The Si/CdTe semiconductor Compton camera of the ASTRO-H Soft Gamma-ray Detector(SGD)
Shin Watanabea,b , Hiroyasu Tajimac , Yasushi Fukazawad , Yuto Ichinohea,b , Shin’ichiro Takedaa , Teruaki Enotoe , Taro
Fukuyamaa,b , Shunya Furuid , Kei Genbaf , Kouichi Haginoa,b , Atsushi Harayamaa , Yoshikatsu Kurodaf , Daisuke Matsuuraf , Ryo
Nakamurad , Kazuhiro Nakazawab , Hirofumi Nodab , Hirokazu Odakaa , Masayuki Ohtaa , Mitsunobu Onishif , Shinya Saitoa,b , Goro
Satog,a , Tamotsu Satoa,b , Tadayuki Takahashia,b , Takaaki Tanakah , Atsushi Togoa,b , Shinji Tomizukaca Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo Sagamihara, Kanagawa, 252-5210, Japanb Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo, 113-0033, Japanc Solar-Terrestrial Environment Laboratory, Nagoya University, Furo, Chikusa Nagoya, Aichi, 464-8601, Japand Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526, Japane Nishina Center, RIKEN, 2-1 Hirosawa, Wako, Saitama, 351-0198, Japanf Nagoya Guidance and Propulsion Systems Works, Mitsubishi Heavy Industry Ltd., 1200 Higashi Tanaka, Komaki, Aichi, 485-8561, Japang Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japanh Department of Physics, Kyoto University, Kitashirakawaoiwake, Sakyo Kyoto, Kyoto, 606-8502, Japan

Abstract
The Soft Gamma-ray Detector (SGD) is one of the instrument payloads onboard ASTRO-H, and will cover a wide energy band
(60–600 keV) at a background level 10 times better than instruments currently in orbit. The SGD achieves low background bycombining a Compton camera scheme with a narrow field-of-view active shield. The Compton camera in the SGD is realized
as a hybrid semiconductor detector system which consists of silicon and cadmium telluride (CdTe) sensors. The design of the
SGD Compton camera has been finalized and the final prototype, which has the same configuration as the flight model, has been
fabricated for performance evaluation. The Compton camera has overall dimensions of 12 cm × 12 cm × 12 cm, consisting of
32 layers of Si pixel sensors and 8 layers of CdTe pixel sensors surrounded by 2 layers of CdTe pixel sensors. The detection
efficiency of the Compton camera reaches about 15% and 3% for 100 keV and 511 keV gamma rays, respectively. The pixel pitchof the Si and CdTe sensors is 3.2 mm, and the signals from all 13312 pixels are processed by 208 ASICs developed for the SGD.
Good energy resolution is afforded by semiconductor sensors and low noise ASICs, and the obtained energy resolutions with theprototype Si and CdTe pixel sensors are 1.0–2.0 keV (FWHM) at 60 keV and 1.6–2.5 keV (FWHM) at 122 keV, respectively.
This results in good background rejection capability due to better constraints on Compton kinematics. Compton camera energy
resolutions achieved with the final prototype are 6.3 keV (FWHM) at 356 keV and 10.5 keV (FWHM) at 662 keV, respectively,
which satisfy the instrument requirements for the SGD Compton camera (better than 2%). Moreover, a low intrinsic backgroundhas been confirmed by the background measurement with the final prototype.
Keywords: Compton camera, ASTRO-H SGD, Gamma-ray detector, Semiconductor detector, CdTe detector, Silicon detector

1. ASTRO-H SGD
ASTRO-H, the new Japanese X-ray Astronomy Satellite[1,2, 3] following the currently-operational Suzaku satellite, aims
to fulfill the following scientific goals:• Revealing the large-scale structure of the universe and its
evolution.
• Understanding the extreme conditions of the universe.
• Exploring the diverse phenomena of the non-thermal universe.
• Elucidating dark matter and dark energy.
In order to fulfill the above objectives, the ASTRO-H satellite
hosts the following four types of instruments: SXT (Soft X-ray
Telescope) + SXS (Soft X-ray Spectrometer), SXT + SXI (Soft

X-ray Imager), HXT (Hard X-ray Telescope) + HXI (Hard Xray Imager)[4, 5, 6, 7, 8, 9, 10, 11] and SGD (Soft Gamma-ray
Detector) [4, 5, 6, 7, 9, 12, 13, 14].
The SGD will cover the energy range of 60–600 keV witha high sensitivity. The SGD utilizes semiconductor detectors using Si and CdTe pixel sensors with good energy resolution (.2 keV) for the Compton camera, which were madepossible by recent progress on the development of low noise
Si gamma-ray sensors[15, 16, 17, 18] and high quality CdTe
sensors[19, 20, 21, 22, 23, 24, 25]. The BGO active shieldprovides a low background environment by rejecting the majority of external backgrounds. Internal backgrounds are rejectedbased on the inconsistency between the constraint on the incident angle of gamma rays from Compton kinematics and thatfrom the narrow FOV (field of view) of the collimator. Thisadditional background rejection by Compton kinematics will
improve the sensitivity by an order of magnitude in the 60–

Preprint submitted to Nuclear Instrumentations and Methods in Physics Research, Section A

July 1, 2014

3. Instrument-level requirements for SGD Compton cameras

600 keV band compared with the currently operating spacebased instruments.
In this paper, we will present the detailed configuration andthe data acquisition system of the SGD Compton camera. We
will also present the performance evaluated for the final prototype, which is equivalent to the flight model.

The ASTRO-H mission-level science objectives describedabove require the SGD to provide spectroscopy up to 600 keV
for over 10 accreting supermassive black holes with fluxes
equivalent to 1/1000 of the Crab Nebula (as measured overthe 2–10 keV band, assuming the spectrum to be a power-law
with spectral index of 1.7). This mission-level science requirement defines the following instrument-level requirements for
the SGD Compton camera:

2. SGD concept and Si/CdTe Semiconductor Compton
CameraThe SGD is based on the concept of narrow FOV Compton
telescopes[5], combining Compton cameras and active welltype shields. The active well-type shield concept originatesfrom the Hard X-ray Detector (HXD)[26] onboard the Suzaku
satellite. The HXD achieves the best sensitivities in the hard
X-ray band, consisting of Si photodiodes and GSO scintillators with BGO active shield and copper passive collimator. The
SGD, however, replaces the Si photodiodes and GSO scintillators with the Compton camera, which provides additional information for the background rejection. Figure 1 shows a conceptual drawing of an SGD unit. A BGO collimator defines a fieldof view of ∼10◦ for high energy photons while a fine collimator
restricts the FOV to .0.6◦ for low energy photons (.150 keV),which is essential to minimize the CXB (cosmic X-ray background) and source confusion. Scintillation light from the BGO
crystals is detected by avalanche photo-diodes (APDs) allowing
for a compact design compared to phototubes.
The hybrid design of the Compton camera module incorporates both Si and CdTe imaging detectors. The Si sensors areused as detectors for Compton scattering since Compton scattering is the dominant process in Si above ∼50 keV comparedwith ∼300 keV for CdTe. The Si sensors also provide better
constraints on the Compton kinematics because of smaller effect of Doppler broadening[27, 28]. The CdTe sensors are used
to absorb gamma rays following Compton scattering in the Si
sensors.

• Effective area for the detector must be greater than 20 cm2
at 100 keV to obtain a sufficient number of photons in areasonable observation time (typically 100 ks).
• Observation energy range must be from 60 keV to
600 keV.
• Energy resolution must be better than 2 keV (FWHM) or
better than 2% (FWHM).
There are the following design constraints on the SGD
Compton camera fro mounting the ASTRO-H:• The size of one camera must be 12 cm × 12 cm × 12 cm
to minimize the size of BGO active shield, since the BGO
is the dominant contributor to the total weight of the SGD.
• The number of Compton cameras must be six in total
ASTRO-H.
• Power consumption must be lower than 6 W for one camera.
The instrument-level requirements and the design constraints
described above guide the designs of the Si sensor, the CdTe
sensor and the readout Application Specific Integrated Circuit(ASIC) for both. The detection area of the Si sensor must belarger than 5 cm × 5 cm. The total thickness of the Si sensor must be about 2 cm, which corresponds to the 50% interaction efficiency for 100 keV photons. Therefore, 32 layersof Si sensors are needed when 0.6 mm thick Si devices areused. In order to satisfy the effective area requirement, CdTe
sensors must cover 50% of the solid angle covered by the Si
sensors. The readout ASICs for the Si and CdTe sensors musthave an internal analog-to-digital converter (ADC) and must be
controllable with digital signals because space in the Compton camera is limited. Moreover, the ASIC must consume lessthan 0.5 mW/channel and have good noise performance of 100–
200 e- (ENC) under the condition that the input capacitance isseveral pF.

G

BGO

E0

Si

E1
Q
E2

CdTe

APD

5 cm

4. SGD Compton Camera Design
BGO

4.1. Overall Design
Based on the design guide described in the last section, theCompton camera consists of 32 layers of Si sensors and 8 layers of CdTe sensors surrounded by 2 layers of CdTe sensors.

Figure 1: Conceptual drawing of an SGD Compton camera unit.

2

(a)

Figure 2 shows a 3D model of the Compton camera structure.
This arrangement allows a placement of the CdTe sensor on theside very close to the stacked Si and CdTe sensors, maximizing
the coverage of the photons scattered by the Si sensors. In addition to sensor modules, the Compton camera holds an ASIC
controller board (ACB) and four ASIC driver boards (ADBs).
The ACB holds a field programmable gate array (FPGA) that
controls the ASICs. The ADB buffers control signals from the
ACB, sends control signals to 52 ASICs, and also provides acurrent limiter to power the ASICs.
The mechanical structure of the Compton camera needs
to hold all components described above within a volume of
12 × 12 × 12 cm3 . Another important requirement for the mechanical structure is sensor cooling. All sensors need to have atemperature that is within 5◦ C of the cold plate interface at thebottom of the Compton camera.
Figure 2 (a) shows the mechanical support structure of theCompton camera. The Compton Camera consists of a stack of
Si and CdTe sensor trays as shown in Figure 2 (b). In addition to the stack, one ”side CdTe sensor module” is installed oneach side as shown in Figure 2 (c). Each ADB is attached tothe side CdTe sensor module and an ACB is attached to the bottom frame. The material of the sensor tray structure employs
polymide.

Al frame

ADB

ADB

Al frame

Al frame

ADB

Al frame!
(top)

Al frame!
(bottom)

Al frame

ADB

(b)
ASIC
ASIC

Si pad

4.2. Readout ASIC for Si and CdTe sensors

ASIC

The main performance requirements for an ASIC are low
noise and low power. In order to satisfy these main requirements, the ASIC was developed based on the VIKING
architecture[29, 30] which is known for good noise performance and has been in various space experiments like
Swift[31], PAMELA[32] and AGILE[33].
Figure 3 shows the circuit diagram for the ASIC developed
for the SGD (and HXI) of ASTRO-H. Each channel consists ofcharge sensitive amplifier followed by two shapers. One shaperwith a short shaping time is followed by a discriminator to forma trigger signal. The other shaper with a long shaping time isfollowed by a sample and hold circuit to hold the pulse heightat the timing specified by an external hold signal.
Many important functionalities are integrated in the ASIC
for the SGD in order to minimize additional components required to read out the signal, as shown in the circuit diagramwith a blue background region. The signals in all channels areconverted to digital values concurrently with Wilkinson-type
analog-to-digital converters (ADCs). The conversion time isless than 100 µs. In order to minimize the readout time, theonly channels that are read out are those above a data thresholdthat can be digitally set for each channel independently fromthe trigger threshold. We usually observe common mode noisefrom this type of ASIC at the level of ∼1 keV. Common modenoise has to be subtracted to accurately apply the threshold forthe zero suppression. The common mode noise level of each
event is detected by taking an ADC value of the 32nd [a halfof total number of channels] pulse height, corresponding to a
median value of all ADC values. With zero suppression, thereadout time is 0.5 µs per ASIC when no data is readout andASIC

Si sensor tray modules
stacked CdTe sensor tray modules(c)side CdTe sensor tray module

Figure 2: (a) 3D model of Compton camera structure. (b) Si and CdTe stack
only. (c) One side CdTe tray module is installed in the stack.

3

(9 + n) µs when we readout n channels. Without zero suppression, the readout time becomes 73 µs per ASIC.
The ASIC produces all necessary analog bias currents andvoltages on the chip by internal Digital to Analog Converters(DACs) except for the main bias current which sets the scale of
all bias currents. Each bit of the registers for all internal DACs
and other functions consists of three flip-flops and a majority
selector for tolerance against single event upset (SEU). If the
majority selector detects any discrepancies among three flipflops, it will set a SEU flag which will be readout as a part of
output data. The ASIC is fabricated on a wafer with an epitaxial layer that will improve immunity against latch up. Table 1
summarizes the specifications.

or 8 ASICs (in the stack tray modules) are daisy chained.
4.3. Si and CdTe Pad Detector
Si and CdTe sensors are pixellated to give two-dimensional
coordinates with a pixel size of 3.2 × 3.2 mm2 . Pixel size ischosen so as to minimize the number of pixels for lower power
consumption while avoiding the pixel size to be the dominant
contribution to the angular resolution of Compton kinematics.
The Si pad devices are manufactured by Hamamatsu Photonics K. K. Each Si sensor has 16 × 16 pixels providing
5.12 × 5.12 cm2 active area. A signal from each pixel on the
Si sensor is brought out to one of the bonding pads at the corner of the sensor by a readout electrode placed on top of the
SiO2 insulation layer with a thickness of 1.5 µm as shown in
Figure 4 (a). The readout electronics are DC-coupled to the
Si sensor. The thickness of each Si device is 0.6 mm and theoperating bias voltage is 230 V. The capacitance per pad is 4–
15 pF, including the capacitance attributed to the traces. The
leakage current is about 2 nA for one device at the temperatureof −20◦ C .
Figure 4 (b) shows the structure of the Si sensor tray module. One tray module consists of two Si pad devices and eight
front end cards (FECs) with one ASIC. Sixteen tray modules
are stacked in one Compton camera. The spectral performanceobtained with Si sensor tray modules is shown in Figure 5.
Figure 5 (a) shows the 241 Am spectrum obtained with one of
the best pixels. The FWHM energy resolution is 0.95 keV for
59.54 keV gamma rays at the operating temperature of −20o C.
Figure 5 (b) shows the distribution of the energy resolutions obtained with the pixels of the Si sensors. This variation of the energy resolutions result from the difference of input capacitancedue to readout traces for readout signal from pixels.
The CdTe pad devices are produced and processed by ACRORAD Co. Ltd. The CdTe sensor has 8 × 8 pixels providing
2.56 × 2.56 cm2 active area as it is difficult to fabricate a CdTe
sensor much larger than 3 × 3 cm2 . CdTe sensors are tiled ina 2 × 2 array for each layer in the bottom and in a 2 × 3 arrayfor each layer on the side to obtain the required active area. In
order to overcome small mobility and short lifetime of carriers in CdTe sensors, we employ a Schottky-barrier diode type
CdTe sensor with Indium (In) anode and Platinum (Pt) cathodeso that we can apply high bias voltage with low leakage current.
The indium electrode functions as a common biasing electrodewhile Pt electrodes form pixels. Titanium is placed on the In
electrode to reduce the resistance. Diode type CdTe sensors
suffer degradation of energy resolution due to charge trapping
over time (i.e. polarization). It is known that the polarization
slows down at lower temperature and the effect of polarization
can be reduced by applying higher bias voltage. For example, it
was found that one week of operation of this type of CdTe sensor shows little polarization effect at <5◦ C and >1000 V/mm.
Moreover, this polarization effect can be recovered by turning
off the bias voltage and the recovery process accelerates at ahigher temperature.
Unlike Si sensors, CdTe sensors cannot have integrated readout electrodes above pixel electrodes on the device. In addition, it is difficult to perform wire-bonding on the electrodes

Table 1: SGD ASIC specifications

Geometrical specifications
Number of channels
64
Input pitch
91 µm
Thickness
0.45 mm
Analog specifications
Power consumption
0.2 mW/channel
Fast shaper peaking time
0.6 µsSlow shaper peaking time
∼3 µs
Noise performance
180 e− (RMS) at 6 pF load
1.5 keV (FWHM) for Si
Threshold1500 e− at 6 pF load
5.4 keV for Si
Threshold range
625 – 6250 e−
Threshold step
208 e−
Dynamic range±100,000 e−
360 keV for Si
Digital specifications
ADC setup time
5 µs
ADC power consumption
0.5–2 mW/channel
5–20 µW/channel at 100 Hz
Data clock speed<10 MHz
Conversion clock speed<10 MHz (external clock)<20 MHz (internal clock)
Conversion time<100 µs (external clock)<50 µs (internal clock)
0.5 µs per ASIC
Readout time (no data)
Readout time (n channels)(9 + n) µs per ASIC

The data input and output circuits on the ASIC are designed
to allow daisy-chaining of multiple ASICs. In one scheme, thedata output of one ASIC can be connected to the input of another ASIC and the ASIC will pass the input data to the output
via a shift register. This scheme is used to set register values. In
another scheme, the outputs of several ASICs can be connected
to a single bus. The output is controlled by passing a token
from ASIC to ASIC. Or, in the case of trigger signal, ASICs
can issue trigger signals at any time since the output circuit isopen-drain FET to allow multiple triggers on the same bus. In
the Compton camera, 6 ASICs (in the side CdTe tray modules)
4

Vdd

ADC
S/H
Chargein

To common
mode noise
detection
Discriminator

10Discriminator
Digitaldelay

CSA

Triggerout

Slow shaper
τ~ 2–4 µs

10-bitADC
latch

64 channels

Digitalcomparator
10

Fast shaper
τ~ 0.5 µs

Vss

10

TA

VA
Hold

Voltageramp

Threshold

10-bit global
counter
Digitalthreshold

ADC
data
Fixed delay

Figure 3: Circuit diagram of the ASIC developed for the SGD. The circuits shown in a blue background are implemented in this development.

(a)

(b)

Wire-bonding
Frame

Si

LSI

Si

LSI

3.6mm
Wire-bonding
Figure 4: (a) Schematic drawing of Si sensor showing layout of pixels and readout traces. (b) 3D model and conceptual illustration of the Si sensor tray module.

5[count/bin]

(a)

(b)
13.9 keV

17.6 keV

241Am

FWHM 0.95
0.95 keV!
FWHM
keV@59.54keV@59.54 keV

21.0 keV
26.3 keV[ADC CH]
Figure 5: (a) 241 Am X-ray/gamma-ray spectrum obtained with a Si sensor of a Si tray module. This spectrum is obtained with one of the best pixels. The FWHM
energy resolution is 0.95 keV for 59.54 keV gamma rays. The operating temperature is −20o C. (b) The distribution of the energy resolutions obtained with the
pixels of the Si sensor tray modules for 59.54 keV gamma rays. The mean energy resolution is 1.5 keV (FWHM).

of the CdTe sensor. In order to address these issues, we employ a separate fanout board to route signal from each pixel tothe corner of the sensor where ASICs are placed. The fanoutboard is made of 0.3 mm thick ceramic (Al2 O3 ) substrate thatallows fine pitch between electrodes to match the input pitchof the ASIC (91 µm). The CdTe sensor and the fanout board
are bonded by using a conductive epoxy as shown in Figure 6.
ASIC and the fanout board are connected by wire bonding. The
readout electronics are DC-coupled to the CdTe sensor.
Figure 7 (a) shows the structure of the stack CdTe sensor
tray module. This CdTe sensor tray module has the same outershape as the Si sensor tray module. The stacked CdTe sensor
tray module consists of two 2 × 2 tiled CdTe sensor arrays andeight FECs. In one Compton camera, four stacked CdTe sensor
tray modules are placed underneath the Si sensor tray modules.

Figure 6: The CdTe pixel sensor devices bonded with the ceramic fanout board.
The left half shows the front with the ceramic fanout board for the pixels andthe right half shows the common anode electrode utilizing Indium.

The side CdTe sensors surrounding the Si and CdTe stackplay important roles in the SGD Compton camera. Since
the SiCdTepart is tall, the side sensors are essential for covering the largesolid angle of the Si sensors. Relatively low energy gammaray photons, such as 100–200 keV, are the main target of the
SGD and these photons are the most likely to be Compton scattered with a large scattering angle. Moreover, these Compton
scatterings with a large scattering angle provide the gamma-ray
polarization information[34, 35]. Figure 7 (b) shows the structure of the side CdTe sensor tray module. This module consistsof two layers. CdTe sensors are tiled in an 2 × 3 array for eachlayer.

4.4. Readout System
The readout system of the SGD Compton camera consists of
the FECs, ADBs, ACB and MIO(Mission I/O) board as shownin Figure 9.
The front-end electronics of the Compton camera consists offour groups of 42 FECs and an ADB, and an ACB. Two FECs
are connected back-to-back at the corner of each Si sensor tray
module and each stack CdTe sensor tray module, and are readout in daisy chain. FECs for the side CdTe sensor tray modules
have six ASICs that are daisy-chained on each board. Forty
FECs from the Si and stack CdTe trays and two FECs from theside CdTe trays are connected to an ADB, which is located on
the side of the Compton camera. Eight FECs (eight ASICs) are
daisy-chained for Si and stack CdTe trays, resulting in sevengroups of ASICs for each side, five for Si and stack CdTe trays(eight ASICs each) and two for side CdTe trays (six ASICs
each). Only digital communication is required between ADBs

The spectral performance obtained with the CdTe sensor tray
is shown in Figure 8. This is a 57 Co spectrum obtained with oneof the best pixels. The FWHM energy resolution is 1.7 keV at
122 keV. The operating temperature is −10o C and the appliedbias voltage is 1000 V.
6

(a)

(b)

Wire-bonding
Frame

CdTe

CdTe

LSI

CdTe

CdTe

LSI

ASIC

Wire-bonding

4.4mm
Frame

Wire-bonding

CdTe

Figure 7: (a) 3D model and conceptual illustration of the stacked CdTe sensor tray module. (b) 3D model and conceptual illustration of the side CdTe sensor tray
module.

and FECs and all digital signals are differential to minimize theelectromagnetic interference. Digital signals that are not usedfrequently are single ended between the ADB and the ACB due
to constraints on the cable pin count. The ADB detects excess current of each ASIC group in order to protect ASICs fromlatch-ups due to highly ironizing radiation or other causes. We
can recover ASICs from latch-ups by cycling the power supply.
ASICs are controlled by an FPGA on the ACB (one board per
Compton camera), and the ACB FPGAs are controlled by theuser FPGA on the MIO board. Communication between theCompton camera and MIO is handled via 3-line (CLK, DATA,STRB) serial protocol on LVDS physical layer. We have two
additional real-time LVDS lines dedicated for trigger and trigger acknowledgement signals.

57Co

FWHM 1.7 keV@122 keV

2500

2000

1500

Figure 10 shows a timing chart representing the readout sequence of the SGD Compton camera. Once the ACB FPGA
has received a trigger signal from an ASIC, the ACB FPGA
sends a trigger to the MIO and prepares the sample hold signal
for ASICs. After receiving the acknowledgement signal from
the MIO, the ACB FPGA holds ASICs’ pulse height signals
with proper delays upon reception of triggers from ASICs, andcontrols analog-to-digital conversion on ASICs and data transfer from ASICs. The ACB FPGA has an internal memory for
one event data, and saves the data transferred from ASICs. The
MIO FPGA receives the signal from the ACB that the event
data has been prepared, and then MIO FPGA read out the data
by using the command and telemetry serial lines. In the MIO,
the data is formatted and stored in the SDRAM, which canbuffer the event data over periods of several seconds. Finally,

1000

500

0

0

100

200

300

400

500

600

700

Figure 8: 57 Co spectrum obtained with the CdTe sensor tray. This spectrum isobtained with one of the best pixels. The FWHM energy resolution is 1.7 keV
at 122 keV. The operating temperature is −10o C and the applied bias voltage is1000 V.

7

the buffered event data in the SDRAM are pulled by a data acquisition computer via the Space Wire network.
The dead-time associated with the event data acquisition is
not short (100 µs – several 100 µs). There are two functions formore effective event data acquisitions. One is ”Trigger pattern
judging”. The trigger patterns of 28 daisy chains are categorized into 16 groups. For each group, we can select whether
the trigger is sent to the MIO or not. By using this function,
we can ignore the charged particle events resulting in no relateddead-time. The other function is FastBGO cancel. FastBGO
is the BGO scintillator signal processed with a relatively shorttime interval (∼ 5 µs) and corresponds to the relatively large energy deposit in one of the BGO shields. With this function, we
can cancel the event data acquisition before AD conversion by
FastBGO signals, and can reduce the dead-time.
It is very important for astronomical observations to measure
the dead-time accurately. To estimate the dead-time, the SGD
Compton camera has a “pseudo trigger” function. This function has been successfully introduced in the HXD onboard theSuzaku satellite[36]. In the SGD Compton camera system, thepseudo triggers are generated by the ACB FPGAs internallyand are then processed in the same manner as usual triggers.
Since the pseudo events are discarded if the pseudo trigger isgenerated while a “real event” is inhibiting other triggers, thedead-time fraction can be estimated by counting a number ofpseudo events, output to the telemetry, and comparing withthe expected counts during the same exposure. Although periodic pseudo triggers were used in the HXD system, the random
pseudo trigger function is developed for the SGD system. A
random pseudo trigger function is based on the pseudorandom
numbers calculated in the FPGA and the expected count rate
can be set up to 422 Hz. Random pseudo trigger function enable us to estimate the dead-time due to the telemetry saturationin the back-end network, which can occur periodically.

Figure 11: Picture of the final prototype of the SGD Compton camera.

gamma rays of 356 keV and 662 keV, respectively. These spectral characteristics satisfy the requirement for the SGD Compton camera, better than 2%(FWHM).
By using the information gained from Compton scattering,
we can obtain a constraint of the incident angle and can select
event data from gamma rays that enter the camera directly fromthe radio isotope. The solid line spectra in Figure 12 are madefrom the event data after incident direction selection. This selection is performed by using the angular resolution measure(ARM) as ∆θ = |θenergy − θgeom |. Here, θenergy is the Comptonscattering angle calculated from the detected energy information, and θgeom is that determined from the detected position
information. The selection criteria for 133 Ba and 137 Cs are∆θ < 8.4◦ and ∆θ < 6.1◦ , respectively. The spectral components besides emission lines are compressed with the selection,
because these components are mainly generated by the gamma
rays scattered by the materials around the camera and the radio isotope. These results provide partial confirmation aboutbackground rejection capabilities of SGD and SGD Compton
camera.

5. Performance evaluation with the final prototype
5.1. Final prototype
Prior to the production of the flight model, we have fabricated the final prototype of the SGD Compton camera. Figure 11 shows a picture of the prototype. The prototype has all
the components installed in the same manner as the flight model
except for some space-qualified electronic parts.
5.2. Spectral Performance
In order to evaluate the spectral performance of the prototype, we performed gamma-ray measurement tests using various radio isotopes. In the measurement tests, the prototype
was operated in the temperature of about −10◦ C. The prototype
camera was cooled in a portable freezer and was irradiated withgamma rays by radio isotopes outside of the freezer.
133
Ba and 137 Cs gamma-ray spectra obtained with the prototype are shown in Figure 12. The dotted line spectra are madefrom the event data that can be used to reconstruct the incident
gamma-ray information using Compton kinematics. The obtained FWHM energy resolution is 6.3 keV and 10.5 keV for

The ∼ 100 hours long measurement tests have been performed. During the tests, we have confirmed that there is no significant variation in performance. Moreover, a thermal vacuum
test simulating the environment in space has been performed.
The final prototype Compton camera was put into a thermal
vacuum chamber with a vacuum pressure of ∼ 2 × 10−3 Pa anda temperature of −15◦ C – −25◦ C, and, the stable operation inthis environment was confirmed.
8

208 ASICs are controlled/readout in28 daisy chains by the CC FPGA.

Command/Telemetry: 3 line serial LVDS interface(10MHz)!
Ack, Trig: realtime LVDS lines
User FPGATrig

CC FPGAAck

SpW FPGA

Space Wire

Command
TelemetrySDRAM

MIO(SpW Digital I/O board)

ASIC Control Board(ACB)

CC2

CC3

CameraPower
Management
Unit

Signals fromBGO Shields

Figure 9: Readout system of the SGD Compton cameras.

5.3. Effective Area
The response functions for gamma-ray photons including theeffective area are built with a Monte Carlo simulator. We haveconstructed the Monte Carlo simulator and have tuned it byadopting the measurement results from the final prototype of
SGD. The details of the Monte Carlo simulators for early phase
prototypes were described in [22, 37, 35, 38], and the details of
the current SGD Monte Carlo simulator and event reconstruction algorithm will be described in a separate publication.
The effective area based on the Monte Carlo simulator isshown in Figure 13. The effective area is defined as the product of the detector geometrical area and the detection efficiency,
which is derived from the Monte Carlo simulation. The combined effective area for the six Compton cameras is estimatedto be larger than 20 cm2 at 100 keV, and this value satisfies
the instrument-level requirements for the SGD Compton camera. The efficiency reaches about 15% and 3% for 100 keV and
511 keV gamma rays, respectively.

Effective Area[cm2]25

6 Compton cameras
requirement

20

15

10

5

0

50 60 70

100

200

300

400 500

1000

Initial Energy[keV]

5.4. Intrinsic backgroundFigure 13: Plot of the effective area for six SGD Compton cameras onboard
ASTRO-H. The effective area is derived from the Monte Carlo simulator adopting the measurement results with the prototype.

Low background is the most important factor for sensitive
observations with the SGD. Therefore, we performed background measurements with the prototype in order to evaluate
the intrinsic background generated by radio isotopes containedin the SGD Compton camera. The measurements were held ina lead cave, which works as a passive shield. This lead cave
9

hold%&me%Si
 #hold%&me%CdTe

reset%wait%&me 




Trigger &Judge

AD conversion

Sample Hold

Data Readoutfrom ASIC to CC
FPGA



Data Transfer toDigital I/O board



"

!

Wait Trigger

Wait Event Data

!

Wait Trigger

!

Figure 10: Timing chart representing the readout sequence of the SGD Compton camera.

2200
2000

1200

133Ba

before rejection

1000

1800

1000

after rejection
by gamma-ray
direction800∆θ < 8.4º

1400
1200

10.5 keV(FWHM)@662 keV

800

counts

6.3 keV(FWHM)@356 keV

1600

counts137Cs

before rejection

600

after rejection
by gamma-ray
direction∆θ < 6.1º

400

600
400

200

200
0
0

0.1

0.2

0.3

0.4

0.5

0
0

0.6

Energy[MeV]

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Energy[MeV]

Figure 12: The gamma-ray spectra of 133 Ba (left) and 137 Cs (right) obtained with the final prototype of the SGD Compton camera. The event data applicable to theCompton calculation are used for the dotted line spectra. The FWHM energy resolutions at 356 keV and 662 keV are 6.3 keV and 10.5 keV, respectively. The solidlines show the spectra obtained after the rejection by gamma-ray incident direction, which are calculated by the information of Compton scattering. The selection
criteria for 133 Ba and 137 Cs are ∆θ < 8.4◦ and ∆θ < 6.1◦ , respectively (∆θ shows the angular resolution measure as ∆θ = |θenergy − θgeom |.)10

References

counts/sec/keV/
1CC
counts/sec/keV

consists of 5 cm thick lead with 1–5 cm of thick copper or brass
layer inside the lead shield. The prototype Compton camera
with the Pb cave was cooled to −10◦ C in a thermostatic chamber.
The obtained spectra are shown in Figure 14 together withthe background spectrum obtained without any shields. Although some emission lines from the thorium and uranium decay series can be seen, the background level per effective area is
as low as several × 10−5 counts/sec/keV/cm2 after the Compton
reconstruction. We have confirmed that there is no strong contaminating background and will perform further measurements
using BGO active shields.

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10-2with Pb cave

212Pb
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0.07

0.1

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0.5 0.6 0.7

1
Energy[MeV]
Total
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Figure 14: Background spectra obtained with the prototype SGD Compton
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6. Summary
We have finalized the design of the SGD Compton
camera.
The Compton camera has an overall size of
12 cm × 12 cm × 12 cm, consisting of 32 layers of Si pixel
sensors and 8 layers of CdTe pixel sensors surrounded by 2 layers of CdTe pixel sensors. The pixel pitch of the Si and CdTe
sensors is 3.2 mm, and the signals from 13312 pixels in total are processed by 208 readout ASICs with low noise and low
power consumption. In order to evaluate the performance of theCompton camera, we have fabricated a final prototype, which
has the same design as the flight model. The final prototype has
been evaluated in the laboratory and the spectral performance,
effective area and low background have been confirmed.
Acknowledgements
This work was supported by JST-SENTAN programand JSPS KAKENHI Grant Number 21684015, 24105007,
24244014, 24244021. The authors would like to thank Ms.
Kirsten Bonson and Dr. Poshak Gandhi for their critical readingof the manuscript.
11

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12