Kokubun, M. et al., "Hard X-ray imager (HXI) for the NeXT mission"

Hard X-ray Imager (HXI) for the NeXT mission
Motohide Kokubuna , Kazuhiro Nakazawab , Shin Watanabea , Yasushi Fukazawad , Jun
Kataokae , Hideaki Katagirid , Tsunefumi Mizunod , Kazuo Makishimab,c , Masanori Ohnoa ,
Goro Satoa , Rie Satoa , Hiroyasu Tajimaf , Tadayuki Takahashia , Toru Tamagawac , Takaaki
Tanakaf , Makoto Tashirog , Hiromitsu Takahashid , Yukikatsu Teradag , Yasunobu Uchiyamaa ,
Yuji Uratag , Kazutaka Yamaokah , Shin’ichiro Takedaa,b , Tetsuichi Kishishitaa,b , Masayoshi
Ushioa,b , Jun’ichiro Katsutaa,b , Shin’nosuke Ishikawaa,b , Hirokazu Odakaa,b , Hiroyuki Aonoa,b ,
Souichiro Sugimotoa,b , Yuu Kosekia , Takao Kitaguchib , Teruaki Enotob , Shin’ya Yamadab ,
Takayuki Yuasab , Tsuyoshi Uedab , Yuichi Ueharab , Sho Okuyamab , Hajimu Yasudad , Sho
Nishinod , Yudai Umekid , Katsuhiro Hayashid , Masayuki Matsuokad , Yuki Ikejirid , Akira Endog
, Yuichi Yajig , Natsuki Kodakag , Wataru Iwakirig , Tomomi Kouzug , Takako Sugasawarag ,
Atsushi Harayamag , Satoshi Nakahirah , and the HXI team
a
ISAS/JAXA, 3-4-1 Yoshinodai Sagamihara Kanagawa, 229-8510, Japan;
Department of Physics, University of Tokyo, 7-3-1 Hongo Bunkyo Tokyo, 113-0033, Japan;
c Cosmic Radiation Laboratory, RIKEN, 2-1 Hirosawa Wako Saitama, 351-0198, JAPAN;
Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama Higashi-Hiroshima
Hiroshima, 739-8526, Japan;
e Department of Physics, Tokyo Institute of Technology, 2-21-1 Ookayama Meguro Tokyo,
152-8551, Japan;
f Stanford Linear Accelerator Center, 2575 Sand Hill Road Menlo Park CA, 94025, USA;
g Department of Physics, Saitama University, 255 Shimo-ohkubo Sakura Saitama Saitama,
338-8570 , Japan;
h Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe
Sagamihara Kanagawa, 229-8558, Japan;
b
d
ABSTRACT
The Hard X-ray Imager (HXI) is one of three focal plane detectors on board the NeXT (New exploration X-ray
Telescope) mission, which is scheduled to be launched in 2013. By use of the hybrid structure composed of
double-sided silicon strip detectors and a cadmium telluride strip detector, it fully covers the energy range of
photons collected with the hard X-ray telescope up to 80 keV with a high quantum efficiency. High spatial
resolutions of 400 micron pitch and energy resolutions of 1-2 keV (FWMH) are at the same time achieved with
low noise front-end ASICs. In addition, thick BGO active shields compactly surrounding the main detection
part, as a heritage of the successful performance of the Hard X-ray Detector (HXD) on board Suzaku satellite,
enable to achive an extremely high background reduction for the cosmic-ray particle background and in-orbit
activation. The current status of hardware development including the design requirement, expected performance,
and technical readinesses of key technologies are summarized.
Keywords: X-ray Astronomy, Hard X-ray Imager, NeXT mission
1. INTRODUCTION
The Hard X-ray Imager (HXI) is one of four onboard instruments of the New exploration X-ray Telescope
(NeXT), which is a next Japanese X-ray astronomical satellite.1, 2 Two identical detectors are located at focal
points of two hard X-ray telescopes (HXTs),3 and the combination of HXT/HXI will realize the first hard X-ray
Send correspondence to M.Kokubun, E-mail: [email protected]
focusing imaging up to ∼80 keV to investigate the non-thermal universe with an unprecedented sensitivity. The
launch year of NeXT is currently scheduled as 2013, and the mission is now in the phase A. Researches and
developments to construct this novel detector have been extensively performed for several years, and almost all
key technologies are ready to be employed with sufficient technical readiness.
In this paper, the current status of detector development are summarized.
2. DESIGN REQUIREMENTS AND CONCEPT OF THE DETECTOR
2.1 Spacecraft Interface
The NeXT-HXT is designed to have a long focal length of 12 m, to obtain a large effective area at higher energy
range. Since this length is too large to be achieved by a single fixed optical bench, an extendable optical bench,
which increases in length of 6 m in the orbit, is employed. As shown in figure 1, a plate having on board the
detector is extended, while the telescopes are mounted on the fixed bench. Therefore, power supply and signal
cables which connect between the detector and spacecraft bus have to be also drawn out, and special cares are
inevitable in designing thermal and mechanical interfaces between the detector and spacecraft bus system.
Figure 1. A schematic drawing of spacecraft cadre after the in-orbit extension. The X-ray mirrors are mounted on top
(left in figure) of fixed optical bench of 6 m, while the detectors are drawn away from the base-plate by extending the
detector plate at the bottom (right).
2.2 Detector Design
Figure 2 shows a schematic view of sensor part of HXI. From both of scientific and technical point of views, there
are many requirements which should be met by the focal-plane imaging detector. First of all, it should achieve
a fine position resolution smaller than 400 µm, which roughly corresponds to a tenth of half-power diameter of
HXT (∼1′ ). This requirement can be fulfilled by a semiconductor device which is segmented with small pitch
electrodes, while it is difficult for scintillator-based detectors. To minimize the power consumption, a strip-type
readout is employed as the electrode structure, instead of pixel-type configuration. A field-of-view at the focal
point of mirror is then linearly correlates with a number of readout channels, namely, the larger geometrical size
of device realizes the wider field-of-view. Based on a trade-off study on a current limitation of material size,
increase of readout channels, and scientific requirements for the field-of-view, the size is determined to be 32×32
mm2 which corresponds to ∼9′ field-of-view and 128 channels readout.
Second requirement is a high detection efficiency covering highest end of the energy range of hard X-ray
photons collected by HXT (∼80 keV). This requirement is obviously too rigid for a Silicon-based detector since
the efficiency rapidly decreases above ∼20 keV even with a multi-layer configuration. Thus, a novel compound
semiconductor, Cadmium Telluride (CdTe) having a larger atomic number than that of Silicon is utilized, and a
Schottky type electrode is employed to make the device fully depleted suppressing the leak current as small as
possible.4 The CdTe detector is located beneath the Silicon devices, so as to form a hybrid formation as shown
in figure 2. The upper four layers of Silicon are fundamental to achieve a low energy threshold, and to reduce
in-orbit activation backgrounds of CdTe by detecting fluorescent X-ray photons. By selecting only one-layer hit
events, these in-orbit activation will be sufficiently reduced. From the viewpoint of focal depth, total five layers
should be located as close as possible, and a length between the bottom CdTe layer and top Si layer is 3 cm.
A total detection efficiency obtained with four layers of 0.5 mm thick Silicon and one layer of CdTe having the
same thickness is shown in figure 3. A higher efficiency than 50 % is acquired up to 80 keV.
BGO+APD
CFRP
Coldplate
Heat-pipe
S/C Baseplate
+ radiator
Figure 2. A schematic view of one unit of sensor part of HXI (left), a close-up view of the imaging sensor consisting
multi-layer Si/CdTe semiconductor devices (upper right), and a photo of prototype detector (upper right). Dissipated
power in the detector is transferred to radiator panels of spacecraft via heat-pipes, and the temperature is controlled by
heaters attached to a cold-plate beneath the detector.
Figure 3. The detection efficiency plot of HXI sensor. An efficiency obtained by the top layer and all of four layers of 0.5
mm thick Silicon, probability of detecting hard X-ray photons which are detected by the CdTe layer after transmission
of DSSD, and a total efficiency of whole detector are shown.
The last and most important requirement for HXI is to achieve a low detector background, which is inevitable
to realize an unprecedented sensitive hard X-ray observations. Since the effective area is solely determined by
HXT, especially at a low energy range, a focusing hard X-ray imaging system automatically realizes a high signalto-noise ratio according to a reduction of geometrical size of sensor device. In addition, an active shield, which is
a crucial technique in case of non-focusing hard X-ray detectors to reject in-orbit background events caused by
cosmic-ray particles and Compton scattering of incident photons, is still play a critical role. As shown in figure
2 similar active shield made of BGO scintillators as that utilized in the Hard X-ray Detector (HXD) onboard
Suzaku,5 fully surrounds the main detection part except the direction of field-of-view. An average thickness of
BGO crystals are designed to be ∼4 cm in order to minimize low-energy protons in the South Atlantic Anomaly
(SAA) arriving at the imaging sensor and to detect annihilation lines (511 keV) emerging from the sensor via β +
radioactive decays. Scintillator signals from individual BGO crystals are read by directly attached Avalanche
Photo-Diodes (APDs) instead of photo-multipliers used in HXD. The expected performances of HXI, based on
the current detector design, are summarized in table 1.
Table 1. The fact sheet of HXI
Parameter
Energy range
Geometrical area
Field-of-view
Detection efficiency
Energy resolution
Detector background
Operation temperature
Value
5-80 keV
32 × 32 mm2
9.1 × 9.1 arcmin2
80 % @60 keV
1-2 keV (FWHM) @ 60 keV
1-3 × 10−4 cts cm−2 s−1 keV−1
-20 ±5 C
3. IN-ORBIT BACKGROUND AND SENSITIVITY
Since the thickness of the active shields is almost the same as those used in Suzaku HXD, and the orbit of NeXT
is also the same as Suzaku, in-orbit environments for the sensor can be considered as an equivalent. Therefore,
an expected in-orbit background can be estimated based on the actual data observed with HXD.6 In HXD, an
energy range of 10−−80 keV is covered with 2 mm thick Si-PIN diodes, and the residual background events of
HXD-PIN are mainly caused by elastic scattering with atmospheric neutrons, and hence can be rescaled with a
difference of volumes of two detectors. This results more than one order of magnitude lower background level,
when normalized by the effective area. In addition to the neutron scattering, the CdTe sensor will be suffered by
induced radio-activities, mainly due to the SAA protons. This component can be estimated with the on-ground
experimental results, in which a CdTe device is irradiated with a monochromatic energy protons at an accelerator
facility.7
In figure 4, thus calculated in-orbit background is shown, together with that achieved in HXD-PIN. Since
most of low energy hard X-ray photons are detected by the top Si-layer, a lower background level can be achieved
below ∼40 keV when events detected by the top layer only are utilized. Line features in the background spectrum
are caused by electron capture (EC) decays of radio-active isotopes induced in the CdTe. When compared with
the level achieved by HXD-PIN, which is the lowest level among past non-imaging hard X-ray detectors, that
of HXI is more than two order of magnitude lower. Based on this background level, a sensitivity limit of HXI,
achieved with an exposure of 100 ks for a continuum emission from a point source, is also shown in figure 4. A
smooth extension from the soft X-ray range will be for the first time realized with HXT-HXI system.
Figure 4. (Upper) Expected in-orbit detector backgrounds normalized with the effective area, for cases of HXD-PIN
detector onboard Suzaku and HXI. The background level expected in the top single layer of HXI is also shown to
demonstrate a lower level below ∼40 keV. (Bottom) Detection limits of the soft X-ray imaging system and hard X-ray
imaging system of NeXT for point sources, shown with that achieved with HXD-PIN.
4. TECHNICAL READINESS OF KEY TECHNOLOGIES
4.1 Double-sided Silicon Strip Detector (DSSD)
The double-sided Silicon Strip Detector (DSSD) used in HXI is a well established detector based on our past
extensive studies.10, 11 Two prototype detectors have been already developed, whose effective sizes are 2.5 cm
and 4.0 cm with a strip pitch of 400 µm and a thickness of 0.3 and 0.5 mm.8, 9 A typical energy resolution of
1.5 keV (FWHM) at 60 keV is achieved when combined with low-noise analog ASICs,12, 13 which are developed
in collaboration with GM-IDEAS. With this fine energy resolution, a lower energy threshold of ∼5 keV is at the
same time achieved. In addition, a compact mounting technology is also established, and four layers of DSSDs
can be assembled with an interval of a few mm, as shown in figure 5.
Figure 5. A photo of compactly stacked four layers of DSSD.
4.2 Double-sided Cadmium Telluride Strip Detector (DSCD)
Figure 6. (Left:) A photo of prototype double-sided CdTe strip device (1.3 cm size), and the prototype detector (2.6 cm)
assembled with two front-side-electronics board containing ASICs (right).
Since the stopping power of the device made with Silicon becomes quite small above 10 keV, another material
has to be employed as the detection part of a hard X-ray detector, otherwise the device thickness has to be as
large as a few mm like that used in HXD-PIN. For this purpose, we have for the first time developed a doublesided CdTe strip detector (DSCD) which has an aluminum electrode in anode and platinum one in cathode.4, 14
As a dimension of the detector, the double-sided strip configuration is more suitable than the pixel (or pad), in
terms of minimizing the power consumption of read-out electronics. A prototype device and detector is shown
In figure 6. The geometrical size of the device is 2.6 cm with a strip pitch of 400 ∼m, and the thickness is 0.5
mm.
Figure 7. (left) A photo of materials located above the DSCD in measuring the shadow hard X-ray image, and an actual
image obtained at 60 keV. A thin washer is becoming transparent at this energy band, while thick nuts and a solder is
kept to be opaque.
The readout electronics is basically the same as the case of DSSD except for a relatively higher bias voltage
(∼500 V) applied to the detector. To demonstrate a high imaging capability of the DSCD, a ”shadow image” is
measured by use of a few radioactive isotopes. As shown in figure 7, two nuts, a washer made of stainless-steel,
and solder bars are placed above the DSCD, and hard X-rays are irradiated to the DSCD through these passive
materials. An example image measured at 60 keV is also shown in figure 7. In this measurement, an energy
resolution of ∼1.8 keV is obtained at 60 keV.
4.3 Avalanche Photo Diode
An active vetoing of cosmic-ray particles with BGO scintillator plays an important role to realize an unprecedented sensitivity in the orbit. Since the shield also works for the Compton suppression, it is significant to achieve
a low energy threshold as much as possible. As a readout sensor of scintillation lights, reverse-type Avalanche
Photo-Diodes (APDs),?, 15 developed in collaboration with Hamamatsu photonics, are employed. When compared with the canonical photo-multiplier tubes (PMTs), a significantly lower high-voltage required helps to
design the flight electronics, in terms of in-orbit discharges. Based on our research and development studies, the
size of APD sensor is optimized as 1×1 cm2 (Fig. 8), and an energy threshold of ∼30 keV can be expected even
when directly coupled with a large BGO crystal, as shown in figure 9. Although both of the APD gain and light
yield of BGO scintillator depend on the temperature, this will not significantly affect the detector performance
by controlling the operation temperature within ∼ 5 degree.
5. SUMMARY
A baseline design of HXI detector is almost finished, taking the spacecraft interfaces into consideration. Most
of key technologies, including the novel double-sided CdTe strip detector, have been already established based
Figure 8. (Left:) A photo of reverse-type APD devices whose sizes are 5×5, 10×10, and 20×20 mm2 , from left to right.
(Right:) A photo of APD attached to a large BGO crystal which is also surrounded by reflecting sheets.
Figure 9. Typical spectra obtained with BGD read by 1×1 cm2 APD, measured by irradiating 60 keV (left) and 662 keV
(right). A narrow peak shown at higher channels than that of 662 keV corresponds to input test pulses.
on our past researches. The expected performance of HXI fully satisfies the scientific requirements such as the
position resolution, energy range, detection efficiency, and low detector background. The development plan is
scheduled according to the master schedule of the NeXT project, and an engineering model will be constructed
this year.
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