PASJ: Publ. Astron. Soc. Japan , 1–??,
c 2011. Astronomical Society of Japan.
°

Solid-state Slit Camera (SSC) on Board MAXI
Hiroshi Tomida,1 Hiroshi Tsunemi,2 Masashi Kimura,2 Hiroki Kitayama,2 Masaru Matsuoka,1 Shiro Ueno,1Kazuyoshi Kawasaki,1 Haruyoshi Katayama,3 Kazuhisa Miyaguchi,4 Kentaro Maeda,4 Arata Daikyuji,5 andNaoki Isobe,61

ISS Project Science Office, Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 2-1 Sengen,Tsukuba, Ibaraki, 305-8505 Japan
tomida.hiroshi@jaxa.jp
2
Department of Earth and Space Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043 Japan
3
Earth Observation Research Center, Japan Aerospace Exploration Agency, 2-1-1, Sengen, Tsukuba, Ibaraki 305-8505
4
Solid State Division, Hamamatsu Photonics K.K., 1126-1 Ichino, Higashi, Hamamatsu, Shizuoka 435-8558, Japan
5
Department of Applied Physics, Miyazaki University, 1-1 Kihanadai-nishi, Miyazaki, Miyazaki 889-2192, Japan
6
Department of Astronomy, Kyoto University, Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan(Received ; accepted )Abstract
Solid-state Slit Camera (SSC) is an X-ray camera onboard the MAXI mission of the International
Space Station. Two sets of SSC sensors view X-ray sky using charge-coupled devices (CCDs) in 0.5–12 keVband. The total area for the X-ray detection is about 200 cm2 which is the largest among the missions of
X-ray astronomy. The energy resolution at the CCD temperature of −70 ◦ C is 145 eV in full width at thehalf maximum (FWHM) at 5.9 keV, and the field of view is 1◦ .5 (FWHM) × 90◦ for each sensor. The
SSC could make a whole-sky image with the energy resolution good enough to resolve line emissions, andmonitor the whole-sky at the energy band of < 2 keV for the first time in these decades.
Key words: instrumentation: detectors, methods: data analysis, space vehicles: instruments, X-rays:
general1.

Introduction

The Monitor of All-sky X-ray Image (MAXI) mission (Matsuoka et al. 2009) is an all-sky monitor (ASM)
launched by Space Shuttle Endeavor on 2009 Jul 16. Afterthe successful installation onto the Japanese Experiment
Module - Exposed Facility (JEM-EF or Kibo EF) on theInternational Space Station (ISS) by astronauts using the
remote manipulator system, MAXI was activated on 2009
Aug 3. The initial checkout was also successfully completed, and MAXI is in the normal observation phase fromMar 2010. In the 1-year observation, MAXI has discovered many transient phenomena in X-ray band such as Xray bursts, Gamma-ray bursts, X-ray flashes, star flares,
outbursts of recurrent nova. MAXI now occupies the essential position in monitoring astronomy.
The SIS (Solid-state Imaging Spectrometer) onboard
the ASCA satellite was the first X-ray photon counting
CCD (charge-coupled device) camera in space (Burke etal. 1991). Its energy resolution was good enough to resolve characteristic K-shell X-rays from heavy elements
such as oxygen, silicon, and iron. The ASCA/SIS openeda new window of the X-ray astronomy. Since then, CCDs
have been standard focal plane detectors of X-ray missions, such as Chandra (Weisskopf et al. 2002), XMMNewton (Lumb et al. 2000), Swift (Gehrels et al. 2004),
and Suzaku (Mitsuda et al. 2007). For the X-ray monitoring with a large field of view, the SXC of HETE2 employed
X-ray CCDs for the first time (Villasenor et al. 2003).

MAXI has two types of X-ray camera, the Gas Slit
Camera (GSC) and the Solid-state Slit Camera (SSC).
The SSC is a CCD camera covering 0.5−12 keV energy
range. The energy band below 2 keV has not been covered by ASMs in these decades, which is achieved withMAXI/SSC. The moderate energy resolution enables us
to make all-sky maps with resolved line emissions. The
X-ray detection area of the SSC is unprecedentedly large
to obtain the enough photon statistics.
This paper describes the design and the results of theground performance test for the SSC. We overview the
SSC system in section 2. Details of the sensor unit andthe onboard data processing are described in section 3 and4, respectively. The results of the CCD screening and thecalibration on the ground are summarized in section 5. Inorbit performance of the SSC is described in Tsunemi etal. (2010).
2.

SSC system

Figure 1 shows the block diagram of the SSC and the
peripheral systems. The main components of the SSC are
two SSC Units (SSCUs). Figure 2 is a photograph and an
exploded view of SSCUs. The SSCU is a sensor part of
the SSC. One SSCU consists of CCD units, preamplifiers,
multiplexers, a collimator and slit unit, and a calibration
source. The ISS orbits around the earth like the moonin which the ISS always faces the same side towards theearth. So MAXI is placed such that it always sees the2

H.Tomida et al.

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X-raysDP interface

power controller

SSCU

commandtemperature

peltier
controller

system part

SSCE

GSC part

SSCE interface

CCD image /HK datavideo signal

drive signal

CCD driver /signal processor

peltier current

Multiplexer/ preampifier

video signal

drive signal

LHP

temperature

CCD / peltier devices

peltier current

radiatorcollimator andslit unit

SSC
data reduction

DP

SSC main components

analog line
digital line

PDAP

power line

Fig. 1. Block diagram of the SSC including peripheral systems and signal flow among the components.

sky direction. Since one of two SSCUs is placed so as tomonitor the zenithal sky, it is called SSC-Z. The otherunit is called SSC-H which sees +20◦ above the horizontal (forward moving) direction of the ISS/MAXI. The SSC
Electronics (SSCE) controls SSCUs. The SSCE generates
CCD drive signals, and digitizes the analog signals from
CCD. The SSCE also controls CCD temperature using
peltier devices embedded in each CCD unit. Heat frompeltier devices in SSCUs is transferred to fixed radiatorpanels by using a Loop Heat Pipe (LHP). The digitizeddata from the SSCE are transferred to the Data Processor(DP). The DP analyzes image data from the SSCE, extracts X-ray events, edits them into the telemetry data,and sends them to the ground via the ISS/JEM-EF. The
DP also relays commands from the ground to the SSCE.
The Power Distributor of Attached Payload (PDAP) supplies electric power to the SSCE in three channels. One
channel is used for the CCD operation, and other two arefor peltier currents in SSC-H and SSC-Z.
3.

SSC Units

The body of the SSCU is made of aluminum. To suppress the light reflection, black inorganic anodized aluminum alloys are used for the surface. The SSCU is244×112×124 mm, and 3.9 kg including a collimator andslit unit.
3.1.

CCD Unit

CCD units used in the MAXI/SSC are fabricated byHamamatsu Photonics K.K.1 using the CCD, FFTCCD4673. A photograph of a CCD unit is shown in Figure 3.
The CCD unit consists of a CCD wafer, peltier devices,
and a base plate made of copper-tungsten. The CCD
1

http://www.hamamatsu.com/

wafer is front-illuminated p-type CCD operated in full
frame transfer mode. The pixel number for the X-ray detection is 1024 × 1024, and the pixel size is 24 µm× 24 µm,
giving the detection area of about 25 mm × 25 mm. We
employ two phase clock for the vertical and horizontaltransfers. The weight is 55 g. The number of pin is 32 for
the CCD operation, and there are two other electric lines
in which the peltier current of 1.2 A flows at the maximum.
In order to obtain a large X-ray detection area, each SSCU
includes 16 CCD units that are aligned in 2×8 array as
shown in Figure 4. The total X-ray detection area of the
SSC (32 CCD units) is about 200 cm2 .
CCD is sensitive for optical and infrared lights that degrade the performances of the X-ray detection. In order toavoid it, aluminum of 0.2 µm thick is coated on the CCD
surface, which makes the structure of the SSCUs quite
simple. The aluminum coat is free from pinhole, which is
confirmed by illuminating the CCD with optical light.
Since the SSC does not have an X-ray mirror, the energy range of the SSC is determined by mainly CCD wafer.
The quantum efficiency for soft X-ray is limited by absorption at the gate structure (dead layer) and aluminum on
the front surface of a CCD wafer. That of hard X-ray islimited by thickness of the depletion layer. The gate structures are made of SiO2 and Si whose designed thickness
are 0.8 and 0.1 µm, respectively. The designed value of depletion layer thickness is about 70 µm. Then the energy
range determined by CCD is 0.5–15keV where the quantum efficiency is larger than 10%. However, the actual energy end of the SSC coverage is limited by dynamic rangeof analog-to-digital converters. Then the energy range of
the SSC including the electronics is 0.5–12 keV.

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Fig. 2. Left is a photograph of SSC units mounted on the aluminum stand before the installation to MAXI. Slit apertures areprotected by cover plates (non-flight item). Multi-layered insulator will cover the SSCUs in the final configuration. Right is an
exploded view of the SSC.

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

readout nodevertical transfer
horizontal transfer

Fig. 3. A bird’s-eye view of a CCD unit in the SSC.

3.2.

Collimator and Slit Unit (CSU)

Each SSCU has a collimator and slit unit (CSU).
Figure 5 is a schematic view of the CSU. Collimators of
the CSU restrict the field of view (FOV) to be a fan-beam.
The collimator sheet pitch determines the narrow FOV to
be 3◦ .0 in bottom-to-bottom (θ1 in Figure 5). A scan of
an object with constant flux forms a triangular response,
whose full width at the half maximum (FWHM) is 1◦ .5.
On the other hand, the combination of 8 CCD units andthe slit determine the long FOV to be 90◦ (θ3 in Figure 5).
One orbit scan corresponds to 90◦ ×360◦ . About 29% of
the entire sky are not covered by SSC in an orbit scan.
It takes about 70 days to cover the unobservable region,
depending on the precession period of the ISS orbit plane.
In addition, SSC cannot observe the region around theSun, which is too bright to block optical light with thealuminum coat on the CCD surface. Then it takes about
a half year to obtain the actual all-sky image.

exposed region of the calibration source

Fig. 4. The upper panel is a photograph of CCD array in
an SSCU. The gap width between adjacent CCDs is 0.4 mm.
The lower panel shows the position of the readout node, thedirection of horizontal and vertical transfer, and the region
exposed to the calibration source. The numbers in the lower
panel (0∼15) are CCDIDs.

The collimator consists of thin sheets made of phosphor
bronze with the thickness of 0.1 mm. There are 24 sheetswith 2.4 mm interval that are placed 5.0 mm above the
CCD units. The surface of the collimator sheet is chemically etched to suppress the X-ray reflection, and platedwith black chromium to suppress the optical light.
The slit of the SSC consists of two tungsten bars withsharp edges. The width between the two edges is 2.7 mm.
The angular resolution depends on the acquisition angle
which is the angle between the incident X-ray direction
and the normal of the slit plane. For larger acquisition angle, angular resolution becomes better, while the effective

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H.Tomida et al.

area becomes smaller by a cosine factor. At the acquisition angle=0◦ .0, the angular resolution (θ2 in Figure 5) is1◦ .5.
3.3.

Calibration Source

Each SSCU has an 55 Fe calibration source. The calibration source is mounted on the CSU so that small partof 2 CCDs (CCDID=0,8) in each SSCU is exposed to Mn
Kα(5.9 keV) and Kβ(6.5 keV) X-rays. The circle area in
Figure 4 shows the exposed region. The main purpose of
the calibration source is to monitor the long-term trendof the gain. The fluxes are high enough to determine thegain with the statistical error of 0.1 % in one-day accumulation.
3.4.

Thermal Design

The energy resolution and the detection limit at thelower energy region of the SSC are limited by various conditions, one of which is a variance of thermally excitedcharges (electrons) in CCD pixel during the exposure andthe charge transfer. The low temperature operation is essential to suppresses the thermal noise. As for the SSC,
one-stage peltier devices are used to cool CCDs. EachCCD unit is equipped with peltier devices as shown in
Figure 3. The CCD wafer in each unit is mechanicallysupported by 12 pairs of peltier devices (24 posts madeof bismuth telluride). There is no other mechanical support, which suppress the heat input through the conduction. Since eight CCD units are serially connected in thepeltier line, there are four electric current lines in SSC,
each of which can be controlled independently. For thecurrent control, we have two operation modes; the current constant mode and the temperature constant mode.
In the former mode, the constant electric current is provided into the peltier line. In the latter mode, the peltier
current is controlled so that the CCD temperature is kept
at the target temperature.
The electrical resistance of the peltier devices in each
CCD unit is about 1 Ω, and the peltier current in standard SSC operation is about 1 A. Hence, peltier devicesof 32 CCDs generate 32 W. In addition, 5 W is required
to operate analog circuits in SSCUs. Then the power consumption of the SSCUs is 37 W that is transferred throughthe LHP to radiator panels on MAXI. It is designed that
the LHP and radiator system could cool SSCUs to around−20◦ C . The hot sides of peltier devices are thermallyconnected to the SSCU body. The peltier devices givethe temperature difference of >40 ◦ C between the CCD
wafer and the SSCU body. Hence, CCDs are to be operated below −60◦ C . The body of SSC-H and SSC-Z
are, both, placed on an aluminum stand, then they arethermally combined to each other. The aluminum standis mounted on the GSC-H unit with a thermal insulator
made of polycarbonate (Figure 2).
The total consuming power of SSC is 90 W in typical;
26 W for CCD operation and 64 W for the CCD cooling. 21 W of 26 W for the CCD operation is consumed
in the SSCE, and 5 W is in SSCUs. 32 W of 64 W for
the CCD cooling is used in the SSCE, and the rest 32 W

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is for peltier devices in the SSCUs. Then SSCE including
DC/DC converters generates 53 W. The SSCE is maintained at about 20 ◦ C by the fluid loop system on the
ISS/JEM.
4.

Onboard Data Processing

4.1.

CCD drive

CCD clocking signals are generated in the SSCE. CCD
has two-dimensional pixel array, while the SSC requires
only one-dimensional position information since the SSC
conducts a scan observation. Hence, we operate the SSC
in parallel-sum mode, that is, charges in multiple pixels
are summed in serial register at the bottom of the imaging
region. The summed charges in the serial register are horizontally transferred to the readout node. The number ofsummed pixels in observation can be selected from 16, 32,
64 by commands. We call this number as the “binning”
parameter that is selected to be 64 for the standard observation. The larger binning gives the better time resolution, that provides better angular resolution in X-ray sky
map. The binning of 1, 2, 4, 8 is used only for diagnosis.
The horizontal pixel clock is 8 µs pixel−1 , and the verticalclock is 100 µs line−1 for binning=1. The video signal from
16 CCDs in a SSCU is processed serially by one readout
electronics. When one CCD is readout, other 15 CCDs
receive no clocking signal and are in the exposure state.
Since it takes 0.232 s to readout one CCD in binning=64,
the read cycle for 16 CCDs is 3.719 s. Clocking voltages
are common for 16 CCDs in each SSCU. During the exposure state, CCDs are left in the flip mode (Miyata et al.
2004), in which the bias voltage of CCDs is periodicallyinverted from +3 V to −9 V to reduce the dark current.
In the ISS orbit, CCDs are exposed to high flux ofcharged particles, which degrades the CCD performance.
We expect the increase of the CTI, dark current, and thenumber of hot pixels. The degradation of the MAXI/SSC
was estimated based on the experiments (Miyata et al.
2002 and Miyata et al. 2003). It is known that the charge
injection (CI) technique was a good way to restore theCTI (Tomida et al. 1997). Uchiyama et al (2009) demonstrated that the increased CTI due to charged particles
could be restored in the orbit operation of the Suzaku/XIS
with CI, and constructed a new method of correcting pulse
heights. We, then, added the CI function to the SSC, andcould select by command whether or not the CI function
is applied.
The CI function has two important parameters: theamount of charge and the injection period that is determined from the binning parameter. In the case of binning=N (N>1), charge is injected at every N rows. The
signal charges in N-1 rows that include no injected charge
are summed and readout in serial register. Whereas theinjected charges are transferred and discarded at the readout node. Since the extra horizontal transfer is requiredin CI, the readout cycle of 16 CCDs is 5.865 s, which is
1.577 times longer than that without CI.
The charge amount can be roughly controlled by com-

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SSC onboad ISS5θ3collimaotor sheets

2.7mm

slit

103mm

θ2

θ1

91.7mm

2.4mm

5.0mm
CCD

CCDs

CCDs

208mm

Fig. 5. Schematic view of a collimator and slit unit (CSU) in an SSCU. The left figure shows the positional relation between theslit and CCDs. The right figure, perpendicular to the left figure, shows the 24 collimator sheets and CCDs.

mands through the clocking voltage. The voltage can bechanged in 0.1 V step, which gives the difference of about
6000 electrons pixel−1 for the injected charge. Since thelarger amount of charge gives the better restoration of the
CTI (Tomida et al. 1997), we set the injected charge as
much as possible with the condition that there is no overflow during the transfer. The charge amount is estimatedto be about 105 electrons pixel−1 . The CTI before thelaunch is negligibly small, so we could not confirm the CI
effect in the ground test. The Suzaku/XIS has a capability
to control the amount of injected charge precisely, which isutilized to measure the CTI (Nakajima et al. 2008, Ozawa
et al. 2009). The SSC, however, cannot precisely control
the charge amount. We use the CI method only for the
CTI improvement.
4.2.

Digital Processing

The CCD images digitized in the SSCE are sent to the
DP. We adopted the data reduction procedure used in thetiming mode of Suzaku/XIS (Koyama et al. 2007). The
dark level, which is the time-averaged pixel data with neither incident X-rays nor charged particles, is determined
for individual pixels and up-dated at every readout time.
The DP searches for the charge deposit pattern of signals, which is called an “event”. The pixel data after the
dark-level subtraction are called “pulse height (PH)”. The
event search is done by referring to the PH. An event isrecognized when both of the following conditions are satisfied : (1) a pixel has a PH between the lower and theupper thresholds defined by command, (2) the PH is alocal maximum among adjacent three pixels in the row.
The MAXI telemetry data are transferred from the
DP to the ground through two physical networks in the
ISS/JEM-EF. One network is MIL-STD-1553B and theother is Ethernet. MIL-STD-1553B has higher reliability and more real-time connection between ISS/JEM-EF
and the ground station than Ethernet. However, the datatransfer rate of MIL-STD-1553B allocated for MAXI is
50 kbps (Ishikawa et al. 2009), while that of the Ethernet
is 600 kbps. Therefore, we designed that Ethernet data of

the SSC are used for the detailed analysis (spectroscopy
and diagnosis of sensors) while the MIL-STD-1553B data
are to search for the transient phenomena quickly.
Each event data of Ethernet telemetry includes X- andY-address (X-ray detected pixel position in CCD), and
the PHs of adjacent 5 pixels (local maximum, two trailing pixels, and two leading pixels). X-address is used todetermine the position of X-ray in the FOV, Y-address isused to correct PH with CTI. If the number of event is
too large, the event data are compressed automatically or
by command. In the compress mode, PHs of five pixels
are replaced with a summed PH according to the pixel
pattern called grade. There are four grades (G0, G1, G2,
and G3), which is the same to the timing mode of the
Suzaku/XIS. Figure 6 shows the four grade patterns. G0
is a pixel pattern that all signal charge (electrons) generated by an incident X-ray photon is confined in one pixel.
PH of the pixels adjacent to the local maximum is smaller
than the value called split threshold. G1 and G2 indicatethat either the leading pixel or the trailing pixel to the
local maximum has more charge than the split threshold.
G3 is that both of the leading pixel and the trailing pixel
to the local maximum have more than the split threshold. The replaced PH of the telemetry in the compressmode is the sum of local maximum and those over the split
threshold. In the compress mode, we can select events to
be downlinked according to the grade pattern. Events of
G3 are discarded in normal operation since G3 events arethought to be generated by charged particles.
For the MIL-STD-1553B telemetry, the DP compresses
every X-ray event into 16 bits. 14 bits are assigned for Xaddress and the summed PH, and 2 bits for the grade. The
grade definition is the same to that of Ethernet, but thegrade selection to be sent to the ground can be selected independently from the Ethernet data. The bit assignmentfor X-address and PH is determined by command. In thestandard operation, 6 bits are assigned for X-address and8 bits for PH, where the accuracy of the position determination (X-address) is 0.36 mm and that for the energy
is 60 eV. The number of SSC events for MIL-STD-1553B

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H.Tomida et al.
G0local maximum
G1
over the split threshold
G2
under the split threshold
G3transfer direction
Fig. 6. Four grade patterns (G0 through G3) used in the
SSC data processing.

telemetry is limited to be less than 255 events for every
16 lines.
5.

Screening Test and Pre-flight Calibration

5.1.

CCD screening

Before assembly of the SSCUs, we had many candidatesof CCD units to be installed. Then we conducted thescreening test to select the CCD units suitable for the SSC.
CCDs were driven by using the E-N A system (Miyata etal. 2001), and cooled down with a pulse tube cooler in a
vacuum chamber. The peltier devices were not activated.
The CCD units for the flight camera are selected from64 candidates provided by Hamamatsu Photonics K.K.
Miyata et al (2004) described the detailed setup of theexperiment and the initial results of 25 CCDs. We report
the final results here.
At first, we determined the proper voltages to drive the
CCDs. There are 16 voltages for the operation of the
MAXI-CCDs. The 16 CCDs in each SSCU have to be
operated with common voltages since we have only one
set of electronics for each SSCU. To search for the proper
voltages, the CTI and the CI ability are mainly evaluated.
From the experiment, we confirmed that 14 of 16 voltages
could be common for 2 SSCUs to obtain the low CTI andgood CI performance, while we could not operate with
common voltages for two which are used to inject charge
correctly. They are ISV (Input Source for the Verticaltransfer) and IGV2 (second Input Gate for Vertical transfer), which are important to control CI. The structure of
the charge injection gate is described in the technical noteof the manufacturer 2 . In the case of ISV, we found that
CCDs were separated into two groups; one is that appropriate voltage of ISV is 3.0 V while the other is 3.8 V. We,
then, decided that CCDs in the 3.8 V group were installed
to SSC-H, and the 3.0 V group were installed to SSC-Z.
The appropriate voltages of IG2V are scattered and cannot be properly grouped. So we designed the SSCE suchthat the IG2V voltage could be set separately for each2

http://sale.hamamatsu.com/assets/applications/
SSD/fft ccd kmpd9002e06.pdf[Vol. ,CCD.
After the operating voltages were fixed, we measured
various parameters as a function of temperature; the darkcurrent, the number of the hot pixel, the number of thedead column, the readout noise, and the energy resolution. The CTI was also examined again. Radio active
55
Fe sources were employed for the experiments. The binning of 1 and 64 are tested. We sorted the flight devices
based on the energy resolution, then we selected 16 CCD
units having the best energy resolution for SSC-H and
SSC-Z, respectively. The readout noise of the selected
CCDs ranges from 6 to 10 electrons in root mean square(RMS) that includes system noise. The readout noise ismeasured from the fluctuation of PH in horizontal overclocked region. The values of CTI are < 1.2×10−5 for all
the selected CCDs, where the CTI is defined as the ratioof the lost charge to the original one in a single verticaltransfer.
The hot pixels and the dead columns are troublesome
since the fluctuation of the dark current is so large depending on the temperature. We, then, check the numberof the hot pixels and the dead columns for the selected
CCDs. We defined the hot pixel that the PH with no
X-ray is significantly higher than that of the surrounding
pixels. The dead column is defined that the event number
is smaller than that of neighbor columns by 5 σ level when
X-ray from 55 Fe are uniformly illuminated. The numbers
are evaluated in binning=1. The total number of deadpixels and dead columns in 32 CCDs were 37 and 15, respectively. About half of the CCDs are free from deadpixels and dead columns. We found that the cosmetics ofthe MAXI-CCDs is excellent.
Figure 7 exhibits the dark current as a function of the
CCD temperature. From the thermal analysis, the CCD
temperature in the normal operation in the ISS is estimated to be about −60 ◦ C (Section 3.4). The dark current at −60 ◦ C is about 0.02 electron sec−1 pixel−1 inbinning=1. In the normal operation, the binning parameter is 64, and the readout cycle time is 5.9 s with CI-on.
The dark current is, then, about 8 electron pixel−1 cycletime−1 , which gives us the noise of about 3 electrons asa fluctuation in typical. This is smaller than the readout
noise, then we concluded that −60◦ C was low enough tooperate the CCDs at the beginning of the in-orbit operation.
Since the X-ray intensity of 55 Fe sources utilized in theabove performance tests was calibrated well, we could estimate the detection efficiency at 5.9 keV. Our experiments showed that the detection efficiency of the selected
CCD units is >89% at least, while the designed depletion
layer thickness of 70µm provides the detection efficiency of
about 91%. We, hence, confirmed the detection efficiency
is high enough to cover the energy range up to 12 keV.
In order to conduct the detailed spectrum analysis, thecalibration at the low energy range is also important. We
plan to measure the detection efficiency from 0.5 to 12 keVusing the Crab nebula in the in-orbit operation.

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Fig. 7. The dark current as a function of the CCD temperature at the screening experiment. The CCD unit employed
is assigned to CCDID=3 in SSC-H.

5.2.

Energy Calibration

The energy calibration of CCD performance with the
SSCE was done before the SSC was installed into the
MAXI flight structure. Fluorescent X-rays from nine materials and Mn-K X-rays from 55 Fe sources were used to
study the energy response of the SSC. The energy range
is 0.52 (oxygen) to 12.5 keV (selenium). Figure 8 (left)
shows a energy spectrum of various fluorescent X-rays.
The configuration of the calibration experiment was: (1)The SSCU to be calibrated is in the vacuum chamber
and the other SSCU and the SSCE are in the atmosphere(room temperature at 1.0 atom). (2) The CSU are removed. (3) CCDs are driven with the same operation
condition as that in the orbit. The binning is 64 and the
CI is on. (4) The SSCU in the vacuum chamber is cooled
to around −30◦ C and peltier devices are controlled so that
CCDs are at the constant temperature. The pulse height
distribution, energy resolution, and the linearity of theenergy scale are investigated in detail.
5.2.1. Pulse Height Distribution
Figure 8 (right) shows the spectrum of 55 Fe sources with
SSC-H/CCDID-3 at −70◦ C . The binning parameter is
64. The spectrum is created from G0 events only. We
can see that Mn Kα and Kβ lines are clearly resolved,
but the spectrum shape cannot be represented by two
Gaussians for Mn-Kα and Kβ. Then we fit the spectrumwith a model employed in the Suzaku/XIS (Koyama etal. 2007) that includes six components : a main Gaussian,a sub Gaussian, a triangle component, a silicon escapepeak, a silicon peak, and a constant component. We,
however, could not determine the parameters of the triangle component since this component is very small. In
this way, we found that the SSC data could be represented well with other five components. The increase be-

Fig. 8. Pulse heights distribution of the SSC-H/CCDID=3.
In the top figure, fluorescent X-rays from Al, Si, Cl, Ti, Mn,
Fe, Ni, Zn are irradiated. The vertical axis is in linear scale.
In the bottom figure, X-rays from 55 Fe (5.895 and 6.490 keV)
are irradiated. The vertical axis is in log scale, and the unit
is count sec−1 keV−1 .

low 1.0 keV in Figure 8 is not created by X-ray but bya background component. The count rate of the component is about 0.1 counts sec−1 keV−1 at 0.5 keV, while
the particle background in the orbit is 0.3 counts sec−1
keV−1 even at the high cut-off-rigidity region (Tsunemi
et al. 2010). Although the origin of this component is still
unclear, we concluded that there is no need to pay attention to the low energy component by applying the normal
background subtraction method.
5.2.2. Energy Resolution
We defined the energy resolution as the width of themain Gaussian peak. The energy resolution of Figure 8 is
136 eV at 5.9 keV in FWHM. Figure 9 shows the distribution of the energy resolution of 32 CCDs at −60 ◦ C and−70 ◦ C . The average of the energy resolution is 149 eV
for −60 ◦ C and 145 eV for −70 ◦ C . Since we apply thesame clocking pulses to all the CCDs in each SSCU, wetuned them so that the averaged performance (energy resolution) becomes the best.
The energy resolution of the CCDs depends on the energy of incident X-rays. Figure 10 shows the energy resolution of SSC-H/CCDID-3 at −70◦ C as a function of X-ray

8

H.Tomida et al.

[Vol. ,

number of CCD

8

6-70degC
4

2

number of CCD

8

6-60degC
4

2

135

140

145

150

155

energy resolution
Fig. 9. Distribution of the energy resolution (FWHM at
5.9 keV) for 32 CCDs in SSCUs at −70 ◦ C (upper) and −60 ◦ C(lower).
Fig. 11. Pulse height of SSC-H/CCDID=3 as a function of
X-ray energy. The lower panel shows the residuals between
the best fit model (linear function) and the data.

5.3.

Fig. 10. Energy resolution of SSC-H/CCDID=3 as a function of X-ray energy.

energy. G0 events are used for the spectrum analysis. The
readout noise of 32 CCDs ranges from 5 to 10 electrons
in RMS, which is the same level to that measured in the
CCD screening.
5.2.3. Energy Scale Linearity
Figure 11 shows the PH peak of SSC-H/CCDID-3 asa function of incident X-ray energies, where PH peak isthe center of the main Gaussian of G0 events. We fitted
the data with a linear function, and the best fit line isshown in Figure 11. We can see that the relation is well
reproduced by a linear function. No significant deviation
can be seen around the silicon K-edge. The residuals is
less than 1.0 % .

Performance of Peltier Devices

The performance of peltier devices was investigated in
the system pre-flight test, where the whole system of
MAXI including the SSCUs, the SSCE, and the DP wereassembled as flight configuration. MAXI structure wasinstalled in a vacuum chamber, and the structure wassurrounded by cooled black panels. Radiator and LHP
worked well. Figure 12 shows the distribution of temperature difference between CCD wafer and aluminum standdescribed in Figure 2. Peltier current was set to 1.0 A, andthe temperature of the aluminum stand was −21◦ C . The
averaged temperature difference was −45.4 ◦ C , which waslarge enough for CCDs to be kept at < −60◦ C when SSCU
bodies were at −20◦ C .
The annealing is a good method of recovering the CCD
performance degraded by charged particles (Janesick2001). Holland et al. (1990) reported that the high temperature above 100◦ C could significantly restore the degraded performance. Since we also designed that the function of LHP can be halted in the orbit by command,
SSCUs could warm up. The SSCU, however, must to be
kept at <+60 ◦ C . The temperature difference of +40 ◦ C
between CCD wafer and SSCU body is required for theannealing at 100◦ C . We, then, tried the reverse currentof peltier devices, and confirmed that the reverse currentof 0.22 A made the temperature difference of about 20 ◦ C. We, then, plan to supply the reverse current of 0.44 A
for the restoration, when the degradation of the CCD performance becomes significant after the several years of operation in the orbit.

No. ]

SSC onboad ISS
6.

15number of CCD

10

5

-50

-48

-46

-44

-42

-40

-38

-36

temperature difference
Fig. 12. Distribution of the temperature difference between
CCD wafer and SSCU body. The temperature of SSCU bodyis −21◦ C , and the peltier current is 1.0 A.

9Summary

We have prepared the SSC consisting of two SSCUs.
Each sensor contains the slit, collimators, and X-ray
CCDs. The slit and collimators of each SSCU define
the size of field of view to be 1◦ .5(FWHM)×90◦ . SSC
has 32 CCDs of FFTCCD-4673 fabricated by Hamamatsu
Photonics K.K. The SSC has total X-ray detection areaof about 200 cm2 with a sensitive X-ray energy range of
0.5−12keV. SSC operation has been optimized as a slit
camera onboard MAXI. CCDs are operated in parallelsum mode. The binning number of 64 is standard, but it
can be changed by commands. CCDs are readout one by
one in each SSCU. The charge injection is implemented tocompensate the performance degradation by charged particle. The working temperature of CCDs is < −60◦ C inorbit by using the peltier devices, LHP, and the radiators.
The energy resolution (FWHM) of the SSC for 5.9 keV
X-rays is 145 eV at the CCD temperature of −70◦ C and
149 eV at −60◦ C .
This work is partly supported by a Grant-in-Aid for
Scientific Research by the Ministry of Education, Culture,
Sports, Science and Technology (16002004 and 19047001).
M. K. is supported by JSPS Research Fellowship for
Young Scientists (22-1677).
References

Fig. 13. An image taken with SSC-H. Mn-K X-rays fromtwo radio-active source of 55 Fe are irradiated. Data are takenwith binning=64. The image is also binned in the horizontal
direction with binning=64 by software so that the ratio of
the horizontal axis to the vertical is the same to a real CCD
configuration.

5.4.

Imaging and Effective Area

The X-ray direction of the SSC events is determinedwith the combination of the CCD and the CSU. The imaging capability of the CCDs was simply checked with the55
Fe source in the energy calibration. Figure 13 is theimage showing two bright spots generated by two radio
active sources of 55 Fe. We find that there is no defectregion for the X-ray detection.
SSC has a large X-ray detection area, but the effective
area for the X-ray collection is limited by the CSU. We,
then, measured the effective area by using an X-ray beamline. In the beam line, the CSU was installed in SSCU
of engineering model, and the parallel X-ray beam wasirradiated into the CCD through the CSU. As the results,
we confirmed the silt width to be 2.70 ±0.05 mm, which
was consistent with the designed value of 2.7 mm. The
collimator sheets reduced the effective area to 89.5% and86.8% of the designed value for the SSC-H and the SSC-Z,
respectively. It was due to the bending and non-uniformalignment of the collimator sheets. These factors are taken
into account for the astronomical analysis.

Burke, B. E., Mountain, R. W., Harrison, D. C., Bautz, M.
W., Doty, J. P., Ricker, G. R., & Daniels, P. J., 1991,
IEEE Trans. ED, 38, 1069
Ishikawa, M., et al. 2009, Transactions of the Japan Society for
Aeronautical and Space Sceinces, Space Technology Japan
7, ists26, 43
Gehrels, N., et al. 2004, ApJ, 611, 1005
Janesick, J. R., 2001, Scientific charge-coupled devices (SPIE
Press, Bellingham, WA), 836
Koyama, K. et al. 2007, PASJ, 59, S23
Lumb, D. H., et al. 2000 Proc SPIE 4140, 2
Matsuoka, M., et al. 2009, PASJ, 61, 999
Mitsuda, K., et al. 2007, PASJ, 59, S1
Miyata, E., Natsukari, C., Akutsu, D., Kamazuka, T.,
Nomachi, M., Ozaki, M., 2001, Nucl. Instrum. and Method,459 157
Miyata, E., Kamiyama, D., Kouno, H., Nemesh, N.,
Tomida, H., Katayama, H., Tsunemi, H., Matsuoka, M.,
2004 Proc SPIE 5165, 366
Miyata, E., et al. 2002, Jpn. J. Appl. Phys. 41, 7542.
Miyata, E., et al. 2003, Jpn. J. Appl. Phys. 42, 4564.
Nakajima et al. 2008, PASJ, 60, S1
Ozawa et al. 2009, PASJ, 61, S1
Tomida, H., et al. 1997, PASJ, 49, 405.
Tsunemi, H., et al. 2010, PASJ, 62, 1371
Uchiyama et al. PASJ, 61, 9.
Villasenor, J. N., et al. 2003, GAMMA-RAY BURST
AND AFTERGLOW ASTRONOMY 2001: A Workshop
Celebrating the First Year of the HETE Mission. AIP
Conference Proceedings, 662, 33.
Weisskopf, M. C., Brinkman, B., Canizares, C., Garmire, G.,
Murray, S., & Van Speybroeck, L. P. 2002, PASP, 114, 1