PASJ: Publ. Astron. Soc. Japan , 1–??, 2011 August 25
c 2011. Astronomical Society of Japan.
°

Gas Slit Camera (GSC) onboard MAXI on ISS
Tatehiro Mihara, 1 Motoki Nakajima, 2 Mutsumi Sugizaki, 1 Motoko Serino, 1 Masaru Matsuoka, 1
Mitsuhiro Kohama, 3 Kazuyoshi Kawasaki, 3 Hiroshi Tomida, 3 Shiro Ueno, 3 Nobuyuki Kawai, 4
Jun Kataoka, 4 ∗ Mikio Morii, 4 Atsumasa Yoshida, 5 Kazutaka Yamaoka, 5 Satoshi Nakahira, 5
Hitoshi Negoro, 6 Naoki Isobe, 7 Makoto Yamauchi, 8 and Ikuya Sakurai, 1 †1
MAXI team, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198
School of Dentistry at Matsudo, Nihon University, 2-870-1, Sakaecho-nishi, Matsudo, Chiba 271-8587
3
ISS Science Project Office, ISAS, JAXA, 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505
4Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551
5Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8558
6
Department of Physics, Nihon University, 1-8-14, Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308
7
Department of Astronomy, Kyoto University, Oiwake-cho, Sakyo-ku, Kyoto 606-8502
8Department of Applied Physics, University of Miyazaki, Gakuen-Kibanadai-Nishi, Miyazaki, Miyazaki 889-2192(TM) tmihara@riken.jp
2(Received 2011 February 10; accepted 2011 March 22)

Abstract
The Gas Slit Camera (GSC) is an X-ray instrument on the MAXI (Monitor of All-sky X-ray Image)
mission on the International Space Station. It is designed to scan the entire sky every 92-minute orbital
period in the 2–30 keV band and to achieve the highest sensitivity among the X-ray all-sky monitors
ever flown so far. The GSC employs large-area position-sensitive proportional counters with the total
detector area of 5350 cm2 . The on-board data processor has functions to format telemetry data as wellas to control the high voltage of the proportional counters to protect them from the particle irradiation.
The paper describes the instruments, on-board data processing, telemetry data formats, and performance
specifications expected from the ground calibration tests.
Key words: instrumentation: detectors – X-rays: general
1.

Introduction

All-sky monitors (ASM) have been playing an important role in the research in X-ray astronomy. Since most
X-ray sources are highly variable, continuous monitoringof large sky fields and fast acknowledgments of flaring
events when they occur are very important for detailed
studies with follow-up observations.
The first dedicated ASM, Ariel-V (Holt et al. 1976)
observed a number of X-ray novae and transients with
two one-dimensional scanning pinhole cameras in 1974–
1980. The Ginga satellite operated in 1987–1991 carried
an ASM that consists of two proportional counters with
six slat collimators rotated by about 16◦ each (Tsunemi
et al. 1989). X-ray novae and transients detected by the
ASM were sometimes followed by observations with themain Large Area Counter (LAC) (Turner et al. 1989).
The gamma-ray burst monitors such as Vela-5B(Conner et al. 1969), CGRO/BATSE (Fishman et al.
1993), and Swift/BAT (Gehrels et al. 2004) also work as
ASMs. Since 1996, RXTE/ASM has monitored X-ray intensities of hundreds of X-ray sources as well as X-ray
novae and transients (Levine et al. 1996). It has three∗†

Present Address:
Research Institute for Science andEngineering, Waseda University, Shinjuku, Tokyo, 169-8555
Present Address: Research Center for Synchrotron Radiation,
Nagoya University, Nagoya, Aichi, 464-8603

cameras consisting of one-dimensional coded masks andproportional counters. These light curves are open to public through the internet1 and used by X-ray astronomers
world-wide. The long-term data are useful, but the detection limit is about 50 mCrab (5σ) per day. Therefore
the targets of RXTE/ASM are mainly on Galactic X-ray
sources.
Monitor of All-sky X-ray Image (MAXI) (Matsuoka et
al. 2009) is a mission onboard Japanese Experimental
Module - Exposed Facility (JEM-EF) on the International
Space Station (ISS). It was designed to achieve the better sensitivity than any ASM flown so far using large-area
proportional counters with a low background in order tomonitor long-term variability of Active Galactic Nuclei(AGN). MAXI carries two kinds of X-ray cameras : Gas
Slit Camera (GSC) and Solid-state Slit Camera (SSC:
Tsunemi et al. 2010; Tomida et al. 2011). Both GSC
and SSC employ slit and collimator optics. The payload
was launched by the space shuttle Endevour on July 16,
2009, then mounted on the port No. 1 on JEM-EF on July
24. After the electric power was turned-on on August 3,
MAXI started nominal observation since August 15, 2009.
This paper describes the design of the GSC instrument
and the expected performance from the ground tests. The
in-orbit performance is described in Sugizaki et al. (2011).
1

http://xte.mit.edu/ASM lc.html

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Camera Design

The MAXI/GSC employs the slit camera optics. The
slit camera has an advantage of being free from the source
contamination over the coded-aperture mask while it has
a disadvantage in the limited slit area. Thus, it is better suited for relatively faint and stable sources such as
AGNs. To achieve the high sensitivity, large-area proportional counters filled with Xe gas are used for the Xray detectors. The total detector area of 5350 cm2 using
twelve gas counters is optimized within the limit of the
payload size (0.8×1.2×1.8 m3 ).
Figure 1 illustrates the schematic drawing of the GSC
camera units on the MAXI payload. The entire GSC systems are composed of six identical units. Each unit consists of a slit and slat collimator and two proportionalcounters with one-dimensional position-sensitivity. The
two counters in each unit are controlled by two individual
data processors (GSC MDP-A/B in section 5) via independent signal paths for the redundancy.
The six camera units are assembled into two groups
whose field of views (FOVs) are pointed toward the tangential direction of the ISS motion along the earth horizon
and the zenith direction. They are named as horizon andzenith modules respectively. Each horizon/zenith module
covers a wide rectangular FOV of 160◦ ×1.5◦ (FWHM)
with an almost equal geometrical area of 10–20 cm2 combining three camera units. Figure 2 shows the crosssection view of each module. The areas of 10◦ from the
FOV edges to both the rotation poles are not covered because these directions are always obstructed by the ISS
structures.
The two FOVs of the horizon and the zenith modules
both scan the almost entire sky in the 92 minutes orbital
period. Any X-ray source is, therefore, observed twice in aorbit. The horizon module precedes the zenith module by21.5 minutes in the normal ISS attitude. The FOV of thehorizon unit is tilted up by 6 degree above the directionof the ISS motion to avoid the earth atmosphere as anallowance for the possible ISS attitude fluctuation. The
FOV of zenith units is set in the plane that includes thezenith and is perpendicular to the direction of motion.
Both FOVs have no earth occultation and use no moving
mechanics.
In the actual in-orbit operation, observation periods arelimited on a low particle-background area in order to protect the counters from the heavy particle irradiation. It reduces the observation duty cycle down to ∼40% (Sugizaki
et al. 2011). The two FOVs are capable of covering thewhole sky even if some of the counters have to be suspended from operation. The sampling period of 92 minutes is suited to the studies of long-term variability (> 1hour) such as AGNs.
3.

Detector Unit

3.1.

Gas counter

The GSC proportional counters employ resistive carbon fibers with a diameter of 10 µm for anode wires.

Fig. 1. Schematic drawing of GSC cameras on the MAXI
payload. The camera numbers are marked in hexadecimal as
0...9, A and B. Six cameras are assembled into two modules
whose field of views are aimed to the horizontal/zenithal directions in orbit. Two cameras in each unit are connectedto the two individual readout systems (MDP-A/B) independently for the redundancy. The arrow in each counter indicates the X direction (DETX) of carbon anodes. Two RBM(Radiation-Belt Monitor) detectors are mounted in the middle of the central units of both the horizon and the zenith
modules.

Higher resistive anodes are preferable for the better position resolution because the thermal noise on the readout
signal is inversely proportional to the anode resistance.
A carbon fiber is better than a nichrome wire in thispoint. Although the traditional carbon-coated quartz hasa larger resistance, it is mechanically weak, thus easy to
break by launch vibration. The carbon fiber anodes in
the GSC were developed at RIKEN and were successfully
used in the HETE/WXM (Shirasaki et al. 2003).
Figure 3 shows a picture of a single GSC proportionalcounter. All the flight counters were manufactured by
Metorex (now a part of Oxford Instruments) in Finland.
The front X-ray window has an area of 192 × 272 mm2 . It
is sealed with a 100-µm-thick beryllium foil. To support
the pressure on the beryllium foil in vacuum that amounts
740 kgW in the whole area, grid structures with a 17mm height are placed every 10.6-mm pitch parallel to theanode wires. The vertical grid is placed only at the centerto keep the open area as large as possible. The maximum
pressure of 1.66 atm is expected at the temperature of50◦ C. Every flight counter was tested to withstand 1.5
times higher than the design pressure, i.e. 2.5 atm. The
bodies of the gas counters were made of titanium, which
has sufficient strength and a heat expansion coefficient
close enough to that of beryllium. The beryllium foil isglued on the body with epoxy.
Figure 4 and 5 show the counter cross-section views.

No. ]MAXI GSC on ISS

3

Fig. 2. Cross-section view of each horizon/zenith module. Three camera units are packed in a rectangular space. The incident
angle φ is defined for each camera, while the source-acquisition angle β is defined for the MAXI payload. The thick outline on thecenter camera indicates the shields made of 0.1-mm lead and 0.1-mm tin sheets installed to block external X-ray.

The gas cell is divided by ground wires into six carbonanode cells for X-ray detection and ten tungsten-anodecells for particle veto. The carbon-anode layer andthe bottom veto-detector layer have depths of 25 mm
and 18 mm, respectively. These sizes are determinedso that the main X-ray detectors and the veto detectors have enough efficiencies for X-rays in the 2–30 keVband and minimum-ionization particles, respectively. The
minimum-ionization energy in the 18-mm thick Xe gas is
30 keV. The carbon anodes and veto anodes are not located at the center of each cell in the vertical direction.
The anode locations, the aspect ratios of these gas cells,
and the spacings of the ground wires are determined sothat the spatial non-uniformity of gas gain is small withineach cell.
The tension of the carbon-anode wire is set to 4 gW,
which is sufficiently smaller than the breakage limit, ∼ 25
gW. All anode and ground wires are fixed via a spring atright end to absorb the difference of the heat expansion
coefficient and keep the wires tight and straight. The
veto anode wires are made of gold-coated tungsten witha 18-µm diameter, which is pulled with a tension of 18
gW. We chose as thin wires as possible for veto anodes
to achieve similarly high gas gain as the carbon anodes
since the same high voltage (HV) is applied to both thecarbon anodes and the veto anodes. The gas-gain ratio ofcarbon anode to the veto anode is 20:1. The ground wires
are made of gold-coated tungsten with a 50 µm diameter.
The tension is about 50 gW.
We tested several kinds of gas mixture and chose a combination of Xe (99%) + CO2 (1%) with a pressure of 1.4
atm at 0◦ C. The amount of CO2 is decreased from WXM
PC (3%) (Shirasaki et al. 2003) in order to reduce the spatial gas-gain non-uniformity (section 8.3), and still keep
the sufficient quenching effect (Mihara et al. 2002).
The position resolution and the energy resolution are
incompatible requirements. The position resolution is primarily determined by the thermal noise on the resistive-

anode wire against the readout signal charge. The higher
gas gain is basically preferred for the better position resolution. However, the high voltage for the best position
resolution is usually in the limited proportionality region
rather than the proportionality range, where the spatial
gain non-uniformity is larger due to the space-charge effect, which also degrades the energy resolution. The operating high voltage (HV = 1650 V and the gas gain of
7,000) is chosen to achieve a sufficient position resolution
and still keep an adequate energy resolution.
For the in-orbit calibration, a weak radioactive isotopeof 55 Fe is installed in every counter, which illuminates asmall spot of about 1 mm in diameter at the right end of
the C2-anode cell (figure 4). Each isotope has a radiationof 30 kBq and its count rate by a GSC counter is about
0.2 c s−1 at the launch time.
3.2.

Front-end electronics

Each GSC counter has six position-sensitive anodes
readout at the both ends (left and right), and two signals for connected veto anodes. A total 14 preamplifiers
are used for the 14 analog signals.
The front-end electronics boards are built in the backside of the proportional counter. It is designed to shieldexternal noises and also to strengthen the counter frame.
The electronics boards include the high-voltage power
supply, HK (House-Keeping) electronics and their connectors. The HK circuit monitors temperatures at eight
points in the camera (HV box, preamplifiers, gas cell,etc.) and HV values. The HV-power supply with a lowpower consumption was manufactured by Meisei Electric
Co. Ltd. Figure 6 illustrates the configuration of the HVconnections in each counter. One HV-power-supply unit
works on one GSC counter. In total, twelve HV-powersupplies are used. The coupling capacitors connecting
the preamplifiers and the anode wires are of 2200 pF,
which have sufficiently lower impedance (< 1 kΩ for 2µs
shaping out signal of the preamplifier) than that of the

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Fig. 3. Proportional counter used in GSC and drawing of the window from the top. See figure 4 and 5 for the cross-section views.
.

Fig. 4. Cross-section view of GSC proportional counter on the plane perpendicular to the anode wires. All anode/ground-wirelocations are shown. The names for anode, C, B, ST and SB, denote Carbon, Bottom, Side Top and Side Bottom, respectively.
The wires of B0 to B5 are connected together in the counter and read out as a single bottom-veto (BV) signal. The same for
ST0, ST1, SB0 and SB1, as a side-veto (SV) signal. All numbers represent the scale in units of mm. Electric potential in the
counter calculated by Garfield is shown in the below. The “Garfield” is a program to simulate gas counters developed in CERN
(http://garfield.web.cern.ch/garfield/).

No. ]MAXI GSC on ISS

5Fig. 5. Cross-section view of GSC proportional counter along the wire direction. Numbers are in units of mm.

carbon anodes (33 kΩ) and still do not accumulate toomuch charge for the preamplifiers.
Since the wire hermetic rods come out from both theanode ends, two front-end circuit boards are placed separately at the side ends. We selected a hybrid-IC, Amptek
A225, for the preamplifier, which is made with a space-use
quality and has a low-power consumption.
The preamplifier gains represented by the ratio of theoutput pulse height to the input charge (Volt/Coulomb)
should be the same between the left and the right of eachcarbon anode. We thus measured gains of all A225 chips
under the temperature condition of −20 to 60◦ C, thenselected pairs whose gains show a similar temperature dependence. The feedback capacities of veto anodes are left
as they are 1 pF at the default, while those of carbon
anodes are modified to 4 pF by adding an external 3 pF
capacitor in order to obtain closer pulse heights for bothsignals from carbon anodes and veto anodes. The ratio
becomes 5:1.
3.3.

Slit and Slat Collimator

Figure 7 illustrates the schematic view of the GSC slit
and slat collimator. The parallel tungsten rods with 3.7mm separation are placed at the top of the slat collimator
constituting the opening slit of the camera. The collimator slats with a 118.4-mm height, placed at 3.1-mm
pitch, constitute the FOV of 1.5◦ in FWHM, which arealigned vertically to the slit rods. The slats are made ofphosphor bronze with 0.1 mm thick. The thickness is determined so that X-rays up to 30 keV are stopped andthe sheets can be flattened when pulled by the tension
springs. These surfaces are chemical-etched and roughened to avoid reflection. 64 slats are installed at the frontof each counter. The “roofs” of the collimator module (asshown in a thick outline in Figure 2) and the both ends
of slat collimators are covered by 0.1-mm lead and 0.1mm tin sheets to shield Cosmic X-ray Background (CXB).
Twice thicker shield made of 0.3-mm lead and 0.1-mm tin
sheets are placed to block the direct path from the space
to the beryllium window.
The collimator transmission was tested in the JAXA
beam facility. The setup of ground calibration is describedin Morii et al. (2006). The transmissions are confirmed
to be within ±5 % of the design. A model of the trans-mission function was constructed based on the measured
data. Figure 7 right panel shows a comparison between
the measured data and the model. The in-orbit alignment
calibration was carried out using celestial X-ray sources,
Sco X-1 and the Crab nebula, and is described in Morii
et al. (2011).
4.

Radiation Belt Monitor (RBM)

The GSC is equipped with RBMs to measure the particle flux which causes background for GSC and SSC, andto protect the instruments from heavy irradiation. Two
RBM detectors are mounted at the central part of boththe horizon and the zenith modules (figure 1).
Each RBM employs a silicon PIN diode (5×5 mm2 , 200µm depth) made by Hamamatsu Photonics for its detector. The detector window is covered by a 50-µm-thick aluminum whose particle transmission is equivalent to thatof the 100-µm-thick beryllium used for the window of themain proportional counters. The FOV is about 80◦ × 50◦ .
The opening angle of 80◦ matches the FOV of the central
camera unit. The lower-level discriminator (LD) can be
set at 4 levels, equivalent to the deposited energies of 30,
50, 200, and 1000 keV. The LD level of 30 keV is sensitive
to both electron (E > 200 keV) and proton (E > 2 MeV).
The LD 50 keV is also sensitive for both electrons andprotons in the same energies, but efficiency is about 50 %for minimum-ionization particles. The LD 200 keV and1000 keV are to detect only protons whose energies below
100 MeV and 30 MeV, respectively. The energy losses ofelectrons and high energy (= minimum-ionization) protons are small and below 200 keV.
The readout electronics are designed to be fast enoughto count up to 105 c s−1 , and do not saturate up to 107 c
s−1 . The count rate limit is high enough so that the RBM
can tolerate the in-orbit rate that is expected to be ∼ 104
c s−1 in the ISS low-altitude orbit.
The two RBMs, one on horizontal module and the otheron the zenithal module, are also considered as the redundancy.2
2

The results obtained in orbit show a slight difference between
the two RBMs for the anisotropy of trapped particles, which isreported in another paper for in-orbit performance (Sugizaki etal. 2011).

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Mihara et al.

[Vol. ,

HV
6800pF

6800pF
6800pF

6800pF
A225

A225

2200pF

carbon anodes

2200pF

C0 L

C0 R

C1 L

C1 R

C2 L

C2 R

C3 L

C3 R

C4 L

C4 R

C5 L

2200pF

2200pF

C5 R

tungsten anodes

SV

470pF
470pF

BV

All resistors are 10MΩ.

Fig. 6. Schematic view of high-voltage connections to anode wires on the front-end circuit board.

S

a
ca n

le
ng

3.7
slit
3.1
collimator
64 sheets

118.4

160

22.5
Be window
anode direction
Fig. 7. (Left): Geometry of GSC slit and slat collimator. All numbers represent the scales in units of mm. Collimator sheets are
0.1-mm thick and placed by 3.1-mm pitch. (Right): Effective area of slit and slat collimator for φ = 5◦ , 35◦ . Points represent datameasured with X-rays at Cu-K line (8.0 keV) in a ground test and solid lines represent the response models constructed for the data.

5.

On-board Data Processing System

5.1.

Mission Data Processor (MDP)The GSC electronics system, named GSC Mission Data
Processor (MDP, hereafter), processes a total of 168channel analog signals from 12 position-sensitive proportional counters. It also have a function to manipulates thereadout event data and to receive and process commands
from the central MAXI data processor (DP). Figure 8 illustrates the block diagram of the MDP data processing.
The system consists of two identical modules (MDP-A
and MDP-B) for the redundancy. Each MDP consists ofsix analog boards and one digital board to process signalsof six counters, a half of the entire twelve counters. Eachanalog board embodies circuits to process signals from one
counter.
We developed a hybrid IC for the GSC analog signal

processing, which is named HIC-MAXI. One HIC-MAXI
package contains a gain amplifier, a shaping amplifier, an
LD and a peak hold circuit for a single analog channel.
Each analog board carries 14 packages. The gain and LD
are adjustable by command to 512 levels (×1.0 − 36.0,
nominal 5.0) and 4 levels (typically equivalent to the Xray energies of 0.25, 0.5, 1.0, 2.0 keV, nominal 1.0 keV),
respectively. The design of the shaping amplifier is optimized for the counter position resolution. It consists of
one-order high-pass filter with a cutoff at 145 kHz andtwo-order low-pass filter at 2.1 MHz. The peak-hold capacitor of 1000 pF is chosen to sharpen the LD-cut threshold. Since a MOS FET, SD215, used to discharge thepeak-hold capacitor was found to be weak for heavy irradiation, it were replaced by the radiation-tolerant version
fabricated by Calogics. Furthermore, the HIC package

No. ]MAXI GSC on ISS

was shielded with a 2-mm-thick iron. It then accepts thetotal dose requirement of 2.8 krad (for 2 years, including the safety factor of 3). All other electric parts havea specification of the radiation hardness tolerant for 10
krad.
Each analog board embodies a 14-bit ADC, AD 7899,
whose conversion time is 2µs. While 8-bit precision is sufficient for the energy resolution of the proportional counter,a 14-bit ADC is required for obtaining a positional value
with a precision of ∼ 0.1 mm precision along the 333 mm
anode wire even for small pulse-height events.
The position along the anode wire is encoded into theratio of two PHAs (Pulse Height Amplitudes) read out
at the both ends. Fourteen channels in each counter areprocessed by a single ADC. Each signal is peak-held, thenis fed to the ADC one by one through a multiplexer.
The digital processing was programmed on the fusetype FPGAs operated at 10-MHz clock frequency, fabricated by ACTEL. They are used on both the analog anddigital boards.
5.2.

Event-data processing

The front-end data processing is started when a LD-hit
signal is issued at any of 14 signal channels. It triggers thepeak-holds of all the 14 analog outputs in each counter.
The coincidence of the 14 LD-hit signals is judged within
the 10-µs window. The readout process is activated if thecoincidence satisfies the required combination. The hitpatterns of the readout events can be changed by commanding. In the nominal configuration, only those eventsin which the LD-hit signal is found only from a single
carbon anode are processed.
The GSC has two event-readout modes: one is the observation mode and the other is diagnostic mode. In thenormal observation mode, two PHA data from a carbon
anode with LD hits are A/D converted, then processed
into the telemetry data. In the diagnostic mode, all the
14 signals are processed. The A/D conversion takes 20µs for two PHA data in the observation mode and 140 µs
for 14 PHA data in the diagnostic mode. The dead time
for the analog processing becomes about 30 µs and 150µs in observation and diagnostic modes, respectively. The
diagnostic mode are used for the detector health check.
The LD-hit count in each anode is always monitored
by LD-hit counters. These count data are stored in the
HK data in the telemetry every second. The counter dataalso includes information of multiple hits (the numbers ofcarbon-carbon or veto-carbon coincidences). The hardware upper-level discriminator is not equipped on the circuit board. Instead, the rate of events whose pulse heightat either end is saturated in A/D conversion are monitored.
5.3.

Event Timing

Each MDP-A/B system is equipped with a 20-kHz freerun clock counter (FRC) with a 16-bit length to measureeach event time with a 50-µs precision. When the eventreadout process is activated by a LD-hit signal, the number of the FRC is latched and included into the event

7

data.
Each event data in the telemetry has a time tag of DP
clock counter (DPTC) that is incremented every second.
The timing of the DPTC and FRCs on GSC MDP-A/B
systems can be calibrated on the ground using data ofan on-board GPS which are also downloaded through thetelemetry.
6.

Telemetry Data

6.1.

DP Event Data Reduction

The MAXI Data Processor (DP) employs a multi-CPU
computer system using MIPS R3081. The DP controls
all payload instruments including GSC by real-time andscheduled commands. It also packages the telemetry data
to be downlinked to the ground.
The DP receives GSC event data from the GSC MDPA/B via RS-422 connection. Each MDP can transfer 2,000
event data every second at the maximum. The data rate
corresponds to 285 events s−1 in the diagnostic mode.
The event data are usually reduced in units of the 64bit format which consists of an camera number (4 bits) an
anode ID number (4 bits), two pulse height data (14 bits×2) at the both anode ends (Left and Right), value of the
20-kHz clock counter at event timing (16 bits), hit pattern of the 14 gas-counter signals (both ends of six carbon
anodes and two sets of veto-counter signals) (8 bits), anddata-readout mode (observation/diagnostic) (4 bits). The
telemetry formatting occurs every second. In the observation mode, each single-event data consists of the 64-bitformat units. In the diagnostic mode, each event consistsof seven 64-bit units; six for the six carbon-anodes andone for the two veto anodes.
Since the telemetry bandwidth between the ISS and theground is limited, there are options to reduce the data
mass. The DP has a function to filter event data withthe software LD and UD, which can be set separately forthe two downlink paths (MIL-1553B and ethernet; see thenext subsection), in each counter. The telemetry event
data also have three reduction modes: 64-bit, 32-bit and16-bit to be able to fit the limited data rate. In the 64
bit mode, the primary 64-bit-format data are transferredwithout any reduction. In the 32 bit mode, each 64-bit
data are reduced to 32 bits which consists of camera number (3 bits), anode number (3 bits), two PHAs (13 bits× 2). In the 16 bit mode, camera number (3 bits), preregistered encoded numbers for position (eg. 8 bits) andenergy (eg. 5 bits) are returned. The bit assignments in
16 bit mode are commandable. Table 1 summarizes the
bit assignments of the three telemetry-reduction modes. 3
6.2.

Data Downlink Flows and Bandwidth

MAXI has two data downlink paths to the ground, the
MIL-STD-1553B (MIL-1553B hereafter) and the ethernet,
3

Prior to the MAXI launch the nominal data-reduction mode for
MIL-1553B downlink was set to the 32-bit mode, which continued for the initial operation phase. It was changed to the64-bit mode on October 30, 2009. The data through the ethernet downlink always takes the 64 bit mode.

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Mihara et al.
A225
14 outputs[Vol. ,

MDP : A System
Analog Board 0DP
Digitalboard

HIC-MAXI

BV

Shape
amp

Peak hold

HIC-MAXI

C0R

HIC-MAXI

BV

...

BV

BV Matrix

Analog Board 1

GSC5

Analog Board 5

GSC6

MDP : B System

M
P
X

G
S
C
F
R
C

14 bitADC

D 1digit=1s
P
T
C 1553B

1digit
=50µs

RS422

16/32/64
bit data

I
S
Ethernet S

command

norm/
diag
event

64bit data

...

...

GSC1

R

Vout C0L

...

...

C0R

Level 0-3

Hold

x1-36
adjustable

GSC0

HIT C0L
C0L
C0R O C0R Anti
...

Gain
amp

C0L

LD

...

Fast
Shape amp

...

D
P
F
R
C

GSCB

1digit=1µs

GPS 1 pps
Fig. 8. Block diagram of event-data readout flows on the GSC Mission Data Processor (MDP).
Table 1. Bit assignments of event data in three telemetry-reduction mode

Reduction Mode
64-bit32-bit16-bit
∗

GSCID
4
3
3

ANODE
4
3
0

PHA
14×2
13×2
8∗ (position) + 5∗ (energy)

GSC-FRC
16
0
0

HIT-PTRN
8
0
0

MODE
4
0
0

: The bit assignment between position and energy data are commandable within the total 13 bits.

whose maximum data-transfer rates are 51 kbps and 600kbps, respectively. While the ethernet has an advantage
in the speed, the MIL-1553B is superior in the connection
reliability. GSC usually uses 50 kbps of the ethernet and
SSC 200 kbps. SSC mainly uses the ethernet for the event
data.
GSC can use about 70% of the MIL-1553B bandwidth
assigned to MAXI. If the event rate exceeds the limit, theoverflow data are stored in the DP buffer. The buffereddata are transferred later when the data rate becomes
lower than the limit. The buffer for each of two readout
systems (A/B) has a volume of 500 kByte and can store
62,000 events in the 64-bit format. The buffer data should
keep the data for 50 seconds typically, which corresponds
to the single-scan duration of a point source. Since theexpected event rate for the Crab nebula is 31 events s−1
for each system, bright sources with up to 40 Crab fluxes
can be observed without any data loss in the 64bit mode.
Note that the brightest X-ray source, Sco X-1, is about
16 Crab in 2–20 keV.
7.

Detector protection

The proportional counter must be operated carefully inorbit not to suffer from any breakdown. GSC has various
protection functions in hardware and software to avoid
undesirably high count rates. The MDP equips a hardware protection function using RBM counter. The DP has

four kinds of protections in software using information of
RBM counts, location of ISS, location of the sun, and the
LD count rates.
7.1.

On-board RBM Protection

A protection function using the RBM count rates is
implemented in the MDP circuit. When either count rateof the two RBMs exceeds the threshold, HVs of all thetwelve counters are reduced to 0 V. The threshold of each
RBM can be changed individually by commanding. The
HV suppression is released when the count becomes lessthan the half of the threshold. These judgments are madeevery 10 seconds. 4
7.2.

DP Software Protection

The DP software can perform more flexible and finer
controls of the counter HVs using information collectedfrom all the subsystems. The DP is always monitoring theparameters relevant to the counter protections describedin the following subsections and reduces the HVs of therelated gas counters to 0 V when any of the parameters
reaches the threshold. The HV suppression is released
automatically when they are recognized to return to thenormal condition. The DP slowly raises the HV not to
4

The thresholds for both horizon and zenith RBMs are set at 100
counts in 10 seconds in the flight operation . The MDP-RBM
flag is high during about 11 % of the whole time, and in 3 % of
the low-lattitude time.

No. ]MAXI GSC on ISS

stress the counters. The nominal rate is an incrementwith a 330-V step every 10 second, which can be changed
by commanding for each counter.
7.2.1. RBM-Count Protection
The DP also offers a protection with RBM, but in amore sophisticated manner than that by the hardware as
described in section 7.1. The accumulation of the RBM
counts and its judgement are carried out every second in
the sliding window. The threshold level and the accumulation period (nominal 20 seconds) are commandable.
The criteria for the HV reduction and the recovery are thesame as the hardware one. When either of the integrated
counts of the two RBMs exceeds their threshold, the HVsof all the 12 counters are reduced to 0 V. The HV suppression is released when both the integrated counts becomeless than the halves of their thresholds.
7.2.2. Radiation-Zone Protection
The Radiation-Zone (RZ) protection is designed to
avoid particle irradiation predicted from the geographical location such as SAA. DP keeps three RZ-map datawith 2.5◦ ×2.5◦ mesh for three different altitudes. WhenISS enters a radiation area defined by the RZ maps, theHVs of all the 12 counters are reduced to 0 V. The ISS location are obtained from the ISS ancillary data, which arebroadcasted on the ISS common network of MIL-1553B.
7.2.3. Sun Protection
Since GSC scans the almost entire sky every orbital
cycle, the direction to the sun is also covered by some GSC
camera units. The Sun protection is designed to avoid adirect sun illumination on the detector, where unfavorable
high count rates area expected. The direction to the sun
is calculated using the ISS ancillary data. If the sun iscloser to the FOVs of some camera units than the limit,
the HVs of these cameras are reduced to 0 V. The limit of
the separation between the sun direction and the camera
FOV can be changed by commands.5
7.2.4. LD-Count Protection
Every anode signal of each camera has a LD-hit counter(subsection 5.1). The LD-count protection is designed to
avoid damages by any unexpected high count rates suchas breakdown. Two kinds of counting methods are implemented to be sensitive to various break modes on thedetector.
1. Integrated High Flag
If a breakdown occurs, the count-rate suddenly increases to a high level. Every second, DP calculates
the integrated number of LD-hit counts of all anodes
in a camera during a given period. If it exceeds thethreshold, the high voltage of the counter is reduced
to 0 V. The integration is done by the sliding window.
2. Continuous High Flag
If a moderate spark occurs, the count-rate becomesslightly high continuously. In such a case the continuous high flag works. If a total LD-hit count of all
5

The limit of the sun angle has been reduced from 30◦ at theinitial operation to 4◦ in the nominal operation to optimize thesky coverage.

9

anodes stays over the threshold for a given period,
the high voltage of the camera is reduced to 0 V.
These thresholds can be set for each counter individually by commanding. In these LD-count protection, theHV suppression is released after a 5-minute wait time.6
8.

Detector performance

8.1.

Energy Band and Efficiency

The efficiency curve is shown in figure 5 in Matsuoka
et al. (2009). It stays higher than 15% in the GSC nominal energy range of 2–30 keV. In the lower energy end,
the efficiency drops due to the absorption in the 100-µmthick beryllium window and varies from 2 to 5% at 1.5
keV according to the X-ray incident angle of 0◦ –40◦ . In
the higher energy range, the efficiency jumps up at theXe K-edge energy of 34.6 keV by a factor of 4.8. X-ray
events whose energy is higher than the K-edge lose Kline energy due to the escape of fluorescence lines. The
escape probability of Xe K-line is 66%, thus it has a significant effect on the counter response. We measured theescape fraction and the complex energy response aroundthe Xe K-edge energy using GSC flight-spare counter at
KEK photon factory7 .
8.2.

Energy response

The GSC gas counter is operated at the nominal HV of1650V, which is chosen to achieve a good position resolution. It is in the limited proportionality region where theenergy-PHA relation is rather distorted. Thus, the detailcalibration is necessary to construct the energy-PHA response matrix. The tests for the energy response calibration were carried out for all the flight counters using X-ray
beams including fluorescence lines from various target elements placed in the X-ray generator. Figure 9 shows theobtained typical energy spectra.
Figure 10 shows the energy-PHA relation derived fromthe calibration-test data. It has a discontinuity at 4.7
keV for Xe L-edge. The relation is well reproduced by
two expedient functions for those below and above theL-edge. The deviations between the data and the model
functions are within 0.6% over the 2–23 keV band.
Figure 11 shows the obtained energy resolution against
the X-ray energy and that of the theoretical limit. The
measured energy resolution is 16 % (FWHM) at 6 keV. It
is close to the theoretical limit in the 3–5 keV band. The
difference is larger at low and high energies. At low energies, a part of the electron cloud is lost by the beryllium
window. At high energies, the spatial gain non-uniformity
becomes effective according to the large mean free path.

6

7

On April 18, 2010, the ISS auxiliary data was stopped due tothe ISS computer trouble, and the sun went into the FOV of
a GSC camera. The integrated high flag became on and the
HV of the camera was shutdown safely. At that time the sunreached one-fourth to the center of FOV. The flux of the sun
was measured 1400 Crab in 2-4 keV.
http://www.kek.jp/intra-e/research/PF.html

10

Mihara et al.

[Vol. ,

30

800

Zn:8.6 keV
Se:11 keVCountsCr:4.5 keV

400

Energy Resolution (%)

Fe:6.4 keV

600

Y:15 keV

Ti:2.6 keV

200

20

Th

eo
r

eti

ca

ll

im
it

10

0

0

500

1000
1500
PHA channel

2000

2

2500

Fig. 9. Energy spectra for K-shell emission lines from Ti (2.4
keV), Cr (4.5 keV), Fe (6.4 kV), Zn (8.6 keV), Se (11 keV),
and Y (15 keV). The data were taken for pencil X-ray beams
irradiated at an off-wire position of counter #0 anode C0 in
the nominal anode HV of 1650 V.

510Energy (keV)

20

30

Fig. 11. Energy resolution (FWHM) against X-ray energy. Circles with 1-σ error bars represent measured datawith K-shell emission lines from the elements same as
those in figure 10. The theoretical limit is estimated as
2.35{W (F + b)/E}1/2 , where the mean ionization energy
W = 21.5 eV, the Fano-factor F = 0.2, and dispersion of gas
gain in electron avalanche b = 0.5 are assumed (Knoll 1989).

3000

PHA channel

Ag
Y

2000

8.3.

Mo

Se
Zn
FeCu
Cr
V
Ti
Ca
Cl

1000

Ratio

0
1.01
1
0.99
0

5

10
15
Energy (keV)

20

25

Fig. 10. Linearity of energy-PHA (Pulse height amplitude)
relation. Circles represent measured data for K-shell emission lines from the elements labeled. The data were takenin the same condition with those of figure 9. Bottom panel
shows residuals from a model expressed by expedient function, a − b exp(cE). The discontinuity at the Xe L-edge (4.70
keV) is implemented in the model.

Gain Spatial Non-uniformity

The gain non-uniformity along the anode wire is measured with a 2 mm pitch using Cu Kα (8.0 keV) and Mo
Kα (17.5 keV) X-ray beams with a diameter of 0.2 mm.
The non-uniformity among each anode cell is ∼ 20 % typically. The data is used in the PHA-PI conversion carriedout on the ground data reduction and also to build theenergy response matrix in the spectral analysis.
The non-uniformity on the vertical plane to the anodewire was measured with slant X-ray beams taking an advantage of the position sensitivity. The method is described in Mihara et al. (2002). Figure 12 shows the obtained gain non-uniformity around the anode wire. The
gain is the highest in the annulus of an about 5-mm radiusfrom the anode. It is lower than the average at the central region around the anode wire and the outer region.
Compared to the distribution of the electric potential infigure 4, the gain has a positive correlation with the fieldstrength in the outer region.
8.4.

Position Response

Since the GSC employs slit-camera optics, the detector
position response is important for determining the direction of incident X-ray photons with a good accuracy. The
data of the position response is taken with a 1-mm pitch
along the carbon anode using X-ray beams of Cu K-line
in the ground calibration tests.
The position is encoded in the ratio of the pulse heightreadouts at the both anode ends. We here define the two
PHA as the left PHAL and the right PHAR and introduce
a position-measure parameter, PM ,PHAR − PHAL
(1)
PHAR + PHAL
The position-measure parameter has an approximately
PM =No. ]MAXI GSC on ISS

0

3000

14

2900

00

2700

-10

2600

-15

2500

ADC channel

DETZ (mm)

2800

2400

-20
2300

-90

-85

-80
-75
DETY (mm)

-70

-65

Position Resolution (mm)

-5

-95

100
0
0

5

−100

DETX (mm)

15

50

V
16

50

10

−5

0
PM

10

V

1

0.5

Fig. 13. Relation between the position at which X-ray is absorbed along the anode wire (DETX) and the position-measure parameter derived from the PHA ratio (PM : see text).
This is for the GSC camera 6, anode 0 measured at the nominal HV of 1650 V using pencil X-ray beam of Cu Kα line .
The data (square) are fitted with a linear function (solid line)in the top panel, and the residuals are shown in the bottom
panel.

linear relation with the event location where the X-ray
is absorbed along the anode wire (figure 13). However,
the relation cannot be exactly linear in the real experiment. Any analytic function cannot successfully reproduce the data obtained in the calibration tests with therequired position accuracy. We thus decided to use thetable-lookup method in the response function based on
the ground calibration tests.

100
Normalized PHA (channel)

1000

Fig. 14. Relation between position resolution (FWHM) andsignal pulse height normalized by read-out amplifier gain.
Squares, filled circle, and triangle points are data taken at
the HV of 1400 V, 1550 V, and 1650 V, respectively. Fourdata points at each HV are the resolutions for Ti-Kα +Kβ(4.5 keV), Fe-Kα (6.4 keV), Cu-Kα (8.0 keV) and Mo-Kα(17.5 keV). The dashed line represents an inversely proportional relation.

8.5.

−0.5

V

2200

Fig. 12. Cross section of gas-gain non-uniformity in a single
carbon-anode cell (C0 cell) on the plane perpendicular to theanode wire. The DETY direction is parallel to the detector
window and the DETZ represent the depth from the the beryllium-window plane. The anode position is (−80, −14.75).
The data are obtained by the slant-beam method using X-ray
beams of Mo K-line (17.5 keV). Two black stripes are unmeasurable region located under the counter-support structure.
The gray scale represents the the pulse heights of the output
signals for the Mo K-line.

residuals [mm]11

Position Resolution and High Voltage

The position resolution should be better than the slit
opening width of 3.7mm to achieve the optimal angularresolution. It is mostly determined by the ratio of thethermal noise to signal charges, which are the photoelectrons multiplied by the avalanche process in the counter.
Figure 14 shows the relation between the measured position resolution and the signal pulse height normalized by
the readout-amplifier gain, obtained from data taken for
X-ray beams of Ti-Kα +Kβ (4.5 keV), Fe-Kα (6.4 keV),
Cu-Kα (8.0 keV) and Mo-Kα (17.5 keV) in three anodevoltages of 1400, 1550 and 1650 V. It is clear that theposition resolution is inversely proportional to the pulse
height. It is because the equivalent noise charge is constant in each anode and the position resolution depends
only on the number of the multiplied signal charges. The
rightmost point, measured for Mo-Kα at 1650 V, is slightly
higher than the line because the mean free path of thephotoelectron (∼ 0.5 mm for 17.5 keV) is not negligible(Tabata et al. 1972).
We determined the nominal anode voltage of 1650 V
based on the results.
8.6.

Anti-coincidence

Figure 15 shows energy spectra and count-rate distributions of the room background events. Those with andwithout anti-coincidence cut are shown. The energy bandof 2–30 keV is used for the plot. The data are obtained
as a part of the final ground tests at Kennedy Space center in 2008 November. The background rate at KSC wasabout 90 % of that in Tsukuba Space Center. The anticoincidence cut reduces the event rate down to about 1/9.
The remaining room background rate is 1.0 × 10−4 c s−1
cm−2 keV−1 in 2–30 keV. The room background mainly
consists of gamma rays such as 40 K. The spectrum shape12

Mihara et al.

tector position resolution of 1–4 mm varying with X-ray
energy at the nominal HV of 1650 V, and the backgroundrejection efficiency by the anti-coincidence technique.

no−anti.
(10−4 counts s−1cm−2 keV−1)

10

anti.
1

2

3

5
10
Energy (keV)

20

30

no−anti.

(10−3 counts s−1 mm−1)

10Authors thank Metorex (Oxford Instrument), Meisei
Electric Co. Ltd., and NEC Co. Ltd., in particular
to Matti Kaipiainen for the development and fabrication of the GSC proportional counters, Isao Tanaka,Nobuyuki Kidachi, Koji Taguchi for the development
and production of the collimators and the electronics,
and Takahiko Tanaka, Hiroshi Mondo for the development of the DP. This research was partially supported by
the Ministry of Education, Culture, Sports, Science andTechnology (MEXT), Grant-in-Aid for Science Research
19047001, 20244015, 21340043, 21740140, 22740120 and
Global- COE from MEXT ”Nanoscience and Quantum
Physics” and ”The Next Generation of Physics, Spun from
Universality and Emergence”.
References

anti.
1

−100

0
DETX (mm)100

Fig. 15. Energy spectra (top) and position distributions
along the anode (DETX) (bottom) of the room backgroundtaken in the final ground test at Kennedy Space center in2008 November. The measurements for counter #0 anode C0
taken with and without anti-coincidence cut, are shown . The
anode-cell length in the DETX direction is −136 to +136 mm
and the width is 32 mm.

does not change with and without the anti-coincidence.
9.

[Vol. ,

Summary

The GSC is one of the X-ray instruments of the MAXI
mission on the ISS. It is designed to scan the entire sky
every 92-minute orbital period in the 2–30 keV band andto achieve the highest sensitivity among the all-sky Xray monitors built so far. It employs large-area positionsensitive proportional counters with a total detector areaof 5350 cm2 and the slit and slat collimators defining
two rectangular FOVs of 1.5◦ (FWHM)×160◦ to optimize
the monitoring of relatively faint sources including AGNs.
The on-board data processor (DP) has functions to format telemetry data for the two download paths different
in the bandwidth and to protect the gas counters fromparticle irradiation in orbit. The ground calibration tests
confirmed the effective area in the energy band of 2–30
keV, the energy resolution of 16% (FWHM) at 6 keV,
the energy response in the limited proportionality region,
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