- Email: [email protected]
Radiation characteristics of scintillator coupled CMOS APS for radiography conditions
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 568 (2006) 124–127 www.elsevier.com/locate/nima
Radiation characteristics of scintillator coupled CMOS APS for radiography conditions Kwang Hyun Kima,, Soongpyung Kimb, Dong-Won Kanga, Dong-Kie Kima a
College of Dentistry, Chosun University, Gwangju, Republic of Korea Deparment of Nuclear Engineering, Chosun University, Gwangju, Republic of Korea
b
Available online 27 July 2006
Abstract Under industrial radiography conditions, we analyzed short-term radiation characteristics of scintillator coupled CMOS APS (hereinafter SC CMOS APS). By means of experimentation, the contribution of the transmitted X-ray through the scintillator to the properties of the CMOS APS and the afterimage, generated in the acquired image even at low dose condition, were investigated. To see the transmitted X-ray effects on the CMOS APS, Fein focusTM X-ray machine, two scintillators of LanexTM Fine and Regular, and two CMOS APS array of RadEyeTM were used under the conditions of 50 kVp/1 mAs and 100 kVp/1 mAs. By measuring the transmitted Xray on signal and Noise Power Spectrum, we analytically examined the generation mechanism of the afterimage, based on dark signal or dark current increase in the sensor, and explained the afterimage in the SC CMOS APS. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Mc; 87.59.Hp; 87.59.Ek Keywords: SC CMOS APS; Transmitted X-ray; NPS; Afterimage
1. Introduction In a digital X-ray imaging system based on scintillator coupled CMOS active pixel sensor imager (hereinafter SC CMOS APS), there are two considerable categories of short and long exposure conditions according to the amount of incident X-ray photons on the SC CMOS APS, depending on the operation mode of X-ray machine. Medical X-ray imaging such as general radiography and mammography come under the short exposure condition. Industrial radiography needs not only short interval Xray exposure between each image, but also continuous exposure and consequently, high exposure of X-ray photons. These conditions influence the degradation of system resolution by the increase of dark signal on the detector [1] showing the effect of cumulative exposure on the image quality for long-term exposure conditions. This paper analyzes temporal noise and side effect for the single shot and relatively high X-ray energy. From the Corresponding author. Tel.: +82 42 869 3861; fax: +82 42 861 8779.
E-mail address: [email protected] (K.H. Kim). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.05.283
simulations and measurements of the direct detection by the transmitted X-ray photons through the scintillator, the generation mechanism of the afterimage has been investigated by using the relation between absorbed energy and dark signal of the detector in detail. 2. Indirect and direct signals on SC CMOS APS Although the CMOS APS based imager shows less radiation effects compared to a CCD based system [2], it is not free from ionizing radiation since the incident radiation makes ionization in the oxide layer of the MOS and photodiode area through the total ionizing dose (TID) mechanism [3]. It has been known that the main radiation effects on the CMOS APS are the increase of dark current from the photodiode, the transistors and the interconnectivity in the pixel. These increases arise predominantly at the interface of the silicon and the silicon dioxide. Certainly, in a low dose rate environment, the dark current is expected to increase very slowly. At high dose rate, threshold voltage shift as well as dark current change become sub-linear with dose and tend to saturate. Other
ARTICLE IN PRESS K.H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 124–127
expected effects are changes in sensitivity, optical responsivity and dynamic range. The capacitance of the photodiode determines the sensitivity of the pixel. The smaller the capacitance, the higher the voltage swings for each collected charge [4]. Others [5] measured and generalized the increased dark current by radiation, resulting in I dark / ðI 0 þ KDÞ eE a =kT
(1)
where I0 is the initial dark current, K a damage factor, D the dose, Ea activation energy, and T temperature. However, in the scintillator-coupled sensor, the absorbed dose of the sensor is highly dependent on the physical properties of the scintillator such as attenuation coefficient (mS) and thickness (tS). In the scintillator coupled CCD [6,7], it was reported that the influence of the transmitted X-rays on the detector only related to signal and noise. From the results, the signal to noise ratio (SNR) decreased as the energy of the X-ray beam increased, indicating great contribution of direct detection to image noise, which was proportional to the number of X-ray photons captured within the sensitive layer. However, it is not easy to distinguish an indirect signal (a signal generated by scintillation light from scintillator) from a direct signal (DS, a signal generated by the direct detection of the transmitted X-rays in a sensor). The indirect signal (S(E)in) comes from the following equation: Z SðEÞin FQðlÞ F0 ðEÞ 1 exp mS tS C eff (2) where F is the fraction of absorbed X-ray energy in the thickness of tS scintillator, Q(l) quantum efficiency of sensor, F0(E) incident X-ray spectrum, and Ceff conversion efficiency of the scintillator. On the other hand, the DS (S(E)dir) is proportional only to the absorbed energy in the sensitive layer of the sensor as follows: Z SðEÞdir ¼ F0 ðEÞ exp mS tS 1 exp mC tC (3) where the first bracketed expression is the amount of the transmitted X-ray from the scintillator, the second one is the amount of the absorbed X-ray in the sensitive layer of tC. To estimate the transmitted X-ray of the scintillators,
125
Monte Carlo simulation was done by using MCNP 4B code [8] and semi-empirical X-ray spectrum code [9] for GADOX based scintillators, LanexTM Fine (34 mg/cm2) and Regular (67 mg/cm2). At the tube operation of 50 kVp and 1 mAs, the amounts of the transmitted X-rays under those conditions were 43% and 28% for Lanex Fine and Regular, respectively. At the condition of 100 kVp and 1 mAs, the amounts of the transmitted X-rays were 69% and 54% for each scintillator, respectively. We measured the indirect signal (S(E)in) and the DS (S(E)dir) in the SC CMOS APS for industrial conditions. As shown in Table 1, Fein focusTM X-ray machine, two LanexTM scintillators: fine and regular, and two CMOS APS imagers of RadEyeTM were used. Each exposure was measured using an ionization chamber of RAD CHEKTM PLUS (models 06-526). Measuring the effect of the direct X-ray on signal, a low-density thin graphite cover was used to block or pass the scintillation light by placing it in front or back of the scintillator to produce a signal (Total Signal, TS) by the scintillation light and the transmitted X-rays or a signal (DS) generated by the transmitted X-rays only, respectively. The measured results of TS, DS, and calculated DS contribution on TS (DS Con.) for each of the industrial conditions are shown in Table 2. Another measurement results of DS contribution on TS are shown in Fig. 1. The transmitted X-ray and the direct detection are increased by exposure time and the result differs slightly from the exposure conditions and experimental conditions. Although the higher X-ray of 100 keV compared to 50 keV at the same current and exposure time, generated the more DS, the contribution of the DS on the TS was very similar. Noise Power Spectrum (NPS) measurements were performed for the imager employing each scintillator up to 50% of pixel saturation. In this measurement all data were acquired at the Source to Imager Distance (SID) of 300 mm. The flood–field data offset was performed by others [10]. 2D surface fitting method (second-order polynomial fitting) [11] was used, which needs subtraction image data from the original image to the fitted image. The results of measured NPS for each application were shown in Fig. 2. The NPS of Lanex fine for the exposure condition of 50 kVp/1 mAs was somewhat poor at low frequency region.
Table 1 Each specification and their properties for experimental set-up X-ray machine
Fein focusTM X-ray machine (Tungsten target/Beryllium window)
X-ray conditions Entrance exposure (at SID 30 cm) Scintillator Scintillator properties
Tube voltage/current: (50 kVp/1 mAs) 0.24R Lanex fineTM Density: 7.34 (g/cm3) Coverage: 34 (mg/cm2) Active area: 25 mm 50 mm Pixel size: 48 mm Noise floor: 150 electrons rms Digitization noise: 250–300 electros rms
CMOS APS imager (RadEyeTM CMOS APS)
Tube voltage/current (100 kVp/1 mAs) 0.43R Lanex regularTM Density: 7.34 (g/cm3) Coverage: 67 (mg/cm2)
ARTICLE IN PRESS K.H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 124–127
126
Table 2 Measured TS and DS and calculated direct signal contribution on total signal (DS con.) X-ray source
Signals
Scint.
50 kVp/1 mAs
Fine Regular
100 kVp/1 mAs
TS
DS
DS Con. (%)
TS
DS
DS Con. (%)
870 876
84 69
9.6 7.9
4200 4127
345 309
8.2 7.5
Direct Signal Cont. (%)
9
8
7 Lanex fine-100kVp Lanex fine-50kVp Lanex regular-100kVp Lanex regular-50kVp
6
5
0
Fig. 3. This image, showing an afterimage of ghost test pattern, was acquired without any X-ray exposure or test pattern.
100 200 300 400 500 600 700 800 900 1000 1100 X-ray Tube Current (µamp.)
Fig. 1. Direct signal contribution on total signal by various changes of Xray tube current.
9.0x10-6
Normalized NPS (mm2)
8.0x10-6
Lanex fine Lanex regular
-6
7.0x10
6.0x10-6
phantom was a Tantalum round type, thickness of 1.5 mm. After single-shot exposure on each test pattern, each digitized image was acquired. Fig. 3 shows an undesirable afterimage of a ghost-like image containing round type Tantalum phantom and discrete device of the electric circuit. After getting an X-ray image for the test patterns under the exposure conditions of 50 kVp and 1 mAs, another dark image that was acquired without X-ray exposure did not reveal any afterimage. On the other hand, after exposure of 100 kVp and 1 mAs, an afterimage was showing each test pattern as a ghost image even at dark image.
5.0x10-6 4.0x10-6
4. Analyses and discussion
3.0x10-6 2.0x10-6 -6
1.0x10
1
2
3 6 4 5 7 Frequency (cycles/mm)
8
9
10
Fig. 2. Measured NPS for two scintillators at 50 kVp/1 mAs.
3. Afterimage generation on SC CMOS APS With the same test conditions, test patterns with various properties of materials, an electric circuit and a phantom, were exposed to acquire each image. The used electric circuit was PCB with a thickness of less than 1 mm and the
The afterimage was analyzed by using each pixel signal of ADC values in the dark image. The afterimage showing each test pattern had different values in pixel by pixel. The pixels, covered with high-density thick material, revealed the less pixel signal than those covered with low density and thin material. This is simply because of the difference of absorption in each pixel and consequently the DS through each test pattern. Since the W value of semiconductor is 3.6 eV and relatively smaller than GADOX of 13 eV, the contribution of the direct X-rays on the sensor to the TS is severe. Those transmitted X-rays may deposit their partial energies in each pixel. The transmitted X-rays generate ionization charges which are proportional to the absorbed energy as shown in Eq. (3) and will be a source of dark signal.
ARTICLE IN PRESS K.H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 124–127
If there are two different objects of t1 and t2 to be projected on the SC CMOS APS imager, the difference of the absorbed dose DD(E) in each pixel is Z DDðEÞ ¼ F0 ðEÞ exp mO1 t1 exp mO2 t2 ð4Þ exp ðmS tS Þ 1 exp ðmC tC Þ . And, assuming the difference of the dark signal or dark current is proportional only to the difference of absorbed dose DD(E), the difference of dark signal in afterimage in each pixel of the sensor as follows: DI dark / I 0 þ KDDðEÞ eE a =kT .
the the the (5)
From the experiments, we first showed the effects of the transmitted X-ray on the SC CMOS APS in industrial conditions of short exposure and analyzed the side effect of afterimage. The contribution of the DS on the TS did not show a large difference since higher incident X-ray energy gives more TS with scintillation light. However, in the viewpoint of the DS only, the higher energy of X-ray beam influences
127
bad noise characteristics and leads a dark image (without any X-ray) to an afterimage showing a previously projected image. References [1] K.H. Kim, et al., Nucl. Instr. and Meth. A 537 (2005) 454. [2] E. Fossum, Proc. SPIE 1990 (1993) 2. [3] A. Holmes-Siedle, et al., Handbook of Radiation Effects, Oxford University Press, Oxford, New York, Tokyo, 1993 (Chapter 4). [4] J. Bogaerts, et al., Ind. Digital Photogr. Appl. 3965 (2000) 157. [5] B.R. Hancock, et al., Total dose testing of a CMOS charged particle spectrometer, IEEE TNS 44(6) (1957). [6] M. Gambaccini, et al., Nucl. Instr. and Meth. A 409 (1998) 508. [7] E. Dubaric, et al., Nucl. Instr. and Meth. A 466 (2001) 178. [8] J.F. Briesmeister, MCNP-A General Monte Carlo N-Particle Transport Code Version 4B, Los Alamos National Laboratory Los Alamos, New Mexico, 1997. [9] P. Hammersberg, et al., Absolute energy spectra for an industrial micro focal X-ray source under working conditions measured with a Compton scattering spectrometer—full spectra data, J. X-ray Sci. Technol. 8(1) (1998). [10] S. Vedantham, et al., Med. Phys. 27 (3) (2000) 558. [11] M.B. Williams, et al., Med. Phys. 26 (7) (1999) 1279.
Nuclear Instruments and Methods in Physics Research A 568 (2006) 124–127 www.elsevier.com/locate/nima
Radiation characteristics of scintillator coupled CMOS APS for radiography conditions Kwang Hyun Kima,, Soongpyung Kimb, Dong-Won Kanga, Dong-Kie Kima a
College of Dentistry, Chosun University, Gwangju, Republic of Korea Deparment of Nuclear Engineering, Chosun University, Gwangju, Republic of Korea
b
Available online 27 July 2006
Abstract Under industrial radiography conditions, we analyzed short-term radiation characteristics of scintillator coupled CMOS APS (hereinafter SC CMOS APS). By means of experimentation, the contribution of the transmitted X-ray through the scintillator to the properties of the CMOS APS and the afterimage, generated in the acquired image even at low dose condition, were investigated. To see the transmitted X-ray effects on the CMOS APS, Fein focusTM X-ray machine, two scintillators of LanexTM Fine and Regular, and two CMOS APS array of RadEyeTM were used under the conditions of 50 kVp/1 mAs and 100 kVp/1 mAs. By measuring the transmitted Xray on signal and Noise Power Spectrum, we analytically examined the generation mechanism of the afterimage, based on dark signal or dark current increase in the sensor, and explained the afterimage in the SC CMOS APS. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Mc; 87.59.Hp; 87.59.Ek Keywords: SC CMOS APS; Transmitted X-ray; NPS; Afterimage
1. Introduction In a digital X-ray imaging system based on scintillator coupled CMOS active pixel sensor imager (hereinafter SC CMOS APS), there are two considerable categories of short and long exposure conditions according to the amount of incident X-ray photons on the SC CMOS APS, depending on the operation mode of X-ray machine. Medical X-ray imaging such as general radiography and mammography come under the short exposure condition. Industrial radiography needs not only short interval Xray exposure between each image, but also continuous exposure and consequently, high exposure of X-ray photons. These conditions influence the degradation of system resolution by the increase of dark signal on the detector [1] showing the effect of cumulative exposure on the image quality for long-term exposure conditions. This paper analyzes temporal noise and side effect for the single shot and relatively high X-ray energy. From the Corresponding author. Tel.: +82 42 869 3861; fax: +82 42 861 8779.
E-mail address: [email protected] (K.H. Kim). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.05.283
simulations and measurements of the direct detection by the transmitted X-ray photons through the scintillator, the generation mechanism of the afterimage has been investigated by using the relation between absorbed energy and dark signal of the detector in detail. 2. Indirect and direct signals on SC CMOS APS Although the CMOS APS based imager shows less radiation effects compared to a CCD based system [2], it is not free from ionizing radiation since the incident radiation makes ionization in the oxide layer of the MOS and photodiode area through the total ionizing dose (TID) mechanism [3]. It has been known that the main radiation effects on the CMOS APS are the increase of dark current from the photodiode, the transistors and the interconnectivity in the pixel. These increases arise predominantly at the interface of the silicon and the silicon dioxide. Certainly, in a low dose rate environment, the dark current is expected to increase very slowly. At high dose rate, threshold voltage shift as well as dark current change become sub-linear with dose and tend to saturate. Other
ARTICLE IN PRESS K.H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 124–127
expected effects are changes in sensitivity, optical responsivity and dynamic range. The capacitance of the photodiode determines the sensitivity of the pixel. The smaller the capacitance, the higher the voltage swings for each collected charge [4]. Others [5] measured and generalized the increased dark current by radiation, resulting in I dark / ðI 0 þ KDÞ eE a =kT
(1)
where I0 is the initial dark current, K a damage factor, D the dose, Ea activation energy, and T temperature. However, in the scintillator-coupled sensor, the absorbed dose of the sensor is highly dependent on the physical properties of the scintillator such as attenuation coefficient (mS) and thickness (tS). In the scintillator coupled CCD [6,7], it was reported that the influence of the transmitted X-rays on the detector only related to signal and noise. From the results, the signal to noise ratio (SNR) decreased as the energy of the X-ray beam increased, indicating great contribution of direct detection to image noise, which was proportional to the number of X-ray photons captured within the sensitive layer. However, it is not easy to distinguish an indirect signal (a signal generated by scintillation light from scintillator) from a direct signal (DS, a signal generated by the direct detection of the transmitted X-rays in a sensor). The indirect signal (S(E)in) comes from the following equation: Z SðEÞin FQðlÞ F0 ðEÞ 1 exp mS tS C eff (2) where F is the fraction of absorbed X-ray energy in the thickness of tS scintillator, Q(l) quantum efficiency of sensor, F0(E) incident X-ray spectrum, and Ceff conversion efficiency of the scintillator. On the other hand, the DS (S(E)dir) is proportional only to the absorbed energy in the sensitive layer of the sensor as follows: Z SðEÞdir ¼ F0 ðEÞ exp mS tS 1 exp mC tC (3) where the first bracketed expression is the amount of the transmitted X-ray from the scintillator, the second one is the amount of the absorbed X-ray in the sensitive layer of tC. To estimate the transmitted X-ray of the scintillators,
125
Monte Carlo simulation was done by using MCNP 4B code [8] and semi-empirical X-ray spectrum code [9] for GADOX based scintillators, LanexTM Fine (34 mg/cm2) and Regular (67 mg/cm2). At the tube operation of 50 kVp and 1 mAs, the amounts of the transmitted X-rays under those conditions were 43% and 28% for Lanex Fine and Regular, respectively. At the condition of 100 kVp and 1 mAs, the amounts of the transmitted X-rays were 69% and 54% for each scintillator, respectively. We measured the indirect signal (S(E)in) and the DS (S(E)dir) in the SC CMOS APS for industrial conditions. As shown in Table 1, Fein focusTM X-ray machine, two LanexTM scintillators: fine and regular, and two CMOS APS imagers of RadEyeTM were used. Each exposure was measured using an ionization chamber of RAD CHEKTM PLUS (models 06-526). Measuring the effect of the direct X-ray on signal, a low-density thin graphite cover was used to block or pass the scintillation light by placing it in front or back of the scintillator to produce a signal (Total Signal, TS) by the scintillation light and the transmitted X-rays or a signal (DS) generated by the transmitted X-rays only, respectively. The measured results of TS, DS, and calculated DS contribution on TS (DS Con.) for each of the industrial conditions are shown in Table 2. Another measurement results of DS contribution on TS are shown in Fig. 1. The transmitted X-ray and the direct detection are increased by exposure time and the result differs slightly from the exposure conditions and experimental conditions. Although the higher X-ray of 100 keV compared to 50 keV at the same current and exposure time, generated the more DS, the contribution of the DS on the TS was very similar. Noise Power Spectrum (NPS) measurements were performed for the imager employing each scintillator up to 50% of pixel saturation. In this measurement all data were acquired at the Source to Imager Distance (SID) of 300 mm. The flood–field data offset was performed by others [10]. 2D surface fitting method (second-order polynomial fitting) [11] was used, which needs subtraction image data from the original image to the fitted image. The results of measured NPS for each application were shown in Fig. 2. The NPS of Lanex fine for the exposure condition of 50 kVp/1 mAs was somewhat poor at low frequency region.
Table 1 Each specification and their properties for experimental set-up X-ray machine
Fein focusTM X-ray machine (Tungsten target/Beryllium window)
X-ray conditions Entrance exposure (at SID 30 cm) Scintillator Scintillator properties
Tube voltage/current: (50 kVp/1 mAs) 0.24R Lanex fineTM Density: 7.34 (g/cm3) Coverage: 34 (mg/cm2) Active area: 25 mm 50 mm Pixel size: 48 mm Noise floor: 150 electrons rms Digitization noise: 250–300 electros rms
CMOS APS imager (RadEyeTM CMOS APS)
Tube voltage/current (100 kVp/1 mAs) 0.43R Lanex regularTM Density: 7.34 (g/cm3) Coverage: 67 (mg/cm2)
ARTICLE IN PRESS K.H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 124–127
126
Table 2 Measured TS and DS and calculated direct signal contribution on total signal (DS con.) X-ray source
Signals
Scint.
50 kVp/1 mAs
Fine Regular
100 kVp/1 mAs
TS
DS
DS Con. (%)
TS
DS
DS Con. (%)
870 876
84 69
9.6 7.9
4200 4127
345 309
8.2 7.5
Direct Signal Cont. (%)
9
8
7 Lanex fine-100kVp Lanex fine-50kVp Lanex regular-100kVp Lanex regular-50kVp
6
5
0
Fig. 3. This image, showing an afterimage of ghost test pattern, was acquired without any X-ray exposure or test pattern.
100 200 300 400 500 600 700 800 900 1000 1100 X-ray Tube Current (µamp.)
Fig. 1. Direct signal contribution on total signal by various changes of Xray tube current.
9.0x10-6
Normalized NPS (mm2)
8.0x10-6
Lanex fine Lanex regular
-6
7.0x10
6.0x10-6
phantom was a Tantalum round type, thickness of 1.5 mm. After single-shot exposure on each test pattern, each digitized image was acquired. Fig. 3 shows an undesirable afterimage of a ghost-like image containing round type Tantalum phantom and discrete device of the electric circuit. After getting an X-ray image for the test patterns under the exposure conditions of 50 kVp and 1 mAs, another dark image that was acquired without X-ray exposure did not reveal any afterimage. On the other hand, after exposure of 100 kVp and 1 mAs, an afterimage was showing each test pattern as a ghost image even at dark image.
5.0x10-6 4.0x10-6
4. Analyses and discussion
3.0x10-6 2.0x10-6 -6
1.0x10
1
2
3 6 4 5 7 Frequency (cycles/mm)
8
9
10
Fig. 2. Measured NPS for two scintillators at 50 kVp/1 mAs.
3. Afterimage generation on SC CMOS APS With the same test conditions, test patterns with various properties of materials, an electric circuit and a phantom, were exposed to acquire each image. The used electric circuit was PCB with a thickness of less than 1 mm and the
The afterimage was analyzed by using each pixel signal of ADC values in the dark image. The afterimage showing each test pattern had different values in pixel by pixel. The pixels, covered with high-density thick material, revealed the less pixel signal than those covered with low density and thin material. This is simply because of the difference of absorption in each pixel and consequently the DS through each test pattern. Since the W value of semiconductor is 3.6 eV and relatively smaller than GADOX of 13 eV, the contribution of the direct X-rays on the sensor to the TS is severe. Those transmitted X-rays may deposit their partial energies in each pixel. The transmitted X-rays generate ionization charges which are proportional to the absorbed energy as shown in Eq. (3) and will be a source of dark signal.
ARTICLE IN PRESS K.H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 124–127
If there are two different objects of t1 and t2 to be projected on the SC CMOS APS imager, the difference of the absorbed dose DD(E) in each pixel is Z DDðEÞ ¼ F0 ðEÞ exp mO1 t1 exp mO2 t2 ð4Þ exp ðmS tS Þ 1 exp ðmC tC Þ . And, assuming the difference of the dark signal or dark current is proportional only to the difference of absorbed dose DD(E), the difference of dark signal in afterimage in each pixel of the sensor as follows: DI dark / I 0 þ KDDðEÞ eE a =kT .
the the the (5)
From the experiments, we first showed the effects of the transmitted X-ray on the SC CMOS APS in industrial conditions of short exposure and analyzed the side effect of afterimage. The contribution of the DS on the TS did not show a large difference since higher incident X-ray energy gives more TS with scintillation light. However, in the viewpoint of the DS only, the higher energy of X-ray beam influences
127
bad noise characteristics and leads a dark image (without any X-ray) to an afterimage showing a previously projected image. References [1] K.H. Kim, et al., Nucl. Instr. and Meth. A 537 (2005) 454. [2] E. Fossum, Proc. SPIE 1990 (1993) 2. [3] A. Holmes-Siedle, et al., Handbook of Radiation Effects, Oxford University Press, Oxford, New York, Tokyo, 1993 (Chapter 4). [4] J. Bogaerts, et al., Ind. Digital Photogr. Appl. 3965 (2000) 157. [5] B.R. Hancock, et al., Total dose testing of a CMOS charged particle spectrometer, IEEE TNS 44(6) (1957). [6] M. Gambaccini, et al., Nucl. Instr. and Meth. A 409 (1998) 508. [7] E. Dubaric, et al., Nucl. Instr. and Meth. A 466 (2001) 178. [8] J.F. Briesmeister, MCNP-A General Monte Carlo N-Particle Transport Code Version 4B, Los Alamos National Laboratory Los Alamos, New Mexico, 1997. [9] P. Hammersberg, et al., Absolute energy spectra for an industrial micro focal X-ray source under working conditions measured with a Compton scattering spectrometer—full spectra data, J. X-ray Sci. Technol. 8(1) (1998). [10] S. Vedantham, et al., Med. Phys. 27 (3) (2000) 558. [11] M.B. Williams, et al., Med. Phys. 26 (7) (1999) 1279.