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Abstract
The light yield, spatial resolution, and image quality of inorganic scintillators (CsI:Tl (CsI), Gd3Al2Ga3O12: Ce (GAGG), Lu3Al5O12: Ce (LuAG), CdWO4 (CWO), Y3Al5O12: Ce (YAG), Bi4Ge3O12 (BGO), and Gd2O2S: Tb (GOS)) were investigated using monochromated synchrotron radiation. For quantitative analysis, the same imaging system composed of a relay lens system and a camera was used for all scintillators. The light yield of GOS was highest, followed in order by CsI, GAGG, CWO, YAG, LuAG, and BGO. The spatial resolution of micro X-ray radiographic images using GAGG and LuAG was highest, followed by using CsI, CWO, and YAG. The image qualities were almost the same, except for GOS. These results indicate that CsI is suitable for micro X-ray radiography and computed tomography with a low X-ray flux source, and GAGG and LuAG are suitable with a high X-ray flux source.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
An X-ray camera, as well as the X-ray source and X-ray optical elements, is a key component for X-ray imaging to determine the spatial and contrast resolution of an image. Direct X-ray cameras (for detecting X-rays directly by using an imaging sensor such as photon counting detector and flat panel detector (FPD)) are mostly used in radiography and X-ray computed tomography (CT) from medical to industrial applications. Indirect X-ray cameras (for converting X-rays to visible light and detecting by using image sensors), however, are mainly used in micro- and fast-radiography and X-ray CT combined with synchrotron radiation (SR). The incident X-rays for indirect X-ray cameras are first converted to visible light by using a scintillator, transferred to a visible light camera by a relay lens system or optical fiber, and detected using a visible light camera (charged-coupled device (CCD) or scientific complimentary metal-oxide semiconductor (sCMOS)) as a two-dimensional image showing the spatial distribution of X-ray intensity. Therefore, the detective efficiency, spatial resolution, and image quality mainly depend on the scintillator, relay lens, and camera, and a high conversion ratio from an X-ray to visible light without blurring the image is required of the scintillator. High resistance against radiation damage is also required of the scintillator to conduct stable measurements.
Many types of inorganic scintillators, not only conventional CsI, Gd2O2S: Tb (GOS), and CdWO4 (CWO) but also newly developed Gd3Al2Ga3O12: Ce (GAGG) and Lu3Al5O12: Ce (LuAG), are commercially available [1–3] and used widely in SR-based micro- and fast-radiography and X-ray CT [4–9]. Many studies on the analysis of the physical properties, such as light yield (conversion ratio), peak emission wavelength, energy resolution, and decay time of these scintillators, have been conducted [10–17], and the typical values have been put on the homepages of material manufacturers [1–3]. However, physical properties, especially light yield, depend on the energy of the incident X-ray (called non-proportional response [18,19]), the temperature of the scintillator, the stray light inside the scintillator, and the self-absorption of visible light, so these values significantly differ from report to report. In addition, analysis of many types of scintillators under the same condition has not been reported. Therefore, it is difficult to select the optimal scintillator for various X-ray imaging methods such as SR-based micro-CT, time-resolved fast radiography, and phase-contrast X-ray imaging.
To overcome the above problem, we conducted a quantitative analysis of the light yield of commercially available inorganic scintillators (CsI, GAGG, LuAG, CWO, YAG, BGO and GOS) under the same measurement condition using monochromated 10-, 20- and 34-keV SR. We also investigated the spatial resolution and quality of micro-radiographic images using these scintillators as well as radiation resistance (coloring).
2. Method
2.1 Method and instrumentation
The light yield of each scintillator was measured using an in-house-developed X-ray micro camera called Kenvy 2, as shown in Fig. 1, composed of a scintillator holder, infinity-correction relay lens system, and sCMOS visible light camera. The camera signal Ic [counts/pixel/sec] is given by
where Io, μ, t, η, Tr, QE, and Scam are the incident X-ray photon flux [x-ray photon/sec/pixel], the linear absorption coefficient of the scintillator [1/mm], the thickness of the scintillator [mm], conversion ratio with self-absorption and the stray light of converted visible light [visible photon/X-ray photon], the transmission efficiency of the lens system, the quantum efficiency of the sCMOS chip for the converted visible light wavelength, and the sensitivity of the sCMOS camera, respectively. The QE and Scam are known depending on the sCMOS camera, μ can be calculated from the elemental composition ratio and density of the scintillator, and Tr can also be calculated from t, the refractive index of the scintillator, focal length, and diameter of the objective lens by the calculation described below. Therefore, η can be obtained quantitively by using the measured Io, and the light yield Nph [ph/keV] can finally be calculated by where E is the X-ray energy.
Fig. 1. Schematic view (upper) and photo (lower) of micro X-ray camera (Kenvy 2) used in study. Camera is composed of scintillator holder, relay lens system, and sCMOS visible light camera.
The penetration depth of the X-ray with a few tens of keV energy is limited to a few tens of microns from the scintillation surface; therefore, the transmission efficiency of the lens system (Tr) can be calculated using the optical configuration shown in Fig. 2. The visible light emitted at the scintillator surface passes through the scintillator, refracted at the boundary of the scintillator and air, and incidents the objective lens. The refractive index of the scintillator is much larger than that of the air, so the visible light path is refracted at a high angle, as shown in Fig. 2; therefore, the effective numerical aperture NA’ is given by
Fig. 2. Optical configuration of scintillator and objective lens. Emitted visible light was refracted to high angle at boundary of scintillator and air because of large refractive index of scintillator.
The relay lens system is composed of an object lens, imaging lens, and tube, as shown in Fig. 1. An infinity-corrected object lens (Edmund Optics, 5X EO HR Infinity Corrected Objective) and imaging lens (Mitutoyo, MT-4 Accessory Tube Lens) were used for Kenvy 2. The magnification, NA, and focal depth of the object lens are x5, 0.225, and 10.9 μm, respectively, and the magnification of the imaging lens is x1. An air cooled sCMOS visible light camera (Andor, Zyla 4.2 [21]) was used for detecting visible light. The pixel size and number of pixels were 6.5 μm2 and 2048 × 2048 pixels, respectively, so the effective pixel size was 1.3 μm2. The gain and system readout rate were set to “low noise and high well capacity” and 200 MHz, and the sensitivity of the sCMOS (Scam) under this condition was 0.55 [e- per camera signal (A/D count)] according to a performance sheet of the camera.
The scintillator was held using a clip holder attached at the tip of a shading barrel, and the distance (focal length) between the scintillator and lens was adjusted using a precise linear positioner (THK Precision, PKVL84F-300U) with a piezo electric translator embedded in the barrel. The whole system was set up in the 2nd experimental hatch of the beamline BL07 of the SAGA Light Source in Japan [22], and quantitative analysis of the light yield was conducted using monochromated SR. The temperature in the hatch was kept at 26°C during the measurement.
For quantitative evaluation of the spatial resolution of micro radiography using Kenvy 2 with each scintillator, the modulation transfer functions (MTFs) of each scintillator at each X-ray energy were measured using five kinds of Au grid meshes. The grid spacing of each Au mesh was 125, 62.5, 25, 16.5, and 12.5 microns (200, 400, 1000, 1500, and 2000 mesh (line/space per inch)), respectively. The thickness of Au of the 200 and 400 mesh was 9 microns, and absorption ratios at X-ray energies of 10, 20, and 34 keV were 83.0%, 74.0%, and 31.5%, respectively. The thickness of Au of the 1000, 1500, and 2000 mesh was 4 microns, and absorption ratios at X-ray energies of 10, 20, and 34 keV were 54.64%, 45.0%, and 14.9%, respectively. Note that the edge part's width (rim) was 500 microns, which was wide enough and uniform for the stray light evaluation described later. The mesh was positioned in front of Kenvy 2 by using a linear table driven by a stepping motor and was close to the scintillator, less than 2 mm, to reduce image blurring caused by the X-ray refraction of the mesh, as shown in Fig. 1.
The transmission of the grids was not zero, and the intensity It of the Au area includes the stray light Is, which made it difficult to evaluate the MTF accurately. Therefore, we calculated the image contrasts V for MTFs by
Fig. 3. Schematic view of Au mesh (left) and line profile (right). Image contrast for MTF was calculated using Imax, Imin, Io, and Ia.
The quality (image uniformly) of the micro-radiographic image was analyzed based on the standard deviation of the camera signal in the center area (50 × 50 pixels) of the image obtained without any samples in the SR path.
The radiation resistance of each scintillator was evaluated based on the coloring caused by exposing the scintillator to white SR (average energy was 15 keV) for 30 min (total dose was 30 MGy) at the optical hatch of the same beamline of SAGA Light Source.
2.2 Scintillator
Quantitative analysis was conducted on commercially available inorganic scintillators; CsI, GAGG, LuAG, CWO, YAG, BGO (OHYO KOKEN KOGYO), and GOS (Hamamatsu Photonics). Table 1 lists the main physical properties of each scintillator. The size of the crystal scintillators (CsI, GAGG, LuAG, CWO, YAG, and BGO) was unified to 10 × 10 × 1 mm3 to prevent the effect of self-absorption and total reflection of emitted visible light, as shown in Fig. 4. The thickness of 1 mm was sufficient to absorb almost all incident X-rays of a few tens of keV; therefore, the absorption ratio (exp(-μt)) could be assumed as 1. The GOS scintillator was a powder-type scintillator and deposited on a 6-mm thick quartz glass plate coated with anti-reflection coating of 430 nm (emitted visible light wavelength of GOS). The thickness of GOS was 10 μm, and the absorption ratios for 10-, 20-, and 34-keV X-rays calculated with elemental composition and electron density were 79.7, 24.9, and 7.4%, respectively [23]. The average powder particle size was 3 μm.
Fig. 4. Photo of scintillators analyzed in this study. Size of crystal scintillators was unified to 10 × 10 × 1 mm3.
Table 1. Main physical properties of each scintillator
3. Results
3.1 X-ray photon flux at BL07 of SAGA Light Source
We first measured the X-ray photon flux Io in the front of Kenvy 2 at BL07 using an ion chamber. The white SR emitted from the super-conducting wiggler of BL07 was monochromated using a double-crystal monochromator using Si (220) X-ray diffraction, cut to form 1 × 1 mm2 by a four-dimensional slit, and irradiated to the ion chamber. The X-ray photon flux of each energy was calculated using Hephaestus software with the detected ionization current, gas composition (85% N2 + 15% Ar) of the ion chamber, and chamber length (33 mm).
Figure 5 shows the obtained Io [ph/sec/mm2]. Note that, the storage ring current decreased gradually from 300 to 100 mA during operation (9 am–9 pm), so the flux was normalized at 200 mA of the ring current. The calculated peak energy of white SR emitted from the wiggler was 10 keV [22]; however, the peak energy was shifted to 12-keV because of absorption of the Be window, Kapton window, and air between the exit port of the beamline and ion chamber. The maximum photon flux at 12 keV reached 2 × 108 counts/sec/mm2, which is 1/3 that at the BL-14C of the Photon Factory in Japan.
3.2 Light yield of each scintillator
The conversion ratio and light yield of each scintillator were calculated as follows.
- 1. Obtain X-ray images without any samples with different exposure times (1, 2, 5, 10, and 20 sec) using 10-, 20-, and 34-keV SR.
- 2. Calculate average Ic by linear least-squares fitting of exposure time (incident X-ray photon number) and camera signal in the center of the images.
- 3. Normalize Ic at 200 mA storage ring current
- 4. Calculate η using Eq. (1) and known values.
- 5. Calculate the light yield Nph using Eq. (2).
Figure 6 (b) shows the calculated light yields of the scintillators for 10-, 20-, and 34-keV X-ray using Eq. (2) with conversion ratios. Every light yield except that of GOS was increased by increasing the X-ray energy, which is called the non-proportional property. The changing ratio of the light yield of GAGG was largest (180% from 10 to 34-keV SR), followed in order by LuAG, BGO, CWO, and YAG, which agreed with the results from a previous report [24]. In addition, the light yield of CsI remained almost the same from 20- to 34-keV SR, which also agreed with previous reports [25,26].
The stray light caused by the total reflection inside the scintillator can be obtained quantitatively from the difference between the measured and theoretical absorption ratio of a sample with known material and thickness. The edge part (rim) of the Au mesh as shown in Fig. 3 can be assumed as an Au foil, and the thickness of the Au mesh was known, so we calculated the stray light ratio from the difference between the measured absorption ratio of Au foil in the acquired Au mesh image and theoretical absorption ratio calculated using the foil thickness and the absorption coefficient.
Figure 7 shows the stray light ratios of each scintillator at each X-ray energy. The ratios of CsI and BGO were about 5%, and other scintillators were 5%-10%, and therefore the effect on the light yields was less than 10%. Except for GOS, the ratio was decreased by half at 34 keV, whereas the ratio for GOS increases significantly. This is probably due to increased X-ray penetration depth into the scintillator and optical plate as the energy increases and the emission area expansion and scattering X-rays.
Figure 8 shows the average light yields from Fig. 6 (b) (blue bars) with subtracting the stray light using the obtained ratio in Fig. 7 and typical light yield [2] (orange bars) of each scintillator with various previously reported values (CsI [15,27], GAGG [2], LuAG [13,14], CWO [12,13], YAG [28], BGO [29][30], and GOS [16]). The previously reported light yields were basically measured using an 662-keV γ-ray emitted from Cs 137, so the average light yields were calculated with non-proportional corrected light yields using reported non-proportional ratios of CsI [25,26], GAGG [15], LuAG [31], CWO [10,32,33], YAG [34], and BGO [35,36], respectively. Note that the non-proportional ratio of GOS could not be determined, so correction was not carried out (the original value was shown).
The results indicate that the light yields of CsI, GAGG, YAG, and BGO were within the range of previously reported values. However, the light yields of LuAG, CWO, and GOS were slightly larger than the reported ones. In any case, the measured light yields were high overall, and the reason may be attributed to the thickness of the scintillator. The emitted visible light passed through the scintillator, and some light was absorbed by the scintillator [27,29]. This phenomenon is called self-absorption, and the absorption ratio increased to tens of percent for cm-sized scintillators used in previous reports [29]. The thickness of the scintillators used in this study was only 1 mm for scintillators except for GOS and 10 μm for GOS, which were thin enough to ignore absorption; therefore, the obtained light yields were high. Note that, GOS powder was not clear but milky white, as shown in Fig. 4, so the obtained light yields were much higher than the reported values.
3.3 Spatial resolution of micro-radiographic images
We then observed Au meshes using each scintillator with 10, 20, and 34-keV SR, and calculated the MTF (image contrast at each grid interval) for quantitative evaluation of micro-radiographic images’ spatial resolution. Figure 9 shows the micro-radiographic images of the center area of Au 400-mesh obtained by 10-, 20-, and 34-keV SR. The image size was 0.26 × 0.26 mm2 (200 × 200 pixels), and exposure time was 20 sec. Fine mesh images were obtained using each scintillator, except GOS, with 10-keV SR; however, image blurring increased with increasing X-ray energy. Images obtained with YAG were terribly blurred, the reason of which is described later. The GOS scintillator was powder of 3 microns, so the sensitivities differed pixel by pixel (1.3 μm) and unevenness contrast was generated. As the result, GOS was found not suitable for micro-radiography.
Fig. 9. Micro-radiographic images of Au grid mesh obtained using each scintillator and 10-, 20-, and 34-keV SR. Spatial resolution decreased with increasing SR energy. GOS was not suitable for micro-radiography because of spatial non-uniformity of sensitivity.
Figure 10 (a), (b), and (c) show the obtained MTF of each scintillator at 10, 20, and 34-keV SR, respectively. These results show that the image contrast of GAGG and LuAG was highest, followed by using CsI, CWO, and YAG. The image contrast of GOS (powder scintillator) is lower than that of the other scintillators. However, since the scintillator is very thin (10 microns), the emission area is fixed to a small area even for high-energy X-ray, so the contrast does not decrease significantly as in other scintillators. The absorption coefficient of YAG was smaller than that of the other scintillators, and the emission area expands with the increasing of the X-ray energy, so the image contrast is getting lower rapidly for the high-energy X-ray region. On the other hand, despite CWO's large absorption coefficient, the image contrast is lower than that of GAGG and LuAG in the higher frequency region. The lower image contrast is assumed to be attributed to the high stray light ratio compared to other scintillators. Note that, the image contrast was not 100% at 0 [cycles/mm] of the spatial frequency, because the stray light's effect was also taken into account in the calculation of the image contrast as indicated in Eq. (5).
Fig. 10. Obtained MTF of each scintillator for (a) 10-, (b) 20-, and (c) 34-keV SR. Image contrast of 1000 mesh for the X-ray's absorption length (d).
Figure 10 (d) shows the image contrast of 1000 mesh for the X-ray's absorption length. As expected from Figs. 10 (a)-(c), the image contrast generally decreases with increasing of the absorption length. There was no difference in this tendency among scintillators, so the decrease in the image contrast was mainly caused by increasing the penetrating depth with increased energy of SR because the emission area in the scintillator became larger than the focal depth of the objective lens for high-energy X-rays.
3.4 Image quality of micro-radiographic images
The quality of micro-radiographic images using each scintillator was evaluated using the camera signals and their standard deviations of the center area (50 x5 0 pixels) in obtained images without any sample. Figure 11 shows the standard deviation at each camera signal of the images obtained from each scintillator and (a) 10-, (b) 20-, and (c) 34-keV SR. The results shown in Fig. 11 (a) indicate that the inclination of GOS was steeper than that of the other scintillators, so the intensity varied pixel by pixel and image quality (uniformity of intensity without sample) was low. In addition, the standard deviations of the other scintillators were proportional to the camera signals (not square root), so the unevenness of intensity seemed to be caused by crystal defects and dislocations and small scratches on the scintillator surface, not shot noise.
Fig. 11. Standard deviation at each camera signal of images obtained using each scintillator with (a) 10-, (b) 20-, and (c) 34-keV SR. Standard deviation of each scintillator except GOS and BGO is on same line, so quality of micro-radiographic images seemed to differ little.
The standard deviations in (b) and (c) were proportional to the square root of the camera signal except for BGO and GOS, so the noise was caused by shot noise because the effect of scratches on the surface decreased due to the deeper penetration depth of the X-ray by increasing the SR energy. On the other hand, the deep penetration depth caused large unevenness of sensitivity in GOS, and the noise was still large compared to the other scintillators. In addition, the large standard deviation of BGO in (c) seemed to be caused by the thermal noise of the sCMOS visible camera because the camera signal was very small (tens of counts). The relationship between the camera signals and standard deviations of all scintillators except GOS and BGO indicated a similar tendency, so the quality of micro-radiographic images from each scintillator seemed to differ little.
3.5 Radiation resistance
The radiation resistance of each scintillator was measured using white SR at an optical hatch of BL07 of SAGA Light Source. Note that GOS was excluded because the high radiation sensitivity of the binder of the powder was known. The total exposure time was set to 30 min (30 MGy), and discoloration of the scintillator was observed every 10 min. Figure 12 shows photos of CsI before exposure (left), after 10-min exposure (middle), and after 30-min exposure. The reddish coloring in the upper areas of the scintillator became darker by increasing radiation. On the other hand, the other scintillators (GAGG, CWO, LuAG, YAG, and BGO) were not colorized by 30 MGy. Therefore, the radiation sensitivity of CsI was higher than that of other scintillators. Note that the uneven pattern in the background area was caused by a paper towel used as table covering.
Fig. 12. photos of CsI before exposure (left), after 10-min exposure (middle), and after 30-min exposure. Reddish coloring in upper area became darker by increasing radiation.
4. Discussion
The NA of the objective lenses of X-ray cameras used in micro-radiography, such as Kenvy 2, are much smaller than that of lenses used in automatic inspection systems; therefore, the emitted visible light could not be transferred efficiently to the visible camera because of Tr below 1%. For example, the NA of the lens used in Kenvy 2 and refractive index of CsI were 0.225 and 1.85, respectively, and the transferring efficiency (Tr x QE x Scam) was calculated as 0.18%. Therefore, the detected camera signal was only 1.8 counts for one X-ray photon of 10-keV even when using CsI, which had the highest light yield (8.0 × 104 ph/MeV). The light yields of the other scintillator were a few fractions of that of CsI, the detection efficiency of X-ray would be below 100%. Therefore, high-light-yield scintillators, such as CsI, are suitable for micro-radiography and CT using low flux X-ray sources such as an X-ray tube and a small SR facility.
If a high-flux X-ray source, such as a 3rd generation SR facility, e.g., Spring-8, is available, spatial resolution can take precedence over detection efficiency. The obtained MTF showed the image contrasts (spatial resolution) of GAGG and LuAG were higher than that of other scintillators, and therefore GAGG and LuAG are suitable for higher-spatial-resolution observation such as with nano-radiography and CT. Note that the penetration depth depends on the elemental components and density of the scintillator and cannot be shortened in principle, so a thin scintillator below 10 μm is suitable for extremely high-resolution observation [37].
Large aperture lenses or fiber optics are generally used for transferring lens system in SR-based X-ray imaging of large field view. The NA of these optical components is much higher than that of objective lenses used in micro-radiography and amounts to nearly 1. As a result, the transferring efficiency increases a few percentages, and the calculated camera signal for one X-ray photon is 30 counts for CsI. Fine observation can be carried out under a very low X-ray flux condition, but the camera signal would soon saturate even with an X-ray tube or small SR facilities. The monochromatic 10-keV SR flux at BL07 of SAGA Light Source was 2 × 108/mm2, as shown in Fig. 5, so the SR flux at one pixel (6.5 μm2) of a fiber-coupled X-ray camera is 2.8 × 105 counts/sec/pixel, which corresponds to a 9 × 106 camera signal. The dynamic range of the camera is normally 16 bits (maximum signal is 65500 counts); therefore, the camera signal saturates with only 0.1-sec exposure. On the other hand, the number of detected X-ray photons is only 2000, and fine observation cannot be carried out under such a low number of X-ray photons. Therefore, scintillators with low light yield, high absorption for high energy, and high radiation resistance, such as GAGG, LuAG, and CWO, are suitable for large-area X-ray cameras using high NA lenses or fiber optics.
5. Conclusion
We quantitatively analyzed the light yield, spatial resolution, image quality, and radiation resistance of inorganic scintillators (CsI, GAGG, LuAG, CWO, YAG, BGO and GOS) by using 10-, 20- and 34-keV monochromated and white SR. The light yield of GOS was highest, followed in order by CsI, GAGG, CWO, YAG, LuAG, and BGO. The spatial resolution of micro X-ray radiography using GAGG and LuAG was highest, followed by using CsI, CWO, and YAG. The image qualities (standard deviation of intensity) were almost the same except for GOS. Only CsI was colorized by white SR after 30-min exposure (30 MGy). These results indicate that CsI is suitable for micro X-ray radiography and CT with low X-ray flux sources such as an X-ray tube and small SR facilities, and GAGG and LuAG are suitable for higher-spatial-resolution radiography with a high X-ray flux source such as large SR facilities.
Acknowledgments
We would like to show our appreciation to Dr. Wataru Yashiro of Tohoku University and Mr. Naoto Tsuchiya of AD Science Inc. for useful discussions. The study was carried out under the approval (proposal no. 190202I) of the Committee of the SAGA Light Source.
Disclosures
The authors declare no conflicts of interest.
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