- Email: [email protected]
Study on response function of CdTe detector
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 610 (2009) 302–306
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Study on response function of CdTe detector Hyunduk Kim a, Gyuseong Cho a, Bo-Sun Kang b, a b
Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea Department of Radiological Science, Catholic University of Daegu, Kyoungsan, Kyoungbuk 712-702, Republic of Korea
a r t i c l e in f o
a b s t r a c t
Available online 27 May 2009
So far the origin of the mechanism of light emission in the sonoluminescence has not elucidated whether it is due to blackbody radiation or bremsstrahlung. The final goal of our study is measuring X-ray energy spectrum using high-sensitivity cadmium telluride (CdTe) detector in order to obtain information for understanding sonoluminescence phenomena. However, the scope of this report is the measurement of X-ray spectrum using a high-resolution CdTe detector and determination of CdTe detector response function to obtain the corrected spectrum from measured soft X-ray source spectrum. In general, the measured spectrum was distorted by the characteristics of CdTe detector. Monte Carlo simulation code, MCNP, was used to obtain the reference response function of the CdTe detector. The X-ray spectra of 57Co, 133Ba, and 241Am were obtained by a 4 4 1.0(t) mm3 CdTe detector at room temperature. & 2009 Elsevier B.V. All rights reserved.
Keywords: CdTe X-ray detector Response Asymmetry
1. Introduction In the X-ray and gamma-ray measurement, semiconductor and scintillator combined detector are widely used. However, the detector selection is made by considering various required detector characteristics such as fast raising time, detection efficiency, energy resolution, etc. NaI(Tl) and HPGe detectors are useful tools for hard X-ray and gamma-ray radiation measurement because NaI(Tl) and HPGe have good efficiency compared to any commercial detectors and provide high resolution. However, they are not suitable for soft X-ray measurement. Recently, many researchers developed a detector that has good detection efficiency and high resolution in soft X-ray spectral range, the so-called soft X-ray detector. Especially, cadmium telluride (CdTe) is a promising material for X-ray and gamma-ray detectors that can operate at room temperature with a high detection efficiency comparable to NaI(Tl) and good energy resolution comparable to an HPGe detector. The prior application fields of the CdTe detector are medical engineering and astrophysics. To date, many researchers have developed and tested various types of detectors that are used in the X-ray range for obtaining an accurate knowledge of energy spectra as well as for astrophysical and medical applications [1–8]. Among those X-ray detectors, cadmium telluride detector is gaining interest recently in many applications because its large band-gap reduces leakage current, and makes room-temperature operation feasible. High-energy resolution, good mobility lifetime product and large stopping
power are attracting many users. It also has a compactness and portability that are outstanding advantages in many applications.In our research, we selected CdTe for the study on sonoluminescence phenomena [9,10]. So far the origin of light emission mechanism in sonoluminescence has not elucidated whether it is due to blackbody radiation or bremsstrahlung. X-ray spectrum analysis might be the interesting measurement that could provide the key information to find unrevealed sonoluminescence mechanism. We investigated the nonlinear behavior of heat transfer within and through the shell of a microbubble by ultrasound
Corresponding author. Tel.: +82 53 850 3437; fax: +82 53 850 3292.
E-mail address: [email protected] (B.-S. Kang). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.05.097
Fig. 1. Schottky-type CdTe shielded inside aluminum dark box.
ARTICLE IN PRESS H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 610 (2009) 302–306
303
injection. We are trying to infer the microbubble temperature using a high-sensitivity CdTe X-ray spectrometer by measuring X-ray spectrum from a sonoluminary microbubble. Recently, we tested a 4 4 1 mm3 high-resolution Schottky CdTe detector to study its response to low-energy gamma-ray source at room temperature. The measured spectrum was generally distorted by the characteristics of CdTe detectors such as hole tailing and pileup effect. Computer simulation was also performed using MCNP to obtain response function of CdTe detectors.
2. Experiment and simulation In the experiment, a CdTe detector of 1 mm thickness and 4 4 mm2 surface area was employed. The detector was a Schottky contact-type CdTe detector made at ACRORAD, which helps the CdTe operating with higher electric field than the one with Ohmic contacts type. It was shielded by 5.5 6 2.7 cm3 of aluminum box as shown in Fig. 1.
Fig. 2. Input geometry for MCNPX simulation.
x 102
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4000
0.1059E+06/ 22 19.03 0.2850E-01 -0.4433E-01 0.1414E-03 0.3765E+06 352.8 234.3 0.5765E-01 4.652 0.2718E-02 0.3560E+05 117.0 262.4 0.1744E-01 4.281 0.1143E-01
0.4835E+05/ 46 Constant 0.1385E+06 267.4 Mean 156.1 0.6434E-02 Sigma 3.837 0.4810E-02
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Fig. 3. The pulse height spectra were measured by CdTe at room temperature. 57Co spectrum was shown in (a). Two 57Co gamma-ray peaks of 122 and 136 keV were found at 230 and 260 channel of MCA, respectively. Two Gaussian peaks of 133Ba appeared at about 60 and 150 channels as shown in (b). 241Am spectrum is shown in (c). Gaussian peak of 59 keV gamma ray appears at the 120 channel.
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H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 610 (2009) 302–306
acquired with a 2048 multichannel analyzer (MCA). A high voltage for biasing CdTe and gain was adjusted so that the 300 channel corresponded to about 122 keV. Computer simulation was performed by MCNPX. Fig. 2 shows the CdTe inside a 5.5 6 2.7 cm3 Al box. F8 tally was used in the MCNPX simulation in order to simulate pulse height spectrum.
x 102 5000
4000
3. Results
3000
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0 0
0.02
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0.06 0.08 Co-57 energy
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Fig. 4. The pulse height spectrum as a function of incident gamma-ray energy of 57 Co has two photo peaks.
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Three gamma-ray spectra were measured at room temperature as mentioned above. The sources emit low-energy gamma rays that are suitable for the experiments. The interested gamma peaks were 45 and 75 keV of 133Ba, 59 keV of 241Am and 122 and 136 keV of 57Co. Fig. 3 shows three different measured gamma-ray pulse height spectra from 57Co, 241Am, and 133Ba. In the 57Co spectrum, Fig. 3(a), the full-energy peaks of 122 and 136 keV were found at 230 and 260 channels of the MCA. Since the relative intensities of 122.06 and 136.47 keV are 85.60% and 10.68%, respectively, the full-energy peak of 122.06 keV is higher than that of 136.47 keV. Hence CdTe was used, and the two component materials of CdTe, Cd and Te, generate both Ka and Kb peaks, so each primary full-energy peak generates four escape peaks as mentioned in previous reports [3]. However, the escape peaks were not shown in our measurements. Two Gaussian peaks of 133Ba were measured at about 60 and 150 channels as shown in Fig. 3(b). In the case of 133Ba, 45 keV is a main energy and has a good relative intensity with 46.9%. The about 150 channel peak is due to 75 keV with a relative intensity of 7.6%. Fig. 3(c) shows the spectrum of 241 Am. The 59.56 keV gamma energy of 241Am generates the fullenergy peak at the 110 channel. The 59.56 keV has a good relative intensity of 35.9%. The CdTe detector shows linear characteristics between channel and energy around 100 keV; therefore the energy calibration was performed using 4 peaks. Measured response energy spectrum of 57Co, and the relationship between energy and sigma of Gaussian fit are shown in Fig. 4 and Fig. 5, respectively. The asymmetry of the gamma peak above 50 keV is derived by the hole tailing effect. Fig. 6 shows the ratio of asymmetry. Hole tailing arises from the short lifetime of the holes due to the density of trapping sites in the crystal. In the case of 241Am, the rate of asymmetry is about 0.5. An effect of asymmetry by hole tailing will be an increase of 0.9 in around 100 keV. In the slope of its ratio, the value was about 0.15–0.13.
4. Conclusion & further study
0.18
0.16 0.06
0.08
0.1
0.12
0.14
Fig. 5. The resolution of CdTe as a function of incident gamma energy.
In order to reduce noise signal, a short BNC connector was used at the connection between CdTe and Preamp. Since CdTe is sensible to visible light, it was shielded from light. The distance between gamma-ray sources and CdTe was 1 cm. 241Am, 133Ba and 57Co standard sources were placed at the upper panel of an aluminum dark box. Weak detection signal from CdTe was amplified using Preamp and Amp. Pulse height spectra were
In order to study the response of a CdTe detector, we analyzed the measured response spectra of 57Co, 133Ba, and 241Am. The response spectra were measured by the 2048 channel MCA data acquisition system. Since the MCA did not have enough channel number, it deteriorated the Gaussian broadening of full-energy peaks. In addition to the broadening effect, Compton continuums under-wrapped the escape peaks. The energy resolution as a function of incident gamma ray was about 4–6%, which was not bad. Hole tailing derived the asymmetry of the full-energy photopeak, which increases the shift of peak center at higher energy. The asymmetry ratio was distributed within 0.5–0.9. For further study, we plan to measure X-ray due to sonoluminescence phenomena. For the comprehensive understanding of CdTe response, a more detailed analysis and measurement on Compton background and photopeak asymmetry should be fulfilled.
ARTICLE IN PRESS H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 610 (2009) 302–306
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Fig. 6. The ratio of asymmetry of full peak as a function of channels that was measured using a CdTe detector. From left side, the linear fit shows the slope of the ratio of asymmetry at 241Am, 133Ba, 57Co(122 keV, 136 keV) peaks.
ARTICLE IN PRESS 306
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Acknowledgements This research was supported by ‘Radiation and Nuclear Medical Engineering Research Center’ at KAIST according to the contract of ‘Electric Power Research Institute’ (EPRI). References [1] E.D. Castro, R. Pani, R. Pellegrini, C. Bacci, Phys. Med. Biol. 29 (1984) 1117. [2] H. Tsutsui, T. Ohtsuchi, K. Ohmori, S. Baba, IEEE Trans. Nucl. Sci. NS-39 (1992).
[3] L. verger, M. Boitel, M.C. Gentet, R. Hamelin, C. Mestais, F. Mongellaz, J. Rustique, G. Sanchez, Nucl. Instr. and Meth. A 458 (2001) 297. [4] M.B. Freitas, F.H.M. Medeiros, E.M. Yoshimura, Mater. Sci. Forum 480 (2005) 53. [5] M. Krumrey, M. Gerlach, F. Scholze, G. Ulm, Nucl. Instr. and Meth. A 568 (2006) 364. [6] M. Moralles, D.A.B. Bonifacio, M. Bottaro, M.A.G. Pereira, Nucl. Instr. and Meth. A 580 (2007) 270. [7] R. Gunnink, R. Arlt, Nucl. Instr. and Meth. A 458 (2001) 196. [8] S. Miyajima, Med. Phys. 30 (2003). [9] Ho-Young Kwak, Joong-Yeone Lee, Sarng Woo Karng, J. Phys. Soc. Japan 70 (2001). [10] Jin-Seok Jeon, Ik_Jun Yang, Sang-Woo Karng, Ho-Young Kwak, Jpn. J. Appl. Phys. 39 (2000).
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Study on response function of CdTe detector Hyunduk Kim a, Gyuseong Cho a, Bo-Sun Kang b, a b
Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea Department of Radiological Science, Catholic University of Daegu, Kyoungsan, Kyoungbuk 712-702, Republic of Korea
a r t i c l e in f o
a b s t r a c t
Available online 27 May 2009
So far the origin of the mechanism of light emission in the sonoluminescence has not elucidated whether it is due to blackbody radiation or bremsstrahlung. The final goal of our study is measuring X-ray energy spectrum using high-sensitivity cadmium telluride (CdTe) detector in order to obtain information for understanding sonoluminescence phenomena. However, the scope of this report is the measurement of X-ray spectrum using a high-resolution CdTe detector and determination of CdTe detector response function to obtain the corrected spectrum from measured soft X-ray source spectrum. In general, the measured spectrum was distorted by the characteristics of CdTe detector. Monte Carlo simulation code, MCNP, was used to obtain the reference response function of the CdTe detector. The X-ray spectra of 57Co, 133Ba, and 241Am were obtained by a 4 4 1.0(t) mm3 CdTe detector at room temperature. & 2009 Elsevier B.V. All rights reserved.
Keywords: CdTe X-ray detector Response Asymmetry
1. Introduction In the X-ray and gamma-ray measurement, semiconductor and scintillator combined detector are widely used. However, the detector selection is made by considering various required detector characteristics such as fast raising time, detection efficiency, energy resolution, etc. NaI(Tl) and HPGe detectors are useful tools for hard X-ray and gamma-ray radiation measurement because NaI(Tl) and HPGe have good efficiency compared to any commercial detectors and provide high resolution. However, they are not suitable for soft X-ray measurement. Recently, many researchers developed a detector that has good detection efficiency and high resolution in soft X-ray spectral range, the so-called soft X-ray detector. Especially, cadmium telluride (CdTe) is a promising material for X-ray and gamma-ray detectors that can operate at room temperature with a high detection efficiency comparable to NaI(Tl) and good energy resolution comparable to an HPGe detector. The prior application fields of the CdTe detector are medical engineering and astrophysics. To date, many researchers have developed and tested various types of detectors that are used in the X-ray range for obtaining an accurate knowledge of energy spectra as well as for astrophysical and medical applications [1–8]. Among those X-ray detectors, cadmium telluride detector is gaining interest recently in many applications because its large band-gap reduces leakage current, and makes room-temperature operation feasible. High-energy resolution, good mobility lifetime product and large stopping
power are attracting many users. It also has a compactness and portability that are outstanding advantages in many applications.In our research, we selected CdTe for the study on sonoluminescence phenomena [9,10]. So far the origin of light emission mechanism in sonoluminescence has not elucidated whether it is due to blackbody radiation or bremsstrahlung. X-ray spectrum analysis might be the interesting measurement that could provide the key information to find unrevealed sonoluminescence mechanism. We investigated the nonlinear behavior of heat transfer within and through the shell of a microbubble by ultrasound
Corresponding author. Tel.: +82 53 850 3437; fax: +82 53 850 3292.
E-mail address: [email protected] (B.-S. Kang). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.05.097
Fig. 1. Schottky-type CdTe shielded inside aluminum dark box.
ARTICLE IN PRESS H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 610 (2009) 302–306
303
injection. We are trying to infer the microbubble temperature using a high-sensitivity CdTe X-ray spectrometer by measuring X-ray spectrum from a sonoluminary microbubble. Recently, we tested a 4 4 1 mm3 high-resolution Schottky CdTe detector to study its response to low-energy gamma-ray source at room temperature. The measured spectrum was generally distorted by the characteristics of CdTe detectors such as hole tailing and pileup effect. Computer simulation was also performed using MCNP to obtain response function of CdTe detectors.
2. Experiment and simulation In the experiment, a CdTe detector of 1 mm thickness and 4 4 mm2 surface area was employed. The detector was a Schottky contact-type CdTe detector made at ACRORAD, which helps the CdTe operating with higher electric field than the one with Ohmic contacts type. It was shielded by 5.5 6 2.7 cm3 of aluminum box as shown in Fig. 1.
Fig. 2. Input geometry for MCNPX simulation.
x 102
x 102
P1 P2 P3 P4 P5 P6 P7 P8
4000
0.1059E+06/ 22 19.03 0.2850E-01 -0.4433E-01 0.1414E-03 0.3765E+06 352.8 234.3 0.5765E-01 4.652 0.2718E-02 0.3560E+05 117.0 262.4 0.1744E-01 4.281 0.1143E-01
0.4835E+05/ 46 Constant 0.1385E+06 267.4 Mean 156.1 0.6434E-02 Sigma 3.837 0.4810E-02
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1750
312.5 0.3916E-02 0.4151E-02
1500 1250 1000 750 500 250 0 50
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150 200 Am-241 channel
250
300
Fig. 3. The pulse height spectra were measured by CdTe at room temperature. 57Co spectrum was shown in (a). Two 57Co gamma-ray peaks of 122 and 136 keV were found at 230 and 260 channel of MCA, respectively. Two Gaussian peaks of 133Ba appeared at about 60 and 150 channels as shown in (b). 241Am spectrum is shown in (c). Gaussian peak of 59 keV gamma ray appears at the 120 channel.
ARTICLE IN PRESS 304
H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 610 (2009) 302–306
acquired with a 2048 multichannel analyzer (MCA). A high voltage for biasing CdTe and gain was adjusted so that the 300 channel corresponded to about 122 keV. Computer simulation was performed by MCNPX. Fig. 2 shows the CdTe inside a 5.5 6 2.7 cm3 Al box. F8 tally was used in the MCNPX simulation in order to simulate pulse height spectrum.
x 102 5000
4000
3. Results
3000
2000
1000
0 0
0.02
0.04
0.06 0.08 Co-57 energy
0.1
0.12
0.14
Fig. 4. The pulse height spectrum as a function of incident gamma-ray energy of 57 Co has two photo peaks.
-2
x 10 0.26
0.24
0.22
0.2
Three gamma-ray spectra were measured at room temperature as mentioned above. The sources emit low-energy gamma rays that are suitable for the experiments. The interested gamma peaks were 45 and 75 keV of 133Ba, 59 keV of 241Am and 122 and 136 keV of 57Co. Fig. 3 shows three different measured gamma-ray pulse height spectra from 57Co, 241Am, and 133Ba. In the 57Co spectrum, Fig. 3(a), the full-energy peaks of 122 and 136 keV were found at 230 and 260 channels of the MCA. Since the relative intensities of 122.06 and 136.47 keV are 85.60% and 10.68%, respectively, the full-energy peak of 122.06 keV is higher than that of 136.47 keV. Hence CdTe was used, and the two component materials of CdTe, Cd and Te, generate both Ka and Kb peaks, so each primary full-energy peak generates four escape peaks as mentioned in previous reports [3]. However, the escape peaks were not shown in our measurements. Two Gaussian peaks of 133Ba were measured at about 60 and 150 channels as shown in Fig. 3(b). In the case of 133Ba, 45 keV is a main energy and has a good relative intensity with 46.9%. The about 150 channel peak is due to 75 keV with a relative intensity of 7.6%. Fig. 3(c) shows the spectrum of 241 Am. The 59.56 keV gamma energy of 241Am generates the fullenergy peak at the 110 channel. The 59.56 keV has a good relative intensity of 35.9%. The CdTe detector shows linear characteristics between channel and energy around 100 keV; therefore the energy calibration was performed using 4 peaks. Measured response energy spectrum of 57Co, and the relationship between energy and sigma of Gaussian fit are shown in Fig. 4 and Fig. 5, respectively. The asymmetry of the gamma peak above 50 keV is derived by the hole tailing effect. Fig. 6 shows the ratio of asymmetry. Hole tailing arises from the short lifetime of the holes due to the density of trapping sites in the crystal. In the case of 241Am, the rate of asymmetry is about 0.5. An effect of asymmetry by hole tailing will be an increase of 0.9 in around 100 keV. In the slope of its ratio, the value was about 0.15–0.13.
4. Conclusion & further study
0.18
0.16 0.06
0.08
0.1
0.12
0.14
Fig. 5. The resolution of CdTe as a function of incident gamma energy.
In order to reduce noise signal, a short BNC connector was used at the connection between CdTe and Preamp. Since CdTe is sensible to visible light, it was shielded from light. The distance between gamma-ray sources and CdTe was 1 cm. 241Am, 133Ba and 57Co standard sources were placed at the upper panel of an aluminum dark box. Weak detection signal from CdTe was amplified using Preamp and Amp. Pulse height spectra were
In order to study the response of a CdTe detector, we analyzed the measured response spectra of 57Co, 133Ba, and 241Am. The response spectra were measured by the 2048 channel MCA data acquisition system. Since the MCA did not have enough channel number, it deteriorated the Gaussian broadening of full-energy peaks. In addition to the broadening effect, Compton continuums under-wrapped the escape peaks. The energy resolution as a function of incident gamma ray was about 4–6%, which was not bad. Hole tailing derived the asymmetry of the full-energy photopeak, which increases the shift of peak center at higher energy. The asymmetry ratio was distributed within 0.5–0.9. For further study, we plan to measure X-ray due to sonoluminescence phenomena. For the comprehensive understanding of CdTe response, a more detailed analysis and measurement on Compton background and photopeak asymmetry should be fulfilled.
ARTICLE IN PRESS H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 610 (2009) 302–306
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Fig. 6. The ratio of asymmetry of full peak as a function of channels that was measured using a CdTe detector. From left side, the linear fit shows the slope of the ratio of asymmetry at 241Am, 133Ba, 57Co(122 keV, 136 keV) peaks.
ARTICLE IN PRESS 306
H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 610 (2009) 302–306
Acknowledgements This research was supported by ‘Radiation and Nuclear Medical Engineering Research Center’ at KAIST according to the contract of ‘Electric Power Research Institute’ (EPRI). References [1] E.D. Castro, R. Pani, R. Pellegrini, C. Bacci, Phys. Med. Biol. 29 (1984) 1117. [2] H. Tsutsui, T. Ohtsuchi, K. Ohmori, S. Baba, IEEE Trans. Nucl. Sci. NS-39 (1992).
[3] L. verger, M. Boitel, M.C. Gentet, R. Hamelin, C. Mestais, F. Mongellaz, J. Rustique, G. Sanchez, Nucl. Instr. and Meth. A 458 (2001) 297. [4] M.B. Freitas, F.H.M. Medeiros, E.M. Yoshimura, Mater. Sci. Forum 480 (2005) 53. [5] M. Krumrey, M. Gerlach, F. Scholze, G. Ulm, Nucl. Instr. and Meth. A 568 (2006) 364. [6] M. Moralles, D.A.B. Bonifacio, M. Bottaro, M.A.G. Pereira, Nucl. Instr. and Meth. A 580 (2007) 270. [7] R. Gunnink, R. Arlt, Nucl. Instr. and Meth. A 458 (2001) 196. [8] S. Miyajima, Med. Phys. 30 (2003). [9] Ho-Young Kwak, Joong-Yeone Lee, Sarng Woo Karng, J. Phys. Soc. Japan 70 (2001). [10] Jin-Seok Jeon, Ik_Jun Yang, Sang-Woo Karng, Ho-Young Kwak, Jpn. J. Appl. Phys. 39 (2000).