Patent Application
20240417885
The present invention relates to the field of crystal growth and, more particularly, to methods of growing advanced cerium-activated lutetium-based oxyorthosilicate scintillation crystals for use in molecular imaging and high energy physics.
Cerium-activated lutetium-yttrium oxyorthosilicate, commonly identified as LYSO:Ce (or simply as LYSO), is among the most technologically advanced group of scintillation crystals thus far created by mankind. Although initially developed for radiation detection and medical imaging (namely, Positron Emission Tomography (PET)) towards the end of the last millennium, LYSO:Ce scintillation crystals have more recently found expanded uses in, among other things, High Energy Physics (HEP) applications (especially in the context of high luminosity particle colliders). Indeed, during the past decade manmade LYSO:Ce scintillation crystals have been formulated and used by scientist to develop cutting-edge high precision electromagnetic calorimeters such as the one designed for the Mu2e experiment [Mu2e collaboration, Mu2e Conceptual Design Report, Tech. Rep. FERMILAB-TM-2545, FERMILAB-DESIGN-2012-03, FERMILAB (2012)] and the CCALT forward calorimeter of the KLOE-2 experiment [M. Cordelli et al., CCALT: A crystal calorimeter for the KLOE-2 experiment, Journal of Physics: Conference Series 293 (2011)].
Unlike other types of scintillation crystals, LYSO:Ce scintillation crystals advantageously possess a high mass density (roughly twice the density of NaI(Tl)), fast scintillation kinetics (roughly six times faster decay time than BGO), and a high light yield (about 40000 ph/MeV). In the field of crystal growth, LYSO:Ce scintillation crystals are known for their excellent scintillation properties. The growth of such crystals, however, is not without its challenges. The process often requires precise control over the growth conditions, including temperature, pressure, and the composition of the growth medium and surrounding atmosphere. Furthermore, the choice of crucible design and its insulation materials can significantly impact the quality and properties of the grown crystal. Finally, the presence of different co-dopants, impurities, and defects are known to play important roles in scintillation crystal dynamics and, therefore, need to be tightly controlled.
In addition, the post-growth treatment of the grown crystal, such as annealing, can also affect the crystal's properties. The annealing process must also be carefully controlled to ensure that the crystal achieves the desired properties.
U.S. Pat. No. 4,958,080 to Melcher, a pioneering patent in the field, teaches a single crystal scintillator composed of cerium-activated lutetium oxyorthosilicate (commonly identified as LSO:Ce, or simply as LSO) having the formula Ce2xLu2(1−x)SiO5. The original LSO crystal were developed for uses in high-energy radiation detectors.
Lutetium oxyorthosilicate (LSO) crystals are known to have a lattice structure composed of lutetium (Lu), silicon (Si), and oxygen (O) in proportions given by the general formula Lu2SiO5; that is, a formula unit with two lutetium atoms, one silicon atom, and five oxygen atoms. It is a 1:1 mole ratio compound in the binary rare-earth silicate system composed of one Lu2O3 mole and one SiO2 mole. Upon mixing, melting, and subsequent solidification of these two initial starting oxides, a new composition is formed:
1 mol [Lu2 O3]+1 mol [SiO2]→1 mol [Lu2 SiO5]
The LSO solid-state composition has a ratio of lutetium to silicon to oxygen of 2:1:5 (Lu2SiO5). That is, two lutetium to each one silicon and five oxygen atoms. In order to attain scintillation attributes, the starting oxide material are doped with small amounts of cerium oxide (CeO2) to produce, after melting and subsequent growing, cerium-activated lutetium oxyorthosilicate crystal Ce:LSO, wherein reduced trivalent cerium Ce3+ ions substitute for trivalent lutetium Lu3+ ions within the overall crystal lattice structure. The substituted trivalent cerium Ce3+ ions act as sensitizers, improving the efficiency of energy transfer and light production of the crystal. The amount of cerium substituted for lutetium, which is assigned a value of x, is the same value that reduces the amount of lutetium, resulting in lutetium's new value, which may be expressed as (1−x). Because cerium replaces some of the lutetium, it may be said that cerium oxide belongs to the “lutetium group,” which, as a whole, still maintains the general 1:1 mole ratio:
(1−x)mol[Lu2 O5]+(x)mol[CeO2]=1 mol Lu group oxides:1 mol Si group oxides
Generally, this may be expressed with the following reaction leading to cerium-activated lutetium oxyorthosilicate (Ce:LSO):
(1−x) mol Lu2 O3+x mol CeO2+1 mol SiO2→1 mol Ce2x Lu2(1−x) SiO5
In this composition, x is defined as 0.001<x<0.0045. Therefore, the entire range of each element's formula unit (f.u.) value claimed within the general composition of the '080 patent to Melcher may be expressed as the following:
Lu1.991<Lu<1.998 Ce0.002<Ce<0.009 Si1 O5
U.S. Pat. No. 6,278,832 to Zagumennyi et al., another pioneering patent in the field, teaches enhanced scintillation crystals composed of cerium-activated lutetium oxyorthosilicate having the formula Lu1−yMeyA1−xCexSiO5 wherein at least two additional variable elements, “Me” and “A” are added (inclusive of divalent and trivalent metals). In some embodiments, the substituted co-doped Ce:LSO and/or Ce:LYSO scintillation crystals include oxygen vacancies () resulting from charge imbalances. The oxygen vacancies are recited to reflect an accurate value of oxygen in the final solid-state crystal composition. When the crystal forms, it must obey the conservation of charge neutrality laws, or, in other words, the total positive ions must equal the total negative ions. Because the moles of oxygen is calculated to be slightly less than 5 for all values of x and y, there must exist some value, z, of oxygen vacancies (
). After accounting for the oxygen vacancies, the resulting value of oxygen is 5−z.
U.S. Pat. No. 6,624,420 to Chai et al., yet another pioneering patent in the field, teaches co-doped scintillation crystals composed of cerium activated lutetium yttrium oxyorthosilicate (Ce:LYSO) having the formula Ce2x(Lu1−yYy)2(1−x)SiO5. In this solid-state composition, yttrium (Y) and cerium (Ce) each replace some of the lutetium (Lu):
(1−y−x−xy) mol Lu2O3+x mol CeO2+(y−xy) mol Y2O3+1 mol SiO2→1 mol Ce2x (Lu1−yYy)2(1−x) SiO5
The range of variables x and y are defined as 0.0001<x<0.05 and 0.0001<y<0.9999. Therefore, the entire range of each element's formula unit (f.u.) value claimed within the general composition of the '420 patent to Chai may be expressed as the following:
Lu0.00019<Lu<1.9996Y0.00019<Y<1.9996Ce0.0002<Ce<0.1Si1O5
U.S. Pat. No. 7,132,060 to Zagumennyi et al., still yet another pioneering patent in the field, teaches various enhanced scintillation crystals composed of cerium-activated lutetium oxyorthosilicate having the formula CexLu2+2y−x−zAzSi1−yO5+y, but wherein at least one additional element denoted as “A” may substitute for other atoms within the overall crystal lattice structure. In addition, and unlike the exact 1:1 mole ratio of starting components taught for melts used to make prior art lutetium oxyorthosilicate crystals, the substituted co-doped Ce:LSO and/or Ce:LYSO scintillation crystals taught by the '060 patent are grown from non-stoichiometric melts wherein the silicon concentration is slightly less than one (Si<1) and the mole ratios of other components relative to silicon is slightly greater than two (i.e., Lu2+2y−x−z+Cex+Az)/Si>2).
Despite recent advancements in crystal growth techniques, there is still a need in the art for new and improved crystal growth methods and devices that can reliably produce high-quality advanced lutetium-based cerium-activated oxyorthosilicate scintillation crystals of uniform composition suitable for molecular imaging and/or high energy physics applications, as well as to the grown crystals themselves. The present invention fulfills this need and provides for further related advantages.
In accordance with certain embodiments of the present invention, a method is provided for growing a boule of co-doped LYSO:Ce,Ca scintillation crystal having enhanced scintillation performance and uniformity. The method involves providing an iridium crucible and loading it with a powdered mixture of specific oxides and a carbonate salt in specific relative amounts. A vacuum is applied to the loaded crucible, which is then pressurized with a specific gas mixture (i.e., mostly argon admixed with a small amount of carbon monoxide) to within a specified atmospheric pressure range (i.e., generally from about 1.02 to 1.05 atm). The loaded and pressurized crucible is then heated to yield a melt, into which a seed crystal is placed to grow an unfinished crystalline boule via the Czochrolaski pulling method. The unfinished boule is then annealed at a specific temperature range for a selected duration with the same specific gas mixture (i.e., mostly argon admixed with a small amount of carbon monoxide) at an elevated atmospheric pressure to yield a finished crystalline boule of co-doped LYSO:Ce,Ca.
In accordance with other embodiments, the method may include adjusting various parameters such as the relative amounts of the starting oxide materials and the carbonate salt, the specific pressure environment, and the heat treatment temperatures and durations. The method generally also involves using iridium crucibles that are in direct contact with one or more zirconia insulation materials. The crystalline boules grown in accordance with the invention preferably contain both Ce3+ ions and Ce4+ ions uniformly distributed throughout the boule in a ratio of about 1:1 (i.e., about 50% trivalent cerium Ce3+ ions to about 50% tetravalent cerium Ce4+ ions), but may, however, numerically range from about 40%-60% trivalent cerium Ce3+ ions to about 40%-60% tetravalent cerium Ce4+ ions.
In an embodiment, the invention is directed to a co-doped cerium and calcium lutetium-yttrium oxyorthosilicate scintillation crystal (substantially free of other ions) represented by the following chemical formula:
Lu(2−y−x−z) Yy Cex Caz SiO5
wherein
In another embodiment, boules of co-doped LYSO:Ce,Ca scintillation crystal are grown from slightly non-stoichiometric melts in which the silicon concentration is slightly less than one (Si<1) and the mole ratios of other components relative to silicon is greater than two (i.e., Lu2−y−x−z+Yy+Cex+Caz)/Si>2). Thus, the invention is also directed to a co-doped cerium and calcium lutetium-yttrium orthosilicate scintillation crystal (substantially free of other ions) represented by the following chemical formula:
Lu(2−y−x−z) Yy Cex Caz Si1−q O5+q
wherein
These and other aspects of the present invention will become more readily apparent to those of ordinary skill in the art when reference is made to the following detailed description in view of the accompanying drawings.
The drawings are intended to be illustrative of certain preferred embodiments of the present invention. Like reference numerals have been used to designate like steps or features throughout the several views of the drawings.
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols or markings have been used to identify like or corresponding steps or elements, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting and are not necessarily to scale. Other embodiments may be utilized, and other changes may be made, without departing from the scope of the invention as defined by the appended claims.
As is appreciated by those skilled in the art, lutetium-based oxyorthosilicate crystals, commonly represented by the generic chemical formula [Lu(1−x)Yx]2SiO5:Ce or more simply as LYSO:Ce and even more simply as just LYSO, generally have a variable yttrium and cerium content depending on the grower/manufacturer and end user prefaces. Both of these co-dopants are related to fundamental properties of the crystals. The yttrium content generally correlates with the mass density (and consequently with the minimum ionizing particle (MIP) deposited energy), while the cerium content generally relates to light yield and decay time. In addition, the stoichiometry of the metal (Lu(1−x)Yx) group is not entirely fixed and depends on the precise crystal growth recipe of each manufacturer (values for x, however, are generally less than 10% of the total metal content). The large difference between the atomic mass of lutetium (174.967 amu) and yttrium (88.906 amu) accounts for significant differences between the densities of lutetium-based oxyorthosilicate crystals having different yttrium content.
There are a number of qualities that make different crystals more or less useful as scintillators, including their light yield, their decay rate, their energy resolution, and their afterglow. In all cases, the physical properties of any substance (including crystals) are determined solely by its chemical composition and the arrangement of the chemical constituent components that make up its composition. The scintillation crystals of the present invention as disclosed herein strike a unique balance of these properties, whereas the methods of the present invention enable the growth of large crystalline boules of a uniform composition possessing these properties. In the context of the present invention, a scintillator's “light yield” is a measure of how bright it becomes when struck by ionizing radiation, a scintillator's “decay rate” describes how quickly the scintillator returns to its unlit state after lighting up, a scintillator's “energy resolution” is essentially a measure of the width of the light beam emitted by the scintillator when it lights up—that is, the energy resolution is the ratio (expressed as a percentage) of the light yield over the wavelength of the light at full width/half maximum (FWHM).
In next generation positron emission tomography (PET) medical scanners, for example, scintillation crystals with higher light yields, faster decay rates, narrower energy resolutions, and lower afterglows are preferred. The co-doped LYSO:Ce,Ca scintillation crystals grown in accordance with the present invention are particularly well-suited for use in PET scanners because of their unique mix of favorable physical properties. In the context of the present invention, a dopant refers to an element or compound that is intentionally added to the crystal lattice structure to modify its properties and/or enhance its sensitivity to certain types of radiation, such as gamma rays. A co-dopant refers to one or more additional elements or compounds that are introduced into the melt/crystal along with the original first dopant. Thus, a co-doped scintillation crystal refers to a scintillation crystal that has been doped with two or more additional elements or compounds.
The scintillation performance of lutetium-based oxyorthosilicate crystals also depends on several other additional factors. For example, and without necessarily prescribing to any specific scientific theory, it is believed that the intensity of the optical absorption peak at 360 nm directly correlates to the concentration of trivalent Ce3+ ions within the crystal, whereas the scintillation light output (LO) directly correlates the concentration of tetravalent Ce4+ ions within the crystal. In accord with the present invention, the ratio between these two ions is controlled, in large part, by precise co-doping of the crystal with precise amounts Ca2+ ions (introduced by adding small amounts of calcium carbonate (CaCO3) as a starting ingredient, which compound favorably decomposes under heat to yield Ca2+ ions, O2−ions, and gaseous CO as follows: CaCO3→Ca2++O2−+CO) while growing the crystal in the manner as herein described.
In some embodiments, the co-doped LYSO:Ce,Ca scintillation crystals of the present invention are substantially free of other metallic ions such as, for example, monovalent lithium ions Li+ and/or bivalent magnesium ions Mg2+ because more than de minimis amounts of these and other types of charged ions interfere with the scintillation process in undesirable and/or unwanted ways.
In accordance with the methods of the present invention, large boules of co-doped LYSO:Ce,Ca scintillation crystals may be grown such that the number trivalent Ce3+ ions and the number of tetravalent Ce4+ ions are about the same. Put differently, the ratio of Ce3+ ions and Ce4+ ions within crystals grown in accordance with the present invention is preferably equal to about 50:50 or about 1:1 (i.e., about 50% trivalent Ce3+ ions to about 50% tetravalent Ce4+ ions), but may numerically range from about 40%-60% trivalent Ce3+ ions to about 40%-60% tetravalent Ce4+ ions (depending on the specifics of the crystal growth recipe and growing/annealing conditions). In the context of the present invention, a large boule refers to any boule having a diameter of greater than 80 millimeters (mm) and a length of greater than 220 millimeters (mm). Again, and with necessarily prescribing to any specific scientific theory, it is believed that scintillation performance is optimized when LYSO:Ce,Ca scintillation crystals are grown such the ratio of Ce3+ ions and Ce4+ ions within crystals are roughly the same.
In view of the foregoing and referring now to the drawings, the present invention in an embodiment is directed to a method of growing a boule of co-doped LYSO:Ce,Ca scintillation crystal, wherein the ratio of Ce3+ ions and Ce4+ ions within the boule are roughly the same. The crystal growth method involves the several different process steps illustrated in
More specifically, and with reference to
As shown in
In addition, and as further shown, a housing 218, positioned about the lower portion 204 of the crystal growing device 200, surrounds and holds in place the iridium crucible 208 and a portion of the two types of insulation (deoxidized zirconia insulation material 214 and zirconia insulation material 216). An induction coil 220 (water cooled) wraps around the housing 218 and is used to inductively and selectively/controllably heat the iridium crucible 208 and it contents.
By growing crystals in a crystal growing device 200 comprising concentric insulation of two different types as described herein, it has been found that iridium losses minimized while the thermal field, created as part of the heating, melting, and annealing process steps, is exceptionally uniform and stable (thereby by allowing the growth of superior crystals—including large crystal boules of co-doped LYSO:Ce,Ca having uniform composition and favorable scintillation dynamics).
With reference again to
Step 104 involves applying a vacuum to the filled iridium crucible 208. This may be done using a vacuum pump, for example. The purpose of this step is to remove any air or other gases from the iridium crucible 208, creating a low pressure environment. This is important because it helps to prevent unwanted reactions between the chemical compounds and the gases in the air.
In step 106, the filled iridium crucible 208 is pressurized with a mixture of gases consisting essentially of about 95% to about 99.9% Ar and, more preferably, from about 0.1% to about 5% CO on a weight percent basis, and more even preferably from about 99.75% Ar to about 0.25% CO on a weight percent basis. The pressure is maintained from about 1.02 to 1.05 atm (during the crystal growth and annealing steps). This step is important because it helps to ensure that the crystal attains a uniform composition and favorable scintillation dynamics.
Step 108 involves heating the pressurized iridium crucible 208 to produce a melt. This is generally done using inductive heating (and within the specialized crystal growing device 200). The temperature is carefully controlled to ensure that the chemical compounds melt but do not otherwise degrade. The result is a molten material that is ready for the next step.
In step 110, a seed crystal secured at the end of a rod is inserted into the melt via the opening 206. The seed crystal provides a template for the molten material to crystallize around, helping to form into a crystalline boule (in accordance with the Czochrolaski melt pulling method as is known in the art). More particularly, as the crystal forms around the seed crystal, the rod is slowly withdrawn from the melted material, a process that results in a cylindrical crystal known as a boule.
Finally, in step 112, the unfinished crystalline boule is annealed. This step is also generally done within the specialized crystal growing device 200. The annealing temperature preferably is constant and ranges from about 1450° C. to about 1500° C. The heat-up time is about 24 hours, the constant temperature time is about 48 hours, and the cool-down time is about 48 hours. Annealing in this way helps to relieve any internal stresses in the boule, improving its structural integrity and optical properties. The result is a finished boule of co-doped LYSO:Ce,Ca scintillation crystal. In addition, and after finishing its growth, the boule may be cut into many smaller crystals referred to as pixels, which, for example, can then be installed into the detector ring of a PET scanner. Such pixels can be tailored, depending on the application, to have a decay time of between 30 to 40 nanoseconds (ns).
Stated somewhat differently, the process of the invention begins by providing a crucible made of iridium. The iridium crucible 208 is then filled with the following compounds and relative amounts on a weight percent basis: Lu2O3 at about 85 to 86 parts, Y2O3 at about 1 to 2 parts, SiO2 at about 13 to 13.5 parts, CaCO3 at about 0.02 parts, and CeO2 at about 0.09 parts. Once the crucible is filled, a vacuum of about 0.1 Pa is applied to the filled iridium crucible 208. The filled iridium crucible is then pressurized with an atmosphere consisting essentially of about 99.75% Ar and 0.25% CO on a weight percent basis, and to an atmospheric pressure ranging from about 1.02 to 1.05 atm. The filled and pressurized iridium crucible 208 is then heated to produce a molten mixture. A seed crystal is then inserted into the molten liquid mixture to initiate the growth of the boule. Next, the unfinished boule is then annealed at a temperature ranging from about 1450° C. to 1500° C. for a selected period of time (e.g., 48 hours) with an atmosphere consisting essentially of about 99.75% Ar and 0.25% CO on a weight percent basis at an atmospheric pressure ranging from about 1.02 to 1.05 atm. The end product of this process, is a single crystal of LYSO:Ce,Ca having a ratio of Ce3+ ions to Ce4+ within the crystal of about 1:1. This particular LYSO:Ce,Ca formulation can be advantageously used in various applications, including as a scintillator in molecular imaging devices and in high energy physics.
For purposes of illustration and not limitation, the following examples more specifically disclose the different amounts (representative weights in grams) of the starting compounds (i.e., essentially pure Lu2O3, Y2O3, SiO2, CaCO3, and CeO2) that were used to successfully grow successive (batch-wise) crystalline boules of co-doped LYSO:Ce,Ca in accordance with the methods and devices of the present invention.
Representative weights (in grams) of the starting oxides added to crucible for different grows.
In the practice of the invention and with reference to the above starting compounds and their respective weight amounts, large boules of co-doped LYSO:Ce,Ca scintillation crystal were grown in accordance with the following steps: load starting oxides into iridium crucible; vacuum to about 0.1 Pa; introduce Argon (containing about 0.25% CO) to about 1.02-1.05 atmospheres; heat up for about 12 hours to melt starting oxides; find the right melt temperature (about 4 hours); insert seed crystal into the melt and begin pull (about 4 hours); crystal shoulder expansion pull (about 24 hours); crystal equal diameter pull (about 124 hours); crystal tail pull (about 4 hours); crystal annealing (about 48 hours); remove the finished crystal.
With respect to detection and quantification of the amount of Ce3+ ions and Ce4+ ions present within an individual LYSO crystal, it has been determined that X-ray Absorption Near Edge Structure (XANES) is presently the most appropriate testing methodology. XANES is a spectroscopic technique used to study the electronic structure and local chemical environment of atoms within a material. It focuses on the X-ray absorption near the ionization energy of core electrons, providing detailed information about the oxidation state and coordination environment of the absorbing atom. XANES can distinguish between different oxidation states of an element like cerium (i.e., Ce3+ ions and Ce4+ ions) because the absorption edge position shifts depending on the oxidation state. XANES measurements typically require synchrotron radiation due to the need for highly intense and tunable X-rays. A representative spectrograph (of a co-doped LYSO:Ce,Ca crystal pixel) is provided as
While the present invention has been described in the context of the embodiments illustrated and described herein, the invention may be embodied in other specific ways or in other specific forms without departing from its scope or essential characteristics. Therefore, the described embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing descriptions, and all changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.
| Number | Date | Country | |
|---|---|---|---|
| 63610346 | Dec 2023 | US |
