Methods of Growing Co-Doped Cerium Calcium Lutetium-Yttrium Oxyorthosilicate Scintillation Crystals and Related Crystals

Abstract

The method involves growing a boule of co-doped LYSO:Ce,Ca scintillation crystal containing Ce.sup.3+ ions and Ce.sup.4+ ions in a ratio of about 1:1, wherein the grown boule is substantially free of Li.sup.+ ions and Mg.sup.2+ ions. This is achieved by providing a specialized crystal growing device having an inner iridium crucible and loading it with specific oxides and a carbonate salt in specific amounts. The loaded iridium crucible is then subjected to a vacuum and then pressurized with a specific gas mixture consisting mostly of Ar admixed with a small amount of CO. The loaded and pressurized iridium crucible is then heated to yield a melt, into which a seed crystal is placed to grow (by way of the Czochrolaski pull method) an unfinished crystalline boule. This unfinished boule is then annealed in the same special pressurized atmosphere to yield the final boule of co-doped LYSO:Ce,Ca scintillation crystal.

Claims

1. A method of growing a boule of LYSO:Ce:Ca scintillation crystal, comprising the steps of: providing an iridium crucible; loading the iridium crucible with the following compounds and relative amounts on a weight percent basis: Lu.sub.2O.sub.3 at about 84 to 87 parts, Y.sub.2O.sub.3 at about 0.1 to 3 parts, SiO.sub.2 at about 11.5 to 15 parts, CaCO.sub.3 at about 0.01 to 0.03 parts, and CeO.sub.2 at about 0.08 to 0.1 parts; applying a vacuum to the loaded iridium crucible; pressurizing the loaded iridium crucible with at least about 95% Ar and less than about 5% CO on a weight percent basis; heating the loaded and pressurized iridium crucible to yield a melt; placing a seed crystal into the melt and growing an unfinished crystalline boule; annealing the unfinished crystalline boule at a temperature of at least about 1450 C. in an atmosphere of at least about 95% Ar and less than about 5% CO on a weight percent basis to yield the boule of LYSO:Ce:Ca scintillation crystal.

2. The method of claim 1 wherein the loading of the iridium crucible is with the following oxides and relative amounts on a weight percent basis: Lu.sub.2O.sub.3 at about 85 to 86 parts, Y.sub.2O.sub.3 at about 1 to 2 parts, SiO.sub.2 at about 13 to 13.5 parts, CaCO.sub.3 at about 0.02 parts, and CeO.sub.2 at about 0.09 parts.

3. The method of claim 2 wherein the Lu.sub.2O.sub.3, Y.sub.2O.sub.3, SiO.sub.2 and CeO.sub.2 are each about 99.999% pure.

4. The method of claim 3 wherein a vacuum of about 1 Pa or less is applied to the loaded iridium crucible before the steps of pressurizing or heating.

5. The method of claim 4 wherein the pressurizing of the loaded iridium crucible is with a gas mixture consisting essentially of about 99.75% Ar and about 0.25% CO on a weight percent basis.

6. The method of claim 5 wherein the pressurizing of the loaded iridium crucible is to a pressure of about 1.02 atm to 1.05 atm.

7. The method of claim 6 wherein the step of annealing is at a temperature of about 1450 C. to 1500 C.

8. The method of claim 7 wherein the step of annealing is for a period of about 48 hours.

9. The method of claim 1 wherein the iridium crucible is surrounded by, and in direct contact with, a deoxidized zirconia insulation material.

10. The method of claim 1 wherein the boule of LYSO:Ce:Ca scintillation crystal contains Ce.sup.3+ ions and Ce.sup.4+ ions in a ratio of about 1:1.

11. A LYSO:Ce:Ca scintillation crystal grown in accordance with the method of claim 1.

12. A cerium and calcium lutetium-yttrium oxyorthosilicate scintillation crystal represented by the following chemical formula:
Lu.sub.(2yxz) Y.sub.y Ce.sub.x Ca.sub.z SiO.sub.5 wherein x is0.0001 and0.02 y iszero and0.1 z is0.0001 and0.005 and wherein Ce.sup.3+ ions and Ce.sup.4+ ions are present within the crystal in a ratio of about 40%-60% Ce.sup.3+ ions to about 40%-60% Ce.sup.4+ ions.

13. The cerium and calcium lutetium-yttrium oxyorthosilicate scintillation crystal of claim 12 wherein the Ce.sup.3+ ions and Ce.sup.4+ ions are present within the crystal in a ratio of about 50% Ce.sup.3+ ions to about 50% Ce.sup.4+ ions.

14. A co-doped cerium and calcium lutetium-yttrium oxyorthosilicate scintillation crystal represented by the following chemical formula:
Lu.sub.(2yxz) Y.sub.y Ce.sub.x Ca.sub.z Si.sub.1q O.sub.5+q wherein x is0.0001 and0.02 y iszero and0.1 z is0.0001 and0.005 q iszero and0.0001 and wherein Ce.sup.3+ ions and Ce.sup.4+ ions are present within the crystal in a ratio of about 40%-60% trivalent Ce.sup.3+ ions to about 40%-60% tetravalent Ce.sup.4+ ions.

15. The co-doped cerium and calcium lutetium-yttrium oxyorthosilicate scintillation of claim 14 wherein the Ce.sup.3+ ions and Ce.sup.4+ ions are present within the crystal in a ratio of about 50% Ce.sup.3+ ions to about 50% Ce.sup.4+ ions.

16. A lutetium-yttrium oxyorthosilicate scintillation crystal co-doped with calcium and cerium, wherein the crystal comprises Ce.sup.3+ ions and Ce.sup.4+ ions in a ratio of about 40-60% Ce.sup.3+ ions to about 40-60% Ce.sup.4+ ions.

17. The lutetium-yttrium oxyorthosilicate scintillation crystal co-doped with calcium and cerium of claim 16 wherein the crystal comprises Ce.sup.3+ and Ce.sup.4+ ions in a ratio of about 50% Ce.sup.3+ ions to about 50% Ce.sup.4+ ions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] 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.

[0030] FIG. 1 illustrates, in a flowchart format, the steps associated with growing a co-doped LYSO:Ce,Ca crystal boule in accordance with certain embodiments of the present invention.

[0031] FIG. 2 illustrates a crystal growing device suitable for growing a co-doped LYSO:Ce,Ca crystal boule in accordance with the present invention.

[0032] FIG. 3 is a representative XANES spectrograph illustrating a cerium atom step edge of a co-doped LYSO:Ce,Ca crystal pixel grown accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0033] 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.

[0034] As is appreciated by those skilled in the art, lutetium-based oxyorthosilicate crystals, commonly represented by the generic chemical formula [Lu.sub.(1x)Y.sub.x].sub.2SiO.sub.5: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.sub.(1x)Y.sub.x) 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.

[0035] 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 upthat 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).

[0036] 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.

[0037] 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 Ce.sup.3+ ions within the crystal, whereas the scintillation light output (LO) directly correlates the concentration of tetravalent Ce.sup.4+ 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 Ca.sup.2+ ions (introduced by adding small amounts of calcium carbonate (CaCO.sub.3) as a starting ingredient, which compound favorably decomposes under heat to yield Ca.sup.2+ ions, O.sup.2ions, and gaseous CO as follows: CaCO.sub.3.fwdarw.Ca.sup.2++O.sup.2+CO) while growing the crystal in the manner as herein described.

[0038] 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.sup.+ and/or bivalent magnesium ions Mg.sup.2+ 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.

[0039] 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 Ce.sup.3+ ions and the number of tetravalent Ce.sup.4+ ions are about the same. Put differently, the ratio of Ce.sup.3+ ions and Ce.sup.4+ 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 Ce.sup.3+ ions to about 50% tetravalent Ce.sup.4+ ions), but may numerically range from about 40%-60% trivalent Ce.sup.3+ ions to about 40%-60% tetravalent Ce.sup.4+ 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 Ce.sup.3+ ions and Ce.sup.4+ ions within crystals are roughly the same.

[0040] 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 Ce.sup.3+ ions and Ce.sup.4+ ions within the boule are roughly the same. The crystal growth method involves the several different process steps illustrated in FIG. 1. In addition, the process steps illustrated in FIG. 1 are preferably carried out using the specialized crystal growing device 200 shown in FIG. 2 (because this novel configuration creates a more uniform and stable thermal field, which thermal field facilitates crystal growth and enables the growth of large crystal boules of exceptional quality and uniformity).

[0041] More specifically, and with reference to FIG. 1 in view of FIG. 2, and starting with step 100, the novel crystal growing process of the present invention begins by supplying an iridium crucible 208. The iridium crucible 208, when filled with measured amounts of the starting compounds (i.e., lutetium oxide Lu.sub.2O.sub.3, yttrium oxide Y.sub.2O.sub.3, silicon dioxide SiO.sub.2, cerium oxide CeO.sub.2, as well as calcium carbonate CaCO.sub.3) is used to melt or otherwise alter its contents. The iridium crucible 208 is used to hold the different starting solid oxides and the carbonate salt (generally in powdered form and 99.999% pure) that, as part of the process, are melted together to form the melt used to grow the boule (not shown). Here, and as shown in FIG. 2, the iridium crucible 208 is a part of (i.e., an inner component of) a larger crystal growing device 200. The iridium crucible 208 can withstand temperatures on the order of 2100 degrees Celsius.

[0042] As shown in FIG. 2, the crystal growing device 200 of the present invention comprises an upper portion 202 and a lower portion 204. As shown, a generally doughnut-shaped iridium lid 210 is fitted on top of the iridium crucible 208 and separates the upper portion 202 from the lower portion 204. An iridium semi-ring 212 is similarly fitted near the top of the upper portion 202 and communicates with an opening 206 (used to add ingredients and for insertion of a seed crystal positioned at the end of a rodnot shown). As shown, the iridium crucible 208 is surrounded by two distinct types of thermal insulation; namely, a deoxidized zirconia insulation material 214 and a zirconia insulation material 216. The deoxidized zirconia insulation material 214 surrounds, and is in direct contact with, the iridium crucible 208, whereas the zirconia insulation material 216 surrounds, and is in direct contact with, the deoxidized zirconia insulation material 214. It has been found, surprisingly, that using two different types of zirconia-based insulation material better protects the iridium crucible 208 from loss of iridium over extended use (thereby reducing costs).

[0043] 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.

[0044] 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 crystalsincluding large crystal boules of co-doped LYSO:Ce,Ca having uniform composition and favorable scintillation dynamics).

[0045] With reference again to FIG. 1, and in step 102, the iridium crucible 208 is initially filled with specific chemical compounds in specific amounts. Here, and in the practice of the invention, the following oxides/carbonate salt and relative amounts (given in ranges) on a weight percent basis are used: Lu.sub.2O.sub.3 at about 85 to 86 parts, Y.sub.2O.sub.3 at about 1 to 2 parts, SiO.sub.2 at about 13 to 13.5 parts, CaCO.sub.3 at about 0.02 parts, and CeO.sub.2 at about 0.09 parts. These starting compounds, in precisely measured amounts, are placed into the iridium crucible 208 in this step.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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).

[0051] 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: Lu.sub.2O.sub.3 at about 85 to 86 parts, Y.sub.2O.sub.3 at about 1 to 2 parts, SiO.sub.2 at about 13 to 13.5 parts, CaCO.sub.3 at about 0.02 parts, and CeO.sub.2 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 Ce.sup.3+ ions to Ce.sup.4+ 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.

[0052] 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 Lu.sub.2O.sub.3, Y.sub.2O.sub.3, SiO.sub.2, CaCO.sub.3, and CeO.sub.2) 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.

EXAMPLES

TABLE-US-00001 Required weight Required weight Required weight of the oxides of the oxides of the oxides (dry result): (wet result): (final result): Lu.sub.2O.sub.3 6883.61200 Lu.sub.2O.sub.3 6906.19500 Lu.sub.2O.sub.3 6906.19500 Y.sub.2O.sub.3 99.79998 Y.sub.2O.sub.3 100.4832 Y.sub.2O.sub.3 100.4832 SiO.sub.2 1056.89180 SiO.sub.2 1057.00000 SiO.sub.2 1057.00000 CaCO.sub.3 1.76942 CaCO.sub.3 1.76942 CaCO.sub.3 1.36084 CeO.sub.2 6.0853 CeO.sub.2 6.0853 CeO.sub.2 6.0853 Total 8048.15850 Total 8071.53292 Total 8071.12434 Lu.sub.2O.sub.3 4623.17330 Lu.sub.2O.sub.3 4638.34000 Lu.sub.2O.sub.3 4638.34000 Y.sub.2O.sub.3 50.1461 Y.sub.2O.sub.3 50.4894 Y.sub.2O.sub.3 50.4894 SiO.sub.2 709.92360 SiO.sub.2 709.92360 SiO.sub.2 710.92360 CaCO.sub.3 0.47220 CaCO.sub.3 0.47220 CaCO.sub.3 0.20670 CeO.sub.2 1.3898 CeO.sub.2 1.3898 CeO.sub.2 1.3898 Total 5385.10500 Total 5400.61500 Total 5401.34950 Lu.sub.2O.sub.3 42.76930 Lu.sub.2O.sub.3 42.76930 Lu.sub.2O.sub.3 85.53860 Y.sub.2O.sub.3 0.4638 Y.sub.2O.sub.3 0.62 Y.sub.2O.sub.3 1.24 SiO.sub.2 6.56670 SiO.sub.2 6.56670 SiO.sub.2 13.13340 CaCO.sub.3 0.00000 CaCO.sub.3 0.00000 CaCO.sub.3 0.02200 CeO.sub.2 0.0000 CeO.sub.2 0.0000 CeO.sub.2 0.0929 Total 49.79980 Total 49.95600 Total 100.02690
Representative weights (in grams) of the starting oxides added to crucible for different grows.

[0053] 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.

[0054] With respect to detection and quantification of the amount of Ce.sup.3+ ions and Ce.sup.4+ 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., Ce.sup.3+ ions and Ce.sup.4+ 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 FIG. 3 illustrating a small signal edge (edge step) at a photon energy of about 5720 eV.

[0055] 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.