ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 541 (2005) 350–356
www.elsevier.com/locate/nima

Development of the X-ray CCD camera for the MAXI mission
Haruyoshi Katayamaa,, Hiroshi Tomidaa, Masaru Matsuokaa,
Hiroshi Tsunemib, Emi Miyatab, Daisuke Kamiyamab, Norbert NemesbaJapan Aerospace Exploration Agency, Tsukuba Space Center, 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan
Department of Earth & Space Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka,
Osaka 560-0043, Japanb

Available online 2 March 2005

Abstract
Monitor of All-sky X-ray Image (MAXI) is an astrophysical payload for the Japanese Experiment Module (Kibo) on
the International Space Station (ISS). MAXI is designed for full-sky monitoring in the 0.5–30 keV X-ray band.
The Solid-state Slit Camera (SSC) is the X-ray CCD Camera for the MAXI mission. We have developed anengineering model (EM) of the SSC. Combined with the EM electronics, we achieved low readout noise ð 5e rmsÞand good energy resolution (150 eV at 5.9 keV).
We also examine the energy scale and the energy resolution as a function of energy using our calibration system.
From these results, we conclude that the performance of the SSC-EM almost fulﬁlls our requirements and that the ﬁnal
performance goals will be obtained in the ﬂight model.
r 2005 Elsevier B.V. All rights reserved.
PACS: 07.85.m; 95.55.n; 95.55.Aq; 95.55.Ka
Keywords: X-ray Astronomy; X-ray CCD; International Space Station; MAXI

1. Introduction
The Monitor of All-sky X-ray Image (MAXI)[1–4] payload has been selected as one of the ﬁrstgeneration payloads that will be attached to theJapanese Experiment Module (Kibo)-Exposed
Facility (JEM-EF), a component of the InternaCorresponding author.

E-mail address: Katayama.Haruyoshi@jaxa.jp(D. Kamiyama).

tional Space Station (ISS). MAXI is currently
planned for launch by an H-IIA Transfer Vehicle(HTV) in 2008 and is expected to have a mission
life of at least two years in orbit. This payload isdesigned for full sky monitoring in the 0.5–30 keV
X-ray band.
MAXI consists of two X-ray detector systems:
the gas slit camera (GSC) [5,4] and the Solid-state
Slit Camera (SSC) [6]. These are combinations of anarrow slit, a collimator, and a one-dimensional
position-sensitive detector. The energy ranges

0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2005.01.075

ARTICLE IN PRESS
H. Katayama et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 350–356

of the GSC and the SSC are 2–30 keV and
0.5–10 keV, respectively. In this paper, we describe
the performance of the engineering model (EM) of
the MAXI/SSC.

2. SSC overview
The SSC is developed by JAXA, Osaka University, and Mesei Electric Co., Ltd. The SSC uses
an X-ray CCD to achieve a high quantum
efﬁciency and a good energy resolution for soft
X-rays. The total imaging area of the SSC is
200 cm2 and it consists of two camera units,
viewing both the horizontal (SSC-H) and zenith(SSC-Z) directions. Fig. 1 shows a schematic viewFig. 1. Schematic view of two camera units of the SSC, where
the colored region represents 16 CCD chips.

351

of the two camera units of the SSC. The ﬁeld ofviews (FOVs) of the two cameras are set almost
perpendicular to each other to achieve complete
sky coverage in spite of breaks in the observations
when the ISS passes through radiation belts such as
the South Atlantic Anomaly. The FOV of SSC-H
is tilted upward by 20 to avoid earth occultationand light reﬂection from the earth in daytime. The
FOV of each camera is restricted to 1:5  90 witha combination of a slat collimator and a slit. Fig. 2(left) shows the slat collimator of the SSC viewed
from the bottom side (CCD side). The collimatorslat is made up of phosphorus bronze (Cu: 93.9%,
Sn: 6.0%, and P: 0.1%), and its surface is etchedand coated black for light shielding.
Each of the SSC cameras consists of 16 CCD
chips. Fig. 2 (right) shows the SSC Engineering
Model (EM) without its collimator. The CCD chip
(FFTCCD–4673) fabricated by Hamamatsu
Photonics K.K. has 1024  1024 pixels with apixel size of 24 mm and covers 25 mm square. The
thickness of the depletion layer is 70 mm: On the(surface of the CCD, an aluminum layer  2000 A
thick has been deposited for light shielding. Thispermits omission of an additional light shield, likean optical blocking ﬁlter, and its associated launchsafety mechanism. The CCD is damaged by soft
protons in orbit, resulting in a decrease of thecharge transfer efﬁciency. The SSC CCD contains
a charge injection register for the purpose ofrecovery from this radiation damage. The effect of
the radiation damage in the two years of MAXI
mission is examined in Ref. [7].

Fig. 2. Left: Slat collimator of the SSC viewed from the bottom side (CCD side). Right: A view of SSC-EM, with the collimator not
installed. On the bottom plate are mounted 7 out of 16 CCDs. The remaining 9 CCDs were installed in Oct. 2004. A white square
represents the size of 1 CCD chip.

ARTICLE IN PRESS
352

H. Katayama et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 350–356

Table 1
Basic properties of the MAXI/SSC
CCD chip (FFTCCD-4673)
Pixel size
Number of pixels
Depletion layer thickness
Operation temperature
Charge transfer rate
CameraEnergy range
Energy resolution
Number of CCDs
Field of view
Effective imaging area

24 mm
1024  1024
70 mm
o60 1C
125 kHz/62.5 kHza
0.5–10 keV
p150 eV at 5.9 keV
16  2
1:5  90  2
100 cm2  2a
The nominal charge transfer rate is 125 kHz, but it can be
reduced to 62.5 kHz if required.

To achieve the required energy resolution(150 eV FWHM at 5.9 keV), the CCDs are cooled
below 60 1C using a combination of a radiatorand a one-stage Peltier cooler. The Peltiercooler, attached directly to each CCD, provides atemperature difference of 40 1C between a hotside and a cold side for 1 W of power. The hot sideof the Peltier cooler is cooled below 20 1C by theloop heat pipe radiation system (LHPRS) [8].
We summarize the basic properties of the
MAXI/SSC in Table 1.

3. Calibration system of SSC
We evaluated the performance of the SSC-EM
with a calibration system constructed at JAXA.
This MAXI calibration system is described inRef. [9]. The SSC calibration system, which uses
ﬂuorescent X-rays, is shown in Fig. 3. The
compact X-ray generator (KEVEX) can be operated from 5 to 10 kV and at a maximum current of
1 mA. The X-ray generator is connected to asecondary target chamber. We employ Al, Cl, Ti,
V, Fe, Ni, and Zn as secondary targets. These
secondary X-rays are then incident on the SSC
installed in a large chamber. The SSC is cooled
down using a cryogenic cooler. Each of the twochambers is evacuated to less than  105 Torr byturbo molecular and rotary pumps. The SSC iscooled to 20 1C in the SSC chamber, while theFig. 3. Calibration system of the SSC. X-rays from the X-ray
generator are incident on a secondary target. Fluorescent Xrays from this secondary target are emitted to the SSC, installed
in a larger chamber as indicated.

SSC Electronics is operated at about 20 1C, thenominal temperature in orbit.

4. Performance of the SSC engineering model
We used 7 CCD chips for the evaluation of the
EM (Fig. 2 (right)). These are almost the same
type as the ﬂight CCD chips, though the thicknesses of depletion layer of these CCDs are thinnerthan those of the ﬂight CCDs.
We ﬁrst examined the basic performance of the
SSC-EM using a 55Fe source. Fig. 4 shows the
spectrum of 55Fe taken with the SSC-EM. The
readout noise is determined from a region ofoversampled pixels. The readout noise and energy
resolution at 5.9 keV for the 7 CCDs are listed inTable 2. The readout noise is 526 e for all CCDs.
These fulﬁll our requirement. The energy resolution of 3 CCDs (CCDID ¼ 4;5,6) are worse thanour requirement. This is mainly due to variation of
the charge transfer efﬁciency of these CCDs.
However, the ﬂight CCD chips, which are notinstalled in the EM but planned to be installed in a
ﬂight model, show energy resolutions below
150 eV [10].

ARTICLE IN PRESS
H. Katayama et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 350–356

1

2

100

200

Energy [keV]
3
4

5

6

counts

103

102

10

300

400

500

600

700

PH[ch]
Fig. 4. Pulse-height spectrum for a 55Fe source taken with the
SSC-EM. Single pixel events (ASCA grade of 0) are selected forthis distribution, as described in the text.

Table 2
Basic performance of the 7 CCDs of the SSC-EM. The
operating temperature is 60 1C
CCDID

Readout noise ðe rmsÞ

FWHM (eV)a

0
1
2
3
4
5
6

5:4
5:7
5:7
5:1
5:3
5:2
5:7

143
145
156
145
194
163
160

a

0:4
0:2
0:2
0:2
0:2
0:2
0:2

4
3
3
5
5
3
3

Full-width at half-maximum at 5.9 keV.

4.1. Energy response (spectrum)
Fig. 5 shows the spectra of ﬂuorescent X-rays
from Al, Cl, Ti, V, Fe, Ni, and Zn taken with the
SSC-EM. These are spectra of single pixel eventswith a split event threshold of 4 times the readout
noise. Except for the Al case, the Ka and Kb
peaks are clearly resolved. In addition to themain peak, we can see a low-energy tail, which
increases towards lower energies. This low-energy

353

tail is considered to originate from photons
absorbed inside the gate insulator of the CCD,
as discussed in an ACIS calibration report [11](Chandra/ACIS) and Ref. [12] (ASTRO-E/XIS).
The MAXI/SSC low-energy tail shows similar
trends with these CCDs, though its level is slightly
higher (about 2 times). One possible explanation isthe difference in the CCD gate thicknesses. Since
the level of the low-energy tail depends on the gate
thickness, a thicker gate leads to a larger lowenergy tail. A quantum efﬁciency measurement,
which will be measured in a next calibration
experiment, will reveal it. We should note thatthe low-energy tail is not due to scattering X-rays
from the vacuum chamber because the level of thelow-energy tail of these spectra is about the sameas that of the 55Fe spectrum (see Fig. 4).
We ﬁtted these spectra with a double Gaussian
added on a constant term to derive the energy scale
and the energy resolution of the SSC-EM.
4.2. Energy scale
We show the energy scale of the SSC-EM in Fig.
6 (left). We ﬁtted the energy scale with a linear
function. The lower panel in Fig. 6 shows theresiduals from the linear energy scale ﬁt. Except
for the Al line, the residuals are within 0.1%. Since
our calibration goal is to determine the energy
scale with 0.1% accuracy, this result almost fulﬁlls
our requirement.
4.3. Energy resolution
Fig. 6 (right) shows the energy resolution of the
SSC-EM as a function of energy. We deﬁne theenergy resolution as the full width at half
maximum (FWHM) of the main peak, and ﬁt it
with the following formula:rﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
F E
FWHM ðeVÞ ¼ w  8 log 2 
þ s20
w
(1)
where w (we ﬁxed 3:65 eV=e ) is the mean
ionization energy per one electron–hole pair, F is
the Fano factor, and s0 is a measure of the system
noise including the readout noise.

ARTICLE IN PRESS
H. Katayama et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 350–356

354

Al
0.6

0.8

Energy [keV]
1 1.2 1.4

Cl
1.6

1.8

10

countscounts

103

102

102

10

10

60 80 100 120 140 160 180 200
PH[ch]V

Energy [keV]
3 3.25 3.5 3.75 4 4.25 4.5 4.75 5 5.25

Ti

Energy [keV]
1.2 1 .4 1.6 1.8 2 2.2 2.4 2 .6 2.8

103

103
counts4

10

125 150 175 200 225 250 275 300 325 350
PH[ch]

350

Energy [keV]NiFe

Energy [keV]
3.5 3.75 4 4.25 4.5 4.75 5 5.25 5.5

102

5.5 5.75 6 6.25 6.5 6.75 7 7.25

400

450
500
PH[ch]

550

600

Energy [keV]

6.5 6.75 7 7.25 7.5 7.75 8 8.25 8.5

103

102

102

102

countscountscounts

103

10

10
10
400

450

500
550
PH[ch]

600

650

625 650 675 700 725 750 775 800 825 850
PH[ch]

750

800

850
900
PH[ch]

950

Energy [keV]

Zn
7.5

8

8.5

9

9.5

counts

103

102

10

850

900

950 1000 1050 1100
PH[ch]

Fig. 5. Fluorescent X-rays spectra from Al, Cl, Ti, V, Fe, Ni, and Zn as measured by the SSC-EM. Except for the Al lines, the Ka andKb peaks are clearly resolved. A solid line represents the best ﬁt to a double Gaussian added on a constant term. Also included in the
Fe and Ni ﬁts are Cr and Fe lines, respectively, which are contamination from the secondary targets. The Kb peak of these lines issuperposed on the tail of the Ka peak of the target lines.

One of the unique scientiﬁc goals of the SSC is
X-ray emission-line mapping of our Galaxy. To
archive this goal, the energy resolution in thesoft X-ray band is important. From thebest-ﬁt parameters ðF ¼ 0:167 0:003 and s0 ¼
7:85 0:19Þ; we estimate the energy resolution at

ARTICLE IN PRESS
H. Katayama et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 350–356
190

1000

800

CCD-9 gain:8.55

180

CCD-9

PH = 0.117 * E - 0.053

170

F=0.167,

σ0=7.853

2000

4000

Energy resolution [eV]900

PH[ch]

700
600
500
400

160
150
140
130
120

300

110
100

χ

residual [%]

200
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
1000

355

2000

3000

4000

5000

6000

7000

8000

9000

6
4
2
0
-2
-4
-6
1000

3000

Energy [eV]

5000

6000

7000

8000

9000

Energy [eV]Fig. 6. Left: Energy scale of the SSC-EM. The data for Ka lines are plotted. The pulse-height is the center of the main peak ﬁtted witha double Gaussian. The lower panel shows the residuals, which are within 0.1% except for the Al line. Right: Energy resolution of the
SSC-EM, with ﬁt goodness plotted in the lower panel.

the oxygen line energy ( 0:53 keV) to be 80 eV
at 0.5 keV, which is higher than the expectedenergy resolution of 60 eV for a readout noise of
5 e : This is represented by the large s0 ; whichsuggests that unknown noise sources are dominant, or there is an uncertainty in the energy
response below the Al line. Although the O VIIð 0:56 keVÞ and O VIII ð 0:65 keVÞ lines can be
resolved even with 80 eV energy resolution at
0.5 keV, in order to investigate this issue, we are
now planning an experiment using a soft X-ray
generator, which can provide X-ray irradiation
below 2 keV.

calibration system. From these results, we conclude that the performance of the SSC-EM almost
fulﬁlls our requirements and that the ﬁnal performance goals will be obtained in the ﬂight model.

5. Conclusion

References

We have developed an engineering model of the
MAXI/SSC. Combined with the EM electronics,
we achieved low readout noise ð 5 e rmsÞ andgood energy resolution (150 eV at 5.9 keV). We
also examined the energy scale and the energy
resolution as a function of energy using our[1] T. Mihara, et al., Adv. Space Res. 25 (2000) 897.
[2] S. Ueno, et al., ASP Conf. Ser. 251: New Century of X-ray
Astronomy (2001) 598.
[3] M. Matsuoka, AAS/High Energy Astrophys. Div. 7(2003).
[4] N. Isobe, et al., Proc. SPIE 5165 (2004) 354.
[5] T. Mihara, et al., Proc. SPIE 4497 (2002) 173.

Acknowledgements
This work is partly supported by the Grant-inAid for Scientiﬁc Research by the Ministry of
Education, Culture, Sports, Science and Technology of Japan (15684002).

ARTICLE IN PRESS
356

H. Katayama et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 350–356[6] E. Miyata, C. Natsukari, D. Akutsu, M. Ohtani,
H. Tsunemi, M. Matsuoka, N. Kawai, Proc. SPIE 4012
(2000) 186.
[7] E. Miyata, et al., Proc. SPIE 4851 (2003) 1080.
[8] M. Muto, H. Nagai, M. Murakami, S. Ueno,
M. Matsuoka, Proceedings of the 32nd International
Conference on Environment System (2002) 2505.
[9] K. Torii, et al., Proc. SPIE 3765 (1999) 636.

[10] E. Miyata, D. Kamiyama, H. Kouno, N. Nemesh,
H. Tomida, H. Katayama, H. Tsunemi, M. Matsuoka,
Proc. SPIE 5165 (2004) 366.
[11] M.W. Bautz, J.A. Nousek, G.P. Garmire, Tech. Rep.,
MIT 1999.
[12] K. Imanishi, H. Awaki, T.G. Tsuru, K. Hamaguchi,
H. Murakami, M. Nishiuchi, K. Koyama, Proc. SPIE 4012
(2000) 137.