The High-Resolution X-ray Microcalorimeter Spectrometer
System for the SXS on ASTRO-H
Kazuhisa Mitsudaa , Richard L. Kelleyi , Kevin R. Boycei , Gregory V. Brownl , Elisa
Costantinim , Michael J. DiPirroi , Yuichiro Ezoec , Ryuichi Fujimotod , Keith C. Gendreaui ,
Jan-Willem den Herderm , Akio Hoshinod , Yoshitaka Ishisakic , Caroline A. Kilbournei , Shunji
Kitamotog , Dan McCammonj , Masahide Murakamie , Hiroshi Murakamig , Mina Ogawaa ,
Takaya Ohashic , Atsushi Okamotob , Stéphane Paltanin , Martin Pohln F. Scott Porteri , Yoichi
Satob , Keisuke Shinozakib , Peter J. Shirroni , Gary A. Sneidermani , Hiroyuki Sugitab , Andrew
Szymkowiakk , Yoh Takeia , Toru Tamagawah , Makoto Tashirof , Yukikatsu Teradaf , Masahiro
Tsujimotoa , Cor de Vriesm , Hiroya Yamaguchih , Noriko Y. Yamasakia
a Institute
of Space and Astronautical Science, JAXA, Sagamihara, Japan; b Aerospace
Research and Development Directorate, JAXA, Tsukuba, Japan; c Tokyo Metropolitan
University, Hachioji, Japan; d Kanazwa University, Kanazawa, Japan; e Tsukuba University,
Tsukuba Japan; f Saitama University, Saitama, Japan; g Rikkyo University, Tokyo, Japan; h
Riken, Wako, Japan; i NASA/Goddard, Greenbelt, MD, USA; j Univ. of Wisconsin, Madison,
WI, USA; k Yale Univ. New Haven, CT, USA; l Lawrence Livermore National Laboratory,
Livermore, CA, USA; m SRON Netherlands Institute for Space Research, Utrecht, Netherlands;
n Geneva University, Geneva, Switzerland
ABSTRACT
We present the science and an overview of the Soft X-ray Spectrometer onboard the ASTRO-H mission with
emphasis on the detector system. The SXS consists of X-ray focusing mirrors and a microcalorimeter array and
is developed by international collaboration lead by JAXA and NASA with European participation. The detector
is a 6 × 6 format microcalorimeter array operated at a cryogenic temperature of 50 mK and covers a 3! × 3! filed
of view of the X-ray telescope of 5.6 m focal length. We expect an energy resolution better than 7 eV (FWHM,
requirement) with a goal of 4 eV. The effective area of the instrument will be 225 cm2 at 7 keV; by a factor of
about two larger than that of the X-ray microcalorimeter on board Suzaku. One of the main scientific objectives
of the SXS is to investigate turbulent and/or macroscopic motions of hot gas in clusters of galaxies.
Keywords: X-ray astronomy, Soft X-ray, High resolution X-ray Spectroscopy
1. INTRODUCTION
The ASTRO-H mission, 6th in series of the ISAS/JAXA’s X-ray astronomy program, is a combination of wide
band X-ray spectroscopy (3-80 keV) provided by multi-layer coating, focusing hard X-ray mirrors (HXT) and
pixel detectors (HXI), and high energy-resolution soft X-ray spectroscopy (0.3-10 keV) provided by the Soft
X-ray Spectrometer (SXS) consisting of thin-foil X-ray optics (SXS-SXT, also called as SXT-S) and the X-ray
Microcalorimeter Spectrometer system (SXS-XCS∗ ). The mission will carry other two instruments, a soft X-ray
telescope + an X-ray CCD camera (SXT-I + SXI) and a soft gamma-ray detector (SGD).
1
The SXS is a recovery mission of the XRS2 onboard Suzaku 3 launched in 2005. It was the first orbiting
x-ray microcalorimeter spectrometer and demonstrated an energy resolution of 6.7 eV at 5.9 keV (Mn Kα) in
orbit and a low background rate of 5 × 10−2 c s−1 cm−2 in the 0.1–12 keV band. However, it could not observe
astronomical sources because of unexpected short lifetime of Liquid He. The spectroscopic capability of X-ray
∗
Further author information: Send correspondence to K.M: E-mail: mitsuda@astro.isas.jaxa.jp
Sometimes the SXS-XCS is called simply the SXS.
Table 1. ASTRO-H SXS key requirments
Energy range
Effective area at 1 keV
Effective area at 6 keV
Energy resolution
Array format
Field of view
Angula resolution
Lifetime
Time assignment resolution
Maximum counting rate
Energy-scale calibration accuracy
Line-spread-function calibration accuracy
Requirement
Goal
0.3 - 12 keV
160 cm2
210 cm2
7 eV
4 eV
6×6
2.9! × 2.9!
1.7’(HPD)
1.3’ (HPD)
3 years
5 years
80 µs
150 c s−1 pixel−1
2 eV
1 eV
2 eV
1 eV
Table 2. ASTRO-H SXS key design parameters
Parameter
Value
SXS XRT (SXT-S, Thin-foil mirrors)∗
Focal length
5.6 m
Diameter of most outer mirror 45 cm
Reflecting surface
Gold
Thermal shield
Al (300 nm) + PET (0.22 µm) with SUS mesh
of a 94 % open fraction
SXS XCS (6 × 6 microcalorimeter array)
Operating temperature
50 mK
Pixel size
814 µm ×814µm
Pixel pitch
832µm
Field of view
3’.05 × 3’.05
X-ray absorber
HgTe, 8µm thickness
Optical Blocking filters
5 filters, polyimide (460 nm) + Al (400nm)
total, Si mesh on two filters
∗
See Okajima et al. (2008)4 for more details of the mirror design.
microcalorimeters is very unique in X-ray astronomy, since no other spectrometers can achieve high energy
resolution, high quantum efficiency, and spectroscopy for spatially extended sources at the same time. The
demand from the science community for spectroscopy with microcalorimeters is even increasing. The following
three scientific objectives are the core science of the ASTRO-H SXS: (1) uncover entire pictures of galaxy-cluster
thermal energy, kinetic energy of intracluster medium, and directly trace the dynamic evolutions of galaxy
clusters, (2) measure the motion of matter governed by gravitational distortion at extreme proximity of black
holes, and reveal the structure of relativistic space-time, and (3) map the distribution of dark matter in galaxy
clusters and the total mass of galaxy clusters at different distances, and thereby study the roles of dark matter
and dark energy in the evolution of galaxy clusters.
The SXS system is being developed by international collaboration lead by ISAS/JAXA and NASA with
European contribution. In this paper, we describe the science and the instruments of the ASTRO-H SXS with
emphasis on the detector system.
2. SXS REQUIREMENTS AND KEY DESIGN PARAMETERS
In Table 1, we summarize the key requirements of the SXS with goals for some of the parameters. In Table 2, key
design parameters to fulfill the requirements/goals are shown. The photon collecting effective area is determined
1000
Effective Area (cm2)
SXS
100
RGS
MEG
HEG
10
LETG
0.5
1
2
Energy (keV)
5
10
Figure 1. Effective areas of high-resolution X-ray spectroscopy missions as functions of X-ray energy. The curve for the
SXS is the present best estimate for a point source, where we assumed to sum all photons detected on the whole array, 1.3’
HPD of the X-ray mirrors, and no contamination of the optical blocking filters. The two crosses show the requirements.
The RGS effective areas is a sum of first order of the two instruments (RGS-1 and RGS-2), and was derived from the RGS
response matrix in SAS v9.0. The effective areas of LETG, MEG and HEG onboard Chandra are, respectively, derived
from the response files for the cycle 12 proposal, and are a sum of first order dispersions in ± directions. (Color on-line)
by the various design parameters of X-ray mirrors, the X-ray absorber thickness of the detector, and the X-ray
transmissions of thermal/optical blocking filters on the X-ray mirrors and inside the Dewar. It is also dependent
on the point spread function of the mirror and the array size of the detector. In Figure 1, the effective area for
a point source is plotted as a function of X-ray energy. The effective are of the SXS is larger than those of high
resolution spectrometers on board Chandra and XMM-Newton at most energies plotted here. Above ∼ 2 keV,
the difference is as large as a factor of five, and a factor of ten at 6 keV.
In Figure 2, we show the resolving power (E/∆E) as a function of energy for two cases, ∆E = 7 eV
(requirement) and 4 eV (goal). In the figure the resolving powers of not only high resolution spectrometers but
also a typical X-ray CCD camera are shown. The SXS has highest resolving power above 2 keV. The broken
lines show a resolving power required to resolve some spectral features of interest. The energy separation of
resonant and inter-combination lines of K emission of He-like ions is shown with a broken line labeled with
∆Res.−I.C(Helike) ). The energy of the emission of each atom is indicated with the labels at the bottom of the
panel. It is found that the SXS can resolve these fine structures of emission for most of ions from O to Fe.
If non X-ray background and continuum emission is negligible
! compared to the line emission, the line-centroid
energy can be determined to a statisitical accuracy of ∼ ∆E/N , where N is the number of photons in the
line. Accuracy of line-centroid determination is also limited by an uncertainty of energy scale calibration, which
will be ∼ 1 eV (Table 1). In Figure 2, we show E/∆E for doppler shift of 100 km s−1 , the shift divided by a
factor of 10, and E/∆E for the calibration accuracy (dot-dash line denoted with 1eV). From these, we find that
the SXS is able to detect doppler shift of emission energy down to a velocity of ∼ 100 km s−1 if we accumulate
V)
',"'&kT=1 ke
'&,!*%$%
+'$."& '/*
#%+
$
&,
'
"*%
)
#%+(!','&+
#
$"
+
$"#
"
* &* 0#
Figure 2. Resolving power of the ASTRO-H SXS as a function of X-ray energy for the two cases, 4 eV resolution (goal) and
7 eV (requirment). The resolving power of high resolution instruments on board Chandra and XMM-newton and typical
resolving power of X-ray CCD cameras are also shown for comparison. The typical energy separations between K emission
of H-like and He-like ions (∆H−Helike ) and between resonant and inter combination lines of He-like ions (∆Res.−I.C(Helike) )
are shown with broken lines, while the emission energies are shown at the bottom of the panel. The broken line denoted
with “Ion thermal motion” is the line broadening due to thermal motion of ions in a kT = 1 keV plasma. The broken
lines indicated with “100 km/s” and “100 km/s (100 photons)” are, respectively, the doppler shift by a bulk motion of
the velocity and a typical detection limit with 100 photons in photon-statistics limit, i.e.continuum emission and non
X-ray background are negligible. The dot-dash line denoted with “1 eV” shows the line shift/broadening detection limit
determined by 1 eV energy-scale or line-spread-function calibration uncertainty. (Color on-line)
more than 100 photons per line for He-like ions of Ar or heavier atoms. A similar argument is valid for line
broadening where line-spread-function uncertainty limits the detectability instead of energy-scale uncertainty.
From Figure 2 we find that thermal broadening of Fe-K line would not be detected for a 1 keV plasma (43 km
s−1 ). However, if kT = 10 keV (130 km s−1 ), it should be possible to detect this with sufficiently high statistics.
In Figure 3, we show a simulation spectrum for the central region of Centaurus cluster of galaxies. The SXS
cannot resolve spatially the central structure of the intra cluster medium (ICM) of the cluster spatially (left top
panel of the fitgure). However, it will resolve the fine structure of emission lines and thus macroscopic motions of
the ICM down to a speed of a few 100 km s−1 by the doppler shift of the line center and/or the line broadening.
3. SXS SYSTEM
In Figure 4 , we show the block diagram of the ASTRO-H SXS X-ray calorimeter spectometer (XCS) system.
The main responsibilities of international partners are indicated with national flags. On the left of the diagram
is the cryogenic Dewar whose outer shell is connected to radiators with heat pipes in order to remove the heat
generated by the mechanical coolers. The microcalorimeter array is mounted inside the detector assembly (DA)
and cooled to 50 mK by the adiabatic demagnetization refrigerator (ADR). The ADR are pre-cooled by superfluid
Centaurus cluster
Chandra image
HPD
3’’
He-like Fe K
Counts s-1 keV-1
6x6 SXS pixels
O
Mg
Z = 0.0114
(3420 km/sec)
Counts s-1 keV-1
Fe-L
Ne
CCD
Response
Si
S
Ar
Ca
Ni
Figure 3. SXS pixel format and half power circle of the SXS X-ray telescope point spread function overlaid on the X-ray
image of Centaurus cluster central region observed with Chandra5 (top left) and simulated SXS energy spectra of the
cluster. The spectrum plotted with a broken line is for a typical X-ray CCD resolution and is multiplied by a factor of 3
for display purpose. (Color on-line)
liquid Helium. The 4 He Joule-Thomson (JT) cooler which is pre-cooled by two sets of double stage Stirling-Cycle
coolers (2ST-PCa and 2ST-PCb), and two shield cooler (2ST-SCa and 2ST-SCb) provide thermal shields for the
liquid He. The ADR and the cryo-cooelrs are controlled by four cooler driver electronics.
Analog signal from the DA is amplified and digitized by the X-ray processing box (Xbox). The digitized data
stream is sent to the Pulse Shape Processor (PSP), where X-ray events are detected and their pulse heights are
determined by applying the optimum digital filtering algorithm. The PSP will also format the science and house
keeping data into telemetry packets and send them to the spacecraft data-handling system. The PSP consists
of the Mission Input/output (MIO) boards and Space Card boards, both of which are commonly used in other
science instruments of ASTRO-H . Hardware logic constructed in a FPGA (Field Programmable Gate Array)
on the MIO board will detect X-ray events from the digitized data stream, and the software running on a CPU
on the Space Card board will perform optimum digital filtering and telemetry formatting.
The Power Supply Unit (PSU) provides extremely low-noise regulated power to the Xbox. It will also provide
a sync signal to the ADR control (ADRC) so that the switching pulses of all the DC-DC converters in the PSU
and in the ADRC are synchronized with one another, which will reduce any beat frequency noises.
The filter wheel Mechanism (FWM) is mounted at a distance of 90 cm from the detector on the lower panel of
the fixed optical bench of the spacecraft. The FWM has six filter positions including open. The choice of filters
are not determined yet, but at least we will have a Be filter to cut low-energy X-rays, a neutral density filter to
reduce X-ray flux without modifying energy spectrum, and an optical blocking filter to observe optically bright
objects. The Be filter will also be employed to protect the detector and optical blocking filters inside the Dewar
X-rays
motor drv, cal source drv
FWM
analog HK
MXS
Aperture
DWR
MD
Valve drv
analog sig.
DA
analog HK
heater
analog HK
ADR
mag. cur.
analog HK
Radiators
Loop HPs
LHe
X-ray processing
Box
(Xbox)
ADR Control
(ADRC)
analog HK
2ST-PCa
AC drv
2ST-SCa
analog HK
AC drv
heater
analog HK
NEA drv
us
stat
He
vent
analog HK
2ST-SCb
NEAC
SpW
SpW
Power Supply
Unit
(PSU)
SpW
analog HK
S/C
BUS
Space
Wire
Router
(SWR)
A/B
SpW
AC drv
HP’’s
MSE
Sync
Pulse Shape
Processor
(PSP1,2)
Pre Cooler
Driver
(PCD)
analog HK
AC drv
2ST-PCb
LVDS
JT Driver
(JTD)
AC drv
JT
NEA
valves
(x4)
S/C
BUS
FWE
Shield Cooler
Driver
(SCD)
C/L
MD
only on SWR-A
Gate valve, He ll, MS vent status
Heater Control
Electronics
(HCE)
S/W
C/L
S/W
C/L
S/W
C/L
S/W
C/L
S/W
C/L
S/W
C/L
S/C Bus
DIST
S/W
C/L
MD
S/W
S/C
BUS
Figure 4. Electrical block diagram of the ASTRO-H SXS.
from micro-meteoroids while the SXS is not observing a source. On the FWM, we will mount commandable,
modulated X-ray sources (MXS), which produce nearly monochromatic fluorescent X-rays as desired. The entire
detector array will be illuminated by the MXS for time intervals as short as 10 ms. We can thereby calibrate the
energy scale of the full array and at the same time screen the data in which the celestial X-ray spectrum is also
exposed to the calibration sources, which produce electron- and photo- loss events. Both the FWM and MXS
are controlled by the electronics, FWE. For more details of the filter wheel system and the MXS, please see de
Vries et al. (2010).6
The SpaceWire protocol is used for communication between the SXS electronics boxes and for communication
with the spacecraft bus. The power distribution unit (DIST), one of the SpaceWire Router (SWR), and the
Dewar are equipped with special GSE interfaces called MD (Manually-Disconnect) connectors. Using these
interface we will be able to operate the ADR and mechanical coolers on ground without space craft power. We
will use this function for the He top-off operation and to keep the Liquid He superfluid during the hold time
before lift off. At that time, the spacecraft will be inside the nose fairing of the launch vehicle. We will access
the MD connectors through an access hole of the nose fairing.
The total mass of the SXS-XCS is estimated to be about 360 kg, and the total power consumed from the
space craft primary power line is estimated to be about 580 W. A large amount of mass and power consumption
are in the cooling system.
(a)
(b)
Figure 5. Layout of the SXS detector array on a Si frame (a), and Mn Kα emission spectrum obtained with a prototype
SXS detector (b).7, 8 (Color on-line)
4. DETECTOR
The calorimeter array for the SXS is being designed and developed with significant heritage from Suzaku XRS2.
However, at the same time, significant improvements are also implemented. Although the array format is the
same 6×6, pixels are larger and maintain a 3’ x 3’ FOV with the longer focal length of the telescope. Furthermore,
by improvement in the X-ray absorber material and also by operating at a lower temperature, a better energy
resolution is obtained. In Figure 5 (b) , we show an example of Mn Kα emission spectrum obtained with a
prototype SXS detector. The FWHM resolution is better than 4 eV. One of the key issue in obtaining a good
energy resolution in orbit is heating of the Si detector frame by charged particle.2 In the SXS detector, a layer
of gold (1.5 µm) will be deposited partially on the Si frame (yellow area in Figure 5 (a)) to obtain a better heat
sinking. More details of the detector and the DA are described in Porter et al. (2010).7
5. CRYOGENICS SYSTEM
A conceptual block diagram of the ASTRO-H SXS cooling chain is shown in Figure 6. A temperature of 50
mK is obtained by the double-stage adiabatic de-magnetization refrigerator (dADR) which consists of a high
temperature stage of 150g of Gadolinium Lithium Floride (GLF) + 3 T (Tesla) superconducting magnet, a low
temperature stage of 300 g of Chromium Potassium Alum (CPA) + 2 T superconducting magnet, and two sets
of gas-gap active heat switches (GGHS). The dADR dumps the heat to the superfluid liquid He (LHe) at 1.2 K.
The third ADR between the 4 He JT cooler and LHe tank is nominally off and the two heat switches (GGHS)
on both sides of the third ADR are also off. Thereby, the 4 He JT cooler provides a 4.5 K thermal shield to
the He tank. The full volume of the He tank is 40". After the He top-off operation, ground hold before lift off,
and initial cool down in orbit, the amount of initial LHe is expected to be > 30". We thus assume 30" for our
base of calculation. Then the LHe lifetime is expected to be 4.5 years in the nominal case (Table 3). The third
ADR can be operated to remove heat from the He tank. If we fully operate it in orbit, the LHe lifetime will be
extended to 6.9 years. The third ADR will be also operated during ground hold before launch. This will also
extend the lifetime of LHe by increasing the initial amount of LHe in orbit. This case has not been studied in
detail yet. In nominal case, we can cool the detector to 50 mK and continue observations even after all LHe has
exhausted. In this cryogen-free operation, the observation efficiency become lower than with LHe, because the
hold time of the ADR until next regeneration would be shorter.
In Table 3, we also show the LHe lifetime for several failure cases of a single mechanical coolers or of
unexpected loss of LHe. In the nominal case, all cyrocoolers are operated at about half maximum power. In
2ST
LHP’’s
2ST
2ST
4He
HP’’s
2ST
JT
LHe
Cryostat
ADR
(GLF)
ADR
(GLF)
ADR
(CPA)
150g, 3T
150g, 3T
300g, 2T
Radiators
Figure 6. Block diagram of the ASTRO-H SXS cooling chain.
contingency cases, we will increase the cyrocooler drive power to compensate for the loss of cooling power and
the heat load through the non-operational cooler. However, if one of the precoolers fails, the remaining precooler
cannot pre-cool the He gas in the JT cooler to a low enough temperature to obtain the JT effect. Thus, in this
case as well as in JT contingency case, cryogen-free operation is not possible. However, the temperature of the
thermal shield, which the JT cooler is supposed to cool, can be kept low enough to preserve the LHe for ∼ 2
years.
In Figure 7, the mechanical layouts of the SXS Dewar are shown. The mass of the Dewar is about 270 kg,
while the total mass of the four cooler driver boxes are estimated to be 40 kg. In nominal case, the four cooler
drivers consumes 480 W of power from the space craft primary power. This includes not only the cooler drive
power but also power conversion efficiency and power used for house keeping and communication. For more
details of the cooling system, see Fujimoto et al. (2010)9 and Shirron et al. (2010).10
ACKNOWLEDGMENTS
We are grateful to the engineers in Sumitomo Heavy Industries (SHI), in NASA/GSFC, in Mitsubishi Heavy
Industrories (MHI), in SRON, and in Geneva University, and scientists and engineers in JAXA and univrsiteis
for the design of the SXS microcalorimeter system, and graduate students who contributed laboratory experiments and design; especially K. Kanao, S. Yoshida, M. Miyaoka, K. Narasaki, S. Tsunematsu, T. Hiroishi,
A. Okabayashi, K. Ootsuka (SHI), J. Lander, J. Crow, A. Mattern, A. Kashani, R. Thorpe, D. McGuinness,
N.Galassi, P. Chen, P. Arsenovic, J. Miko, T. Bialas, K. Mil, M. Chiao, K. Kawaja, I. Linares, C. Masters, T.
Sullivan (NASA/GSFC), K. Masukawa, K. Matsuda, Y. Kuroda (MHI), H. Aarts, M. Frericks, P. Laubert, P.
Lowes (SRON), D. Haas, and R. Dubosson (Geneva), M. Nomachi (Osaka University), H. Ogawa, K. Minesugi,
Detector Assembly
Shield cooler (2ST)
dADR
Aperture Assembly
He tank
Dewar HK
connectors
He ll/
vent lines
Detector
and ADR
connectors
Shield cooler (2ST)
Support struts
Pre coolers (2STx2)
JT cooler
Cooling fans for
ground operation
JT compressors
Figure 7. External view (left) and cutaway drawing (right) of the ASTRO-H SXS cryogenic Dewar. The gate valve which
will be mounted on the top of the Dewar outer shell is not shown. Two shield coolers are mounted on the side of the outer
shell, while the JT cooler and its pre coolers are mounted on the bottom. The detector assembly (DA) and the ADR
are installed as a unit on the He tank from the upper side. The third ADR is on the side of the DA but is not shown
here because it is in the unseen part of the Dewar in the cutaway drawing. The outer diameter of the Dewar outer shell
excluding mechanical coolers and other sticking-out structures is 950 mm. The height of the Dewar is 1292 mm including
the support struts but excluding the gate valve. The total mass of the Dewar is about 270 kg.
Table 3. Expected LHe lifetime and observation lilfetime
Case
Nominal
Cooler powersa
LHe lifetime (y)
Observation (y)
JT=90W
4.5
6.9b
> 4.5c
> 6.9bc
PC=50W×2
SC=50W×2
2.6
> 2.6c
1 SC contingency
JT=90W
PC=50W×2
SC=90W×1
2.1
2.1
JT contingency
JT=0W
PC=50W×2
SC=90W×2
2.1
2.1
1 PC contingency
JT=0W
PC=90W×1
SC=90W×2
0
> 0c
Unexpected He loss JT=90W
PC=50W×2
SC=90W×2
a
Power supplied to the cooler. The conversion efficiency of the cooler driver electronics
is not include.
b
If we operate the 3rd ADR in nominal case.
c
Observation can be continued without LHe using the 3rd ADR as long as the
remaining mechanical coolers are working.
K. Ishimura, N. Iwata, T. Irikado, I. Funak, N. Yamanishi (ISAS/JAXA), K. Ishikawa, Y. Abe (Tokyo Metropolitan University), H. Seta, Y. Shimoda, T. Yasuda, M. Asahina (Saitama University), T. Hagihara, I. Mitsuishi,
H. Yositake (ISAS/JAXA), and T. Yuasa (University of Tokyo). We would also like to thank members of Akari
and SPICA teams, in particular, T. Nakagawa, and the SMILES group at ISAS/JAXA for the development of
mechanical coolers.
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