Abstract:
A method of fabricating a scintillator includes forming a green part comprised of a nanometer-sized powder, sintering the green part at a first temperature for a first time period, and sintering the green part at a second temperature for a second time period.

Description:
The present application is a continuation of and claims priority to U.S. patent application Ser. No. 11/623,723 filed Jan. 16, 2007, the disclosure of which is incorporated herein. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to imaging methods and apparatus, and more particularly, to methods and apparatus that provide for improvements in x-ray detector fabrication. 
     X-ray detectors typically include a photodiode portion and a scintillator portion. An x-ray enters the detector and impinges the scintillator material, wherein photons of visible light are created. The visible light then leaves the scintillator material and impinges a photodiode. The photodiodes are polled, returning attenuation measurements. This data is then used to create images. 
     Currently most of the scintillator ceramics are made from wet chemical processes. The wet chemical processes include dissolving all the ingredients in acid to make a homogeneous solution and the coprecipitation to convert the solution into a slurry. For instance, some scintillator ceramics are made with the oxalate coprecipitation processes. First Y2O3, Gd2O3, Eu2O3, and other dopants are dissolved in nitric acid to prepare a nitrate solution. The nitrate solution is mixed with an oxalic acid solution through dual flow and mixing. During the mixing, the nitrate reacts with the oxalic acid to form insoluble oxalate (a mixture of yttrium oxalate, gadolinium oxalate, europium oxalate, and the oxalate of other dopants). Then the oxalate is filtered and washed with DI water (deionized). The wet cake is dried after filtration. The dried powder is then milled and calcined to form an oxide powder. The oxide powder is dry compacted and sintered into a transparent ceramic. The process for Lu—Tb—Al—O based garnet scintillator is very similar. A sulfate or nitrate solution is first prepared, then the solution is mixed with ammonium hydroxide solution to form the Lu—Tb—Al hydroxide gel as a slurry. The slurry is filtered, washed, dried, milled, and calcined sequentially afterwards. 
     One problem of this process is the agglomeration of powder and the complexity of the processes. Therefore, it is desirable to have a nanometer sized powder process for the ceramic scintillators to achieve a relatively high transparent scintillator material with a relatively lower sintering temperature and to reduce defects and manufacturing cost. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a method includes fabricating an energy detector using a sol-gel process. 
     In another aspect, a detector includes a nanometer sized powder sintered with a grain size of less than 10μ. 
     In yet another aspect, a CT system includes a x-ray source configured to emit x-rays, a x-ray detector positioned to receive x-rays emitted by the source, and a computer operationally coupled to the source and detector, the detector comprising a nanometer sized powder sintered with a grain size of less than 10μ. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary diagnostic imaging system. 
         FIG. 2  illustrates a package/baggage inspection system. 
         FIG. 3  illustrates a method of fabricating an x-ray detector. 
         FIG. 4  illustrates a pixelated scintillator pack without a reflector. 
         FIG. 5  is a view of the pixelated scintillator pack shown in  FIG. 4  looking from the photon exit direction and illustrating reflector material between pixels. 
         FIG. 6  is a cross-sectional view also illustrating the scintillator pack of  FIG. 4  with a reflector positioned between pixels and a top reflector. 
         FIG. 7  illustrates a process wherein a mold is provided and within the mold, the pixelated scintillator array of  FIGS. 4-6  is placed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     There are herein described methods and apparatus useful for imaging systems such as, for example, but not limited to an x-ray system. The apparatus and methods are illustrated with reference to the figures wherein similar numbers indicate the same elements in all figures. Such figures are intended to be illustrative rather than limiting and are included herewith to facilitate explanation of an exemplary embodiment of the apparatus and methods of the invention. Although, described in the setting of an x-ray system, it is contemplated that the benefits of the invention accrue to all diagnostic imaging systems, all current modalities and/or any modality yet to be developed in which scintillators and reflectors are used. 
       FIG. 1  illustrates an imaging system  10  with an associated display  20 . Imaging system  10  can be of any modality, but in one embodiment, system  10  is a CT system. In another embodiment, system  10  is a dual modality imaging system such as a combined CT/PET system and data can be acquired in one modality (e.g., CT) and the processed data can be transferred to the other modality (e.g., PET). Display  20  can be separate from system  10  or integrated with system  10 . System  10  includes an acquisition device such as an x-ray radiation detector. It is contemplated that the benefits of the invention accrue to human and non-human imaging systems such as those systems typically employed in small animal research. Also, it is contemplated that the benefits of the invention accrue to non-medical imaging systems such as those systems typically employed in an industrial setting or a transportation setting, such as, for example, but not limited to, a baggage scanning CT system for an airport or other transportation center as shown in  FIG. 2 . 
     Referring now to  FIG. 2 , a package/baggage inspection system  30  includes a rotatable gantry  40  having an opening  50  therein through which packages or pieces of baggage may pass. The rotatable gantry  50  houses a high frequency electromagnetic energy source  60  aligned with an attenuation filter  70  as well as a detector assembly  80 . A conveyor system  90  is also provided and includes a conveyor belt  100  supported by structure  110  to automatically and continuously pass packages or baggage pieces  120  through opening  50  to be scanned. Objects  120  are fed through opening  50  by conveyor belt  100 , imaging data is then acquired, and the conveyor belt  100  removes the packages  120  from opening  50  in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages  120  for explosives, knives, guns, contraband, and the like. 
       FIG. 3  illustrates a method  128  that can include the step of making a precursor solution  130 , the step of stabilizing the precursor solution  132 , and heating the stabilized solution to form a gel  134 . Method  128  also can include drying the gel at step  136 , milling the dried gel at step  138 , and calcining the milled dried gel at a relatively low temperature at step  140 . Additionally method  128  can include the step  142  of forming an x-ray detector. The forming may be done using a dry forming process  144  or a wet forming process  146  as described in more detail below. 
     Herein described are methods and apparatus that facilitate the making of a nanometer-structured ceramic scintillator. The herein described methods and apparatus apply to all ceramic scintillator materials with a cubic structure. The scintillator compositions covered in this disclosure include ones based on Y2O3—Gd2O3—Eu2O3, GGG based on Gd2O3—Ga2O3:Cr, and Lu—Tb—Al—O:Ce system ceramic scintillators. First, a nanometer ceramic powder with very low agglomeration is produced by a sol-gel (solution/gelatin) method. Then the nanometer powder is sintered into a transparent ceramic at a relatively low temperature. In the sol-gel process, the chemicals have to be carefully selected for making the precursor solution and gel. In the Lu—Tb—Al—O—Ce system, one example of the starting materials is lutetium acetate hydrate (&gt;99.99%) (Lu(O2CCH3)3.xH2O), terbium acetate hydrate (&gt;99.99%) (Tb(O2CCH3)3.xH2O), cerium nitrate (&gt;99.99%) (Ce(NO3)3.6H2O), and aluminum formate hydrate (&gt;99.99) (Al(O2CH)3.3H2O) with the proper ratio (for example Lu0.8Tb2.17Ce0.03Al5O12) are dissolved in hot DI water (deionized). The certain amount of formic acid, ethylene glycol, and isobutyric acid are added to stabilize the solution. The solution is heated to about 60° C. to 80° C. to remove some water and increase the viscosity by polymerization. Once the solution becomes a transparent gel with proper viscosity, it can be moved into a furnace for drying at about 100° C. to 200° C. The dried powder is then ball milled or jet milled to prevent hard agglomeration. After milling, the powder is calcined at about 600° C. to 900° C. The lower calcining temperature avoids hard agglomeration. This process yields a nanometer-sized ceramic powder that is highly sinterable and highly flow-able. The powder is now ready for further processing. 
     In the Y—Gd—Eu—O:Pr system, one example of the starting material is yttrium acetate hydrate (&gt;99.99%) (Y(O2CCH3)3.xH2O), Gadolinium acetate hydrate (&gt;99.99%) (Gd(O2CCH3)3.xH2O), and europium acetate hydrate (&gt;99.99%) (Eu(O2CCH3)3.xH2O), and Pr(NO3)3.xH2O (&gt;99.99%). The alkoxides of a desired ratio are mixed together and dissolved in DI water. Then the praseodymium nitrate can be added into the solution. A certain amount of ethylene glycol and nitric acid can be added to make a transparent solution. The solution is then heated to about 60° C.-80° C. for the polymerization. Once the solution becomes a transparent gel with proper viscosity, it can be moved into a furnace for drying at about 100° C. to 200° C. The dried powder can then be ball milled or jet milled to prevent hard agglomeration. After milling, the powder may be calcined at about 600° C. to 900° C. The lower calcining temperature avoids hard agglomeration. This process yields a nanometer-sized ceramic powder that is highly sinterable and highly flow-able. 
     For the GGG scintillator, the starting materials is gadolinium acetate hydrate (&gt;99.99%) (Gd(O2CCH3)3.xH2O), gallium acetate hydrate (&gt;99.99%) (Ga(O2CCH3)3.xH2O), and chromium acetate hydrate (Cr(O2CCH3)3.xH2O). The raw materials with the proper ratio to yield Gd3—xCrxGa5O12 (x=0.01-0.05) are dissolved in hot DI water. An amount of formic acid, ethylene glycol, and isobutyric acid are added to stabilize the solution. The solution is heated at about 60 to 80° C. to dry the water and increase the viscosity by polymerization. Once the solution becomes a transparent gel with the proper viscosity, it can be moved into a furnace for drying at about 100° C. to 200° C. The dried powder can then be ball milled or jet milled to prevent hard agglomeration. After milling, the powder can be calcined at about 600° C. to 900° C. The lower calcining temperature avoids hard agglomeration. This process yields a nanometer-sized ceramic powder that is highly sinterable and highly flow-able. The powder is now ready for further processing. 
     Once the nanometer sized powder is obtained, it can be formed into a ceramic green part by either dry compact method or wet cast method. 
     For the dry compact method, the ceramic blocks can be sintered in a hydrogen or a vacuum furnace. One goal of the sintering is to achieve a relatively high transparency at the lowest temperature possible. In order to achieve that, the grain growth has to be controlled. For a normal ceramic system such as laser ceramic, a sintering aid is added to restrict the grain growth so densification can occur without trapping the pores. Due to the special requirements of the scintillators, the sintering aid can not be used. Without the sintering aid, one needs to ensure that the pores stay at the grain boundaries so that any gas inside the pores can diffuse out quickly. Toward that end, a two stage sintering method was developed to achieve the high densification without any significant grain growth and to achieve a nanometer-structured ceramic scintillator. First, the ceramic is heated to the highest temperature of the process and held at that temperature for very short time, then the ceramic is cooled down to a lower temperature and held for much longer time. For instance, the dry-compacted and further iso-pressed Y—Gd—Eu—O:Pr ceramic is heated to about 1850° C. to 1950° C. and held for about 10 minutes to 1 hour, typically 30 minutes; Then the ceramic can be cooled down to about 1600° C. to 1700° C. and held for about 5 hours to 10 hours. The sintering is done in a hydrogen atmosphere. The higher temperature hold is to provide energy to grow necks between particles and pin the pores between grain boundaries. The low temperature hold is to provide sufficient energy for the pores to diffuse out of the ceramic through grain boundaries while preventing significant grain growth. Typically the grain size can be less than 1 to 2 microns, compared to normal sintering process that leads to over 20 micron grain size. Note, the herein described methods and apparatus provide a nanometer sized powder sintered with a grain size of less than 10μ. 
     For the Lu—Tb—Al—O:Ce system ceramic scintillator, the dry-compacted ceramic blocks are further iso-pressed to increase green density. The ceramic blocks can then be sintered in a vacuum. First, the ceramic blocks can be heated to about 1650° C. to 1750° C. and held for about 15 minutes to 1 hour, typically 30 minutes. Then the ceramic blocks are cooled down to about 1500° C. to 1600° C. and held for 5 to 10 hours. The mechanisms of control grain growth and achieving full densification are the same as described above. The similar process applies to GGG ceramic scintillator. 
     The alternative method to the dry compact method is the wet cast method. The nanometer sized-ceramic powder can be mixed with DI water and a small amount of dispersant to form a slurry. Then the slurry can be cast into a mold with required dimensions. The slurry can be allowed to dry in the mold and then taken out for further processing. After drying, the green ceramic blocks can be heated in oxygen to about 600° C. to remove all the dispersant and water and other additives. The sintering process as described above can be performed afterwards. 
     One important aspect of the herein described methods and apparatus is to achieve nanometer sized ceramic powder with no hard agglomeration and sintering the ceramic without significant grain growth. It provides for full densification and the relatively high transparency of the resulting scintillator while making it possible to sinter at a relatively low temperature and without relatively high pressure methods such as hot pressing and hot iso-static pressing. This provides better performance and lower manufacturing cost. 
       FIG. 4  illustrates a pixelated scintillator pack  150  including a plurality of pixels  152 . Note there is no reflector yet.  FIG. 5  is a view of pixelated scintillator pack  150  looking from the photon exit direction, and illustrates reflector material  154  (which is white in the drawing) between pixels  152 .  FIG. 6  is a cross-sectional view also illustrating scintillator pack  150  with reflector  154  positioned between pixels  152  and a top reflector  156 . Note, energy enters into the top of scintillator pack  150  as viewed looking straight on in  FIG. 6  as x-ray energy and this energy is converted to visible photons by the scintillation material within a pixel  152 , and these visible photons then impinge a photodiode that would be on the bottom side of pack  150  viewing  FIG. 6  straight on. 
       FIG. 7  illustrates a process  200  wherein a mold  202  is provided and within the mold, pixelated scintillator array  150  is placed. Reflector material  154  in a slurry state is then poured into mold  202 . As shown in  FIG. 7 , reflector material  154  may be delivered using a beaker  206 . However, any delivery system may be employed including pipes and robots. The reflector  154  is then solidified in the mold  202 . Afterwards, the entire assembly is taken out of the mold and machined to the desired geometry (final scintillator pack). 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     Technical effects include that the herein described methods and apparatus allow for a nanometer-structure ceramic scintillator with relatively low defects and more uniform properties. The herein described methods and apparatus allow for a low sintering temperature based on nanometer-powder process. The herein described methods and apparatus allow for a relatively high transparency of the ceramic scintillator that leads to high light output and more uniform spectral performance. 
     Exemplary embodiments are described above in detail. The assemblies and methods are not limited to the specific embodiments described herein, but rather, components of each assembly and/or method may be utilized independently and separately from other components described herein. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.