Abstract:
A radiation detection system may include a detector. The detector may include a scintillator to convert ionizing radiation, which originates externally to the detector, into visible light, a sensor configured to detect the visible light from the scintillator, and a light source. The radiation detection system may further include a controller programmed to control the light source to expose the scintillator to a light to saturate traps in the scintillator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation of U.S. National Stage application Ser. No. 14/897,881, filed Dec. 11, 2015, which is based on International Application No. PCT/US14/42099, filed Jun. 12, 2014, and claims the benefit of priority to U.S. Provisional Patent Application No. 61/835,072, filed Jun. 14, 2013, the entire disclosures of which are incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is related to high-energy imaging systems and high-energy photon detectors. 
       BACKGROUND 
       [0003]    Scintillation materials are scientifically and economically significant in conjunction with photodetectors to detect high-energy photons, electrons and other particles in various applications, including medical imaging, geological exploration, homeland security, and high-energy physics. In radiation detectors/imaging devices, scintillation material (for example, cerium-doped scintillators) may be used. Certain characteristics are desirable in these scintillators, in order to maximize their value in these applications. In general, high scintillation light yield, fast scintillation kinetics (both in decay time and rise time), good energy resolution, a high degree of proportionality, and relative insensitivity to ambient light exposure are desired. 
         [0004]    To these ends, it is desirable to obtain a composition of scintillator free or relatively free of electron/hole traps and other defects that may impede the scintillation process, and/or reduce the undesirable effects of traps and defects in scintillators. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  illustrates a radiation detector according to an embodiment of the invention. 
           [0006]      FIG. 2  illustrates characteristics of a radiation detector according to an embodiment of the invention. 
           [0007]      FIG. 3  illustrates characteristics of a radiation detector according to an embodiment of the invention. 
           [0008]      FIG. 4  illustrates characteristics of a radiation detector according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]      FIG. 1  illustrates a radiation detector  100  according to an embodiment of the invention. 
         [0010]    According to an embodiment, the radiation detector  100  may include a scintillator  110 , a light source  120 , and a sensor or sensors  140 . 
         [0011]    The scintillator  110  may include various scintillation materials capable of converting non-visible radiation (incoming radiation) into visible light (emitted radiation). The sensor  140  may be placed in adjacent or in close proximity to the scintillator  110 , such that any converted visible light may be detected or measured by the sensor  140 . The light source  120  may be placed in adjacent or in close proximity to the scintillator  110 , such that light from the light source  120  may interact with defects in the scintillator  110  to minimize interference on the conversion of non-visible radiation into visible light caused by the defects. 
         [0012]    The scintillator  110  may be or may include a material that, when interacting with ionizing radiation, has a measurable response that may ultimately be used to produce an image. In some embodiments, the material is a scintillation material that produces photons of light when excited by the incoming radiation. 
         [0013]    The measured quantity of light at the sensor  140  may be referred to as ‘light yield’ or ‘light output’, and is usually reported either as relative to a known standard or in absolute numbers as photons per MeV. 
         [0014]    In some embodiments, a radiation detector  100  may be suitable for use in a variety of radiation detection applications including medical imaging applications such as positron emission tomography (PET), time-of-flight positron emission tomography (TOF-PET), X-ray computed tomography (X-ray CT) or single-photon emission computed tomography (SPECT), and any other multimodality systems (PET-CT, PET-MRI, PETSPECT). 
         [0015]    The light source  120  may expose all portions of the scintillator  110  to light of various selected wavelengths to stabilize any longtime light output drift. The drift may be caused by the defects in the scintillation material of scintillator  110 . The exposure of the scintillator  110  to light of certain wavelengths may take place inside the radiation detector  100  to maintain defects in the scintillator  110  in their saturated energy states. The saturated defects may have significantly lower influence or interference on the scintillation process. The exposure to light may be done during the normal work condition of the radiation detector  100 , as well as during calibration or its idle time, continuously or intermittently or in pulses. 
         [0016]    In some embodiments, in order to optimize performance of the radiation detector  100  for any given application, a specific light wavelength or multiple bands/ranges of wavelengths, intensity, and exposure time/duration/frequency from the light source  120  may be selected or tuned. The selection of light wavelengths, intensity, and exposure time/duration/frequency of the light source  120  may be done during design of the radiation detector  100 . Alternatively, tuning of light wavelengths, intensity, and exposure time/duration/frequency of the light source  120  may be done on the fly during operation or calibration, to adjust for the performance changes, such as caused by environmental conditions. The tuning may be controlled by a controller (not shown) that controls the light source  120 , and the tuning may be controlled based upon measurements of performance of the radiation detector  100 . 
         [0017]    The radiation detector  100  may further include a filter  130  between the scintillator  110  and the sensor  140 , which may filter out light from the light source  120 , to increase signal contrast. Because the light from the light source  120  may transmit through the scintillator  110  and reach the sensor  140 , the sensor  140  may detect the light from the light source  120 . The detected light from the light source  120  may be generally considered as noise. To allow the sensor  140  to detect the light from the scintillation process in the scintillator  110  and not detect (or detect less of) the light from the light source  120 , the filter  130  may filter out the light from the light source  120 . Since the light from the light source  120  may be tuned/selected with specific wavelengths, the filter  130  may be similarly tuned/selected to filter/block out the specific wavelengths of the light from the light source  120 , such that the light from the light source  120  may be reduced in intensity after passing through the filter  130  and upon reaching the sensor  140 . 
         [0018]    In an embodiment, the light from the light source  120  may be generated by a light emitting diode (LED) or other light source of desired wavelength in the radiation detector  100  housing such that the scintillator  110  is exposed to the light from the light source  120 . Or the scintillator  110  may be surrounded with a phosphor or other material that emits light at the desired wavelengths when excited by the normal emission of the scintillator itself. The light exposure should be sufficient in duration and/or intensity to saturate the defect centers in the scintillator  110 , so that the defect centers are substantially incapable of trapping charge carriers and the scintillation process may be substantially unimpeded. 
         [0019]    The light source  120  may also include a light pipe or waveguide or reflective surfaces, designed to channel light to the scintillator  110 . 
         [0020]    X-ray or other ionizing radiation sources may be implemented as part of the light source  120  into the radiation detector  100 , and/or designed as a part of a x-ray generator, or radiation source. In certain embodiments of the presently disclosed subject matter, the radiation source may be removed from the vicinity of the radiation detector  100  or shielded from the radiation detector  100  to control the exposure time and amount of energy absorbed in the volume of the scintillator  110 . 
         [0021]    In an embodiment, the radiation detector  100  may expose every portion of the scintillator  110  with non-visible radiation (incoming radiation, such as x-ray) for a duration during a calibration phase, to saturate all of the defects of the scintillator  110 , and then during an operation phase, the same non-visible radiation (incoming radiation, such as x-ray) may pass through a sampling material (for example a biological sample) to generate an image in the non-visible radiation, which would be detected or measured by the radiation detector  100  with saturated defects. During the calibration phase, the light source  120  may channel or direct the non-visible radiation (incoming radiation, such as x-ray) with relatively uniform intensity to every portion of the scintillator  110 . During the operation phase, the light source  120 &#39;s non-visible radiation may pass through the sampling material and then into the scintillator  110 . 
         [0022]    In an embodiment, the scintillator  110  may be garnet-type scintillators, such as Gadolinium Gallium garnet crystals, for example doped with Cerium (Ce) (generally referred to as GGAG or GAGG scintillators). In some embodiments, the presently disclosed subject matter incorporates gadolinium gallium garnet crystals or ceramics into the radiation detector  100  as the scintillation material. However, other material may also be used. Scintillating material compositions may include, but are not limited to, Gd 3 Ga 3 Al 2 O 12 , Gd 3 Ga 2 Al 3 O 12 , Gd 3 Ga 1 Al 4 O 12 , and other variants. These scintillators may be doped with an activator/dopant, such as cerium or praseodymium, with or without a codopant. 
         [0023]    For initial evaluation of the potential of these gallium garnets according to the presently disclosed subject matter, cerium-doped Gd 3 Ga 3 Al 2 O 12  (GGAG) in single-crystal form was used as an example material in the scintillator  110 . However, the presently disclosed subject matter is not limited to this precise composition, nor is it limited to cerium as an activator, nor is it limited to single crystal form. Additional codopants, both aliovalent and isovalant with respect to the activator, may also be used in these compositions. 
         [0024]    Gadolinium Gallium garnets are a promising class of scintillator  110 , with high density and potentially good scintillation properties. However, materials with garnet or pervoskite structures may have vacancy defects and antisite defects, which may cause interference in the scintillation properties of these materials. 
         [0025]    Radiation detectors, such as those used in imaging applications, may rely on consistent performance of the scintillator over the lifetime of the radiation detectors. The hardware, electronics, and software of imaging equipment is designed with the expectation that scintillator light yield will remain relatively constant within a narrow range, and deviations from this expectation may result in degraded signals or images detected. 
         [0026]    This required consistent scintillator performance may, however, be difficult or impossible to achieve in crystalline materials alone with lattice defects, including vacancy and antisite defects. 
         [0027]    Scintillator  110  may include materials with defect trap centers, such that the concentration of trapped charge carriers may change over time, which consequently makes the light output dependent on time, i.e. the light yield may diminish over time. This diminished light yield may cause unstable and degrading scintillator performance in medical imaging devices. 
         [0028]    Therefore, according to some embodiments of the presently disclosed subject matter, a method and an apparatus are provided to re-saturate these traps to restore the diminished light yield by exposing the scintillator crystals to appropriate specific wavelengths of light, x-rays or any other type of ionizing radiation, creating a radiation detector  100  with stable light yield during the operations of the radiation detector  100 . 
         [0029]    In addition, some other defect trap centers may produce an afterglow effect that does not necessarily compete with the scintillation mechanism or cause a time-dependent light output, but does produce a background noise signal of additional undesirable photons in the scintillator  110 . These “afterglow” defect trap centers may be filled during exposure to light of specific wavelengths. The filled “afterglow” defect trap centers may cause the afterglow effect. These defect trap centers may be emptied/detrapped by low temperature heating of the crystal or may be emptied optically, by irradiation of the crystal with certain wavelengths leading to an optical bleaching phenomena. Contrary to the trap centers that cause diminishing light output, it is preferable that the afterglow defect trap centers be empty instead of filled, to reduce the afterglow effect. 
         [0030]    In an embodiment, the light source  120  (or a second light source near the scintillator  110 ) may further generate light to interact with afterglow defects in the scintillator  110  to minimize afterglow caused by the afterglow defects, by for example, tuning the light (for specific wavelengths, intensity, and duration) to empty/detrap any afterglow defects in all portions of the scintillator  110 . 
         [0031]      FIG. 2  illustrates characteristics of a radiation detector according to an embodiment of the invention, by illustrating the thermo-luminescence (TL) glow curve of GAGG:Ce crystal in a range of temperature. 
         [0032]      FIG. 2  shows a glow band around 300 degrees K, which may be referred to as a room temperature trap center. 
         [0033]    The TL spectrum measured for GAGG:Ce crystals (5×5×5 mm 3  test sample) shows several glow bands (with glow peaks) and thus the presence of a number of trap centers from 10 degrees K to 500 degrees K. The defect trap centers responsible for the glow peaks seen at 300 K may be primarily responsible for affecting the room temperature scintillation properties in a scintillator. 
         [0034]    The defect trap centers at room temperature may be thermally depopulated (i.e., emptied/detrapped) either by heating the sample or keeping the sample in the dark for a long time. These emptied defect trap centers then may interfere with scintillation process. 
         [0035]    During sample testing, the scintillation light output was decreased by ˜45% by heating the crystal (5×5×5 mm 3 ) at 500° C. in air. 
         [0036]    The light output was also found to decrease with time by keeping the samples in the dark at room temperature as shown in  FIG. 3 . 
         [0037]      FIG. 3  illustrates characteristics of a radiation detector according to an embodiment of the invention. 
         [0038]    As illustrated in  FIG. 3 , keeping the crystal sample in darkness in room temperature may cause the relative light output to decrease over time. The relative light yield is presented in channel numbers in comparison to the performance of a standard Bismuth germanium oxide (BGO) crystal whose light output was set at reference channel number  100 . 
         [0039]    If not prevented, the decrease in light yield over time may lead to reduced performance in the radiation detector  100 . Therefore, it is desirable to maintain the light yield at, or restore it to, the original value. 
         [0040]    To determine the proper wavelength(s) of light from the light source  120  for restoring the light output with light exposure, the samples (5×5×5 mm 3 ) were heated at 300° C. in order to empty the charge carrier traps, reducing the measurable light yield. Subsequently, the restoration of light yield was tested by monitoring the photoluminescence (PL) emission intensity, as a representation of light yield, after the samples were exposed to light of different wavelengths. 
         [0041]      FIG. 4  illustrates characteristics of a radiation detector according to an embodiment of the invention, specifically the effect of the light exposure of different wavelengths on the restoration of PL intensity after GAGG:Ce crystal samples were heated. The horizontal axis of  FIG. 4  represents wavelengths of incoming radiation used to produce photoluminescence for the measurements. 
         [0042]    As shown in  FIG. 4 , the PL intensity was decreased significantly in a sample after the charge carrier traps were emptied (heated at 300° C. for a duration), as compared to the “before annealing” measurements. The samples were then individually exposed to light of different wavelengths, and then tested via photoluminescence in order to observe the ability of each specific wavelength of light energy to re-fill the defect traps. 
         [0043]    The PL emission intensity was restored to the original value after exposing the samples to light at 445 nm (from for example Ce 4f energy band to 5d energy band transitions) for 10 minutes. 
         [0044]    Similarly, these results hold true for scintillation light yield by heating the crystals to 500° C., emptying the traps, and then measuring the light yield scintillation from radiation of a Cs-137 source. The measured light yield was reduced by ˜45%. The crystals were then exposed to light of 445 nm wavelength for 30 minutes, and the original light yield of scintillation was restored. 
         [0045]    Table 1 below shows the relative scintillation light output of GAGG:Ce crystal annealed at different temperatures. The light output was measured immediately after annealing without exposing the crystals to light, and then measured again after exposing the samples to ambient light for 21 hrs. The reported light yield is relative to a BGO reference standard for which the light yield was set to 100. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Relative scintillation light output of GAGG:Ce 
               
               
                 crystal annealed at different temperatures 
               
             
          
           
               
                   
                 Before exposing 
                 After exposing to 
               
               
                   
                 to light 
                 light for 21 hrs 
               
               
                   
                   
               
             
          
           
               
                   
                 As grown (no anneal) 
                 279 
                 306 
               
               
                   
                 Annealed at 900° C. 
                 163 
                 306 
               
               
                   
                 Annealed at 1100° C. 
                 160 
                 295 
               
               
                   
                 Annealed at 1300° C. 
                 166 
                 310 
               
               
                   
                   
               
             
          
         
       
     
         [0046]    Therefore, a radiation detector  100  with a light source  120  which exposes the scintillator  110  with light at 345 nm or 445 nm wavelengths (which may ionize Ce 3+ ), may maintain substantially constant or stable light yield over the device operation lifetime by saturating defect trap centers and preventing the defect trap centers from impeding the scintillation process. 
         [0047]    As illustrated in  FIG. 4 , the PL intensity measured after the samples were exposed to light of 445 nm wavelength was higher than the sample “before annealing”. This is due to the contribution of TL emissions during testing. 
         [0048]    Although the invention has been described above with reference to specific embodiments, the invention is not limited to the above embodiments and the specific configurations shown in the drawings. For example, some components shown may be combined with each other as one embodiment, or a component may be divided into several subcomponents, or any other known or available component may be added. Those skilled in the art will appreciate that the invention may be implemented in other ways without departing from the spirit and substantive features of the invention. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.