- Email: [email protected]
CdTe hybrid pixel detector for imaging with thermal neutrons
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 563 (2006) 238–241 www.elsevier.com/locate/nima
CdTe hybrid pixel detector for imaging with thermal neutrons Jan Jakubeka,, Giovanni Mettivierb, Maria Cristina Montesib, Stanislav Pospisila, Paolo Russob, Jiri Vacikc a
Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horska 3a/22, CZ 12800 Praha 2, Czech Republic b Dipartimento di Scienze Fisiche, Universita` di Napoli Federico II, and INFN, Sezione di Napoli, Via Cinthia, I-80126 Napoli, Italy c Nuclear Physics Institute, Academy of Science of the Czech Republic, Rez near Prague, CZ-25068, Czech Republic Available online 28 February 2006
Abstract We present tests of a thermal neutron imaging system based on a high-resolution hybrid pixel device Medipix2. The system consists of the Medipix2 readout chip bump bonded to a 1 mm thick CdTe detector (featuring 256 256 square pixels with 55 mm pitch). The CdTe detector is used as direct counter of thermal neutrons. Neutrons interact with cadmium nuclei by means of the 113Cd(n,g)114Cd reaction producing 558 keV gamma rays and internal conversion electrons. The spatial resolution of the imager measured by projection of an opaque edge made of cadmium was 450 mm. This value appears in good agreement with Monte–Carlo simulations we made. Neutron radiograms of two test samples are given. r 2006 Elsevier B.V. All rights reserved. PACS: 87.59.Hp; 87.59.Jq Keywords: Neutron radiography; Pixel detector; Semiconductor detector
1. Introduction
converter design:
The hybrid silicon pixel device Medipix2 developed at CERN [1] was originally designed for position sensitive single X-ray photon detection. The device consists of a semiconductor detector chip (Si, GaAs, CdTe, HgI, etc.) bonded to a readout chip. The detector chip is equipped with a single common backside electrode and a front side matrix of electrodes (256 256 square pixels with pitch of 55 mm). Each element of the matrix (pixel) is connected to its respective preamplifier, double discriminator and digital counter integrated on a readout chip. To adapt a pixel detector for detection of slow neutrons, it is necessary to use a suitable converter material. The following nuclear reactions with high cross-sections of thermal neutron absorption were considered for the
6
Corresponding author. Tel.: +420 22435 9181; fax: +420 22435 9392.
E-mail address: [email protected] (J. Jakubek). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.01.134
Li þ n
10
Bþn
! að2:05 MeVÞ þ 3 Hð2:72 MeVÞ ! að1:47 MeVÞ þ 7 Lið0:84 MeVÞ þ gð0:48 MeVÞ
10
Bþn
113
ð93:7%Þ 7
! að1:78 MeVÞ þ Lið1:01 MeVÞ ð6:3%Þ
Cd þ n ! 114 Cd þ gð0:56 MeV; . . .Þ þ conv: electrons
155
Gd þ n ! 156 Gd þ gð0:09; 0:20; 0:30 MeV; . . .Þ þ conv: electrons
157
Gd þ n ! 158 Gd þ gð0:08; 0:18; 0:28 MeV; . . .Þ þ conv: electrons
The design of the converter can be based on various approaches as illustrated in Fig. 1(a)–(c). Options (a) and (b) in Fig. 1 are already tested and published [2]. Basic properties of such neutron imagers are summarized in Table 1. Option (c) expects a semiconductor
ARTICLE IN PRESS J. Jakubek et al. / Nuclear Instruments and Methods in Physics Research A 563 (2006) 238–241
239
-5 -5
0 Converter
5
Detector chip Bump-bonding
0
Readout chip
(a) 5
Neutron beam
-5
Converter
m] X [m
3D Sensor chip 0
-5
Readout chip 5
(b)
0
Y [mm]
5 n
Sensor chip with conv. material
e
Fig. 2. The point spread function (PSF) obtained from simulation of gamma radiation emitted after neutron capture.
Readout chip (c)
2.1. Monte Carlo simulations
Fig. 1. (a) Coated detector–the suitable neutron converter material is deposited on the sensor’s surface. (b) Stuffed detector—the suitable neutron converter material is filled into 3D structures in the sensor. (c) The sensor itself contains the material sensitive to neutrons.
Table 1 Results of adaptation of Medipix2 device with Si sensor for detection of slow neutrons Converter place-ment
Converter type
Spatial resolution (mm)
Detection efficiency(%)
Remarks
Surface layer
6
100 50 1700
3 1.5 5
— — — MC Simulation,
100
Not tested
100
40
LiF B 113 Cd 10
155,157
Stuffed
6
LiF
Gd
High background MC Simulation
material sensitive to neutrons. A good candidate seems to be CdTe crystal. 2. Pixel device with CdTe sensor as a detector of slow neutrons Thanks to the high neutron capture cross-section of Cd, the 1 mm thick CdTe sensor is practically opaque for slow neutrons. The CdTe sensor with a pixelated electrode is bump bonded to Medipix2 readout chip. When a neutron is captured by Cd nucleus a 558 keV photon is emitted. About 3% of photons are converted to electrons of the same energy by the internal conversion mechanism.
Interactions of gamma photons and conversion electrons in the CdTe sensor were simulated by MCNP [3]. A pencil beam of slow neutrons shining perpendicular to surface of thick CdTe crystal was simulated. The crystal volume was divided to pixels (55 55 mm). Gammas and electrons were generated along the path of the neutron beam (with exponentially decreasing intensity). Energy loses of each gamma or electron in the CdTe sensor were simulated and compared with the threshold value. If energy loses in one pixel were higher than the threshold the particle was detected and the count of events in the pixel was incremented. 2.1.1. Gamma interactions Interactions of 10000 photons with energy of 558 keV in a CdTe crystal have been simulated by MCNP. Results are shown in Figs. 2 and 3. Width of columns in histograms is equal to Medipix2 pixel size (55 mm). The spatial resolution simulated with gamma photons is 480 mm in terms of FWHM1 of LSF.2 2.1.2. Interactions of conversion electrons Results of simulations with electrons are in Figs. 4 and 5. Width of columns in both histograms is equal to Medipix2 pixel size (55 mm) The spatial resolution in terms of FWHM of LSF via detection of electrons of the internal conversion is about 200 mm. 1 2
Full-Width at Half-Maximum. Line Spread Function.
ARTICLE IN PRESS J. Jakubek et al. / Nuclear Instruments and Methods in Physics Research A 563 (2006) 238–241
-6 m
-4 m
-2 m
0
2m
4m
6m
1.5
50
40
40
30
30
20
20
10
10
Y
50
0
-6
-4
-2
0 X [mm]
2
4
6
-1 mm
0.5
-40
-6
-1 mm
Fig. 4. The point spread function obtained from simulation of detection of conversion electrons emitted after neutron capture.
Y [count]
20
40
60
-4
-2
0
2
4
6
200 µ 400 µ 600 µ 800 µ
1
0.6
0.6 0. Simulation 1. Measurement
0.4
0.4 0.2
-4
-2
0 X [mm]
2
4
6
Fig. 7. Comparison of the measured (black) and simulated edge response functions.
2.2. Experiments
300
300
200
200
100
100
0
0 X [pixels]
0.8
-6
0
0
-20
0.8
0
-800 µ -600 µ -400 µ -200 µ
Full area = 19.6%
0.2
1 mm
1 mm
20
Fig. 6. The complete simulation of line spread function of the imager composed by 1 mm thick CdTe sensor and Medipix2 readout chip. 10000 neutrons were simulated.
1
50
0
0.5
0
50
-20
1
-60
Rel. signal
Count
-40
1
0
Fig. 3. The line spread function (LSF) obtained from simulation of gamma radiation. FWHM is about 480 mm.
0
-60
Peak area = 8.1% Relative signal [%]
240
-800 µ -600 µ -400 µ -200 µ
0 X [m]
Tests of Medipipx2 device with CdTe sensor as a neutron imager have been performed at the horizontal channel of the LVR-15 nuclear research reactor at Nuclear Physics Institute of the Czech Academy of Sciences at Rez near Prague [4]. The neutron beam intensity was about 107 neutrons/cm2 s (at reactor power of 6.6 MW). The beam cross-section was 4 mm (height) x 60 mm (width).
0 200 µ 400 µ 600 µ 800 µ
Fig. 5. The line spread function obtained from simulation of detection of conversion electrons. Width of histogram columns is equal to Medipix2 pixel size (55 mm).
2.1.3. Combination The expected behaviour of CdTe as a slow neutron detector is given by a linear combination of both previous results with weights given by probabilities of both mechanisms. The combined LSF is displayed in Fig. 6. The estimated spatial resolution is about 320 mm which is about 6 pixels. The estimated detection efficiency is about 20% but only 8% is usable for imaging (see areas in Fig. 6).
2.2.1. Spatial resolution The edge response function has been measured using projection of a straight edge of 1 mm thick Cadmium plate.3 The results are shown in Figs. 7 and 8. The spatial resolution of the imager in terms of FWHM of LSF is 8.1 pixels which is 445 mm. 2.2.2. Sample objects Neutron radiograms of several sample objects have been taken by the Medipix2 device with CdTe sensor and 3
The cadmium plate was parallel to the detector plane. The edge was slightly rotated (11) with respect to the detector columns to allow oversampling.
ARTICLE IN PRESS J. Jakubek et al. / Nuclear Instruments and Methods in Physics Research A 563 (2006) 238–241
Rel. signal
-100
-50
0
50
3. Conclusion
100
40 m
40 m FWHM = 8.1
20 m
20 m
0
0 -100
-50
0 50 Position [pixels]
241
100
Fig. 8. The line spread function obtained by numerical differentiation of the edge response function.
The simulations and measurements of CdTe pixelated sensor as a neutron imager have been done. The CdTe sensor in combination with Medipix2 chip can be used for the direct imaging with slow neutrons. However, its imaging performance is not good. Particularly:
The spatial resolution is 450 mm, The detection efficiency is about 8%.
Other converter and sensor types (see Ref. [2]) can offer better results from the point of view of either image quality (resolution) either detection efficiency (see Table 1.).
Acknowledgments This work has been carried out in framework of the Medipix-2 collaboration based at CERN and has been supported in part by the Ministry of Education, Youth and Sports of the Czech Republic under Research Projects 1P04LA211 and AV0Z20710524. Fig. 9. A photograph and comparison of two neutron transmission radiograms of a fish head. The first was taken by the CdTe detector, the second by the Si detector coated by 6LiF layer.
References [1] Medipix colaboration: http://www.cern.ch/MEDIPIX/ [2] J. Jaku˚bek, T. Holy´, E. Lehmann, S. Pospı´ sˇ il, J. Uher, J. Vacı´ k, D. Vavrˇ ı´ k, Properties of Neutron Pixel Detector Based on Medipix-2 Device. Book of Abstracts, Nuclear Science Symposium IEEE 2004, 16–22th October 2004, Roma, Italy, Published in the Conference Proceedings on CD-Rom [3] MCNP home page at http://laws.lang.gov/x5/MCNP [4] Department of Neutron Physics of the Nuclear Physics, Institute of the Academy of Sciences of the Czech Republic: http://omega.ujf.cas.cz/
Fig. 10. A photograph and comparison of two neutron transmission radiograms of blanc cartridge.
compared to radiograms measured by the Medipix2 device with a Si sensor coated by 6LiF converter layer. Results are shown in Figs. 9 and 10.
Nuclear Instruments and Methods in Physics Research A 563 (2006) 238–241 www.elsevier.com/locate/nima
CdTe hybrid pixel detector for imaging with thermal neutrons Jan Jakubeka,, Giovanni Mettivierb, Maria Cristina Montesib, Stanislav Pospisila, Paolo Russob, Jiri Vacikc a
Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horska 3a/22, CZ 12800 Praha 2, Czech Republic b Dipartimento di Scienze Fisiche, Universita` di Napoli Federico II, and INFN, Sezione di Napoli, Via Cinthia, I-80126 Napoli, Italy c Nuclear Physics Institute, Academy of Science of the Czech Republic, Rez near Prague, CZ-25068, Czech Republic Available online 28 February 2006
Abstract We present tests of a thermal neutron imaging system based on a high-resolution hybrid pixel device Medipix2. The system consists of the Medipix2 readout chip bump bonded to a 1 mm thick CdTe detector (featuring 256 256 square pixels with 55 mm pitch). The CdTe detector is used as direct counter of thermal neutrons. Neutrons interact with cadmium nuclei by means of the 113Cd(n,g)114Cd reaction producing 558 keV gamma rays and internal conversion electrons. The spatial resolution of the imager measured by projection of an opaque edge made of cadmium was 450 mm. This value appears in good agreement with Monte–Carlo simulations we made. Neutron radiograms of two test samples are given. r 2006 Elsevier B.V. All rights reserved. PACS: 87.59.Hp; 87.59.Jq Keywords: Neutron radiography; Pixel detector; Semiconductor detector
1. Introduction
converter design:
The hybrid silicon pixel device Medipix2 developed at CERN [1] was originally designed for position sensitive single X-ray photon detection. The device consists of a semiconductor detector chip (Si, GaAs, CdTe, HgI, etc.) bonded to a readout chip. The detector chip is equipped with a single common backside electrode and a front side matrix of electrodes (256 256 square pixels with pitch of 55 mm). Each element of the matrix (pixel) is connected to its respective preamplifier, double discriminator and digital counter integrated on a readout chip. To adapt a pixel detector for detection of slow neutrons, it is necessary to use a suitable converter material. The following nuclear reactions with high cross-sections of thermal neutron absorption were considered for the
6
Corresponding author. Tel.: +420 22435 9181; fax: +420 22435 9392.
E-mail address: [email protected] (J. Jakubek). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.01.134
Li þ n
10
Bþn
! að2:05 MeVÞ þ 3 Hð2:72 MeVÞ ! að1:47 MeVÞ þ 7 Lið0:84 MeVÞ þ gð0:48 MeVÞ
10
Bþn
113
ð93:7%Þ 7
! að1:78 MeVÞ þ Lið1:01 MeVÞ ð6:3%Þ
Cd þ n ! 114 Cd þ gð0:56 MeV; . . .Þ þ conv: electrons
155
Gd þ n ! 156 Gd þ gð0:09; 0:20; 0:30 MeV; . . .Þ þ conv: electrons
157
Gd þ n ! 158 Gd þ gð0:08; 0:18; 0:28 MeV; . . .Þ þ conv: electrons
The design of the converter can be based on various approaches as illustrated in Fig. 1(a)–(c). Options (a) and (b) in Fig. 1 are already tested and published [2]. Basic properties of such neutron imagers are summarized in Table 1. Option (c) expects a semiconductor
ARTICLE IN PRESS J. Jakubek et al. / Nuclear Instruments and Methods in Physics Research A 563 (2006) 238–241
239
-5 -5
0 Converter
5
Detector chip Bump-bonding
0
Readout chip
(a) 5
Neutron beam
-5
Converter
m] X [m
3D Sensor chip 0
-5
Readout chip 5
(b)
0
Y [mm]
5 n
Sensor chip with conv. material
e
Fig. 2. The point spread function (PSF) obtained from simulation of gamma radiation emitted after neutron capture.
Readout chip (c)
2.1. Monte Carlo simulations
Fig. 1. (a) Coated detector–the suitable neutron converter material is deposited on the sensor’s surface. (b) Stuffed detector—the suitable neutron converter material is filled into 3D structures in the sensor. (c) The sensor itself contains the material sensitive to neutrons.
Table 1 Results of adaptation of Medipix2 device with Si sensor for detection of slow neutrons Converter place-ment
Converter type
Spatial resolution (mm)
Detection efficiency(%)
Remarks
Surface layer
6
100 50 1700
3 1.5 5
— — — MC Simulation,
100
Not tested
100
40
LiF B 113 Cd 10
155,157
Stuffed
6
LiF
Gd
High background MC Simulation
material sensitive to neutrons. A good candidate seems to be CdTe crystal. 2. Pixel device with CdTe sensor as a detector of slow neutrons Thanks to the high neutron capture cross-section of Cd, the 1 mm thick CdTe sensor is practically opaque for slow neutrons. The CdTe sensor with a pixelated electrode is bump bonded to Medipix2 readout chip. When a neutron is captured by Cd nucleus a 558 keV photon is emitted. About 3% of photons are converted to electrons of the same energy by the internal conversion mechanism.
Interactions of gamma photons and conversion electrons in the CdTe sensor were simulated by MCNP [3]. A pencil beam of slow neutrons shining perpendicular to surface of thick CdTe crystal was simulated. The crystal volume was divided to pixels (55 55 mm). Gammas and electrons were generated along the path of the neutron beam (with exponentially decreasing intensity). Energy loses of each gamma or electron in the CdTe sensor were simulated and compared with the threshold value. If energy loses in one pixel were higher than the threshold the particle was detected and the count of events in the pixel was incremented. 2.1.1. Gamma interactions Interactions of 10000 photons with energy of 558 keV in a CdTe crystal have been simulated by MCNP. Results are shown in Figs. 2 and 3. Width of columns in histograms is equal to Medipix2 pixel size (55 mm). The spatial resolution simulated with gamma photons is 480 mm in terms of FWHM1 of LSF.2 2.1.2. Interactions of conversion electrons Results of simulations with electrons are in Figs. 4 and 5. Width of columns in both histograms is equal to Medipix2 pixel size (55 mm) The spatial resolution in terms of FWHM of LSF via detection of electrons of the internal conversion is about 200 mm. 1 2
Full-Width at Half-Maximum. Line Spread Function.
ARTICLE IN PRESS J. Jakubek et al. / Nuclear Instruments and Methods in Physics Research A 563 (2006) 238–241
-6 m
-4 m
-2 m
0
2m
4m
6m
1.5
50
40
40
30
30
20
20
10
10
Y
50
0
-6
-4
-2
0 X [mm]
2
4
6
-1 mm
0.5
-40
-6
-1 mm
Fig. 4. The point spread function obtained from simulation of detection of conversion electrons emitted after neutron capture.
Y [count]
20
40
60
-4
-2
0
2
4
6
200 µ 400 µ 600 µ 800 µ
1
0.6
0.6 0. Simulation 1. Measurement
0.4
0.4 0.2
-4
-2
0 X [mm]
2
4
6
Fig. 7. Comparison of the measured (black) and simulated edge response functions.
2.2. Experiments
300
300
200
200
100
100
0
0 X [pixels]
0.8
-6
0
0
-20
0.8
0
-800 µ -600 µ -400 µ -200 µ
Full area = 19.6%
0.2
1 mm
1 mm
20
Fig. 6. The complete simulation of line spread function of the imager composed by 1 mm thick CdTe sensor and Medipix2 readout chip. 10000 neutrons were simulated.
1
50
0
0.5
0
50
-20
1
-60
Rel. signal
Count
-40
1
0
Fig. 3. The line spread function (LSF) obtained from simulation of gamma radiation. FWHM is about 480 mm.
0
-60
Peak area = 8.1% Relative signal [%]
240
-800 µ -600 µ -400 µ -200 µ
0 X [m]
Tests of Medipipx2 device with CdTe sensor as a neutron imager have been performed at the horizontal channel of the LVR-15 nuclear research reactor at Nuclear Physics Institute of the Czech Academy of Sciences at Rez near Prague [4]. The neutron beam intensity was about 107 neutrons/cm2 s (at reactor power of 6.6 MW). The beam cross-section was 4 mm (height) x 60 mm (width).
0 200 µ 400 µ 600 µ 800 µ
Fig. 5. The line spread function obtained from simulation of detection of conversion electrons. Width of histogram columns is equal to Medipix2 pixel size (55 mm).
2.1.3. Combination The expected behaviour of CdTe as a slow neutron detector is given by a linear combination of both previous results with weights given by probabilities of both mechanisms. The combined LSF is displayed in Fig. 6. The estimated spatial resolution is about 320 mm which is about 6 pixels. The estimated detection efficiency is about 20% but only 8% is usable for imaging (see areas in Fig. 6).
2.2.1. Spatial resolution The edge response function has been measured using projection of a straight edge of 1 mm thick Cadmium plate.3 The results are shown in Figs. 7 and 8. The spatial resolution of the imager in terms of FWHM of LSF is 8.1 pixels which is 445 mm. 2.2.2. Sample objects Neutron radiograms of several sample objects have been taken by the Medipix2 device with CdTe sensor and 3
The cadmium plate was parallel to the detector plane. The edge was slightly rotated (11) with respect to the detector columns to allow oversampling.
ARTICLE IN PRESS J. Jakubek et al. / Nuclear Instruments and Methods in Physics Research A 563 (2006) 238–241
Rel. signal
-100
-50
0
50
3. Conclusion
100
40 m
40 m FWHM = 8.1
20 m
20 m
0
0 -100
-50
0 50 Position [pixels]
241
100
Fig. 8. The line spread function obtained by numerical differentiation of the edge response function.
The simulations and measurements of CdTe pixelated sensor as a neutron imager have been done. The CdTe sensor in combination with Medipix2 chip can be used for the direct imaging with slow neutrons. However, its imaging performance is not good. Particularly:
The spatial resolution is 450 mm, The detection efficiency is about 8%.
Other converter and sensor types (see Ref. [2]) can offer better results from the point of view of either image quality (resolution) either detection efficiency (see Table 1.).
Acknowledgments This work has been carried out in framework of the Medipix-2 collaboration based at CERN and has been supported in part by the Ministry of Education, Youth and Sports of the Czech Republic under Research Projects 1P04LA211 and AV0Z20710524. Fig. 9. A photograph and comparison of two neutron transmission radiograms of a fish head. The first was taken by the CdTe detector, the second by the Si detector coated by 6LiF layer.
References [1] Medipix colaboration: http://www.cern.ch/MEDIPIX/ [2] J. Jaku˚bek, T. Holy´, E. Lehmann, S. Pospı´ sˇ il, J. Uher, J. Vacı´ k, D. Vavrˇ ı´ k, Properties of Neutron Pixel Detector Based on Medipix-2 Device. Book of Abstracts, Nuclear Science Symposium IEEE 2004, 16–22th October 2004, Roma, Italy, Published in the Conference Proceedings on CD-Rom [3] MCNP home page at http://laws.lang.gov/x5/MCNP [4] Department of Neutron Physics of the Nuclear Physics, Institute of the Academy of Sciences of the Czech Republic: http://omega.ujf.cas.cz/
Fig. 10. A photograph and comparison of two neutron transmission radiograms of blanc cartridge.
compared to radiograms measured by the Medipix2 device with a Si sensor coated by 6LiF converter layer. Results are shown in Figs. 9 and 10.