Scintillator materials used in X-ray detectors require high X-ray absorption efficiency and light output, low afterglow, transparency and spectral response matching with photoelectric converters. The performance of different scintillators varies greatly. In order to cooperate with the specific CCD, we should consider the luminous efficiency, spectral response and other properties comprehensively, and select suitable scintillator materials.

The mean absorption depth of X-ray radiation in the scintillator depends on photon energy and the material. the advantage of the material transparency decreases with the thickness of the imaging plate. If the scintillator is thinner~ the mean absorption depth is lower and the created image is sharper due to less blurring of the image due to less lateral spread of the scintillation photons. Hence~ the thinner the imaging plate is~ the better is the resolution achieved in the image. On the other hand, the detection efficiency decreases with scintillator thickness.

The X-ray detector based on CCD consists of scintillator, optical coupler and CCD array. It covers a thin layer of scintillator on the linear array of CCD. The scintillator converts X-ray into visible light sensitive to CCD, and then converts CCD into electrical signal reflecting X-ray distribution in space.

Types of scintillator crystal applied in x-ray detection

Ce:YAG

Wavelength(Max. emission) (nm)550
Decay time (ns)70
Light yield (photons/keV)35
Refractive index1.82@550nm
Radiation length (cm)3.5

The yttrium aluminum garnet(YAG) of chemical formula Y3Al5O12 activated by cerium is a fast scintillator with excellent mechanical and chemical resistance and is commercially available,The decay time is around 70 ns that makes possible a high counting rate and the quantum yield is estimated to be between 40 and 50 ph/keV,Its non-hygroscopic nature and short decay time is advantageous compared to the standard CsI:Tl scintillator in spite of a smaller light output,The wavelength of maximum emission (around 550 nm) is well suited to the sensitivity of the photo- cathode of the PMT module.

The experiments proved that the YAG:Ce and LuAG:Ce screens are suitable for imaging with high spatial resolution.The resolution of the presented imaging system is about one micrometer.

References
[1] An X-ray counting system based on YAG Ce scintillator
[2] Thin YAG:Ce and LuAG:Ce single crystal imaging plates used for high spatial resolution in X-ray imaging systems
[3] Similarity of trap state and thermoluminescence processes of Ce YAG for X-ray and UV irradiation
[4] High-resolution imaging of biological and other objects with an X-ray digital camera
[5] High-resolution application of YAG Ce and LuAG Ce imaging detectors with a CCD X-ray camera

Ce:YAP

Wavelength(Max. emission) (nm)370
Decay time (ns)28
Light yield (photons/keV)25
Light output relative to Nal(Tl)  (%)60-70
Refractive index1.95@370nm

Among the properties that make a YAP(Ce) crystal an excellent scintillator are (i) its fast decay time of 30 ns, (ii) its high density of 7.4 g/cm3, and (iii) its high light yield about 30% relative to that in a NaI(Tl) scintillator. Ytrium orthoaluminate (YAlO3: Ceor YAP: Ce) are fast emitting scintillators employed mainly in PET and animal PET detectors.

The x-ray luminescence efficiency (XLE) of LYSO:Ce, YAP:Ce, GSO:Ce and BGO as determined by the experimental data for x-ray tube voltages between 22–42kVp(mammography).Points: measured data, line: fitted curve.

The x-ray luminescence efficieny (XLE) of LYSO:Ce, YAP:Ce, GSO:Ce and BGO as determined by the experimental data for x-ray tube voltages between 50–140 kVp (general radiography).Points: measured data, line: fitted curve.

Energy spectra of a 1-mm thick crystal (J) and a 5-mm thick crystal (K) ofYAP:Ce. The X-ray energy was 50 keV

References
[1] A YAP(Ce) imager operated in high energy X-ray region
[2] Comparative evaluation of single crystal scintillators under x-ray imaging conditions
[3] Comparative Investigation of Ce Doped Scintillators in a Wide Range of Photon Energies Covering X-ray CT, Nuclear Medicine and Megavoltage Radiation Therapy Portal Imaging Applications
[4] ISPA tubes with scintillating YAP Ce windows X- and γ- ray imaging
[5] High-resolution application of YAG Ce and LuAG Ce imaging detectors with a CCD X-ray cameraMcps-range photon-counting X-ray computed tomography system utilizing an oscillating linear-YAP(Ce) photon detector
[6] Properties of a YAP Ce detector for high-energy X-ray counting experiments

Ce:LYSO

Wavelength(Max. emission) (nm)410
Decay time (ns)40
Light yield (photons/keV)25
Light output relative to Nal(Tl)  (%)75
Refractive index1.82@410nm

Lutetium–yttrium-based scintillators, such as LYSO:Ce, have a high effective atomic number, are non-hygroscopic, fast emitting materials, and promising candidates for use in positron emission imagers.

LYSO:Ce is a mixed LSO/YSO (5–10%) non-hygroscopic crystal that offers high density (7.1 g/cm3), high light output (30000 ph/MeV), good energy resolution (10%) and short decay time (40 ns). Although LYSO:Ce and LSO:Ce demonstrate similar behavior, as far as the decay scheme is concerned , LYSO:Ce appears to be of approximately 20% higher light yield than LSO under low-energy (35 kV) X-ray excitation.

The x-ray luminescence efficiency (XLE) of LYSO:Ce and GSO:Ce as determined by the experimental data for x-ray tube voltages between 22–42 kVp (mammography). Points: measured data, line: fitted curve

The x-ray luminescence efficiency (XLE) of LYSO:Ce and GSO:Ce as determined by the experimental data for x-ray tube voltages between 40–140 kVp (general radiography). Points: measured data, line: fitted curve

References
[1] A comparative study of the luminescence properties of LYSO Ce, LSO Ce, GSO Ce and BGO single crystal scintillators for use in medical X-ray imaging
[2] Comparative evaluation of single crystal scintillators under x-ray imaging conditions
[3] Comparative Investigation of Ce Doped Scintillators in a Wide Range of Photon Energies Covering X-ray CT, Nuclear Medicine and Megavoltage Radiation Therapy Portal Imaging Applications
[4]Evaluation of the light emission efficiency of LYSO Ce scintillator under X-ray excitation for possible applications in medical imaging
[5] Improving Ce3+ doped scintillating materials for medical imaging applications
[6]  Investigation of luminescence emission properties of LYSO Ce and LuYAP Ce single crystal scintillators under x-ray exposure for use in medical imaging
[7] Luminescence Properties of LYSO ce and GSO Ce Single Crystal Scintillators Under X-Ray Excitation for Use in Medical Imaging Systems
[8] Scintillator efficiency study with MeV x-rays

Ce:GAGG

Wavelength(Max. emission) (nm)520
Wavelength range (nm)475-800
Decay time (ns)90
Light yield (photons/keV)50
Refractive index1.9@540nm

Ce doped Gadolinium Aluminum Gallium Garnet (Gd3Al2Ga3O12:Ce or GAGG:Ce) is a recently developed mixed scintillator crystal with high density (6.69 gcm-3), fast scintillation response (~90 ns) and high light yield (~46000 ph/MeV). Compared to many current efficient scintillators, GAGG:Ce is non hygroscopic (like LaBr3:Ce, LaCl3:Ce, CsI and NaI:Tl crystals) and it does not have natural radioactivity (like Lutetium based materials). GAGG:Ce emits optical photons at 520nm and has an effective atomic number equal to 54.4.

Variation of the X-ray luminescence efficiency (XLE) of the GAGG:Ce crystals with Xray tube voltage

Variation of the detector quantum gain (DQG) of the GAGG:Ce crystals with X-ray tube voltage

References
[1] X-ray Luminescence Efficiency of GAGG Ce Single Crystal Scintillators for use in Tomographic Medical Imaging System
[2] Light Emission Efficiency of GAGG Ce Single Crystal Under X-ray Radiographic Conditions

Ce:LuAG

Wavelength (Max. Emission) (nm)535
Wavelength range (nm)475-800
Decay time (ns)70
Light yield (photons/keV)25
Refractive index1.84@633nm

The high-resolution imagingsystem is a combination of a high-sensitivity digital CCD camera and an optical system with a thin scintillator-imaging screen. The screen can consist of LuAG:Ce inorganic scintillator.

[Ref1]The light emission efficiency (the number of visible photons per keV) and the spatial resolution of both types of screens are compared. In the first experiment, the light detected by the CCD is averaged in a squared ROI of 200_200 pixels in the center of the CCD. The YAG:Ce gives a value 17,507 (number of electrons in CCD pixel per second) and the LuAG:Ce gives the value 26,452, which is about 1.51 times the value of YAG:Ce. The LuAG:Ce single crystal is more dense compared to YAG:Ce (density: 6.73–4.57 g/cm3) and the X-rays are absorbed stronger by LuAG (1.7 times more of X-ray radiation (photons) is absorbed in the range between 1 and 40 keV, as calculated using X-ray radiation attenuation coefficients). The LuAG:Ce screen has higher conversion efficiency than the YAG:Ce screen, so that the signal-to-noise ratio of the image is better for the use in the imaging system.

[Ref2] The image was taken using the LuAG:Ce 20 mm screen. The effective pixel size of the CCD camera was 0.74mm. The X-ray microfocus source was operated at 40kV/ 2mA. The image acquisition time was 5s and the averaging was performed with samples of 25 images.The measured intensity of the light generated by LuAG:Ce is about1.51times the value of YAG:Ce after conversion of the photons flux from the source. The light was detected by the CCD and averaged in a squared ROI of 200×200pixels. The LuAG:Ce single crystal is more dense compared to YAG:Ce (density:6.73–4.57g/cm3) and the X-rays are absorbed more strongly by LuAG (an average of 1.7 times more of X-ray radiation is absorbed in the range between 1 and 40keV).

Absorption of X-ray radiation in 20mm thick laye rof YAG and LuAG X-ray Form Factor.

References
[1] High-resolution application of YAG Ce and LuAG Ce imaging detectors with a CCD X-ray camera
[2] High-resolution imaging of biological and other objects with an X-ray digital camera
[3]  Thin YAG Ce and LuAG Ce single crystal imaging plates used for high spatial resolution in X-ray imaging systems

CsI

Wavelength(Max. emission) (nm)410
Decay time (ns)40
Light yield (photons/keV)25
Light output relative to Nal(Tl)  (%)75
Refractive index1.82@410nm

The most important features of CsI(Tl) is its high-light yield (≥104 photons/MeV) and its emission spectrum having the maximum at about 550 nm, well compatible to amorphous and crystalline silicon photodiodes. the light emission efficiency of the CsI:Tl single-crystal scintillator was investigated as a function of crystal thickness and X-ray tube voltage in the energy range used in X-ray mammography and in general X-ray imaging.

[Ref1] The absolute luminescence efficiency, which was experimentally determined in a wide range of X-ray energies employed in X-ray imaging (40–140 kV) and in mammographic imaging (22–49 kV). The crystals were irradiated by X-rays using: (i) A Philips Optimus X-ray unit with a tungsten anode target and 2mm Al filter and (ii) a General Electric Senographe DMR X-ray mammography unit equipped with a molybdenum anode target and molybdenum filter, using an experimental set-up previously described.

[Ref2] To address these needs we have developed a prototype fast x-ray imaging system, using a structured CsI(TI) scintillator coupled to a fast-frame 1K x 1K CCD. The system has been successfully employed to capture 1024 x 64 pixel x -ray images at a rate of 1000 frames per second (fps) with a 12 bit dynamic range. The system exceeds the capabilities of the current high speed x-ray imaging systems which typically operate at the rate of 30 fps. The high x-ray conversion efficiency of CsI(TI) (59,000 optical photons MeV) makes these screens an excellent choice for the current application of x-ray diffraction where certain important diffraction peaks are often weak and require imaging screens with a high S/N ratio for proper identification.

Variation of absolute luminescence efficiency(AE) of CsI:Tl crystals for Mammography(22–49kVp) X-ray tube voltages. AE units: μWs/mRm2.Points: measured data, line:fitted curve

Variation of absolute luminescence efficiency(AE) of CsI:Tl crystals for Mammography(40–140kVp) X-ray tube voltages. AE units: μWs/mRm2.Points: measured data, line:fitted curve

Variation of absolute luminescence efficiency (AE) of CsI:Tl crystals relative to the different thicknesses used in this study

References
[1] A systematic study of the performance of the CsI Tl single-crystal scintillator under X-ray excitation
[2] high-speed-xray-imaging-camera-using-structured-csitl-scintillator
[3] Scintillator efficiency study with MeV x-rays
[4] Structured CsI(Tl) scintillators for X-ray imaging applications
[5] Validation of columnar CsI x-ray detector responses obtained with hybridr , a CPU-GPU Monte Carlo code for coupled x-ray, electron, and optical transport

CdWO4

Wavelength(Max. emission) (nm)490
Decay time (ns)14000
Light yield (photons/keV)12~15
Light output relative to Nal(Tl) (%)50
Refractive index2.2-2.3

Cadmium tungstate (CdWO4, CWO) crystal is one of the candidate for such application since it has high effective Z number, high density, higher light yield than BGO, and very low afterglow ). Radiation hardness of CWO crystal is as high as 105 rad. Because of such advantage, CWO scintillator is being used extensively in X-ray CT.

[Ref1] A high-resolution x-ray detector array for x-ray imaging is described,It consists of  a discrete array of  CdWO4  scintillators and a Reticon 1024S photodiode array with a fiber-optic faceplate. It developed a very high resolution discrete scintillator array for use in a fan beam x-ray imaging system.

References
[1] High-resolution digital x-ray detector utilizing a discrete array of CdWO4 scintillators and a self-scanned photodiode array
[2] Large Size CdWO4 Crystal for Energetic X- and γ Ray Detection
[3]  X-RAY STUDY OF CHARACTERIZATION OF THE PbWO4 AND CdWO4 SINGLE CRYSTALS

GOS

Wavelength(Max. emission) (nm)510
Decay time (us)5.5
Relative light output (%)80
Afterglow (%)< 0.01
Luminescence intensity (keV)27.5

References
[1] Phosphors for X-ray detectors in computed tomography