- Email: [email protected]
Imaging detector development for nuclear astrophysics using pixelated CdTe
Nuclear Instruments and Methods in Physics Research A 623 (2010) 434–436
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Imaging detector development for nuclear astrophysics using pixelated CdTe J.M. A´lvarez a,, J.L. Ga´lvez a, M. Hernanz a, J. Isern a, M. Llopis a, M. Lozano b, G. Pellegrini b, M. Chmeissani c a
Institut de Ciencies de l’Espai (CSIC-IEEC), Campus UAB, E-08193 Barcelona, Spain Centro Nacional de Microelectronica (IMB-CNM(CSIC)), Campus UAB, E-08193 Barcelona, Spain c Institut de Fı´sica d’Altes Energies (IFAE), Campus UAB, E-08193 Barcelona, Spain b
a r t i c l e in f o
a b s t r a c t
Available online 7 March 2010
The concept of focusing telescopes in the energy range of lines of astrophysical interest (i.e., of energies around 1 MeV) should allow to reach unprecedented sensitivities, essential to perform detailed studies of cosmic explosions and cosmic accelerators. Our research and development activities aim to study a detector suited for the focal plane of a gray telescope mission. A CdTe/CdZnTe detector operating at room temperature, that combines high detection efficiency with good spatial and spectral resolution is being studied in recent years as a focal plane detector, with the interesting option of also operating as a Compton telescope monitor. We present the current status of the design and development of a gray imaging spectrometer in the MeV range, for nuclear astrophysics, consisting of a stack of CdTe pixel detectors with increasing thicknesses. We have developed an initial prototype based on CdTe ohmic detector. The detector has 11 11 pixels, with a pixel pitch of 1 mm and a thickness of 2 mm. Each pixel is stud bonded to a fanout board and routed to an front end ASIC to measure pulse height and rise time information for each incident gray photon. First measurements of a 133Ba and 241Am source are reported here. & 2010 Elsevier B.V. All rights reserved.
Keywords: Gamma-ray astrophysics Laue lens detector Stacked detector CdTe pixel detector
1. Introduction In the last years we have been working on feasibility studies of future instruments in the gray range, from several keV up to a few MeV. The innovative concept of focusing gray telescopes should allow to reach unprecedented sensitivities and angular resolution, thanks to the decoupling of collecting area and detector volume. High sensitivities are essential to perform detailed studies of cosmic explosions and cosmic accelerators, e.g., Supernovae, Classical Novae, Supernova Remants (SNRs), Gamma-Ray Bursts (GRBs), Pulsars, Active Galactic Nuclei (AGN). In collaboration with other institutes, mainly in Europe, we have proposed gray missions based on a Focusing Telescope: MAX project, submitted to CNES in 2004 [1], GRI mission proposal, submitted to ESA Cosmic Vision Programme 20015-2025, in 2007 [2], and the ongoing DUAL project [3]. A focusing telescope mission would be composed of two spacecrafts in formation flight. The optics spacecraft would carry a Laue diffraction lens, able to focus the incoming grays into a focal spot at a given distance ð 100 mÞ. The detector spacecraft would carry a position sensitive detector in the focal plane of the lens, to collect the focused grays.
Cadmium Telluride (CdTe) and Cadmium Zinc Telluride (CdZnTe) are very attractive materials for a gray imaging spectrometer for astrophysical applications. Their high detection efficiency and the advantage of operating at room temperature, have motivated their use in past and current soft gray space missions. However, for those applications that requiring a high resolution spectrometers, semiconductors such as Si or Ge are still more desirable. The considerable amount of charge loss in CdTe and CdZnTe limits their spectral properties, although a significant improvement has been done in the last years. A summary of the technique for improving energy resolution can be found in Ref. [4] and the references therein. CdTe/CdZnTe detectors have been extensively studied in the photoelectric regime (10–300 keV), but the study in the Compton regime (essential to detect grays with energies up to 1 MeV) is not standard at all, which has become an important topic in recent years. Two different approaches are being studied, to extend the CdTe/CdZnTe application to the MeV range and overcome the well known incomplete charge collection problem:
Stacked thin layers of CdTe to get high efficiency with high energy resolution (e.g., Takahashi and Watanabe [4]).
Three-dimensional position sensitivity in a monolithic CdTe Corresponding author. Tel.: + 34 93 581 4779; fax: + 34 93 581 4363.
E-mail address: [email protected] (J.M. A´lvarez). 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.03.027
using a thick detector to get high energy resolution with high efficiency (e.g., He [5]).
´ lvarez et al. / Nuclear Instruments and Methods in Physics Research A 623 (2010) 434–436 J.M. A
435
Fig. 1. Geant4 mass model view of the calorimeter for a Compton Camera prototype, showing several layers of CdTe pixel detector with increasing thickness.
2. CdTe stacked detector with increasing thickness Our R&D project, funded by the Spanish Ministry of Science (MICINN), proposed the development of a calorimeter, for a Compton Camera prototype stacking several layers of pixelated CdTe detectors with different thicknesses: 0.5, 1.0, 2.0, 4.0, and 8.0 mm, in order to achieve good energy resolution as well as high detection efficiency in the energy range from 150 keV to 1 MeV. In this configuration (Fig. 1), soft grays are absorbed by the top thin layers while hard grays are absorbed by the bottom thick layers. Here we report first measurements obtained with a prototype CdTe pixel detector with 2 mm thickness. Our developments are following the initial idea of stacking thin CdTe detectors to form Compton Cameras reported by Takahashi [6], and first discussed as an application for gray lens experiment in Ref. [7]. First demonstration of the results of CdTe Compton Camera with several layers of thin CdTe pixels has been published in Ref. [8]. Unlike in these developments, our prototype will require a charge-loss correction as we increase the thickness of the CdTe detector. We have been using the Geant4 Monte Carlo code [9] to study the response of the focal plane detector, based on CdTe/CdZnTe, for a focusing telescope [10]. Currently we are using this code to optimize the design of our prototype. Parameters such as the number of layers, thicknesses, distance between layers and size of the pixel are being studied. Besides, in order to determine the direction of incident grays by the Compton scattering, multiple interactions (Compton events) should be reconstructed in the proposed configuration, both inside a single layer and between layers.
Fig. 2. Image of the 12.15 mm 12.15 mm 2 mm CdTe pixel detector which is stud bonded to the fanout board (bottom of the CdTe). At the upper end of the fanout board we can see the paths that have been wire bonded to 121 input channels of the readout electronics ASIC.
detector is attached to the fanout board the pads are wirebonded to 121 input channels of the NUCAM chip. A view of the assembly is shown in Fig. 2. The readout electronics is based on the NUCAM ASIC (Application Specific Integrated Circuit) developed by the Rutherford Appleton Laboratory (RAL) [11]. This 128 channel low-noise ASIC carries out the pulse-height and rise-time measurement for every detected interaction. Data conversion is enhanced by the ADC converter integrated in the chip. A data acquisition program, based on LabVIEW, is used to control the ASIC and store the data. 3.2. Experimental setup The detector is cooled down in order to decrease the leakage current and allow us to apply much higher bias voltage than at room temperature. The CdTe detector and its readout system are set inside an aluminum box with the required connections. The container is emptied out of air to avoid any possible condensation in the system during the cooling process. After reaching a stable operating temperature of 10 1C, a bias voltage of 400 V was applied to the common cathode of the pixelated detector. Measurements are performed with two gray sources, 133Ba and 241Am (activities 1 mCi and 10 mCi respectively). The isotopes are placed at a distance of 30 mm from the center of the detector in the cathode side. An exposure time of 5 h is adopted to acquire the data.
3. Test of the 2 mm thick CdTe pixel detector 3.3. First measurements 3.1. The 11 11 CdTe pixel detector The basic structure of CdTe detector is Pt/CdTe/Pt with ohmic contacts for electron collection. The CdTe monocrystal dimensions are 12.15 mm 12.15 mm 2 mm and was manufactured by Acrorad, Japan. The anode side was divided into 11 11 pixels with a pixel pitch of 1 mm. A guard ring with a width of 0.5 mm surrounds the pixels in order to reduce the leakage current caused mainly by the edge effects of the detector. A fanout board, consisting of a glass susbtrate with metal tracks, was designed to route the signal of each pixel to the front end ASIC. A thin layer of Ni/Au was deposited in the bump pads of the fanout in order to ensure a good stud bonding connectivity of the pixels. Once the
Our initial measurements aim to study the spectral performance and imaging capability of the 2 mm thick CdTe pixel detector with the NUCAM ASIC. The readout chip is evaluated by applying a calibration signal to each input channel of the ASIC. The ADC values are measured according to a test pulse amplitude for the 128 channels. The observed behavior in all channels is consistent with the expected characteristics of the NUCAM ASIC. The gain and offset of each channel are obtained from this calibration for later correction. All measurements are carried out at the temperature of 10 1C and with a bias voltage of 400 V. The time constant of the shaping is set to 7:5 ms. In these detector all 121 channels are properly connected. Although a high degree
´ lvarez et al. / Nuclear Instruments and Methods in Physics Research A 623 (2010) 434–436 J.M. A
436
200
Channel45
almost doubles the others. The 133Ba and 241Am spectra obtained for one pixel are shown in Fig. 3. A different threshold level was set in both cases depending on the energy range. The measured photo peak energy resolution (FWHM) is 9.2% and 2.6% at 59.5 and 356 keV respectively. Even with a bias voltage of 400 V and a thickness of 2 mm, the effects of incomplete charge collection should be taken into account as a cause of degradation of the energy resolution. A significant improvement is expected after a rise time discrimination will be applied. A thorough treatment of data including rise time discrimination, as well as new measures, will be required before the spectra for all 121 pixel can be properly added.
356keV
180 133Ba
160
counts / keV
140 120 100 303keV
80
276keV
60 40
4. Conclusions
384keV
20 0 150
200
300 E (keV)
250
350
400
450
Channel45
500
59keV
241Am
counts / keV
400
Current status of the design and development of a gray imaging spectrometer for nuclear astrophysics is presented. Three different 11 11 CdTe pixel detectors have been already implemented and first measurements with a 133Ba and 241Am source have been done. A measure of the depth of interaction of the gamma radiation within the detector will be determined by measuring the charge collection time with the NUCAM ASIC. We will study the improvement in energy resolution after applying this correction to the 2 mm thickness prototype.
300
Acknowledgments
200
The authors would like to thank P. Seller and his group for providing us with the NUCAM ASIC and for their technical support. This work was supported by project AYA2008-01839 of the Spanish MICINN.
100 References
0
0
20
40
60
80 E (keV)
100
120
140
Fig. 3. Spectra of 133Ba and 241Am obtained with one pixel of the CdTe detector of thickness 2 mm. The applied bias voltage is 400 V and the operating temperature is 10 1C. A different threshold level is set in both cases depending on the energy range, clearly visible in the low energies cuts in the 241Am spectra. The measured energy resolution (FWHM) is 9.2% and 2.6% at 59.5 and 356 keV respectively.
of homogeneity is observed in the response of the 121 pixels, we found some differences in this preliminary data processing. Thus, for example the FWHM of the 356 keV peak, in some channels
[1] P. von Ballmoos, et al., MAX—a gamma-ray lens for nuclear astrophysics, SPIE Proceedings, vol. 5168, 2004. ¨ [2] J. Knodlseder, et al., Experimental Astronomy 23 (2009) 121. [3] /http://www.astro.isas.jaxa.jp/conference/gamma2009www.astro.isas.jaxa.jp/conference/gamma2009S. [4] T. Takahashi, S. Watanabe, IEEE Trans. Nucl. Sci. NS-48 (4) (2001) 950. [5] Z. He, et al., Nucl. Instr. and Meth. A 422 (1998). [6] T. Takahashi, et al., SPIE, vol. 4851, 2003, pp. 1228–1235. [7] T. Takahash, Experimental Astronomy 20 (2006) 317. [8] K. Ohnuki, et al., SPIE, vol. 5922, 2005, pp. 78–88. [9] S. Agostinelli, et al., Nucl. Instr. and Meth. A 506 (2003) 250. [10] L. Natalucci, J.M. A´lvarez, et al., CdZnTe for hard x-ray and low energy gray focusing telescope, in: SPIE, vol. 7011, 2008. [11] P. Seller, et al., in: IEEE Nuclear Symposium Conference Record, vol. 6, 2006, pp. 3786–3789.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Imaging detector development for nuclear astrophysics using pixelated CdTe J.M. A´lvarez a,, J.L. Ga´lvez a, M. Hernanz a, J. Isern a, M. Llopis a, M. Lozano b, G. Pellegrini b, M. Chmeissani c a
Institut de Ciencies de l’Espai (CSIC-IEEC), Campus UAB, E-08193 Barcelona, Spain Centro Nacional de Microelectronica (IMB-CNM(CSIC)), Campus UAB, E-08193 Barcelona, Spain c Institut de Fı´sica d’Altes Energies (IFAE), Campus UAB, E-08193 Barcelona, Spain b
a r t i c l e in f o
a b s t r a c t
Available online 7 March 2010
The concept of focusing telescopes in the energy range of lines of astrophysical interest (i.e., of energies around 1 MeV) should allow to reach unprecedented sensitivities, essential to perform detailed studies of cosmic explosions and cosmic accelerators. Our research and development activities aim to study a detector suited for the focal plane of a gray telescope mission. A CdTe/CdZnTe detector operating at room temperature, that combines high detection efficiency with good spatial and spectral resolution is being studied in recent years as a focal plane detector, with the interesting option of also operating as a Compton telescope monitor. We present the current status of the design and development of a gray imaging spectrometer in the MeV range, for nuclear astrophysics, consisting of a stack of CdTe pixel detectors with increasing thicknesses. We have developed an initial prototype based on CdTe ohmic detector. The detector has 11 11 pixels, with a pixel pitch of 1 mm and a thickness of 2 mm. Each pixel is stud bonded to a fanout board and routed to an front end ASIC to measure pulse height and rise time information for each incident gray photon. First measurements of a 133Ba and 241Am source are reported here. & 2010 Elsevier B.V. All rights reserved.
Keywords: Gamma-ray astrophysics Laue lens detector Stacked detector CdTe pixel detector
1. Introduction In the last years we have been working on feasibility studies of future instruments in the gray range, from several keV up to a few MeV. The innovative concept of focusing gray telescopes should allow to reach unprecedented sensitivities and angular resolution, thanks to the decoupling of collecting area and detector volume. High sensitivities are essential to perform detailed studies of cosmic explosions and cosmic accelerators, e.g., Supernovae, Classical Novae, Supernova Remants (SNRs), Gamma-Ray Bursts (GRBs), Pulsars, Active Galactic Nuclei (AGN). In collaboration with other institutes, mainly in Europe, we have proposed gray missions based on a Focusing Telescope: MAX project, submitted to CNES in 2004 [1], GRI mission proposal, submitted to ESA Cosmic Vision Programme 20015-2025, in 2007 [2], and the ongoing DUAL project [3]. A focusing telescope mission would be composed of two spacecrafts in formation flight. The optics spacecraft would carry a Laue diffraction lens, able to focus the incoming grays into a focal spot at a given distance ð 100 mÞ. The detector spacecraft would carry a position sensitive detector in the focal plane of the lens, to collect the focused grays.
Cadmium Telluride (CdTe) and Cadmium Zinc Telluride (CdZnTe) are very attractive materials for a gray imaging spectrometer for astrophysical applications. Their high detection efficiency and the advantage of operating at room temperature, have motivated their use in past and current soft gray space missions. However, for those applications that requiring a high resolution spectrometers, semiconductors such as Si or Ge are still more desirable. The considerable amount of charge loss in CdTe and CdZnTe limits their spectral properties, although a significant improvement has been done in the last years. A summary of the technique for improving energy resolution can be found in Ref. [4] and the references therein. CdTe/CdZnTe detectors have been extensively studied in the photoelectric regime (10–300 keV), but the study in the Compton regime (essential to detect grays with energies up to 1 MeV) is not standard at all, which has become an important topic in recent years. Two different approaches are being studied, to extend the CdTe/CdZnTe application to the MeV range and overcome the well known incomplete charge collection problem:
Stacked thin layers of CdTe to get high efficiency with high energy resolution (e.g., Takahashi and Watanabe [4]).
Three-dimensional position sensitivity in a monolithic CdTe Corresponding author. Tel.: + 34 93 581 4779; fax: + 34 93 581 4363.
E-mail address: [email protected] (J.M. A´lvarez). 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.03.027
using a thick detector to get high energy resolution with high efficiency (e.g., He [5]).
´ lvarez et al. / Nuclear Instruments and Methods in Physics Research A 623 (2010) 434–436 J.M. A
435
Fig. 1. Geant4 mass model view of the calorimeter for a Compton Camera prototype, showing several layers of CdTe pixel detector with increasing thickness.
2. CdTe stacked detector with increasing thickness Our R&D project, funded by the Spanish Ministry of Science (MICINN), proposed the development of a calorimeter, for a Compton Camera prototype stacking several layers of pixelated CdTe detectors with different thicknesses: 0.5, 1.0, 2.0, 4.0, and 8.0 mm, in order to achieve good energy resolution as well as high detection efficiency in the energy range from 150 keV to 1 MeV. In this configuration (Fig. 1), soft grays are absorbed by the top thin layers while hard grays are absorbed by the bottom thick layers. Here we report first measurements obtained with a prototype CdTe pixel detector with 2 mm thickness. Our developments are following the initial idea of stacking thin CdTe detectors to form Compton Cameras reported by Takahashi [6], and first discussed as an application for gray lens experiment in Ref. [7]. First demonstration of the results of CdTe Compton Camera with several layers of thin CdTe pixels has been published in Ref. [8]. Unlike in these developments, our prototype will require a charge-loss correction as we increase the thickness of the CdTe detector. We have been using the Geant4 Monte Carlo code [9] to study the response of the focal plane detector, based on CdTe/CdZnTe, for a focusing telescope [10]. Currently we are using this code to optimize the design of our prototype. Parameters such as the number of layers, thicknesses, distance between layers and size of the pixel are being studied. Besides, in order to determine the direction of incident grays by the Compton scattering, multiple interactions (Compton events) should be reconstructed in the proposed configuration, both inside a single layer and between layers.
Fig. 2. Image of the 12.15 mm 12.15 mm 2 mm CdTe pixel detector which is stud bonded to the fanout board (bottom of the CdTe). At the upper end of the fanout board we can see the paths that have been wire bonded to 121 input channels of the readout electronics ASIC.
detector is attached to the fanout board the pads are wirebonded to 121 input channels of the NUCAM chip. A view of the assembly is shown in Fig. 2. The readout electronics is based on the NUCAM ASIC (Application Specific Integrated Circuit) developed by the Rutherford Appleton Laboratory (RAL) [11]. This 128 channel low-noise ASIC carries out the pulse-height and rise-time measurement for every detected interaction. Data conversion is enhanced by the ADC converter integrated in the chip. A data acquisition program, based on LabVIEW, is used to control the ASIC and store the data. 3.2. Experimental setup The detector is cooled down in order to decrease the leakage current and allow us to apply much higher bias voltage than at room temperature. The CdTe detector and its readout system are set inside an aluminum box with the required connections. The container is emptied out of air to avoid any possible condensation in the system during the cooling process. After reaching a stable operating temperature of 10 1C, a bias voltage of 400 V was applied to the common cathode of the pixelated detector. Measurements are performed with two gray sources, 133Ba and 241Am (activities 1 mCi and 10 mCi respectively). The isotopes are placed at a distance of 30 mm from the center of the detector in the cathode side. An exposure time of 5 h is adopted to acquire the data.
3. Test of the 2 mm thick CdTe pixel detector 3.3. First measurements 3.1. The 11 11 CdTe pixel detector The basic structure of CdTe detector is Pt/CdTe/Pt with ohmic contacts for electron collection. The CdTe monocrystal dimensions are 12.15 mm 12.15 mm 2 mm and was manufactured by Acrorad, Japan. The anode side was divided into 11 11 pixels with a pixel pitch of 1 mm. A guard ring with a width of 0.5 mm surrounds the pixels in order to reduce the leakage current caused mainly by the edge effects of the detector. A fanout board, consisting of a glass susbtrate with metal tracks, was designed to route the signal of each pixel to the front end ASIC. A thin layer of Ni/Au was deposited in the bump pads of the fanout in order to ensure a good stud bonding connectivity of the pixels. Once the
Our initial measurements aim to study the spectral performance and imaging capability of the 2 mm thick CdTe pixel detector with the NUCAM ASIC. The readout chip is evaluated by applying a calibration signal to each input channel of the ASIC. The ADC values are measured according to a test pulse amplitude for the 128 channels. The observed behavior in all channels is consistent with the expected characteristics of the NUCAM ASIC. The gain and offset of each channel are obtained from this calibration for later correction. All measurements are carried out at the temperature of 10 1C and with a bias voltage of 400 V. The time constant of the shaping is set to 7:5 ms. In these detector all 121 channels are properly connected. Although a high degree
´ lvarez et al. / Nuclear Instruments and Methods in Physics Research A 623 (2010) 434–436 J.M. A
436
200
Channel45
almost doubles the others. The 133Ba and 241Am spectra obtained for one pixel are shown in Fig. 3. A different threshold level was set in both cases depending on the energy range. The measured photo peak energy resolution (FWHM) is 9.2% and 2.6% at 59.5 and 356 keV respectively. Even with a bias voltage of 400 V and a thickness of 2 mm, the effects of incomplete charge collection should be taken into account as a cause of degradation of the energy resolution. A significant improvement is expected after a rise time discrimination will be applied. A thorough treatment of data including rise time discrimination, as well as new measures, will be required before the spectra for all 121 pixel can be properly added.
356keV
180 133Ba
160
counts / keV
140 120 100 303keV
80
276keV
60 40
4. Conclusions
384keV
20 0 150
200
300 E (keV)
250
350
400
450
Channel45
500
59keV
241Am
counts / keV
400
Current status of the design and development of a gray imaging spectrometer for nuclear astrophysics is presented. Three different 11 11 CdTe pixel detectors have been already implemented and first measurements with a 133Ba and 241Am source have been done. A measure of the depth of interaction of the gamma radiation within the detector will be determined by measuring the charge collection time with the NUCAM ASIC. We will study the improvement in energy resolution after applying this correction to the 2 mm thickness prototype.
300
Acknowledgments
200
The authors would like to thank P. Seller and his group for providing us with the NUCAM ASIC and for their technical support. This work was supported by project AYA2008-01839 of the Spanish MICINN.
100 References
0
0
20
40
60
80 E (keV)
100
120
140
Fig. 3. Spectra of 133Ba and 241Am obtained with one pixel of the CdTe detector of thickness 2 mm. The applied bias voltage is 400 V and the operating temperature is 10 1C. A different threshold level is set in both cases depending on the energy range, clearly visible in the low energies cuts in the 241Am spectra. The measured energy resolution (FWHM) is 9.2% and 2.6% at 59.5 and 356 keV respectively.
of homogeneity is observed in the response of the 121 pixels, we found some differences in this preliminary data processing. Thus, for example the FWHM of the 356 keV peak, in some channels
[1] P. von Ballmoos, et al., MAX—a gamma-ray lens for nuclear astrophysics, SPIE Proceedings, vol. 5168, 2004. ¨ [2] J. Knodlseder, et al., Experimental Astronomy 23 (2009) 121. [3] /http://www.astro.isas.jaxa.jp/conference/gamma2009www.astro.isas.jaxa.jp/conference/gamma2009S. [4] T. Takahashi, S. Watanabe, IEEE Trans. Nucl. Sci. NS-48 (4) (2001) 950. [5] Z. He, et al., Nucl. Instr. and Meth. A 422 (1998). [6] T. Takahashi, et al., SPIE, vol. 4851, 2003, pp. 1228–1235. [7] T. Takahash, Experimental Astronomy 20 (2006) 317. [8] K. Ohnuki, et al., SPIE, vol. 5922, 2005, pp. 78–88. [9] S. Agostinelli, et al., Nucl. Instr. and Meth. A 506 (2003) 250. [10] L. Natalucci, J.M. A´lvarez, et al., CdZnTe for hard x-ray and low energy gray focusing telescope, in: SPIE, vol. 7011, 2008. [11] P. Seller, et al., in: IEEE Nuclear Symposium Conference Record, vol. 6, 2006, pp. 3786–3789.