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Radiation effects on the silicon semiconductor detectors for the ASTRO–H mission
Nuclear Instruments and Methods in Physics Research A 699 (2013) 225–229
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Radiation effects on the silicon semiconductor detectors for the ASTRO–H mission Katsuhiro Hayashi a,n, InChun Park a, Kyohei Dotsu a, Issei Ueno a, Sho Nishino a, Masayuki Matsuoka a, Hajimu Yasuda a, Yasushi Fukazawa a, Takashi Ohsugi a, Tsunefumi Mizuno a, Hiromitsu Takahashi a, Masanori Ohno a, Satoru Endo b, Takaaki Tanaka c, Hiroyasu Tajima d, Motohide Kokubun e, Shin Watanabe e, Tadayuki Takahashi e, Kazuhiro Nakazawa f, Yukio Uchihori g, Hisashi Kitamura g a
Department of Physical Science, Hiroshima University, 1-3-1, Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan Mechanical System Engineering, Graduate School of Engineering Hiroshima University, 1-4-1, Kagamiyama, Higashi-hiroshima, Hiroshima 739-8527, Japan KIPAC, Stanford University, Stanford, CA 94305, USA d Cosmic-ray Research Facility, Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan e ISAS/JAXA, 3-4-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 229-8510, Japan f Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan g Department of Accelerator and Medical Physics National Institute of Radiological Sciences 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan b c
a r t i c l e i n f o
abstract
Available online 8 June 2012
Hard X-ray Imager (HXI) and Soft Gamma-ray Detector (SGD) onboard the 6th Japanese X-ray satellite, ASTRO–H, utilize double-sided silicon strip detectors (DSSD) and pixel array-type silicon sensors (Sipad), respectively. The DSSD with a 3.4 cm 3.4 cm area has an imaging capability in the lower energy band for the HXI covering 5–80 keV. The Si-pad consists of 16 16 pixels with a 5.4 cm 5.4 cm area and measures a photon direction with the Compton kinematics in 10–600 keV. Since the ASTRO–H will be operated in a low earth orbit, these detectors will be damaged by irradiation of cosmic-ray protons mainly in the South Atlantic Anomaly. In order to evaluate damage effects of the sensors, we have carried out irradiation tests with 150 MeV proton beams and 60Co gamma-rays with a total dose of 10– 20 years irradiation level. In both experiments, the leakage current has increased by 0:21:1 nA=cm2 under an expected operation temperature at 15 1C, which resulted in the noise level within a tolerance of 20 years. In this report, we present a summary of the basic performance of silicon detectors, and radiation effects on them by the irradiation tests. & 2012 Elsevier B.V. All rights reserved.
Keywords: ASTRO–H Silicon detector Radiation damage
1. Introduction ASTRO–H, the 6th Japanese X-ray satellite [1,2] following the currently operational Suzaku satellite, is scheduled to be launched in 2014. ASTRO–H aims to reveal high-energy phenomena, such as supernova remnants, black holes, pulsars and clusters of galaxies. Four observation systems with covering energy band from 0.3 keV to 600 keV will be installed. One of them is a hard X-ray imaging system with covering 5–80 keV. It consists of a hard X-ray telescope (HXT) and a combination of silicon (Si) & cadmium telluride (CdTe) double-sided, two dimensional strip detectors (HXI, Hard X-ray Imager) [4,5] located at the focal plane with a focal length of 12 m. For the highest energy band from 10 keV to 600 keV, soft gamma-ray spectrometer (SGD, Soft
n
Corresponding author. E-mail address: [email protected] (K. Hayashi).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.05.088
Gamma-ray Detector) utilizing a narrow filed of view Si and CdTe Compton camera [6,7] is onboard. HXI is composed of four layers of double-sided Si strip detectors overlaid on a CdTe double-sided strip detector surrounded by BGO (Bi4Ge3O12) active shields. The hybrid structure of Si and CdTe enables us to detect hard X-rays in 5–80 keV energy range with high efficiency. The sensitivity of HXI is higher by almost two orders of magnitude than that of Suzaku-HXD (Hard X-ray Detector) [3], thanks to the light focusing system with HXT and the BGO active shield. SGD is a narrow filed of view Compton camera composed of 32 layers of Si pixel sensors as scatterers, and eight layers of CdTe pixel sensors as absorbers. The multi-layer of Si is an excellent scatterer to determine the photon direction by the Compton kinematics. The narrow filed of view by a collimator and rejections of background events by the BGO active shield improve the sensitivity by nearly one order of magnitude than that of IBIS onboard INTEGRAL satellite [8]. The requirements of energy resolution in FWHM are o 2 keV at 60 keV for HXI and o 2 keV at 100 keV for SGD.
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K. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 699 (2013) 225–229
In the orbit of ASTRO–H (altitude: 550 km, orbital inclination: 311), the sensors suffer radiation damage mainly by cosmic-ray protons ð 150 MeVÞ in the South Atlantic Anomaly (SAA) with a level of 1 krad=yr [9,10]. Dominant effects of radiations on the sensors are divided into two damage processes [11]. One is surface damage; holes generated in the surface SiO2 layer by gamma-ray or particle irradiations are trapped in defects close to the interface, resulting in accumulation of positive charges in the SiO2 layer. The other is bulk damage; lattice atoms are displaced from original positions by interactions with high energy particles. These defects create new trap levels in the forbidden band and the thermal electron transition probability from the valence band into the conduction band increases so that leakage current increases. The energy resolution becomes worse due to the increase in shot noise. In previous studies, bulk damage has been investigated accurately by proton irradiation tests through developments of Si sensors for accelerator experiments and almost consistent results have been obtained [17–19]. On the other hand, surface damage depends on a surface design proper to each detector [16], and we therefore need to investigate the surface damage for DSSD and Sipad with some improvement configurations (see Section 2). Accordingly, we carried out a proton irradiation test in order to confirm the consistency of bulk damage with previous studies, and a 60Co gamma-ray irradiation test, which mainly gives damage to a surface layer, in order to evaluate the surface damage. In this paper, we report specifications and basic performance of the Si sensors with some improvement configurations for the ASTRO–H mission in Section 2. In Section 3, we describe proton and gamma-ray irradiation tests and discuss effects on the Si detectors by the irradiation in Section 4. Finally, we summarize results in Section 5.
Fig. 1. Schematic design of the DSSD. The p- and n-type strips are implanted on an n-type bulk Si. Each n-strip is surrounded by an atoll-shaped p-stop for individualization of the n-strips.
Fig. 2. Overall view of the Si-pad. The p-type 16x16 matrix array are implanted on an n-type bulk Si. The lines from each pad to the corners of sensor indicate signal read out lines.
2. Specifications and basic performance of the Si detectors We have developed Si sensors for HXI and SGD in a collaboration with Hamamatsu Photonics [12–15]. The specifications of Si sensors for the flight model of HXI and SGD are shown in Table 1. For the HXI, double-sided, two dimensional Si strip detector (DSSD) with an n-type bulk is developed [5]. The strip pitch of 250 mm is chosen to meet an angular resolution of 0.07 arcminute for the 12 m focal length [5]. The total number of 128 strips with the strip width of 150 mm and a DC-coupled electrode on each Table 1 Specifications of DSSD and Si-pad. DSSD Size Strip pitch Strip width Bulk thickness Number of strips Capacitance Leakage current Bias voltage Energy resolution
3.4 3.4 cm2 250 mm (both sides) 150 mm (both sides) 500 mm 128 o 10 pF=strip o 2 nA=strip @20 1C o 300 V o 2 keV @59.5 keV (FWHM) with ASIC
Si-pad Size Pixel pitch Pixel size Bulk thickness Number of pixels Capacitance Leakage current Bias voltage Energy resolution
5.4 5.4 cm2 3.2 mm 3.14 mm2 600 mm 256 (16 16) o 10 pF=pad o 2 nA=pad @20 1C o 300 V o 2 keV @59.5 keV (FWHM) with ASIC
side are implanted. The n-strip on the ohmic side is surrounded by an atoll-shaped p-stop with the width of 25 mm for each strip as shown in Fig. 1. For the flight model, a DC-coupled aluminum (Al) electrode is attached on the p-stop to reduce the Johnson noise, then the total noise level is reduced by 30%. For the SGD, a pixel-array-type Si sensor (Si-pad) is developed [7]. The Si-pad has 16 16 matrix array with a pixel size of 3.14 mm2 which is implanted on an n-type bulk Si with a DCcoupled electrode as shown in Fig. 2. The pixel size is determined to minimize power consumption while keeping a proper angular resolution by the Compton kinematics. A signal from each pad is brought out to one of Al-bonding pads at the corner of sensor by a readout line on the SiO2 layer. For the flight model, the thickness of SiO2 layer is increased from 1 mm to 1:5 mm in order to reduce the capacitance between the Si bulk and readout electrodes. This capacitance reduction improves the energy resolution of each pad by 10230%. Followings are basic performance of these flight model Si detectors before irradiation. The leakage current obtained at 20 1C at the full depletion voltage are 1.3 nA/strip and 0.9 nA/ pad for DSSD and Si-pad, respectively, which are within the requirements as shown in Table 1. Fig. 3 shows spectra of 241 Am 59.5 keV X-ray at 15 1C detected by each side of the DSSD. Signals of the silicon sensors are fed into a low noise charge-sensitive amplifier CLEAR PULSE CP-5102 and ORTEC 571 amplifier for a pulse shaping with 3 ms. The energy resolution (FWHM) for the p-side and n-side was 1:23ð 70:04stat Þ keV and 1:8ð 70:1stat Þ keV, respectively. We also measured a spectrum of the Si-pad and obtained the energy resolution (FWHM) of 1:56ð 70:06stat Þ keV.
K. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 699 (2013) 225–229
104
104 Pside
Nside
103
103
59.5 keV
Counts
59.5 keV
Counts
227
102
102
10
10
1
1 0
10
20
30
40
50
60
70
80
0
10
20
30
Energy (keV) Fig. 3.
241
40
50
60
70
80
Energy (keV)
Am spectrum at 15 1C obtained by DSSD for the p-side (left) and n-side (right).
3. Proton and gamma-ray irradiation tests
50
As described in Section 1, we carried out proton and 60Co gamma-ray irradiation tests in order to study radiation effects on the sensors due to bulk and surface damages. The maximum fluence level for the proton irradiation test was 10-years operation level. For the 60Co gamma-ray irradiation test, we set the maximum dose up to 20 krad (equivalent to 20-years operation level), in order to investigate damage effects with a severe condition.
45
The proton irradiation test was held at National Institute of Radiological Sciences (NIRS), Japan. The proton energy is chosen to be 150 MeV, a typical proton energy in SAA. The total fluence of proton irradiation was 3.8 109 cm 2, estimated for the radiation fluence of 10 years operation in the ASTRO–H orbit [10] with the 3 cm thickness BGO crystals surrounding. In the beam tests, protons were counted one by one with a plastic scintillator located between the beam nozzle and detectors. The proton beam profile was 1 cm in diameter. Both detectors were irradiated with the full bias voltage. After the irradiation, we continued monitoring the leakage current in order to see annealing effects at the room temperature, however, the annealing observed for the leakage current reached a stable value in a few minutes. Fig. 4 shows the stable leakage current of the irradiated strips (pads) before and after irradiation. The current level within the spot area increased by 6 (73) pA/strip for DSSD (p-side) and 27 ( 73) pA/pad for Si-pad at 15 1C. The low current increase in DSSD is due to the limited beam size (f 1 cm) for the irradiated strip with the length of 3.2 cm. Scaling the spot size to the full strip length, the current increases become 19 ( 710) pA/strip. The increase in leakage current at 15 1C are 0.24 (70.12) nA/cm2 and 0.26 ( 70.03) nA/cm2 for DSSD and Si-pad, respectively. Radiation damage evaluated by the increase in the leakage current at the temperature of 251 are 3 ( 71) 10 8 nA/cm and 7 (72) 10 8 nA/cm for DSSD and Si-pad, respectively, which are compatible with those of silicon micro strip detectors developed for accelerator experiments [17,18].
Leakage Current (pA)
3.1. Proton irradiation
40 35 30 25 DSSD (p−side) Si−pad
20 15 10
Ch A
Ch A
Ch B
Ch B
Ch C
Ch C
5 0
0
0.5
1
1.5
2
2.5
3
3.5
4
Proton Irradiated Number (x 109 cm−2) Fig. 4. Leakage current at 15 1C before and after the proton irradiation test.
temperature of 15 1C, which is an expected operating temperature in the orbit. The leakage current was monitored until the change rate of current level became less than 1% per day, which took a couple of days. We repeated this set of measurements with the total radiation dose of 2, 4 (or 6), 10 and 20 krad levels. Fig. 5 shows dose dependence of the leakage current for a strip (or pad) at the full depletion voltage at the temperature of 15 1C for DSSD (p-side) and Si-pad. The leakage current increased up to 91 pA and 30 pA at 20 krad dose for the strip of DSSD and the pad of Si-pad, respectively. The increase in leakage current density at 15 1C at the 20 (10) krad dose is 1.1 (0.46) nA/cm2 and 0.29 (0.15) nA/cm2 for DSSD and Si-pad, respectively, which are comparable to the results of proton irradiation test within a factor of two, if we consider the current density at 10 krad dose level. The increase rate of leakage current at 25 1C evaluated by the temperature dependency (equation (1), see Section 4) are 2.2 nA/cm2/krad and 0.54 nA/cm2/krad for DSSD and Si-pad, respectively, which are as large as that of Si micro-strip sensor for GLAST (Gamma-ray Large Area Space Telescope) [21].
3.2. Gamma-ray irradiation
4. Discussion
We carried out a 60Co gamma-ray irradiation test at the 60Co radiation research facility of Faculty of Engineering, Hiroshima University. During the irradiation, these sensors were biased with the full depletion voltage in order to set a similar operational condition in the orbit. The sensors were irradiated almost uniformly. After the irradiation, these sensors were kept at the
As described in Section 1, proton and gamma-ray radiations create new trap levels in the forbidden band. We have evaluated energy levels of defects due to the gamma-ray irradiation. Fig. 6 shows temperature dependency of the leakage current for a channel of DSSD (p-side) and Si-pad. The relation between the leakage current, Jgen, and the temperature is expressed as the
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140
Si−pad
120 Leakage Current (pA)
Table 3 Noise level at 15 1C for a channel of DSSD (p-side) and Si-pad before and after 20 krad irradiation.
DSSD (p−side)
DSSD (p-side) shot noise (keV) total (keV) Si-pad shot noise (keV) total (keV)
80 60
20 krad
0.37 1.23 70.04stat
0.68 1.37 7 0.03stat
0.20 1.56 70.06stat
0.41 1.64 7 0.07stat
40 20 0
0
5
10
15
20
Dose (krad) Fig. 5. Dose dependency of the leakage current for a channel of DSSD (p-side) and Si-pad at 15 1C for the gamma-ray irradiation.
10
Leakage Current (nA)
0 krad
100
1
10−1 DSSD Si−pad
10−2
0 krad 20 krad 0 krad 20 krad
band gap by the irradiation and becomes a dominant source of the leakage current. Finally, we evaluated the effects on the noise level due to the gamma-ray irradiation. Noise sources expected are shot noise, capacitance noise, Johnson noise. The shot noise can be ffi pffiffiffiffiffiffiffiffiffiffiffiffiand expressed as 2qAIt, where q is the electron charge, A is a typical shape factor assuming that an electronic circuit has a CR–RC shaper ð 0:97Þ, I is the leakage current in A and t is the shaping time of the readout electronic circuit in seconds. Listed in Table 3 are the typical noise level observed for DSSD and Si-pad channels. The shot noise after the irradiation calculated from the leakage current was twice of the noise level before irradiation. The total noise (dominated by the noise of capacitance induced) is evaluated by measurements of the 241Am spectra using the same readout amplifier discussed in Section 2. After irradiation of 20 krad, the noise level increased slightly to be 1.37 keV and 1.64 keV for DSSD (p-side) and Si-pad, respectively, at 15 1C. Relatively, the increase in noise level due to the gamma-ray irradiation is not significant.
5. Conclusion 10−3 −20
−15
−10
−5 0 5 10 Temperature (degree C)
15
20
25
Fig. 6. Temperature dependency of the leakage current for a channel of DSSD (pside) and Si-pad for the gamma-ray irradiation. The lines are the best fit model expressed as Eq. (1) and the parameters shown in Table 2.
Table 2 Defect level (Et Ei ) for a channel of DSSD (p-side) and Si-pad before and after 20 krad gamma-ray irradiation.
We have developed Si sensors (DSSD and Si-pad) for the X-ray observatory ASTRO–H. The basic performance of Si sensors meets the requirements for the mission. We also evaluated radiation tolerance of these Si sensors. Beam tests with 150 MeV protons and 60Co gamma-ray were conducted for a total dose of 10–20 years operation level. The leakage current increases by 0.2 to 1.1 nA/cm2 at 15 1C. The increase in noise level is not significant.
Dose (krad)
Et Ei (eV)
w2 /dof
Acknowledgments
DSSD (p-side) 0 20
0.057 0.01 0.027 0.01
3.15/3 2.17/3
Si-pad 0 20
0.207 0.01 0.097 0.01
0.27/3 2.53/3
We would like to express our thanks to Prof. Endo and the staff member at radiation research facility of Faculty Engineering, Hiroshima University for the 60Co gamma-ray irradiation test. We also appreciate Prof. Uchihori, Kitamura and other members at NIRS for the proton irradiation test. The proton irradiation test was carried out as a part of Research Project with Heavy Ions at NIRS-HIMAC with the project number of 258.
formula below [20] J gen ¼ J0 T 2
expðEg =ð2kTÞÞ exp½ðEt Ei Þ=kT þ r exp½ðEt Ei Þ=kT
ð1Þ
where r ¼ sp =sn , the ratio of the hole and electron capture cross section, Eg ¼ 1:12 eV, the band gap energy of Si, Et is the trap energy level, Ei is the intrinsic Fermi level, k is Boltzmann’s constant, and T is the absolute temperature. We assumed that r is zero, indicating the electron capture being dominant [20]. By fitting the data in Fig. 6 with Eq. (1) under the assumption of r ¼ 0, we obtained an average defect energy level, Et Ei . This fits to equation (1) and the parameters obtained are shown in Fig. 6 and Table 2, respectively. A new trap level is created near the center of
References [1] T. Takahashi, et al., Proceedings of SPIE 7011 (2008) 14T. [2] T. Takahashi, K. Mitsuda, R.L. Kelley, et al., Proceedings of SPIE 7732 (2010) 77320Z. [3] T. Takahashi, et al., Publications of the Astronomical Society of Japan 59S (2007) 35T. [4] M. Kokubun, et al., SPIE 7011 (2008) 21K. [5] M. Kokubun, et al., SPIE 7732 (2010) 33K. [6] H. Tajima, et al., IEEE Transactions on Nuclear Science 53 (2005) 2749–2757. [7] H. Tajima, et al., SPIE 7732 (2010) 34T. [8] P. Ubertini, et al., A&A 411L (2003) 131U. [9] R.P. Johnson, et al., IEEE Transactions on Nuclear Science 45 (1998) 927–930. [10] T. Dachev, et al., Advances in Space Research 37 (2006) 1672–1677.
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[11] J.R. Srour, Radiation Effects on and Dose Enhancement of Electronic Materials, ISBN 0815510071. [12] Y. Fukazawa, et al., Nuclear Instruments and Methods in Physics Research Section A 541 (2005) 342–349. [13] K. Nakazawa, et al., Nuclear Instruments and Methods in Physics Research Section A 573 (2007) 44–47. [14] S. Takeda, et al., Nuclear Instruments and Methods in Physics Research Section A 579 (2007) 859–865. [15] H. Yasuda, et al., SPIE 6706 (2007) 67060T. [16] K. Iniewski, Radiation Effects in Semiconductors, ISBN 978-1-4398-2695-9.
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[17] P. Borgeaud, et al., Nuclear Instruments and Methods in Physics Research Section A 211 (1983) 363. [18] T. Ohsugi, et al., Nuclear Instruments and Methods in Physics Research Section A 265 (1988) 105. [19] T. Kondo, et al., Proceedings of the Summer Study on the Design and Utilization of the Superconducting Super Collider, in: R. Donaldson, J.G. Morfin (Eds.), Snowmass, Co., 1984, p. 612. [20] J. Kaneko, et al., IEEE Transactions on Nuclear Science 49 (2002) 1593–1597. [21] S. Yoshida, et al., SLAC-PUB-9643, March 2003.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Radiation effects on the silicon semiconductor detectors for the ASTRO–H mission Katsuhiro Hayashi a,n, InChun Park a, Kyohei Dotsu a, Issei Ueno a, Sho Nishino a, Masayuki Matsuoka a, Hajimu Yasuda a, Yasushi Fukazawa a, Takashi Ohsugi a, Tsunefumi Mizuno a, Hiromitsu Takahashi a, Masanori Ohno a, Satoru Endo b, Takaaki Tanaka c, Hiroyasu Tajima d, Motohide Kokubun e, Shin Watanabe e, Tadayuki Takahashi e, Kazuhiro Nakazawa f, Yukio Uchihori g, Hisashi Kitamura g a
Department of Physical Science, Hiroshima University, 1-3-1, Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan Mechanical System Engineering, Graduate School of Engineering Hiroshima University, 1-4-1, Kagamiyama, Higashi-hiroshima, Hiroshima 739-8527, Japan KIPAC, Stanford University, Stanford, CA 94305, USA d Cosmic-ray Research Facility, Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan e ISAS/JAXA, 3-4-1, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 229-8510, Japan f Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan g Department of Accelerator and Medical Physics National Institute of Radiological Sciences 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan b c
a r t i c l e i n f o
abstract
Available online 8 June 2012
Hard X-ray Imager (HXI) and Soft Gamma-ray Detector (SGD) onboard the 6th Japanese X-ray satellite, ASTRO–H, utilize double-sided silicon strip detectors (DSSD) and pixel array-type silicon sensors (Sipad), respectively. The DSSD with a 3.4 cm 3.4 cm area has an imaging capability in the lower energy band for the HXI covering 5–80 keV. The Si-pad consists of 16 16 pixels with a 5.4 cm 5.4 cm area and measures a photon direction with the Compton kinematics in 10–600 keV. Since the ASTRO–H will be operated in a low earth orbit, these detectors will be damaged by irradiation of cosmic-ray protons mainly in the South Atlantic Anomaly. In order to evaluate damage effects of the sensors, we have carried out irradiation tests with 150 MeV proton beams and 60Co gamma-rays with a total dose of 10– 20 years irradiation level. In both experiments, the leakage current has increased by 0:21:1 nA=cm2 under an expected operation temperature at 15 1C, which resulted in the noise level within a tolerance of 20 years. In this report, we present a summary of the basic performance of silicon detectors, and radiation effects on them by the irradiation tests. & 2012 Elsevier B.V. All rights reserved.
Keywords: ASTRO–H Silicon detector Radiation damage
1. Introduction ASTRO–H, the 6th Japanese X-ray satellite [1,2] following the currently operational Suzaku satellite, is scheduled to be launched in 2014. ASTRO–H aims to reveal high-energy phenomena, such as supernova remnants, black holes, pulsars and clusters of galaxies. Four observation systems with covering energy band from 0.3 keV to 600 keV will be installed. One of them is a hard X-ray imaging system with covering 5–80 keV. It consists of a hard X-ray telescope (HXT) and a combination of silicon (Si) & cadmium telluride (CdTe) double-sided, two dimensional strip detectors (HXI, Hard X-ray Imager) [4,5] located at the focal plane with a focal length of 12 m. For the highest energy band from 10 keV to 600 keV, soft gamma-ray spectrometer (SGD, Soft
n
Corresponding author. E-mail address: [email protected] (K. Hayashi).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.05.088
Gamma-ray Detector) utilizing a narrow filed of view Si and CdTe Compton camera [6,7] is onboard. HXI is composed of four layers of double-sided Si strip detectors overlaid on a CdTe double-sided strip detector surrounded by BGO (Bi4Ge3O12) active shields. The hybrid structure of Si and CdTe enables us to detect hard X-rays in 5–80 keV energy range with high efficiency. The sensitivity of HXI is higher by almost two orders of magnitude than that of Suzaku-HXD (Hard X-ray Detector) [3], thanks to the light focusing system with HXT and the BGO active shield. SGD is a narrow filed of view Compton camera composed of 32 layers of Si pixel sensors as scatterers, and eight layers of CdTe pixel sensors as absorbers. The multi-layer of Si is an excellent scatterer to determine the photon direction by the Compton kinematics. The narrow filed of view by a collimator and rejections of background events by the BGO active shield improve the sensitivity by nearly one order of magnitude than that of IBIS onboard INTEGRAL satellite [8]. The requirements of energy resolution in FWHM are o 2 keV at 60 keV for HXI and o 2 keV at 100 keV for SGD.
226
K. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 699 (2013) 225–229
In the orbit of ASTRO–H (altitude: 550 km, orbital inclination: 311), the sensors suffer radiation damage mainly by cosmic-ray protons ð 150 MeVÞ in the South Atlantic Anomaly (SAA) with a level of 1 krad=yr [9,10]. Dominant effects of radiations on the sensors are divided into two damage processes [11]. One is surface damage; holes generated in the surface SiO2 layer by gamma-ray or particle irradiations are trapped in defects close to the interface, resulting in accumulation of positive charges in the SiO2 layer. The other is bulk damage; lattice atoms are displaced from original positions by interactions with high energy particles. These defects create new trap levels in the forbidden band and the thermal electron transition probability from the valence band into the conduction band increases so that leakage current increases. The energy resolution becomes worse due to the increase in shot noise. In previous studies, bulk damage has been investigated accurately by proton irradiation tests through developments of Si sensors for accelerator experiments and almost consistent results have been obtained [17–19]. On the other hand, surface damage depends on a surface design proper to each detector [16], and we therefore need to investigate the surface damage for DSSD and Sipad with some improvement configurations (see Section 2). Accordingly, we carried out a proton irradiation test in order to confirm the consistency of bulk damage with previous studies, and a 60Co gamma-ray irradiation test, which mainly gives damage to a surface layer, in order to evaluate the surface damage. In this paper, we report specifications and basic performance of the Si sensors with some improvement configurations for the ASTRO–H mission in Section 2. In Section 3, we describe proton and gamma-ray irradiation tests and discuss effects on the Si detectors by the irradiation in Section 4. Finally, we summarize results in Section 5.
Fig. 1. Schematic design of the DSSD. The p- and n-type strips are implanted on an n-type bulk Si. Each n-strip is surrounded by an atoll-shaped p-stop for individualization of the n-strips.
Fig. 2. Overall view of the Si-pad. The p-type 16x16 matrix array are implanted on an n-type bulk Si. The lines from each pad to the corners of sensor indicate signal read out lines.
2. Specifications and basic performance of the Si detectors We have developed Si sensors for HXI and SGD in a collaboration with Hamamatsu Photonics [12–15]. The specifications of Si sensors for the flight model of HXI and SGD are shown in Table 1. For the HXI, double-sided, two dimensional Si strip detector (DSSD) with an n-type bulk is developed [5]. The strip pitch of 250 mm is chosen to meet an angular resolution of 0.07 arcminute for the 12 m focal length [5]. The total number of 128 strips with the strip width of 150 mm and a DC-coupled electrode on each Table 1 Specifications of DSSD and Si-pad. DSSD Size Strip pitch Strip width Bulk thickness Number of strips Capacitance Leakage current Bias voltage Energy resolution
3.4 3.4 cm2 250 mm (both sides) 150 mm (both sides) 500 mm 128 o 10 pF=strip o 2 nA=strip @20 1C o 300 V o 2 keV @59.5 keV (FWHM) with ASIC
Si-pad Size Pixel pitch Pixel size Bulk thickness Number of pixels Capacitance Leakage current Bias voltage Energy resolution
5.4 5.4 cm2 3.2 mm 3.14 mm2 600 mm 256 (16 16) o 10 pF=pad o 2 nA=pad @20 1C o 300 V o 2 keV @59.5 keV (FWHM) with ASIC
side are implanted. The n-strip on the ohmic side is surrounded by an atoll-shaped p-stop with the width of 25 mm for each strip as shown in Fig. 1. For the flight model, a DC-coupled aluminum (Al) electrode is attached on the p-stop to reduce the Johnson noise, then the total noise level is reduced by 30%. For the SGD, a pixel-array-type Si sensor (Si-pad) is developed [7]. The Si-pad has 16 16 matrix array with a pixel size of 3.14 mm2 which is implanted on an n-type bulk Si with a DCcoupled electrode as shown in Fig. 2. The pixel size is determined to minimize power consumption while keeping a proper angular resolution by the Compton kinematics. A signal from each pad is brought out to one of Al-bonding pads at the corner of sensor by a readout line on the SiO2 layer. For the flight model, the thickness of SiO2 layer is increased from 1 mm to 1:5 mm in order to reduce the capacitance between the Si bulk and readout electrodes. This capacitance reduction improves the energy resolution of each pad by 10230%. Followings are basic performance of these flight model Si detectors before irradiation. The leakage current obtained at 20 1C at the full depletion voltage are 1.3 nA/strip and 0.9 nA/ pad for DSSD and Si-pad, respectively, which are within the requirements as shown in Table 1. Fig. 3 shows spectra of 241 Am 59.5 keV X-ray at 15 1C detected by each side of the DSSD. Signals of the silicon sensors are fed into a low noise charge-sensitive amplifier CLEAR PULSE CP-5102 and ORTEC 571 amplifier for a pulse shaping with 3 ms. The energy resolution (FWHM) for the p-side and n-side was 1:23ð 70:04stat Þ keV and 1:8ð 70:1stat Þ keV, respectively. We also measured a spectrum of the Si-pad and obtained the energy resolution (FWHM) of 1:56ð 70:06stat Þ keV.
K. Hayashi et al. / Nuclear Instruments and Methods in Physics Research A 699 (2013) 225–229
104
104 Pside
Nside
103
103
59.5 keV
Counts
59.5 keV
Counts
227
102
102
10
10
1
1 0
10
20
30
40
50
60
70
80
0
10
20
30
Energy (keV) Fig. 3.
241
40
50
60
70
80
Energy (keV)
Am spectrum at 15 1C obtained by DSSD for the p-side (left) and n-side (right).
3. Proton and gamma-ray irradiation tests
50
As described in Section 1, we carried out proton and 60Co gamma-ray irradiation tests in order to study radiation effects on the sensors due to bulk and surface damages. The maximum fluence level for the proton irradiation test was 10-years operation level. For the 60Co gamma-ray irradiation test, we set the maximum dose up to 20 krad (equivalent to 20-years operation level), in order to investigate damage effects with a severe condition.
45
The proton irradiation test was held at National Institute of Radiological Sciences (NIRS), Japan. The proton energy is chosen to be 150 MeV, a typical proton energy in SAA. The total fluence of proton irradiation was 3.8 109 cm 2, estimated for the radiation fluence of 10 years operation in the ASTRO–H orbit [10] with the 3 cm thickness BGO crystals surrounding. In the beam tests, protons were counted one by one with a plastic scintillator located between the beam nozzle and detectors. The proton beam profile was 1 cm in diameter. Both detectors were irradiated with the full bias voltage. After the irradiation, we continued monitoring the leakage current in order to see annealing effects at the room temperature, however, the annealing observed for the leakage current reached a stable value in a few minutes. Fig. 4 shows the stable leakage current of the irradiated strips (pads) before and after irradiation. The current level within the spot area increased by 6 (73) pA/strip for DSSD (p-side) and 27 ( 73) pA/pad for Si-pad at 15 1C. The low current increase in DSSD is due to the limited beam size (f 1 cm) for the irradiated strip with the length of 3.2 cm. Scaling the spot size to the full strip length, the current increases become 19 ( 710) pA/strip. The increase in leakage current at 15 1C are 0.24 (70.12) nA/cm2 and 0.26 ( 70.03) nA/cm2 for DSSD and Si-pad, respectively. Radiation damage evaluated by the increase in the leakage current at the temperature of 251 are 3 ( 71) 10 8 nA/cm and 7 (72) 10 8 nA/cm for DSSD and Si-pad, respectively, which are compatible with those of silicon micro strip detectors developed for accelerator experiments [17,18].
Leakage Current (pA)
3.1. Proton irradiation
40 35 30 25 DSSD (p−side) Si−pad
20 15 10
Ch A
Ch A
Ch B
Ch B
Ch C
Ch C
5 0
0
0.5
1
1.5
2
2.5
3
3.5
4
Proton Irradiated Number (x 109 cm−2) Fig. 4. Leakage current at 15 1C before and after the proton irradiation test.
temperature of 15 1C, which is an expected operating temperature in the orbit. The leakage current was monitored until the change rate of current level became less than 1% per day, which took a couple of days. We repeated this set of measurements with the total radiation dose of 2, 4 (or 6), 10 and 20 krad levels. Fig. 5 shows dose dependence of the leakage current for a strip (or pad) at the full depletion voltage at the temperature of 15 1C for DSSD (p-side) and Si-pad. The leakage current increased up to 91 pA and 30 pA at 20 krad dose for the strip of DSSD and the pad of Si-pad, respectively. The increase in leakage current density at 15 1C at the 20 (10) krad dose is 1.1 (0.46) nA/cm2 and 0.29 (0.15) nA/cm2 for DSSD and Si-pad, respectively, which are comparable to the results of proton irradiation test within a factor of two, if we consider the current density at 10 krad dose level. The increase rate of leakage current at 25 1C evaluated by the temperature dependency (equation (1), see Section 4) are 2.2 nA/cm2/krad and 0.54 nA/cm2/krad for DSSD and Si-pad, respectively, which are as large as that of Si micro-strip sensor for GLAST (Gamma-ray Large Area Space Telescope) [21].
3.2. Gamma-ray irradiation
4. Discussion
We carried out a 60Co gamma-ray irradiation test at the 60Co radiation research facility of Faculty of Engineering, Hiroshima University. During the irradiation, these sensors were biased with the full depletion voltage in order to set a similar operational condition in the orbit. The sensors were irradiated almost uniformly. After the irradiation, these sensors were kept at the
As described in Section 1, proton and gamma-ray radiations create new trap levels in the forbidden band. We have evaluated energy levels of defects due to the gamma-ray irradiation. Fig. 6 shows temperature dependency of the leakage current for a channel of DSSD (p-side) and Si-pad. The relation between the leakage current, Jgen, and the temperature is expressed as the
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140
Si−pad
120 Leakage Current (pA)
Table 3 Noise level at 15 1C for a channel of DSSD (p-side) and Si-pad before and after 20 krad irradiation.
DSSD (p−side)
DSSD (p-side) shot noise (keV) total (keV) Si-pad shot noise (keV) total (keV)
80 60
20 krad
0.37 1.23 70.04stat
0.68 1.37 7 0.03stat
0.20 1.56 70.06stat
0.41 1.64 7 0.07stat
40 20 0
0
5
10
15
20
Dose (krad) Fig. 5. Dose dependency of the leakage current for a channel of DSSD (p-side) and Si-pad at 15 1C for the gamma-ray irradiation.
10
Leakage Current (nA)
0 krad
100
1
10−1 DSSD Si−pad
10−2
0 krad 20 krad 0 krad 20 krad
band gap by the irradiation and becomes a dominant source of the leakage current. Finally, we evaluated the effects on the noise level due to the gamma-ray irradiation. Noise sources expected are shot noise, capacitance noise, Johnson noise. The shot noise can be ffi pffiffiffiffiffiffiffiffiffiffiffiffiand expressed as 2qAIt, where q is the electron charge, A is a typical shape factor assuming that an electronic circuit has a CR–RC shaper ð 0:97Þ, I is the leakage current in A and t is the shaping time of the readout electronic circuit in seconds. Listed in Table 3 are the typical noise level observed for DSSD and Si-pad channels. The shot noise after the irradiation calculated from the leakage current was twice of the noise level before irradiation. The total noise (dominated by the noise of capacitance induced) is evaluated by measurements of the 241Am spectra using the same readout amplifier discussed in Section 2. After irradiation of 20 krad, the noise level increased slightly to be 1.37 keV and 1.64 keV for DSSD (p-side) and Si-pad, respectively, at 15 1C. Relatively, the increase in noise level due to the gamma-ray irradiation is not significant.
5. Conclusion 10−3 −20
−15
−10
−5 0 5 10 Temperature (degree C)
15
20
25
Fig. 6. Temperature dependency of the leakage current for a channel of DSSD (pside) and Si-pad for the gamma-ray irradiation. The lines are the best fit model expressed as Eq. (1) and the parameters shown in Table 2.
Table 2 Defect level (Et Ei ) for a channel of DSSD (p-side) and Si-pad before and after 20 krad gamma-ray irradiation.
We have developed Si sensors (DSSD and Si-pad) for the X-ray observatory ASTRO–H. The basic performance of Si sensors meets the requirements for the mission. We also evaluated radiation tolerance of these Si sensors. Beam tests with 150 MeV protons and 60Co gamma-ray were conducted for a total dose of 10–20 years operation level. The leakage current increases by 0.2 to 1.1 nA/cm2 at 15 1C. The increase in noise level is not significant.
Dose (krad)
Et Ei (eV)
w2 /dof
Acknowledgments
DSSD (p-side) 0 20
0.057 0.01 0.027 0.01
3.15/3 2.17/3
Si-pad 0 20
0.207 0.01 0.097 0.01
0.27/3 2.53/3
We would like to express our thanks to Prof. Endo and the staff member at radiation research facility of Faculty Engineering, Hiroshima University for the 60Co gamma-ray irradiation test. We also appreciate Prof. Uchihori, Kitamura and other members at NIRS for the proton irradiation test. The proton irradiation test was carried out as a part of Research Project with Heavy Ions at NIRS-HIMAC with the project number of 258.
formula below [20] J gen ¼ J0 T 2
expðEg =ð2kTÞÞ exp½ðEt Ei Þ=kT þ r exp½ðEt Ei Þ=kT
ð1Þ
where r ¼ sp =sn , the ratio of the hole and electron capture cross section, Eg ¼ 1:12 eV, the band gap energy of Si, Et is the trap energy level, Ei is the intrinsic Fermi level, k is Boltzmann’s constant, and T is the absolute temperature. We assumed that r is zero, indicating the electron capture being dominant [20]. By fitting the data in Fig. 6 with Eq. (1) under the assumption of r ¼ 0, we obtained an average defect energy level, Et Ei . This fits to equation (1) and the parameters obtained are shown in Fig. 6 and Table 2, respectively. A new trap level is created near the center of
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