- Email: [email protected]
Scintillation characteristics and imaging performance of CsI:Tl thin films for X-ray imaging applications
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 604 (2009) 224–228
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Scintillation characteristics and imaging performance of CsI:Tl thin films for X-ray imaging applications Bo Kyung Cha, Jeong-Hyun Shin, Jun Hyung Bae, Chae-hun Lee, Sungho Chang, Hyun Ki Kim, Chan Kyu Kim, Gyuseong Cho Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejoen 305-701, Republic of Korea
a r t i c l e in f o
a b s t r a c t
Available online 10 February 2009
We have manufactured thallium-doped cesium iodide (CsI:Tl) scintillator thin films by the thermal deposition method. The scintillation characteristics of the CsI:Tl thin films were studied by X-rayinduced luminescence for different Tl doping concentrations between 0.05 and 1.0 mol%. The wavelength of the main emission peak was about 550 nm and the light intensity was increased and the emission peak shifted toward the long wavelength for higher Tl concentration in the X-ray luminescence case. X-ray diffraction (XRD) and scanning electron microscopy (SEM) for observation of structural properties was used to investigate the relationship between the microstructure affected by the evaporation condition and post-heat treatment, and the scintillation properties of samples. The imaging performance of the various CsI:Tl films fabricated will also be evaluated by an X-ray radiographic test after coupling to a CCD sensor. & 2009 Elsevier B.V. All rights reserved.
Keywords: Scintillation properties CsI:Tl scintillator X-ray imaging
1. Introduction In the last decade, thallium-doped cesium iodide (CsI:Tl) scintillator has been widely used in X-ray imaging detectors for medical and industrial applications because of its high scintillation efficiency and proper emission wavelength (550 nm) highly matching silicon-based photo-sensors. The scintillation mechanism and properties of CsI:Tl single crystals were already wellknown since the 1960s [1]. However, those of the CsI:Tl with polycrystalline structure are still under investigation [2]. In this work, CsI:Tl scintillator thin films according to deposition condition such as different doped Tl concentrations, evaporation pressure, substrate temperature and post-heat treatment were prepared by the thermal evaporation method onto glass substrates. The scintillation properties, such as emission spectrum, light output of CsI:Tl thin films, were observed by an X-ray exposure test. In addition, signal response, signal-to-noise ratio (SNR) as X-ray exposure and X-ray imaging performance were obtained and evaluated by direct coupling with a CCD camera system. 2. Materials and methods 2.1. CsI:Tl scintillator thin films preparation CsI:Tl scintillator thin films onto glass substrates supported on a heat holder above 24 cm distance from a tantalum crucible Corresponding author. Tel.: +82 42 350 3821; fax: +82 42 350 5861.
E-mail address: [email protected] (G. Cho). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.01.177
with the mixture of CsI and TlI powder were fabricated by physical vapor deposition. The microstructures and scintillation properties of CsI:Tl are affected by process conditions such as evaporation pressure, evaporation rate, substrate temperature, etc. and Tl doping concentration, post-heat treatment, respectively [3]. Therefore, a number of samples were prepared according to evaporation pressure with 10 2 and 10 5 Torr vacuum atmosphere, 35 and 250 1C substrate temperatures during the evaporation process and the post-heat treatment was carried out at high vacuum, 250 1C temperature condition for 2 h [4].
2.2. Measurement of the microstructures and light output The microstructures and crystal structures of CsI:Tl samples as evaporation condition and post-heat treatment were observed by FE-SEM (JEM-2100F HR) and a high-resolution X-ray diffractometer (RIGAKU Ultima IV) with an analysis range 2y of 20–901. The scintillation properties were investigated by means of a UV–visible spectrometer (Spectra Academy, K-MAC) using an optical cable for emission spectra measurement of the CsI:Tl scintillator in dark box and the lens-coupled CCD camera system (Andor DV-434) connected to the PCI controller card and corresponding software for relative light output of samples by same X-ray exposure. A pulse-type X-ray source was exposed at 30 mA s beam current and 80 kVp peak acceleration voltage (LISTEM, BRS-2) with 4.3 mm spot size and an inherent 0.8 mm Al filter. The distance between X-ray source and CsI:Tl samples was about 40 cm.
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Fig. 1. SEM images of cross-section (top) and surface (bottom) section: (a) 10 5 Torr evaporation pressure, 35 1C substrate temperature; (b) 10 250 1C substrate temperature; and (c) 10 2 Torr evaporation pressure, 35 1C substrate temperature.
2.0 CsI thin film CsI:T1 0.05 mol% CsI:T1 0.1 mol% CsI:T1 0.3 mol% CsI:T1 0.5 mol% CsI:T1 0.7 mol% CsI:T1 1.0 mol%
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Fig. 3. XRD patterns of the CsI:Tl scintillator.
2.3. Evaluation of X-ray imaging performance The prepared CsI:Tl samples combined with the lens-coupled CCD camera were used for X-ray response, signal-to-noise ratio measurement as a function of X-ray exposure doses checked by a calibrated ion chamber(RAD CHEKTM PLUS) and spatial resolution using X-ray images of an object.
3. Results and discussions 3.1. Microstructure The microstructures of evaporated CsI:Tl films in vacuum from the different evaporation conditions are shown in Fig. 1. The surface section of Fig. 1(a) shows the dense structure with some cracks at high vacuum and 35 1C substrate temperature.
As the substrate temperature at high vacuum condition is increased, the cracks disappeared as shown in Fig. 1(b). The microstructure of the square-shaped needles was seen at 10 2 Torr evaporation pressure and 35 1C substrate temperature conditions in Fig. 1(c) [5]. Figs. 2 and 3 show the XRD patterns according to doped Tl concentration and different evaporation conditions. The evaporated CsI:Tl thin films at 10 5 Torr pressure and 35 1C substrate temperature have X-ray peaks at (11 0), (2 0 0), (2 1 0), (2 2 0), (3 1 0), (2 2 2), (3 2 0), and (4 0 0), and are consistent with the cubic crystal structure. According to Tl doping concentration in CsI thin films, XRD peaks were little changed. But the intensity of XRD pattern peaks shows a little difference. The XRD pattern peaks as different evaporation conditions with the same doped Tl concentration are shown in Fig. 3. X-ray peaks at (2 0 0), (2 2 0), and
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(4 0 0) appeared with a change in evaporation pressure from 10 5 to 10 2 Torr using the same doped Tl concentration. Moreover, X-ray peaks at (2 0 0) and (3 1 0) disappeared and X-ray peak at (2 2 0) appeared as the substrate temperature increased from 35 to 250 1C maintained at 0.3 mol% Tl concentration and 10 5 Torr evaporation pressure [2].
3.2. Spectrum and light output of a CsI:Tl scintillator The intensities of the X-ray-induced luminescence (XL) spectra, emitted by X-ray exposure, are shown as the doped Tl concentration in CsI:Tl thin films in Fig. 4. The strong emission peak with 550–560 nm wavelength is shown in the spectra of the CsI:Tl thin films. After CsI:Tl thin films with different Tl concentrations are prepared by heat treatment, the intensities of the films increased and the strong emission peak shifted a little because of Tl diffusion effect in CsI:Tl thin films [6]. The XL spectra of the deposited CsI:Tl films at different evaporation pressures and substrate temperatures using the same Tl concentration are shown in Fig. 5.
The light output of the samples was measured by pixel value averaged over the region of interest (ROI) of X-ray images acquired with a 80 kVp, 30 mA s X-ray source. The relative light output of the CsI:Tl thin films according to different Tl concentrations and heat treatments is shown in Fig. 6. As shown in the figure, the highest light output among the CsI:Tl thin films was observed in case of the sample with 1.0 mol% Tl concentration and post-heat treatment at 250 1C temperature, 10 5 Torr vacuum conditions for 2 h. The light output of the CsI:Tl thin films at different evaporation conditions such as evaporation pressure, substrate temperature and post-heat treatment is shown in Fig. 7. The CsI:Tl scintillator deposited with 10 5 Torr pressure, 35 1C substrate temperature and post-heat treatment showed the highest light output. 3.3. X-ray imaging performance The X-ray response and signal-to-noise ratio were evaluated by means of a CsI:Tl scintillator-coupled CCD camera system and an X-ray generator. The signal response in terms of analog digital units was measured by pixel values averaged over region of interest areas in X-ray images. The X-ray response curves of the
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Fig. 5. The XL spectra of the CsI:Tl scintillator at different evaporation conditions and heat treatment.
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CsI:Tl thin films with different Tl concentrations, evaporation conditions, and post-heat treatment as X-ray exposure are shown in Figs. 8 and 9. The X-ray exposure dose was increased by changing the beam current of the X-ray source fixed at a 50 kVp tube voltage and measured by a calibrated ion chamber. As the X-ray exposure increases, the signal of the CsI:Tl scintillator was linearly increased in case of both Figs. 8 and 9.
The signal-to-noise ratio in terms of the important evaluation factor in the X-ray imaging system was measured as X-ray exposure. The signal-to-noise ratio was measured by dividing the average output signal by the standard deviation over a ROI in the X-ray images. The signal-to-noise ratio of the CsI:Tl thin films with different Tl concentrations and evaporation conditions is shown in Figs. 10 and 11, respectively. The X-ray images acquired by CsI:Tl thin films with different Tl doping concentrations, evaporation conditions, and lens-coupled CCD camera system consisting of 1024 1024 active pixels with 13 mm pixel pitch and 13.3 13.3 mm2 active area at the same X-ray exposure conditions with 80 kVp, 30 mA s are shown in Fig. 12. The CsI:Tl thin film (Fig. 12(c)) deposited at 1.0 mol% Tl concentration, high vacuum, and room temperature conditions shows the highest image contrast. And the CsI:Tl thin film (Fig. 12(e)) with a square columnar structure shows the X-ray image with high resolution because of the reduction of the lateral spreading of light photons and promotion of the directional light channeling in the columnar structure [2].
4. Conclusion In this work, we fabricated the Tl-doped CsI scintillation thin films by the vacuum evaporation method and evaluated the
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Fig. 12. X-ray images of a memory chip: (a) 0.05 mol% Tl/10 5 Torr pressure/35 1C temperature; (b) 0.3 mol% Tl/10 5 Torr pressure/35 1C temperature; (c) 1.0 mol% Tl/ 10 5 Torr pressure/35 1C temperature; (d) 0.3 mol% Tl/10 2 Torr pressure/35 1C temperature; (e) 0.3 mol% Tl/10 5 Torr pressure/250 1C tempeature.
scintillation characteristics and the X-ray imaging performance as Tl doping concentration, evaporation condition such as evaporation pressure, substrate temperature, and post-heat treatment for X-ray imaging detectors. The CsI:Tl thin film deposited at high vacuum, room substrate temperature conditions showed high light output under X-ray exposure. And the light output of the CsI:Tl thin films was increased by post-heat treatment at high vacuum for 2 h. The CsI:Tl thin film with columnar structure was observed at low vacuum and room substrate temperature for X-ray image with high resolution. In conclusion, the CsI:Tl thin films with high light output and spatial resolution can be fabricated by changing evaporation conditions such as the Tl concentration, evaporation pressure, substrate temperature, and post-heat treatment and will be used for X-ray imaging applications such as medical and industrial fields with low X-ray exposure dose.
Acknowledgments This work was supported by Nuclear R&D Program of MEST through (M20701030002-08N0103-00210).
References [1] R. Gwin, R.B. Murray, Phys. Rev. 131 (1963) 508. [2] A. Fedorov, A. Lebedinsky, O. Zelenskaya, Nucl. Instr. and Meth. A 564 (2006) 328. [3] B. Schmitt, M. Fuchs, E. Hell, W. Knu¨pfer, P. Hackenschmied, A. Winnacker, Nucl. Instr. and Meth. B 191 (2002) 800. [4] M. Nakayama, N. Ando, J. Hirai, H. Nishimura, J. Lumin. 108 (2004) 359. [5] A.L.N. Stevels, A.D.M. Schrama-de pauw, Philips Res. Rep. 29 (1974) 340. [6] B.J. Kim, G. Cho, B.K. Cha, D.K. Kim, J. Whang, J.W. Park, C.H. Kim, M. Jae, J.K. Kim, in: SCINT 2005 Book of Abstracts, Institute for Scintillation Materials, Ukraine, 2005, p. 260.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Scintillation characteristics and imaging performance of CsI:Tl thin films for X-ray imaging applications Bo Kyung Cha, Jeong-Hyun Shin, Jun Hyung Bae, Chae-hun Lee, Sungho Chang, Hyun Ki Kim, Chan Kyu Kim, Gyuseong Cho Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejoen 305-701, Republic of Korea
a r t i c l e in f o
a b s t r a c t
Available online 10 February 2009
We have manufactured thallium-doped cesium iodide (CsI:Tl) scintillator thin films by the thermal deposition method. The scintillation characteristics of the CsI:Tl thin films were studied by X-rayinduced luminescence for different Tl doping concentrations between 0.05 and 1.0 mol%. The wavelength of the main emission peak was about 550 nm and the light intensity was increased and the emission peak shifted toward the long wavelength for higher Tl concentration in the X-ray luminescence case. X-ray diffraction (XRD) and scanning electron microscopy (SEM) for observation of structural properties was used to investigate the relationship between the microstructure affected by the evaporation condition and post-heat treatment, and the scintillation properties of samples. The imaging performance of the various CsI:Tl films fabricated will also be evaluated by an X-ray radiographic test after coupling to a CCD sensor. & 2009 Elsevier B.V. All rights reserved.
Keywords: Scintillation properties CsI:Tl scintillator X-ray imaging
1. Introduction In the last decade, thallium-doped cesium iodide (CsI:Tl) scintillator has been widely used in X-ray imaging detectors for medical and industrial applications because of its high scintillation efficiency and proper emission wavelength (550 nm) highly matching silicon-based photo-sensors. The scintillation mechanism and properties of CsI:Tl single crystals were already wellknown since the 1960s [1]. However, those of the CsI:Tl with polycrystalline structure are still under investigation [2]. In this work, CsI:Tl scintillator thin films according to deposition condition such as different doped Tl concentrations, evaporation pressure, substrate temperature and post-heat treatment were prepared by the thermal evaporation method onto glass substrates. The scintillation properties, such as emission spectrum, light output of CsI:Tl thin films, were observed by an X-ray exposure test. In addition, signal response, signal-to-noise ratio (SNR) as X-ray exposure and X-ray imaging performance were obtained and evaluated by direct coupling with a CCD camera system. 2. Materials and methods 2.1. CsI:Tl scintillator thin films preparation CsI:Tl scintillator thin films onto glass substrates supported on a heat holder above 24 cm distance from a tantalum crucible Corresponding author. Tel.: +82 42 350 3821; fax: +82 42 350 5861.
E-mail address: [email protected] (G. Cho). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.01.177
with the mixture of CsI and TlI powder were fabricated by physical vapor deposition. The microstructures and scintillation properties of CsI:Tl are affected by process conditions such as evaporation pressure, evaporation rate, substrate temperature, etc. and Tl doping concentration, post-heat treatment, respectively [3]. Therefore, a number of samples were prepared according to evaporation pressure with 10 2 and 10 5 Torr vacuum atmosphere, 35 and 250 1C substrate temperatures during the evaporation process and the post-heat treatment was carried out at high vacuum, 250 1C temperature condition for 2 h [4].
2.2. Measurement of the microstructures and light output The microstructures and crystal structures of CsI:Tl samples as evaporation condition and post-heat treatment were observed by FE-SEM (JEM-2100F HR) and a high-resolution X-ray diffractometer (RIGAKU Ultima IV) with an analysis range 2y of 20–901. The scintillation properties were investigated by means of a UV–visible spectrometer (Spectra Academy, K-MAC) using an optical cable for emission spectra measurement of the CsI:Tl scintillator in dark box and the lens-coupled CCD camera system (Andor DV-434) connected to the PCI controller card and corresponding software for relative light output of samples by same X-ray exposure. A pulse-type X-ray source was exposed at 30 mA s beam current and 80 kVp peak acceleration voltage (LISTEM, BRS-2) with 4.3 mm spot size and an inherent 0.8 mm Al filter. The distance between X-ray source and CsI:Tl samples was about 40 cm.
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Fig. 1. SEM images of cross-section (top) and surface (bottom) section: (a) 10 5 Torr evaporation pressure, 35 1C substrate temperature; (b) 10 250 1C substrate temperature; and (c) 10 2 Torr evaporation pressure, 35 1C substrate temperature.
2.0 CsI thin film CsI:T1 0.05 mol% CsI:T1 0.1 mol% CsI:T1 0.3 mol% CsI:T1 0.5 mol% CsI:T1 0.7 mol% CsI:T1 1.0 mol%
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2.3. Evaluation of X-ray imaging performance The prepared CsI:Tl samples combined with the lens-coupled CCD camera were used for X-ray response, signal-to-noise ratio measurement as a function of X-ray exposure doses checked by a calibrated ion chamber(RAD CHEKTM PLUS) and spatial resolution using X-ray images of an object.
3. Results and discussions 3.1. Microstructure The microstructures of evaporated CsI:Tl films in vacuum from the different evaporation conditions are shown in Fig. 1. The surface section of Fig. 1(a) shows the dense structure with some cracks at high vacuum and 35 1C substrate temperature.
As the substrate temperature at high vacuum condition is increased, the cracks disappeared as shown in Fig. 1(b). The microstructure of the square-shaped needles was seen at 10 2 Torr evaporation pressure and 35 1C substrate temperature conditions in Fig. 1(c) [5]. Figs. 2 and 3 show the XRD patterns according to doped Tl concentration and different evaporation conditions. The evaporated CsI:Tl thin films at 10 5 Torr pressure and 35 1C substrate temperature have X-ray peaks at (11 0), (2 0 0), (2 1 0), (2 2 0), (3 1 0), (2 2 2), (3 2 0), and (4 0 0), and are consistent with the cubic crystal structure. According to Tl doping concentration in CsI thin films, XRD peaks were little changed. But the intensity of XRD pattern peaks shows a little difference. The XRD pattern peaks as different evaporation conditions with the same doped Tl concentration are shown in Fig. 3. X-ray peaks at (2 0 0), (2 2 0), and
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(4 0 0) appeared with a change in evaporation pressure from 10 5 to 10 2 Torr using the same doped Tl concentration. Moreover, X-ray peaks at (2 0 0) and (3 1 0) disappeared and X-ray peak at (2 2 0) appeared as the substrate temperature increased from 35 to 250 1C maintained at 0.3 mol% Tl concentration and 10 5 Torr evaporation pressure [2].
3.2. Spectrum and light output of a CsI:Tl scintillator The intensities of the X-ray-induced luminescence (XL) spectra, emitted by X-ray exposure, are shown as the doped Tl concentration in CsI:Tl thin films in Fig. 4. The strong emission peak with 550–560 nm wavelength is shown in the spectra of the CsI:Tl thin films. After CsI:Tl thin films with different Tl concentrations are prepared by heat treatment, the intensities of the films increased and the strong emission peak shifted a little because of Tl diffusion effect in CsI:Tl thin films [6]. The XL spectra of the deposited CsI:Tl films at different evaporation pressures and substrate temperatures using the same Tl concentration are shown in Fig. 5.
The light output of the samples was measured by pixel value averaged over the region of interest (ROI) of X-ray images acquired with a 80 kVp, 30 mA s X-ray source. The relative light output of the CsI:Tl thin films according to different Tl concentrations and heat treatments is shown in Fig. 6. As shown in the figure, the highest light output among the CsI:Tl thin films was observed in case of the sample with 1.0 mol% Tl concentration and post-heat treatment at 250 1C temperature, 10 5 Torr vacuum conditions for 2 h. The light output of the CsI:Tl thin films at different evaporation conditions such as evaporation pressure, substrate temperature and post-heat treatment is shown in Fig. 7. The CsI:Tl scintillator deposited with 10 5 Torr pressure, 35 1C substrate temperature and post-heat treatment showed the highest light output. 3.3. X-ray imaging performance The X-ray response and signal-to-noise ratio were evaluated by means of a CsI:Tl scintillator-coupled CCD camera system and an X-ray generator. The signal response in terms of analog digital units was measured by pixel values averaged over region of interest areas in X-ray images. The X-ray response curves of the
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CsI:T1 0.05 mol% CsI:T1 0.05 mol% & Heat treatment CsI:T1 0.1 mol% CsI:T1 0.1 mol% & Heat treatment CsI:T1 0.3 mol% CsI:T1 0.3 mol% & Heat treatment CsI:T1 0.5 mol% CsI:T1 0.5 mol% & Heat treatment CsI:T1 0.7 mol% CsI:T1 0.7 mol% & Heat treatment CsI:T1 1.0mol% CsI:T1 1.0 mol% & Heat treatment
20
10
0
C
0
20
Fig. 7. Light output of the CsI:Tl scintillator.
CsI:T1 0.05 mol% CsI:T1 0.05 mol% & Heat treatment CsI:T1 0.1 mol% CsI:T1 0.1 mol% & Heat treatment CsI:T1 0.3 mol% CsI:T1 0.3 mol% & Heat treatment CsI:T1 0.5 mol% CsI:T1 0.5 mol% & Heat treatment CsI:T1 0.7 mol% CsI:T1 0.7 mol% & Heat treatment CsI:T1 1.0 mol% CsI:T1 1.0 mol% & Heat treatment
80
100
200
30 -5
100
0 0
20
40 60 80 X-ray exposure (mR)
100
SNR (Signal to noise ratio)
300 Signal (ADU)
60
Fig. 10. Signal-to-noise ratio of CsI:Tl samples.
400
CsI:T1 0.3 mol% sub temp. 30°C/pressure 10 torr -5 CsI:T1 0.3 mol% sub temp. 30°C/pressure 10 torr & Heat treatment -2 CsI:T1 0.3 mol% sub temp. 30°C/pressure 10 torr -2 CsI:T1 0.3 mol% sub temp. 30°C/pressure 10 torr & Heat treatment -5 CsI:T1 0.3 mol% sub temp. 30°C/pressure 10 torr -5 CsI:T1 0.3 mol% sub temp. 30°C/pressure 10 torr & Heat treatment
20
10
0
Fig. 8. Signal response of CsI:Tl samples.
0
20
40
60
80
100
X-ray exposure (mR)
400
Fig. 11. Signal-to-noise ratio of CsI:Tl samples.
-5
CsI:T1 0.3 mol% sub temp. 30°C/pressure 10 torr -5 CsI:T1 0.3 mol% sub temp. 30°C/pressure 10 torr & Heat treatment -5 CsI:T1 0.3 mol% sub temp. 30°C/pressure 10 torr -5 CsI:T1 0.3 mol% sub temp. 30°C/pressure 10 torr & Heat treatment -5 CsI:T1 0.3 mol% sub temp. 250°C/pressure 10 torr -5 CsI:T1 0.3 mol% sub temp. 250°C/pressure 10 torr & Heat treatment
300 Signal (ADU)
40
X-ray exposure (mR)
200
100
0 0
20
40
60
80
100
X-ray exposure (mR) Fig. 9. Signal response of the CsI:Tl samples.
CsI:Tl thin films with different Tl concentrations, evaporation conditions, and post-heat treatment as X-ray exposure are shown in Figs. 8 and 9. The X-ray exposure dose was increased by changing the beam current of the X-ray source fixed at a 50 kVp tube voltage and measured by a calibrated ion chamber. As the X-ray exposure increases, the signal of the CsI:Tl scintillator was linearly increased in case of both Figs. 8 and 9.
The signal-to-noise ratio in terms of the important evaluation factor in the X-ray imaging system was measured as X-ray exposure. The signal-to-noise ratio was measured by dividing the average output signal by the standard deviation over a ROI in the X-ray images. The signal-to-noise ratio of the CsI:Tl thin films with different Tl concentrations and evaporation conditions is shown in Figs. 10 and 11, respectively. The X-ray images acquired by CsI:Tl thin films with different Tl doping concentrations, evaporation conditions, and lens-coupled CCD camera system consisting of 1024 1024 active pixels with 13 mm pixel pitch and 13.3 13.3 mm2 active area at the same X-ray exposure conditions with 80 kVp, 30 mA s are shown in Fig. 12. The CsI:Tl thin film (Fig. 12(c)) deposited at 1.0 mol% Tl concentration, high vacuum, and room temperature conditions shows the highest image contrast. And the CsI:Tl thin film (Fig. 12(e)) with a square columnar structure shows the X-ray image with high resolution because of the reduction of the lateral spreading of light photons and promotion of the directional light channeling in the columnar structure [2].
4. Conclusion In this work, we fabricated the Tl-doped CsI scintillation thin films by the vacuum evaporation method and evaluated the
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Fig. 12. X-ray images of a memory chip: (a) 0.05 mol% Tl/10 5 Torr pressure/35 1C temperature; (b) 0.3 mol% Tl/10 5 Torr pressure/35 1C temperature; (c) 1.0 mol% Tl/ 10 5 Torr pressure/35 1C temperature; (d) 0.3 mol% Tl/10 2 Torr pressure/35 1C temperature; (e) 0.3 mol% Tl/10 5 Torr pressure/250 1C tempeature.
scintillation characteristics and the X-ray imaging performance as Tl doping concentration, evaporation condition such as evaporation pressure, substrate temperature, and post-heat treatment for X-ray imaging detectors. The CsI:Tl thin film deposited at high vacuum, room substrate temperature conditions showed high light output under X-ray exposure. And the light output of the CsI:Tl thin films was increased by post-heat treatment at high vacuum for 2 h. The CsI:Tl thin film with columnar structure was observed at low vacuum and room substrate temperature for X-ray image with high resolution. In conclusion, the CsI:Tl thin films with high light output and spatial resolution can be fabricated by changing evaporation conditions such as the Tl concentration, evaporation pressure, substrate temperature, and post-heat treatment and will be used for X-ray imaging applications such as medical and industrial fields with low X-ray exposure dose.
Acknowledgments This work was supported by Nuclear R&D Program of MEST through (M20701030002-08N0103-00210).
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