes correspond to coupling of light arriving at the photonic crystal slab from outside.
With regard to the distribution of energy of a “guided mode”, under the light line, within the photonic crystal slab, embedded in-between upper and lower claddings, it is observed that the mode's electromagnetic field takes a maximum inside the high-index layer and decays exponentially away from it in the claddings. In short, this corresponds to light produced within the PhC being directed within and around the PhC slab towards its lateral directions. This corresponds to light being produced within the photonic crystal slab. For this to happen, the PhC slab material should be able to produce photons.
Another manner of interaction is the Fabry-Perot interference between the light launched into it and the light circulating in the resonator. Constructive interference occurs if the two beams are in phase, leading to resonant enhancement of light inside the resonator. If the two beams are out of phase, only a small portion of the launched light is stored inside the resonator. The stored, transmitted, and reflected light is spectrally modified compared to the incident light. The way these modes and interaction mechanisms are relevant to the disclosure will be further explained in the next paragraphs.
In general, scintillation is a property common in various crystals and organic compounds. A non-exhaustive list of those includes BGO, LSO, LYSO, GSO and other oxides, NaI, CsI and other Iodides, BaF2 and other fluorides, LaBr3 and other bromides, LuAP, LuAG and/or GGAG and other garnets, quantum dots such as CdSe and other selenides organic scintillators, organic glass scintillators and materials with perovskite structure. Organic scintillators usually composite materials combining a plastic substrate such as PVT (polyvinyltoluene) or PS (polystyrene) and a material that provides organic fluorescent emitters, called fluor agents, such as polyphenyl hydrocarbons, oxazole and oxadiazole aryls, especially, n-terphenyl (PPP), 2,5-diphenyloxazole (PPO), 1,4-di-(5-phenyl-2-oxazolyl)-benzene (POPOP), 2-phenyl-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD), and 2-(4′-tert-butylphenyl)-5-(4″-biphenylyl)-1,3,4-oxadiazole (B-PBD). Such components are collectively called organic scintillators in the art. Such scintillators can be used in the proposed disclosure, alone or in combination. This means that in
PhC slabs cam comprise high refractive index materials such as the aforementioned scintillators but can also comprise various semiconductors or metals. A non-exhaustive list of such includes Si, SiO, SiC, GaP, GaAs, Nb2O5, Au, Ag or a combination thereof.
Due to the complex geometry, it is not easy to fabricate 3-dimensional photonic crystals for infrared or optical wavelength ranges. That is why attention is turned to structures with periodicity in x and y direction with the material's property remains invariable along the z direction, this class of design is named PhC slab or planar PhC. Along the thickness direction, the energy of light is contained inside the slab by index guiding, thanks to the high average refractive index of component materials used.
In order to obtain Fabry-Perot interference in case of a homogeneous slab, the thickness of the slab is tuned to make the light with specific incidence angles stay in phase. In case of a photonic crystal slab, a similar tuning can also be useful based on the effective refractive index of the mixture which is calculated by high frequency homogenization.
According to other embodiments, the PhC slab can be constructed following a concept of metascintillators. In this concept, the PhC slab includes scintillating materials so that it can play a dual role of both producing scintillation light 205 and guiding the emitted photons towards the photon-detector setup through guided mode, as mentioned earlier. In particular, quantum dots such as CdSe or lead halide perovskite nanocomposite scintillators can be nano-structured and can have a reasonably high refractive index suitable for fabricating PhC slabs.
Moreover, further enhancement of lateral energy flux can be achieved through leaky mode interaction, with a wave source (scintillation photon production onset) located out-of-plane. This occurs when the impinging wave (scintillation light) has the same characteristics (frequency & transverse wave vector) as one of the leaky modes of the PhC slab. With the preferred embodiment of a fast scintillator placed inside the PhC slab, the optimal confinement and directional guiding of wave from the in-plane wave source is realized by the guided modes of PhC slab.
The PhC slabs may have different designs depending on the desired effect and wavelength.
The dimensions of the periodic pattern (periodicity) and the dimensions of the features are of the order of magnitude of the wavelength of the light that is meant to be guided. In particular, such features can have dimensions between about 10 nm and about 1000 nm. Consequently, in the case of PhC slabs for use in the UV, visible, and near IR domain, the features are fabricated in the PhC slab using processes involving some form of nano-fabrication.
When a periodic pattern is to be designed nano-structuration is necessary; such process usually starts with the fabrication of a master mold that is then used to either etch, nano-imprint, structure, hot-depose the materials that form the PhC slab. This particular embodiment corresponds to the energy flux alteration of
According to one or more embodiments, PhC slabs can have a design that is suitable not only for enhancing the lateral flux of optical energy, but also enhancing the flux predominantly in one specific lateral direction over the second lateral direction or the axial direction. so that the population of photons to be detected is further enhanced and the statistical distribution of the photons is improved.
The enhancement of flux in one specific lateral direction can be achieved through various ways, such as, for instance, an asymmetry in the periodic pattern, the orientation or the shape of the features. In that case, the enhancement of flux in one specific lateral direction can also be achieved by placing PhC slabs in a periodic sequence along the axial direction.
The directionality of energy flux inside the PhC slab can also be achieved via the self-collimation effect whose principal mechanism is shown with in the example depicted in
The device presented in
In the case of a quasi-monolithic design, PhC slabs are arranged at regular intervals of few millimeters apart, confining the optical space over one dimension.
In the case of a monolithic design, the scintillator has only its external surfaces covered with PhC slabs, so that photons produced through gamma interactions are predominantly directed towards the photon-detection setups, instead of being reflected back, on scintillator edge interactions.
While the figure describes a symmetric structure of stacks of different PhC slabs, with the same order and number of layers on each side of the scintillating material, other embodiments might have a different configuration, in particular configurations with different number of PhC slab and technical effect on each of the stacks.
This arrangement is possible for different photon-detector assemblies, such as photon-detector surfaces coupled to one, two or four surfaces of the scintillator assembly. The main feature of interest is that the PhC slab, as described before, is substantially perpendicular to the detection surface. The reason for this is that the purpose of the slab is to enhance optical energy flux towards the lateral directions.
Another preferred embodiment of the proposed disclosure corresponds to PhC slabs configured to interact with scintillation light in such manner as to provide an altered signature for different interaction depth. As the average distance travelled by the totality of scintillation photons is analogous to that between the event onset 203 and photon-detector setup 207, even approximate knowledge of this information can greatly improve the statistical information available per scintillation event. In this sense, depth-of-interaction asymmetries in the photonic slab design that provide different spatial spread for guided photons, depending on the actual depth can further improve the timing resolution of perpendicular PhC slab loaded radiation detectors.
In conclusion, the proposed disclosure includes a variety of embodiments of different characteristics but with the same mission of improving the time resolution of the radiation detector either through improving the statistical characteristics of light incident to the photon-detector setup, include ultrafast scintillators to assist fast photon production or a combination of those features.
Light travelling along the lateral dimensions of the PhC slab has less likelihood to reflect at the conventional edges of the scintillator, meaning that its travel path is shortened and becomes more easily retraceable. In other words, the statistical spread of photon arrival times at the photon-collection device is smaller. As the photon direction towards the detection surface has a practically right angle to the exit surface of the scintillator, such photonic slabs not only shorten the travel path but also enhance extraction efficiency. On top of that, PhC slabs can function as barriers, confining photons within smaller optical volumes, further improving the timing characteristics of scintillation-based detectors with perpendicular PhC slabs. This can be synergistically exploited in the case of metascintillators, as fast light emitted at lower wavelengths does not leak into different materials where it can be easily reabsorbed. Perpendicular PhC slabs are thus addressing practically every aspect of timing resolution deterioration in radiation detectors, while providing useful building blocks to facilitate photon read-out.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21153601.6 | Jan 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/051655 | 1/25/2022 | WO |
