Doped cesium barium halide scintillator films
09720105 · 2017-08-01
Assignee
Inventors
Cpc classification
International classification
Abstract
Strontium halide scintillators, calcium halide scintillators, cerium halide scintillators, cesium barium halide scintillators, and related devices and methods are provided.
Claims
1. A scintillator comprising, a ternary scintillator compound comprising a dopant and cesium barium halide, wherein the scintillator is in the form of a microcolumnar scintillator film or a noncolumnar polycrystalline, and wherein the dopant alters the scintillator's afterglow such that the scintillator comprises less than 0.05% afterglow at 2 ms following X-ray excitation.
2. The scintillator of claim 1, wherein the dopant is Eu, Pr, Ce, Tl, Tb, Yb, or any combination thereof.
3. The scintillator of claim 1, wherein the scintillator has a thickness of less than about 20 microns, less than about 500 microns, or less than about 1 cm.
4. The scintillator of claim 1, wherein the scintillator has a thickness greater than about 1 cm, greater than about 3 cm, or greater than about 5 cm.
5. The scintillator of claim 1, wherein the scintillator is in the form of an amorphous microcolumnar scintillator film or a crystalline microcolumnar scintillator film.
6. The scintillator of claim 1, wherein the scintillator further comprises a coating layer comprising doped or undoped cesium iodide.
7. A hot wall evaporation method of making a ternary scintillator, the method comprising: providing an evaporation chamber having a first end portion with a substrate positioned in a holder, and a second end portion with a first source boat separate from a second source boat, and one or more chamber walls at least partially disposed between the first and second end portions; positioning a dopant salt in the first source boat and a source salt in the second source boat; and depositing a scintillator film comprising a source and a dopant on a surface of the positioned substrate; wherein the film is deposited by a process comprising applying heat to the evaporation chamber so as to vaporize the dopant and the source for film deposition while maintaining a temperature relationship of T.sub.wall>T.sub.source>T.sub.substrate for at least a portion of the deposition process.
8. The method of claim 7, wherein T.sub.source is between about 300° C. and about 750° C.
9. The method of claim 7, wherein the scintillator film comprises doped cesium barium halide.
10. The method of claim 9, wherein the dopant comprises Eu, Pr, Ce, Tl, Tb, Yb, or any combination thereof.
11. The method of claim 7, wherein the dopant and the source are included in melt-growth polycrystals positioned in one source boat at the second end portion.
12. The method of claim 7, wherein the scintillator film comprises doped strontium halide.
13. The method of claim 12, further comprising rotating at least one of the first or second source boats comprising source material around a system axis as well as around an axis that is normal to the surface of the source material.
14. The method of claim 13, wherein the at least one of the first or second source boats rotates at a rate of about 1 to 2 rotations per second.
15. A deposition method of making a doped cesium barium halide scintillator, the method comprising: providing a deposition apparatus comprising a vacuum chamber having a substrate at a first end, and a first source boat comprising cesium iodide and a second source boat comprising barium iodide at a second end of the chamber; and depositing a scintillator film comprising doped cesium barium halide on the substrate.
16. The method of claim 15, wherein the apparatus comprises a third source boat comprising europium iodide.
17. The method of claim 16, wherein the apparatus further comprises a fourth source boat comprising thallium iodide.
18. The method of claim 15, wherein the dopant comprises Eu, Pr, Ce, Tl, Tb, Yb or any combination thereof.
19. The method of claim 15, wherein the dopant material and the source material are included in melt growth polycrystals positioned in one source boat at the second end portion.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
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DETAILED DESCRIPTION OF THE INVENTION
(16) The present invention relates to scintillator fabrication methods and scintillators. More specifically, the present invention provides a variety of scintillators, including strontium halide scintillators, calcium halide scintillators, cerium halide scintillators and cesium barium halide scintillators. Related devices and methods of using the scintillators described herein are also provided.
(17) The methods and structures described herein can be used to make a variety of scintillator compositions. For example, the present invention provides, but is not limited to, strontium halide scintillators, calcium halide scintillators, cerium halide scintillators, or cesium barium halide scintillators. Halides can include fluoride, chloride, bromide and/or iodide. The scintillators can include strontium iodide-based (e.g., SrI.sub.2) scintillator compositions. In some embodiments, the scintillators include cerium bromide (e.g., CeBr.sub.3) scintillator compositions. Cesium barium iodide-based (e.g., CsBa.sub.2I.sub.5) scintillator compositions are also provided. Calcium iodide scintillators (e.g., CaI.sub.2, CaI.sub.2:Eu, and CaI.sub.2:Tl) are further provided herein. Other halide scintillators, include, but are not limited to, doped SrBr.sub.2, CaBr.sub.3, and CeCl.sub.3.
(18) The scintillator compositions of the present invention can optionally include a “dopant.” In certain embodiments, the scintillators described herein will typically include a dopant. Dopants can affect certain properties, such as physical properties (e.g., brittleness, etc.) as well as certain scintillation properties (e.g., afterglow, etc.) of the scintillator composition. The dopant can include, for example, europium (Eu), praseodymium (Pr), cerium (Ce), thallium (Tl), terbium (Tb), ytterbium (Yb), and mixtures of any of the dopants. The amount of dopant present will depend on various factors, such as the application for which the scintillator composition is being used; the desired scintillation properties (e.g., emission properties, timing resolution, etc.); and the type of detection device into which the scintillator is being incorporated. For example, the dopant is typically employed at a level in the range of about 0.1% to about 20%, by molar weight. In some embodiments, the amount of dopant is in the range of about 0.1% to about 100%, or about 0.1% to about 5.0%, or about 5.0% to about 20%, by molar weight. In one embodiment, the scintillators can include SrI.sub.2:Eu scintillator compositions. In another embodiment, the scintillators can include CsBa.sub.2I.sub.5:Eu scintillator compositions. As will be appreciated by one of ordinary skill in the art, cesium barium iodide is also termed barium cesium iodide (or europium-doped barium cesium iodide, or Ba.sub.2CsI.sub.5:Eu). Doped cesium barium halide scintillator films of the present invention can also include dual-doped scintillator films, e.g., Ba.sub.2CsI.sub.5:Eu+Tl. In some embodiments, the scintillators can include CaI.sub.2, CaI.sub.2:Eu, or CaI.sub.2:T1 scintillator compositions.
(19) The scintillators described herein (e.g., SrI.sub.2:Eu, CaI.sub.2:Eu, CeBr.sub.3, or CsBa.sub.2I.sub.5:Eu scintillators) can be produced in a variety of different forms. As shown in
(20) The present invention further includes doped cesium barium halide scintillators (e.g., CsBa.sub.2I:Eu) in various forms, including an amorphous microcolumnar structure form or a crystalline microcolumnar structure form. The doped ternary scintillator, CsBa.sub.2I:Eu, can exist as a mono-phase that is not a solid solution of Cs, Ba, I, but rather a compound with chemical bonding amongst Cs, Ba, and I, as well as Eu. The ternary compound structure can be unstable under certain conditions. Accordingly, to produce an optimally functioning scintillator, a scintillator in the mono-phase can be produced in which, e.g., Cs, Ba, I and Eu can be kinetically trapped in an arrangement that generates excellent scintillation properties. For example, Ba.sub.2CsI.sub.5 is formed in-part from the reaction of BaI.sub.2 and CsI binary compounds. However, while not being limited to any particular theory, when energy is applied over a long period of time on such ternary compounds, the thermodynamically stable product will result, which are BaI.sub.2 and CsI, not Ba.sub.2CsI.sub.5. Binary compounds can exhibit higher lattice stability owing, e.g., to their simplicity in bond formation. Hence, ternary compounds can be less stable and difficult to form if the thermodynamics prefer more stable binary compounds. For Ba.sub.2CsI.sub.5, the melting points of CsI and BaI.sub.2 are 630° C. and 740° C. However, when a mono-phase of Ba.sub.2CsI.sub.5 is synthesized from the reaction of the two binary phases, the melting point of the ternary phase is ˜600° C., which is lower than its constituent binary phases. With a lower melting point than CsI, Ba.sub.2CsI.sub.5 would be expected to have a higher vapor pressure than CsI. However, via indirect experimentation it was discovered that the vapor pressure for Ba.sub.2CsI.sub.5 was lower, not higher, than CsI. Typically, to improve vapor pressure of a material, the temperature can be increased. However, increased temperature for Ba.sub.2CsI.sub.5, e.g., beyond its melting point can cause phase separation into BaI.sub.2 and CsI. Owing in part to this discovery, the methods (e.g., hot wall evaporation, thermal evaporation or physical vapor deposition) of making doped cesium barium halide scintillator films described herein account for this lower vapor pressure and allow for fabrication of the doped cesium barium halide scintillator films. The methods can also include other ways to address the properties of doped cesium barium halide scintillator films. For example, a finely powdered charge of Ba.sub.2CsI.sub.5:Eu can be sublimated from a customized boat that facilitates a large surface area and will provide enough vaporization compared to single planar surface of liquid-melt. In addition, as described further herein, the present invention further includes making doped cesium barium halide polycrystals (e.g., Ba.sub.2CsI.sub.5:Eu polycrystals) that have a monophase prior to vaporization, which can be used to generate monophase films (e.g., Ba.sub.2CsI.sub.5:Eu films).
(21) The scintillators described herein can be produced in a variety of thicknesses and spatial areas. Thicknesses of the films can be designed for certain imaging applications. For example, thin films (e.g., less than 20 μm) can be used to detect alpha particles, while limiting interference from detection of gamma rays. Thickness of the films can also be tailored to detect, for example, gamma rays, while also allowing for sufficient light transmission. Spatial resolution for, e.g., microcolumnar scintillators, can also be tailored by thickness of the films. In some embodiments, scintillators can be produced as a thin film, e.g., films having a thickness ranging from about 10 μm to about 1 cm. In certain embodiments, the scintillators can be thick films having a thickness of 1 cm or greater, and can be referred to as “slabs.” In some embodiments, scintillators can have thickness of less than about 20 microns, less than about 500 microns, or less than about 1 cm. In certain embodiments, the scintillators can have a thickness greater than about 1 cm, greater than about 3 cm, or greater than about 5 cm. The scintillators described herein can be freestanding films, e.g., where the deposited film can be removed from the substrate after deposition. The scintillators can be deposited over small to wide areas on the order, for example, of mm.sup.2 or cm.sup.2 (e.g., up to 50×50 cm.sup.2).
(22) Scintillator compositions and assemblies of the present invention can further include one or more reflective coatings, e.g., formed on a substrate surface or otherwise coupled with a scintillator. In one embodiment, a reflective coating can be formed on a substrate surface prior to deposition of the scintillator, such as in the case of opaque substrates. One embodiment includes the use of substrates such as alumina that are themselves white in color and act as an excellent reflector. Another embodiment includes the use of substrates that are themselves photodetectors (e.g., SSPMs, amorphous silicon arrays, CCDs, and CMOS devices). For graphite-like substrates, reflective metal coatings can be formed. In addition to being highly reflective, such coatings may be required to withstand high process temperatures, maintain adhesion to the substrate during and after deposition, and/or be chemically inert with the scintillator or suitable for coating with a chemically inert material, such as an organic polymer or resin (e.g., Parylene C). For transparent substrates a reflective coating can be applied atop the scintillator film directly or after deposition of one or more other coatings, such as a Parylene coating. As noted above, various coating technologies can be utilized for forming coatings with the required optical and/or protective properties.
(23) In some embodiments, a protective coating can be deposited or placed over the scintillator film to protect it, e.g., from air and/or moisture. For example, a Ba.sub.2CsI.sub.5:Eu scintillator film is hygroscopic and can degenerate when exposed to moisture. In one embodiment, a parylene film can be deposited on the scintillator. In an alternative embodiment, another less hygroscopic scintillator can be coated on the Ba.sub.2CsI.sub.5:Eu scintillator film. For instance, a doped or undoped cesium halide scintillator film (e.g., that is tens of microns in thickness) can be deposited near the end of the deposition cycle of the Ba.sub.2CsI.sub.5:Eu scintillator film. A source boat including cesium halide can, e.g., be evaporated during the deposition cycle to deposit the cesium halide over the Ba.sub.2CsI.sub.5:Eu scintillator film. In some embodiments, a parylene film can also be added on top of the cesium halide film. Experimental evidence has further shown, e.g., that a top layer of CsI of a few microns is transparent to the light emitted in the scintillator and also continues to grow along the same columnar geometry as the scintillator.
(24) The present invention includes methods and structures for fabricating the scintillators described herein. In some embodiments, the scintillators described herein can be grown using a vapor deposition technique, co-evaporation in a hot wall evaporation (HWE) apparatus. In certain embodiments, the hot wall evaporation techniques can include hot wall epitaxial growth of the scintillators described herein. Hot wall evaporation techniques, as described herein, include a vacuum deposition technique where scintillator film is efficiently deposited on a surface of a substrate. In the simplest form the HWE apparatus includes a chamber or cylinder positioned in a vacuum, heated, with an evaporation source “boat” or reservoir at one end (typically the bottom in an upright positioned chamber) and a temperature controlled substrate at the other (typically the top in an upright positioned chamber). In certain embodiments, the disclosed methods make use of salts and vapor deposits them, e.g., simultaneously, on a suitable substrate using two independent sources. Under different deposition conditions, the material grows in the desired form. In one embodiment, two source boats are used to accomplish co-evaporation of a salt and a dopant salt for deposition of a scintillator film on a substrate surface. In some embodiments, the HWE techniques can include three or more source boats. Each boat can contain a particular source material (a.k.a. charge) of interest to produce the scintillators described herein. The source boats can be positioned separate (e.g., laterally spaced) from each other.
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(28) For HWE, the heated cylinder wall serves to enclose, deflect and effectively direct the vapor from the source to the substrate where molecules are deposited with a shallow impinging angle. With the substrate being the coolest part in the system (e.g., compared to the cylinder wall and source material), molecules adhere solely or primarily to the substrate and do not substantially accumulate on the hot walls, making efficient use of the source material. To ensure thermodynamic equilibrium the relationship between the substrate temperature and that of the source and the heated wall should be: T.sub.wall>T.sub.source>T.sub.substrate. After deposition, the deposited scintillator material can be annealed in a variety of atmospheric conditions. Annealing can be used to enhance the performance of the scintillators described herein.
(29) An advantage of HWE is that it preserves the composition of the grown film with reference to the evaporants even though they have relatively large differences in vapor pressures and sticking coefficients. This is due to the fact that HWE takes place under conditions of thermodynamic equilibrium, which allows the high vapor pressures of various compounds to be maintained. As a result, the dissociation of various constituents does not present a problem for film growth. As a matter of fact, the interaction of components with each other on the substrate surface can lead, under favorable growth conditions, to the formation and growth of the well-structured films.
(30) Deposition of microcolumnar films involves methods where the evaporated material be incident on the substrate at a grazing angle. HWE creates this condition through minimized mean free path for the vaporized molecules due to the large density of evaporated material, and through efficient reflection of molecules from walls, which are maintained at the highest temperature in the setup. Thus, the thermodynamic equilibrium and atmosphere conducive to growth created by the HWE process allows deposition of stoichiometrically balanced films with well-separated columnar morphology.
(31) Another aspect of HWE is its high (close to 100%) deposition efficiency, as the substrate is the coldest part of the evaporation environment. As a result, vapors that impinge on HWE system parts, including the hot walls, are deflected and mostly condense only on the relatively cool substrate. Consequently, material loss is at a minimum, enhancing the deposition efficiency to 95% or more (and greatly simplifying apparatus cleaning and maintenance).
(32) A feature of HWE for thick film deposition is that the growth rate is an order of magnitude higher than that of conventional physical vapor deposition (PVD) systems. For a single evaporant material system the deposition rate is proportional to the impingement rate (Ø) of atoms on the substrate at constant temperature, and is governed by the equation (1):
Ø=n(kT/2Πm)½ (1)
where n is the number of evaporant molecules per unit volume, k is the Boltzmann constant, T is the source temperature, and m is the mass of the molecule. For hot wall evaporation, a two-evaporant system, this equation still holds, since the vapor phases of the constituent compounds are in equilibrium with the source materials. Therefore, the two evaporant deposition process is basically very similar to that for a single evaporant material. As the source temperature T is very high and the substrate is the coldest part in the evaporator, the impingement rate of molecules and, hence, the film growth rate is an order of magnitude higher than with conventional systems. The growth rate is related to the impingement rate by the following equation (2):
Growth rate={Ø*Average thickness of a monolayer}/{Surface density of the scintillator composition} (2)
The process of material growth can consist of a series of events that begin with the physical adsorption of a fraction of the incident molecules on the substrate or by forming a stable nucleus by interaction with the other adsorbed molecules. This process of nucleation and growth is typical for the formation of a film of one material on a substrate of a different material. In HWE, no nucleation takes place, but growth occurs by direct adsorption of the molecules on low energy sites, such as kinks on an atomic ledge on the substrate. Under these conditions, even when the growth rate is very high, a monolayer-by-monolayer deposition is obtained resulting in excellent stoichiometry of the films.
(33) In some embodiments, there can be additional modifications to the methods and apparatus for making strontium halide scintillators (e.g., SrI.sub.2:Eu). For example, SrI.sub.2:Eu is a highly corrosive material, so a cold trap is used to protect vacuum pumps. Also, chamber cleaning after deposition is important. Substrate cooling is necessary, since wall temperatures are high. A planetary system can be used which rotates at least one substrate around a system axis as well as around an axis that is normal to the surface of the substrate. A rate of about 1 to 2 rotations per second would be used. Deposition parameters (e.g., temperatures) are adjusted accordingly to achieve a high growth rate of about 10 to about 25 microns per minute, with good thickness uniformity (say, 5% to 10%). For example, gamma detectors use thick sensors for the sake of detection efficiency, so high growth rates are desirable.
(34) In certain embodiments, temperature control for making strontium halide can occurs in three stages: (1) Ramp-up: From ambient room temperature to a range of about 500° C. to about 700° C., about 3 hours, at a slow rate in order to allow and achieve the necessary dehydration of source materials before beginning deposition. Otherwise, moisture in the source materials can vaporize rapidly, causing undesirable sputtering of source materials; (2) Deposition: Temperatures are held at the desired level; e.g., within the range of about 500° C. to about 700° C. Deposition proceeds for a period of time that depends upon the desired scintillator thickness; (3) Ramp-down: The temperature is slowly decreased to ambient room temperature, from about 5 to 10 hours, in order reduce stress on the deposited film. Vacuum is achieved and held between about 10^-4 to about 10^-7 Torr. Source materials are heated to temperatures that are at least (e.g., by 5% to 10%) their melting points; e.g., for SrI.sub.2:Eu, the SrI.sub.2 source would be heated to about 500° C., and the EuI.sub.2 source would be heated to about 875° C. The evaporation rates for SrI.sub.2 and EuI.sub.2 to produce SrI.sub.2:Eu with 2% to 5% Eu doping, for example, can be about 25 microns/minute for SrI.sub.2 and about 0.5 to about 1.25 microns per minute for EuI.sub.2.
(35) Similar processes can be used to make calcium halide scintillators. Material vaporization temperatures for the calcium halide source and dopant materials can be slightly above (e.g., by 5% to 10%) respective melting points. For CaI.sub.2:Eu and CaI.sub.2:Tl, the melting point for CaI.sub.2 is about 783° C., EuI.sub.2 about 875° C., and TlI about 300° C. For Ba.sub.2CsI.sub.5:Eu, the process is the same, except a vaporization temperature between about 500° C. and about 600° C. can be used for the source material, polycrystalline Ba.sub.2CsI.sub.5:Eu, which can be prefabricated by melt growth from an admixture of BaI.sub.2, CsI and EuI.sub.2.
(36) The present invention further includes other deposition methods for making doped cesium barium halide scintillator films. For example, a variety of physical vapor deposition apparatus and methods (e.g., sputtering or electron beam vaporization) can be used. Thermal deposition techniques can also be used. In one embodiment, the methods can include providing a deposition apparatus comprising a vacuum chamber having a substrate at a first end and scintillator source boat at a second end of the chamber; positioning a cesium barium halide source material and a dopant material at the second end; and depositing a scintillator film comprising doped cesium barium halide on the substrate. The source material and dopant material can be provided in a variety of ways. For example, a single crystal and/or polycrystals of doped cesium barium halide, e.g., Ba.sub.2CsI.sub.5:Eu, can be synthesized using vertical Bridgeman growth processes. The crystal(s) can then be evaporated and deposited on the substrate to form the doped cesium barium halide scintillator film. Alternatively, the source material (e.g., CsI and BaI.sub.2) can be co-evaporated in separate boats along with the dopant material (e.g., EuI.sub.2) and subsequently made to react on a heated substrate to form the monophase of CsBa.sub.2I.sub.5:Eu. Temperatures for initiating the reaction can range, e.g., from about room temperature (25° C.) to about 450° C.
(37) As set forth above, scintillator compositions of the present invention may find use in a wide variety of applications. In one embodiment, for example, the invention is directed to a method for detecting energetic (e.g., ionizing) radiation (e.g., gamma-rays, X-rays, neutron emissions, alpha particles, beta particles and the like) with high energy resolution using a detector based on a scintillator described herein. In certain embodiments, the microcolumnar form of scintillators described herein can be used for high spatial resolution imaging.
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(39) A data analysis, or computer, system thereof can include, for example, a module or system to process information (e.g., radiation detection information) from the detector/photodetectors in an invention assembly and can include, for example, a wide variety of proprietary or commercially available computers, electronics, or systems having one or more processing structures, a personal computer, mainframe, or the like, with such systems often comprising data processing hardware and/or software configured to implement any one (or combination of) the method steps described herein. Any software will typically comprise machine readable code of programming instructions embodied in tangible media such as a memory, a digital or optical recording medium, optical, electrical, or wireless telemetry signals, or the like, and one or more of these structures may also be used to transmit data and information between components of the system in any of a wide variety of distributed or centralized signal processing architectures.
(40) The detector assembly typically includes material formed from the scintillator compositions described herein (e.g., SrI.sub.2:Eu, CaI.sub.2:Eu, CeBr.sub.3, or CsBa.sub.2I.sub.5:Eu scintillators). The detector further can include, for example, a light detection assembly including one or more photodetectors. Non-limiting examples of photodetectors include photomultiplier tubes (PMT), photodiodes, PIN detectors, charge coupled device (CCD) sensors, image intensifiers, avalanche detectors and the like. Choice of a particular photodetector will depend in part on the type of radiation detector being fabricated and on its intended use of the device. In certain embodiments, the photodetector may be position-sensitive. Detectors can further include imaging devices that can acquire images at high frame rates, such as frame rates that are faster than about 30 frames per second, about 100 frames per second, or about 1000 frames per second.
(41) The detector assemblies themselves, which can include the scintillator and the photodetector assembly, can be connected to a variety of tools and devices, as mentioned previously. Non-limiting examples include nuclear weapons monitoring and detection devices, well-logging tools, and imaging devices, such as nuclear medicine devices (e.g., PET). Various technologies for operably coupling or integrating a radiation detector assembly containing a scintillator to a detection device can be utilized in the present invention, including various known techniques. In certain embodiments, the radiation detector comprises a scintillator described herein formed on a substrate that is optically coupled to the photodetector. Similarly, scintillator screens including a scintillator described herein can be included in a radiation detector such that the screen is optically coupled to the photodetector.
(42) The detectors may also be connected to a visualization interface, imaging equipment, or digital imaging equipment (e.g., pixilated flat panel devices). In some embodiments, the scintillator may serve as a component of a screen scintillator. Energetic radiation, e.g., X-rays, gamma-rays, neutron, originating from a source, would interact with the scintillator and be converted into light photons, which are visualized in the developed film. The film can be replaced by amorphous silicon position-sensitive photodetectors or other position-sensitive detectors, such as avalanche diodes and the like. In some embodiments, neutrons can be indirectly detected by coupling (e.g., incorporating into or contacting) an absorbing converter material, such as but not limited to lithium, boron or gadolinium, into/with a scintillator described herein, and then detecting emissions (e.g., X-rays and/or alpha particles) produced by interactions between the neutrons and the absorbing converter material.
(43) The methods of the present invention further include methods of performing radiation detection. The methods of performing radiation detection can include providing a detection device comprising a scintillator composition including a scintillator described herein; and a photodetector assembly operably (e.g., optically) coupled to the scintillator composition; and positioning the device such that a radiation source is within a field of view of the scintillator composition so as to detect emissions from the source. Emissions from the source can include x-rays, gamma-rays, neutrons, alpha particles, beta particles, or a combination thereof. In certain embodiments, a material (e.g., a patient, plant, animal, object, liquid, or gas) can be positioned between the radiation source and the scintillator composition. In some embodiments, the radiation source includes a material (e.g., a patient, plant, animal, object, liquid, or gas). In another embodiment, a material of interest (e.g., a patient, plant, animal, object, liquid, or gas) may scatter energetic radiation to the scintillator. The methods of radiation detection may also include X-ray and gamma ray astronomy and cosmic ray detection (e.g., in salt mines). In certain embodiments, a material to be analyzed can be positioned between the radiation source and the scintillator. In some embodiments, the radiation source includes a patient. In some embodiments, the detector can be positioned such that the radiation source is in the field of view of the scintillator. Alternatively, the radiation source can be positioned in the field of view of the scintillator contained in the detector. Also, both the radiation source and the detector can be moved at the same time such that the radiation source is in the field of view of the scintillator.
(44) Imaging devices, including medical imaging equipment, such as PET and SPECT (single-photon emission computed tomography) devices, and the like, represent other potential applications for the invention scintillator compositions and radiation detectors. Furthermore, geological exploration devices, such as well-logging devices, were mentioned previously and represent an important application for these radiation detectors. The assembly containing the scintillator usually includes, for example, an optical window at one end of the enclosure/casing. The window permits radiation-induced scintillation light to pass out of the scintillator assembly for measurement by the photon detection assembly or light-sensing device (e.g., photomultiplier tube, etc.), which is coupled to the scintillator assembly. The light-sensing device converts the light photons emitted from the scintillator into electrical pulses that may be shaped and digitized, for example, by the associated electronics. By this general process, gamma rays can be detected, which in turn provides an analysis of geological formations, such as rock strata surrounding the drilling bore holes.
(45) In applications of a scintillator composition, including those set forth above (e.g., nuclear weapons monitoring and detection, imaging, and well-logging and PET technologies), certain characteristics of the scintillator are desirable, including its light output (higher is can be preferred), rise time (faster can be preferred) and decay time (shorter can be preferred), timing shape (e.g., fixed or varying, depending upon dopant concentration used to analyze scintillation events), energy resolution (finer/lower % can be preferred), spatial resolution (finer, e.g., higher, can be preferred), and suitable physical properties. The present invention is expected to provide scintillator materials which can provide the desired high light output and initial photon intensity characteristics for demanding applications of the technologies. Furthermore, the scintillator materials are also expected to be produced efficiently and economically, and also expected to be employed in a variety of other devices which require radiation/signal detection (e.g., gamma-ray, X-ray, neutron emissions, and the like).
(46) The following examples are provided to illustrate but not limit the invention.
EXAMPLES
Example 1
Fabrication of SrI2:Eu (Europium-Doped Strontium Iodide) Scintillator Films
(47) This example describes fabrication of polycrystalline microcolumnar scintillator films of europium-doped strontium iodide (SrI.sub.2:Eu). A wide range of uniform thickness SrI.sub.2:Eu films can be made ranging from less than 1 mm to greater than 1 cm. Films having diameters of 7 cm or more can also be fabricated. The hot wall evaporation methods described herein can be used to make the SrI.sub.2:Eu films, and since SrI.sub.2:Eu is highly deliquescent the apparatuses and methods can provide encapsulation techniques for protecting the materials. Scintillation properties can be optimized for a variety of applications.
(48) A variety of hot wall evaporation deposition conditions can be used.
(49) Gamma ray spectra were compared between a 1.2 mm thin film of SrI.sub.2:Eu and a single crystal of LaBr.sub.3:Ce. The shaping time for LaBr.sub.3:Ce was 0.25 microseconds, and the shaping time for SrI.sub.2:Eu was 4 microseconds. The gamma source was Co-57 (122 keV). The gamma light yield for the LaBr.sub.3:Ce single crystal was 52,000 ph/MeV and 59,500 ph/MeV for the 1.2 mm thin film of SrI.sub.2:Eu.
Example 2
Fabrication of CeBr3 Scintillator Films
(50) This example describes growth of CeBr.sub.3 films using hot wall evaporation, the method that can produce relatively inexpensive large area, thick films in desired shapes or sizes. The CeBr.sub.3 material could be grown in polycrystalline form suitable for spectroscopic applications, or even in the microcolumnar form desirable for high spatial resolution gamma ray imaging. Films produced using this approach possess all of the excellent properties of their melt-grown crystal counterparts.
Example 3
Pre-synthesis of Ba2CsI5:Eu Polycrystals for Fabrication of CsBa2I5:Eu Scintillator Films
(51) Vertical Bridgman Process:
(52) This example describes pre-synthesis of polycrystals of europium-doped, ternary inorganic compound Ba.sub.2CsI.sub.5. Pre-synthesis Ba.sub.2CsI.sub.5:Eu polycrystals used a simplified vertical Bridgman's method. The polycrystals were grown from CsI, BaI.sub.2 and EuI.sub.2 powder beads. The quartz ampoules containing this admixture were evacuated, sealed and then heated in the furnace up to 700 C, beyond its melting point. This provided for the formation of a homogenous liquid melt which was conducive for the formation of Ba.sub.2CsI.sub.5:Eu singular- or mono-phase. Several such growth runs were performed to synthesize “Ba.sub.2CsI.sub.5:Eu pre-synthesis charges” with varying concentrations of Eu.sup.2+ in the range of 1 to 10% (mole %), to study the effect of Eu dopant concentration on scintillation properties.
(53) Light Yield (Pre-Synthesis):
(54) As the Ba.sub.2CsI.sub.5:Eu scintillators in their polycrystalline formats with variations in Eu concentrations were fabricated, we performed detailed characterizations of their light yield. The optical signal produced by a specimen when irradiated with a .sup.57Co source (122 keV photons) were measured using a PMT coupled to standard NIM electronics (Canberra #2003 preamplifier, Canberra #2020 amplifier), and the spectrum were recorded using an Oxford Instruments MCA. A commercial single-crystal LaBr.sub.3:Ce scintillator with known light yield of 55,000 photons/MeV available at RMD was used as a standard for comparison. By comparing the 122 keV photopeaks in all cases, the relative light output of various Ba.sub.2CsI.sub.5:Eu specimens was determined. The light output of Ba.sub.2CsI.sub.5:Eu polycrystals varied as Eu concentrations changed. The highest light output was estimated to be ˜70,000 ph/Mev at 5% Eu by mole %, taking into account the ratio between the centroid positions and quantum efficiencies of the super bi-alkali PMT for the LaBr.sub.3:Ce reference and Ba.sub.2CsI.sub.5:Eu crystals. This result gave insight into the stoichiometry in Ba.sub.2CsI.sub.5:Eu films for achieving a bright material.
(55) Afterglow (Pre-Synthesis):
(56) Due to the high speed nature of CT imaging, the afterglow of the scintillator is a important parameter for CT imaging. To minimize artifacts arising from excessive persistence, afterglow of a scintillator should be minimal. Therefore, the afterglow measurements of Ba.sub.2CsI.sub.5:Eu are useful in ascertaining its use for CT applications. These measurements were performed by exciting the polycrystal specimens by means of a Golden Engineering XRS-3 source, which provides X-ray pulses 20 ns FWHM, with nominal maximum photon energy of 250 kVp. The scintillation response from the specimens is passed through a 0.2-m McPherson monochromator, detected by a Hamamatsu R2059 photomultiplier, and re-corded by a Tektronix TDS220 digital storage oscilloscope. The pre-synthesized polycrystals demonstrated low afterglow, which is important for CT applications. For example, the polycrystals showed the afterglow results after 2 ms interval following an X-ray pulse, for different specimens as a function of its Eu dopant concentration. The trend seems to show that as the Eu concentration increases, the afterglow increases as well. The afterglow of 0.3% for Ba.sub.2CsI.sub.5:5% Eu is notable considering the current standard CsI:Tl whose afterglow is nearly 5-10% at 2 ms.
(57) X-Ray Diffraction Studies:
(58) One aspect of the pre-synthesis polycrystals is the formation of Ba.sub.2CsI.sub.5:Eu “mono-phase”. Phase identification was performed on the pre-synthesis polycrystals of Ba.sub.2CsI.sub.5:Eu by powder X-ray diffraction (XRD). The measurement was performed with a Bruker Nonius FR591 rotating anode X-ray generator equipped with a Cu target at a 50 kV and 60 mA electron beam.
(59) XRD patterns for three polycrystal samples were obtained as well as the standard diffraction pattern of Ba.sub.2CsI.sub.5 calculated from single-crystal data. PC100A is as-fabricated sample, while PC100B-V specimen was subjected to high-vacuum (10.sup.−6 Torr) for 24 hours post-fabrication. The sample PC100A-VH sample was annealed at 100-125° C. in high-vacuum for 1 hour. The peak positions of all three samples match well with the pattern derived from the single crystal structure. No impurities corresponding to reactants or undesired products were visible, thus confirming that the 3 samples are Ba.sub.2CsI.sub.5. However, the degree of crystallinity differed from sample to sample, and as expected, annealed sample (PC100A-VH) exhibited better crystallinity than the rest. While not being bound by any particular theory, this can be explained by the presence of free iodine in the crystal matrix of Ba.sub.2CsI.sub.5. The source of free iodine is the anion part of EuI.sub.2, which is added during the pre-synthesis fabrication by Bridgman's process. It is also possible for factory-shipped BaI.sub.2 to have excess iodine. In any case, the excess iodine precipitates on the grain boundaries and hinders crystallization of Ba.sub.2CsI.sub.5 during the crystal growth. Hence as a result, it can be seen that post-fabrication annealing helps de-gas the excess iodine in vacuum and improves its crystallinity for producing highly crystalline mono-phase Ba.sub.2CsI.sub.5:Eu. From the standard reference it is deduced that single-crystal Ba.sub.2CsI.sub.5:Eu has a monoclinic P2.sub.1/c space group with cell parameters a=10.541 Å, b=9.256 Å c=14.637 Å; β=90.194°.
(60) Compositional Analysis (Polycrystals):
(61) The preliminary qualitative compositional analysis was carried out using X-ray fluorescence (XRF) microscopy. It is a non-destructive technique that utilizes the photoelectric interaction of radiation with matter to produce a signal that is proportional to elemental concentration. The basic principle is that ionizing radiation induces photoelectric absorption in the test material for the elements of interest, followed by emission of characteristic X-rays that can be detected, isolated and counted. We have adapted a portable device, LeadTracer-ROHS™ from RMD Instruments Inc. originally designed for the screening of lead and mercury, to quantify the Cs, Ba, I and Eu concentrations in Ba.sub.2CsI.sub.5:Eu scintillator. It uses a .sup.57Co source to excite the sample with 122 keV gamma rays, and then detects the resulting X-ray fluorescence from the sample using a CdTe detector that provides ˜700 eV energy resolution. The compositional analysis showed the formation of the “mono-phase” of Ba.sub.2CsI.sub.5 in the pre-synthesis step.
Example 3
Fabrication of CsBa2I5:Eu Scintillator Films with Physical Vapor Deposition
(62) Ba.sub.2CsI.sub.5:Eu is a scintillator, well suited for next-generation radiography. Ba.sub.2CsI.sub.5:Eu has an estimated light yield of ˜100,000 ph/MeV, and its X-ray excited emission wavelength, detected around 430 nm, can be tailored to match the response of CMOS, CCDs or a-Si:H flat panel detectors. The higher density of Ba.sub.2CsI.sub.5:Eu (5.04 gm/cm.sup.3) affords higher stopping power than CsI:Tl (4.53 gm/cm.sup.3). As-deposited films of Ba.sub.2CsI.sub.5:Eu, fabricated via physical vapor deposition, exhibit highly structured morphology, which can be optimized either for high spatial resolution or high light output. Its fast decay and low afterglow make Ba.sub.2CsI.sub.5:Eu ideally suited for high frame-rate applications.
(63) The next generation of medical, security, industrial and scientific X-ray imaging systems demands a high performance scintillator with outstanding properties of high sensitivity, high resolution, high brightness, high speed, and favorable energy of light emission. Here, this example reports on Ba.sub.2CsI.sub.5:Eu, a novel high-performance scintillator well suited for such radiography, fabricated as a thin film via physical vapor deposition (PVD) onto a suitable (typically radiolucent) substrate. Ba.sub.2CsI.sub.5:Eu is a bright scintillator with an estimated light yield of ˜100,000 photons/MeV. While the peak X-ray excited emission wavelength of both crystalline and thin film Ba.sub.2CsI.sub.5:Eu are 430 nm, the emission spectrum of the thin film form shows significant broadening compared to the crystal sample. This implies that the emission properties can be tailored to match the sensitivities of photodetectors such as CMOS, CCDs or a-Si:H flat panels. The density of Ba.sub.2CsI.sub.5:Eu (5.04 gm/cm.sup.3) is higher than traditional sensors such as CsI:Tl (4.5 gm/cm.sup.3), and thus affords higher stopping power. The PVD-deposited films exhibit structured cross-sections, which can be optimized either for high spatial resolution in amorphous microcolumnar structure (AMS) form or for high light output in crystalline microcolumnar structure (CMS) form. The decay characteristics of Ba.sub.2CsI.sub.5:Eu demonstrate <0.05% afterglow at 2 ms following X-ray excitation, making it ideally suited for high frame rate applications such as fluoroscopy and cone beam CT (CBCT).
(64) This example describes a scintillator, Ba.sub.2CsI.sub.5:Eu, (
(65) Ba.sub.2CsI.sub.5:Eu is fabricated via the physical vapor deposition (PVD) method of thermal evaporation in microcolumnar form, a structure that minimizes the traditional tradeoff between spatial resolution and detector X-ray absorption efficiency. Films fabricated in crystalline microcolumnar structure form can be used to retrofit pixelated scintillators in existing CT systems. Amorphous microcolumnar structure forms can be directly deposited onto flat panel detector arrays suitable for new dedicated, organ-specific cone beam CT (CBCT) or radiography systems. Both forms can be grown in dimensions suitable for conventional photodiode coupling or for coupling to large-format CMOS, CCDs or a-Si:H flat panel detectors, and their thickness can be tailored to maximize absorption efficiency for 140 kVp X-rays typically used in medical CT.
(66) Ba.sub.2CsI.sub.5:Eu is a new material and is not yet available commercially in a form amenable to our vapor deposition growth needs. Consequently, in order to pursue PVD fabrication, melt-grown crystals were pre-synthesized at using a vertical Bridgman furnace. Several growth runs were performed to synthesize these crystals, with varying concentrations of Eu.sup.2+ dopant.
(67) This example describes a vapor-deposited ternary compound in a microcolumnar format. This material, in the described CMS and AMS morphologies, is expected to be a breakthrough for the radiography community in general and medical imaging in particular. Additionally, owing to its fast decay and low afterglow, Ba.sub.2CsI.sub.5:Eu is well suited for high speed radiography, fluoroscopy or CBCT applications. This material has the potential to provide high spatial resolution along with high brightness and enhanced X-ray absorption, enabling development of detectors with high detective quantum efficiency (DQE(f)).
(68) This example focuses on the fabrication and characterization of a novel scintillator, Ba.sub.2CsI.sub.5:Eu, in microcolumnar forms that possess traits to qualify as the next generation sensor for radiography. Practical and cost-effective designs and processes have been identified to fabricate Ba.sub.2CsI.sub.5:Eu films suitable for use in X-ray imaging and to integrate them into current detectors, which make it a promising, high-performance scintillator for demanding applications.
Example 4
Fabrication of CsBa2I5:Eu Scintillator Films with Hot Wall Evaporation and Thermal Evaporation
(69) This portion of the example describes a Ba.sub.2CsI.sub.5:Eu scintillator in a crystalline microcolumnar (CMS) and amorphous microcolumnar (AMS) format, made using hot wall evaporation. An HWE apparatus similar to the apparatus in
(70) Light Yield:
(71) The light yield for CMS sample was measured using the super bi-alkali PMT setup as described previously for the polycrystalline samples. The light yield was estimated in reference to the commercial LaBr.sub.3:Ce standard and by accounting for the detector quantum efficiency for its emission wavelength (380 nm) with respect to Ba.sub.2CsI.sub.5:Eu(450 nm).
(72) This portion of the example describes fabrication of microcolumnar Ba.sub.2CsI.sub.5:Eu scintillator films using thermal evaporation. The resulting microcolumnar structure conserves and promotes channeling of the scintillation light by means of total internal reflection, thereby suppressing lateral light spread within the film. This allows, e.g., fabrication of a thicker film for higher X-ray absorption without sacrificing spatial resolution for cone beam CT (CBCT) applications.
(73) Thermal Evaporation:
(74) The schematic for the thermal evaporation used for fabrication of AMS Ba.sub.2CsI.sub.5:Eu films is shown
(75) Microcolumnar Morphology:
(76) Scanning electron microscopy (SEM) was performed on the cross-section of a freshly cleaved AMS Ba.sub.2CsI.sub.5:Eu film.
(77) Spatial Resolution Measurements:
(78) The spatial resolution of the scintillators were evaluated by measuring a pre-sampled line spread function (LSF) followed by a fast Fourier transform (FFT) of the LSF to obtain the modulation transfer function (MTF(f)). A 10 μm wide tantalum slit oriented at a <1° angle relative to the CCD pixel row (or column) direction was imaged to obtain the pre-sampled LSF with a sampling interval of 0.7 μm or less. The acquired MTF data showed that a 50 μm thick Ba.sub.2CsI.sub.5:Eu AMS film exhibited resolution of 6 LP/mm (˜80 μm) spatial resolution as a function of spatial frequency.
(79) Emission Spectrum:
(80) The X-ray excited emission spectra of various scintillator samples was measured using an existing setup, with the sample under investigation excited by the 8 keV Cu Kα line. To generate the required flux at the sample, the X-ray generator was operated at 40 kV with 20 mA current. The resulting scintillation light was collected in a MacPherson 0.2 m monochromator (model 234/302) that separates the light into its wavelength components. The intensity of the selected wavelength was registered using an RCA model C31034 photomultiplier tube (PMT). The operation of the whole instrument, including the X-ray trigger, the rotation of the monochromator to select the wavelengths, and the data acquisition and analysis was software controlled.
(81) As shown in
(82) X-Ray Characterization:
(83) Light output measurements were made by coupling AMS Ba.sub.2CsI.sub.5:Eu films measuring 50 μm thickness (20 mg/cm.sup.2) to an electron multiplying CCD via a 3:1 fiber optic taper. The film was exposed to a uniform flood field of 70 kVp X-rays and resulting analog-to-digital unit values (ADUs) were averaged over a pre-defined region of interest. Similar measurement was repeated on commercial Gd.sub.2O.sub.2S (GOS) screen (MinR-2000, 34 mg/cm.sup.2) for the sake of comparison. The as-deposited AMS film measured 450 ADUs compared to 1800 ADUs for MinR under similar X-ray conditions. This is primarily due to very low (<5%) quantum efficiency of the CCD to 450 nm wavelength of Ba.sub.2CsI.sub.5:Eu compared to 55% QE for the GOS 540 nm light. By correcting for quantum efficiencies of the CCD at different emissions and by accounting for X-ray absorptions arising from different mass thicknesses of the two scintillators, the estimated brightness for Ba.sub.2CsI.sub.5:Eu film is 13,500 ADUs compared to MinR's 3400 ADUs. This result implies that Ba.sub.2CsI.sub.5:Eu scintillator's light output is almost 4 times greater than commercially available GOS screens. The as-deposited samples were annealed in Ar environment for 40 hours at 160 C. Annealing process remove defects in the films, remove any non-uniformity in dopant distribution and improve the microcolumnar morphology. And as a result, a 11% increase in the ADU was registered following annealing of the film.
(84) Dual Doping with Tl+Eu:
(85) Using the schematic for the thermal evaporation described above, fabrication of dual doped AMS Ba.sub.2CsI.sub.5 films were produced. Dual doping can induce a red shift emission spectrum for the Ba.sub.2CsI.sub.5 host from 450 to 550 nm, to, e.g., improve its sensitivity to CCD, a-Si:H, or SiPM detectors. The dopants selected to achieve this effect were thallium and europium. The deposition was carried out by evaporating polycrystals of Ba.sub.2CsI.sub.5:5% Eu and thallium iodide (TlI) beads in two independently controlled tantalum crucibles. The substrates were loaded on the planetary tool plates, which permitted angular deposition on their surfaces and the ambient substrate temperatures were maintained at 50-60° C., all of which favored AMS growth. The resulting films were 60 μm thick and exhibited microcolumnar morphology and line-pair resolution. The spectral emission of dual-doped Ba.sub.2CsI.sub.5:Eu,Tl films is shown in
(86) Light output measurements were made by coupling AMS Ba.sub.2CsI.sub.5:Eu,Tl films measuring 60 μm thickness (24 mg/cm.sup.2) to an electron multiplying CCD via a 3:1 fiber optic taper. The film was exposed to a uniform flood field of 70 kVp X-rays, in a setup as described previously. The as-deposited AMS film measured 2880 ADUs compared to 1800 ADUs for MinR 2000 standard GOS screen under similar X-ray conditions. Dual doped film and MinR 2000 practically have the same emission wavelength and hence do not need any correction for quantum efficiencies. However, by correcting for X-ray absorptions arising from different mass thicknesses of the two scintillators, the estimated brightness for dual doped Ba.sub.2CsI.sub.5:Eu,Tl film is 2 times greater than commercially available GOS screens. A more orchestrated approach is needed to understand the collective effect of multiple activators in any given scintillator host, nevertheless dual doping of Ba.sub.2CsI.sub.5 is a promising pathway to engineer its emission wavelength for improving its sensitivity to various detectors such as CCD, a-Si:H, and SiPMs etc.
Example 5
Imaging with CsBa2I5:Eu Scintillator Films
(87) X-Ray Radiographic Imaging:
(88) To illustrate the X-ray imaging capability, Ba.sub.2CsI.sub.5:Eu films were coupled to an EMCCD via a 3:1 taper. The X-ray source-to-detector distance was 45 cm, and the imaged object was placed in contact with the CCD front end. Thus, there is no optical magnification in registered images and resolution reflects true resolution of the film (EMCCD has a very high intrinsic resolution of 39 μm, or ˜13 lp/mm).
(89) Dynamic X-Ray Imaging:
(90) Dynamic X-ray imaging with a Ba.sub.2CsI.sub.5:Eu film was evaluated. To this end, dynamic images of a moving toy were acquired by our custom developed CMOS X-ray camera capable of providing 1 k×1 k pixel images at 2000 frames per second. With a reduced field of view, this detector can acquire images at up to 120,000 fps. As such this is an ideal tool to evaluate scintillator screens for high frame rate imaging such as the current CBCT application. During this period, the RMD AMS Ba.sub.2CsI.sub.5:Eu film was pressure coupled to this detector and images were acquired at 1000 fps using a Hamamatsu microfocus X-ray source operating at 100 kVp and 100 μA. A single frame of a sequence of images was acquired at 1000 fps. As is evident from the image, the camera (film) shows excellent dynamic range as the plastic gears are clearly visible against the high contrast stainless steel components. Even the plastic gear system inside the rectangular metal frame is clearly visible.
(91) It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof may be suggested to persons skilled in the art and are included within the spirit and purview of this application and scope of the appended claims. Numerous different combinations of embodiments described herein are possible, and such combinations are considered part of the present invention. In addition, all features discussed in connection with any one embodiment herein can be readily adapted for use in other embodiments herein. The use of different terms or reference numerals for similar features in different embodiments does not necessarily imply differences other than those which may be expressly set forth. Accordingly, the present invention is intended to be described solely by reference to the appended claims, and not limited to the preferred embodiments disclosed herein.