Wide band X-ray Imager (WXI) and Soft Gamma-ray
Detector (SGD) for the NeXT Mission
T. Takahashia,b , A. Awakic , T. Dotania , Y. Fukazawad , K. Hayashidae , T. Kamaef ,J. Kataokag , N. Kawaig , S. Kitamotoh , T. Kohmurai , M. Kokubunb , K. Koyamaj ,K. Makishimab , H. Matsumotoj , E. Miyatae , T. Murakamik , K. Nakazawaa , M. Nomachie ,M. Ozakia , H. Tajimaf , M. Tashirol , T. Tamagawam , Y. Teradam , H. Tsunemie , T. Tsuruj ,K. Yamaokan , D. Yonetokuk , and A. Yoshidan
a Institute of Space and Astronautical Science, JAXA, Sagamihara, Kanagawa, 229-8510,

Japan;b Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo, 113-0033, Japan;c Department of Physics, Ehime University, Matsuyama, Ehime, 790-8577, Japan;d Department of Physics, Hiroshima University, Higashi-Hiroshima, Hiroshima, 739-8526,

Japan;e Department of Physics, Osaka University, Toyonaka, Osaka, 560-0043, Japan;
f Stanford Linear Accelerator Center, Stanford, CA 94309-4349, USA;g Department of Physics, Tokyo Institute of Technology, Meguro-ku, Tokyo, 152-8551, Japan;
h Department of Physics, Rikkyo University, Toshima-ku, Tokyo, 171-8501, Japan;
i Department of Physics, Kogakuin University, Hachioji, Tokyo, 192-0015, Japan;
j Department of Physics, Kyoto University, Sakyo-ku, Kyoto, 606-8502, Japan;

k Department of Physics, Kanazawa University, Kanazawa, Ishikawa, 920-1192, Japan;l Department of Physics, Saitama University, Saitama, 338-8570, Japan;m RIKEN, Wako, Saitama, 351-0198, Japan;n Department of Physics, Aoyama Gakuin University, Sagamihara, Kanagawa, 229-8551, Japan

ABSTRACTThe NeXT mission has been proposed to study high-energy non-thermal phenomena in the universe. The highenergy response of the super mirror will enable us to perform the first sensitive imaging observations up to 80
keV. The focal plane detector, which combines a fully depleted X-ray CCD and a pixellated CdTe detector,
will provide spectra and images in the wide energy range from 0.5 keV to 80 keV. In the soft gamma-ray band
upto ∼ 1 MeV, a narrow field-of-view Compton gamma-ray telescope utilizing several tens of layers of thin
Si or CdTe detector will provide precise spectra with much higher sensitivity than present instruments. The
continuum sensitivity will reach several × 10−8 photons/s/keV/cm2 in the hard X-ray region and a few × 10−7
photons/s/keV/cm2 in the soft γ-ray region.
Keywords: Hard X-ray, gamma-ray, X-ray Astronomy, Gamma-ray Astronomy, CdTe, Compton Camera

1. INTRODUCTION
The hard X-ray and gamma-ray bands have long been recognized as important windows for exploring the energetic
universe. It is in these energy bands that non-thermal emission, primalily due to accelerated high energy particles,
becomes dominant. However, by comparison with the soft X-ray band, where the spectacular data from the
XMM-Newton and Chandra satellites are revolutionizing our understanding of the high-energy universe, thesensitivities of hard X-ray missions flown so far, or currently under construction, have not dramatically improved
over the last decade. In order to study the energy content of non-thermal emission and to draw a more complete
picture of the non-thermal universe, observations by highly sensitive missions are important.
Further author information: (Send correspondence to T. Takahashi) E-mail: takahasi@astro.isas.jaxa.jp

Figure 1. Artist’s drawing of the NeXT satellite. The focal length of the HXT is 12 m.

The NeXT (Non-thermal Energy eXploration Telescope) mission proposed in Japan is a successor to the
Astro-E2 mission, and is optimised to study the high-energy non-thermal universe (Fig.1).1, 2 NeXT will carry
three hard X-ray telescopes (HXTs) for the wide band X-ray imager (WXI), one soft X-ray telescope (SXT)
for the soft X-ray spectrometer (SXS), and a soft γ-ray detector (SGD). With the first imaging observations
up to 80 keV, NeXT is expected to achieve two orders of magnitude improvement in the sensitivity in the hard
X-ray region. The SGD utilizes a new concept of a narrow-FOV Compton telescope3, 4 to improve the sensitivity
in the soft γ-ray region for matching with the sensitivity of the HXT/WXI combination. The extremely low
background brought by the SGD will allow us to measure the precise γ-ray spectrum upto 1 MeV. Furthermore,
as described in the separate paper,5 the SGD will be sensitive to the polarization of the incident γ-rays, andwill potentially be the first hard X-ray polarimeter in orbit. In addition to these high energy instruments, thesoft X-ray spectrometer (SXS) will map out the velocity field in objects by means of X-ray line profiles. The
SXS is based on a transition edge sensor (TES) and has both spectral and spatial resolution much better thanthe X-ray calorimeter (XRS) onboard the Astro-E2.6 In this paper, we describe the current ideas about the
WXI and the SGD.

2. WIDE-BAND X-RAY IMAGER (WXI)
The non-imaging instruments flown so far were essentially limited to studies of sources with 10-100 keV fluxes ofat best >10−12 − 10−11 erg cm−2 s−1 . This limitation is due to the presence of high un-rejected backgrounds fromparticle events and Cosmic X-ray radiation, which increasingly dominate above 10 keV. Imaging, and especially
focusing instruments have two tremendous advantages: first, the volume of the focal plane detector can be mademuch smaller than for non-focusing instruments, and second, the residual background, often time-variable, canbe measured simultaneously with the source, and can be reliably subtracted.
A depth-graded multi-layer mirror, referred to as a super mirror, reflects X-rays not only by total externalreflection but also by Bragg reflection. A super mirror consists of a stack of multi-layers with different sets ofperiodic length and number of layer pairs. The current mirror design of the HXT for NeXT7, 8 is based on acarbon/platinum coating, which is adopted in the InFOCµs project by Nagoya University and NASA’s GSFC.9
With three HXT units, effective areas of 1100 cm2 at 20 keV and 230 cm2 at 60 keV can be achieved with a
focal length of 12 m.1 Fig. 2 compare the effective area of the three HXTs with XMM-Newton and Astro-E2.
The effective area of the Soft X-ray Telescope (SXT) and the SGD are also shown in the figure.
In order to match the energy range covered by the super mirror (0.5 − 80 keV), the focal plane detector isrequired to cover a very wide energy band. To this end, we have proposed a new focal plane detector based on

Figure 2. Effective area of the HXT and the SXT as compared to XMM/Newton and Astro-E2. Effective area of the
gamma-ray detectors are also shown for the HXD/Astro-E2 and the SGD/NeXT.

Figure 3. Conceptual drawing of the WXI proposed for the NeXT mission. The WXI is composed of two sub-instruments:
the soft X-ray Imager (SXI) and the hard X-ray Imager (HXI). The CdTe pixel detector in the HXI is placed beneaththe X-ray CCD in the SXI. In the WXI, soft X-rays are absorbed in the CCD, while hard X-rays penetrate through the
CCD and are absorbed in the CdTe pixel detector. The Si strip detector is considered as an option to shield fluorescencelines from CdTe.

Table 1. Specification for the HXI and the SXI1

HXI

SXI

Energy Range
Energy Resolution
Trigger Threshold
Pixel Size
Detector Size
Timing Resolution
Operating Temperature
Energy Range
Energy Resolution
Pixel Size
Detector Size
Operating Temperature8 – 80 keV
0.5 – 1 keV (FWHM)∼ 8 keV200 ∼ 500 µm
20 mm × (20 – 30) mm< 100 µs
−50 – 0 degree
0.5 – 20 keV
130 eV (FWHM at 6keV)
27 × 27 µm2
42 mm × 42 mm−90 degree

an idea of combining a fully depleted CCD and a CdTe (Cadmium Telluride) pixel detector3, 10 as the WXI.11, 12
Figure 3 is a schematic design of the WXI showing the concept. The WXI is composed of two sub-instruments;
the soft X-ray Imager (SXI) and the hard X-ray Imager (HXI). Specifications of the SXI and the HXI are listed
in Table 1. The SXI is based on an X-ray CCD with very thin dead layer in the device and the HXI is based ona fine-pitch CdTe pixel detector. In the WXI, soft X-rays will be absorbed in the CCD, while hard X-rays will
penetrate through the CCD and be absorbed in the CdTe pixel detector. Semiconductor detectors with highmass absorption coefficient, such as CdTe, are crucial for the detection of hard X-ray photons. For CdTe, evena detector with a thickness of 0.5 mm provides a good detection efficiency for the hard X-ray region coveredby the HXT. As shown in Fig. 3, the CdTe detector mounted in the HXI is shielded by a BGO (Bi4 Ge3 O12 )scintillator. This shield is indispensable, as the non X-ray background is the dominant source of the backgroundin the energy range of the HXI. A thickness of ∼2 cm for BGO will be required, not only for shielding against
background photons but also for reducing the number of particles that reach the CdTe detector and give rise toactivation.
The SXI detects soft X-rays below 10-20 keV with the high position resolution of 27 µm. It will be basedon the technology which has been accumulated for the Japanese MAXI mission.13 The SXI utilizes a single,
large-format CCD which covers 12×12 arcmin2 at the focal plane. An energy resolution of 130 eV (FWHM) at 6
keV is expected at the operating temperature of −90 deg. SXI utilizes a thinned CCD, whose undepleted layer
is largely removed. This is necessary to make it transparent to hard X-rays. If we adopt a backside-illuminated
CCD, no extra process is necessary as the undepleted layer is already removed. The CCD support and coolingstructures are configured near the rim of the CCD so as not to interfere with the transmission of hard X-rays.
Thus the transparent portion to hard X-rays is smaller than the size of the CCD itself. In order further to reduce
the undepleted layer and to improve the high energy response, the development of fully depleted p-channel CCD
fabricated on n-type high-resistivity silicon is now under way.14
The current goal for the CdTe detector in the HXI is a pixel detector with both a fine position resolution of
200−250 µm and a high energy resolution better than 1 keV (FWHM), in the energy range from 5 keV to 80
keV. Since significant progress has been made recently on the development of CdTe technologies,15 it is now
possible to fabricate a single crystal CdTe device with the size of 2×2 cm2 . Signals from the individual pixel
electrodes formed on the surface of the CdTe wafer are fed into the readout circuit built in the ASIC. To realize
the fine pitch CdTe pixel detector for the HXI, a low noise front-end ASIC with more than several thousand
independent channels will be the key technology. Development of a simple and robust connection technology isalso necessary, because high compression and/or high ambient temperature would damage the CdTe. In order
to cooperate with the BGO shield, the detector should have fast timing resolution of 10 to 100 µs, such that we
can veto the events when there is a hit in the shield. Taking these requirements into consideration, we have beenworking on high performance CdTe detectors for both planar and pixel configuration.16, 17 Based on the CdTe
wafers manufactured from the single crystal grown by the Traveling Heater Method (THM-CdTe) and with theFigure 4. Efficiency of the SXI with 300 µm thick Si CCD and the HXI for 0.5 mm and 1.0 mm thick CdTe detector.
Effects of a possible dead layer in the Si CCD are not included in the calculation.

Figure 5. Schematic drawing of the WXI. The SXI and the HXI will be operated at different temperatures.

technology for gold-stud bump bonding, we have developed CdTe detectors with high energy resolution.18, 19
With NeXT, we expect to achieve an area of about 750 cm2 at 30 keV with a typical angular resolution of30”. Fig. 4 shows the efficiency of the WXI with a 300 µm thick X-ray CCD and a 0.5 or 1.0 mm CdTe pixel
detector. In the calculation, effects of a possible dead layer in the CCD are not taken into account. By assuming
a background level of 1×10−4 counts/s/cm2 /keV, in which a non X-ray background is dominant, the source
detection limit in 100 ksec would be roughly 10−14 erg cm2 s−1 in terms of the 10–80 keV flux for a power-law
spectrum with a photon index of 2. This is about two orders of magnitude better than present instrumentation.

Table 2. Specification of the SGD1

SGD

Energy Range
Energy Resolution
Geometrical Area
Effective Area

Opening Angle(Angular Resolution)
Operating Temperature

10 keV – 1 MeV
2 keV (FWHM, 40 keV)
625 cm2
525 cm2 (100 keV)
110 cm2 (500 keV)
100 cm2 (Compton mode, 100 keV)
40 cm2 (Compton mode, 500 keV)
0.6 × 0.6 deg2 (< 100 keV)
4 × 4 deg2 (> 100 keV)
1.5 deg (RMS, Compton mode, 500 keV)
−20 – 0 degree

Table 3. Specification of the DSSD and the CdTe pixel detector for the SGD1DSSDCdTe pixel detectors

strip pitch
thickness
number of strips
active area
energy resolution
pixel size
thickness
number of pixels
active area
energy resolution

0.4 mm
0.5 mm125
5 × 5 cm2
1.5 keV (FWHM)
2 × 2 mm2
0.5 mm (thin), 5 mm (thick)
25 × 25
5× 5 cm2
1.5 keV (FWHM)

3. NARROW FOV COMPTON TELESCOPE
Highly sensitive observations in the energy range above the HXT/WXI bandpass is crucial to study the spectrumof accelerated particles. The SGD outperforms previous soft-γ-ray instruments in background rejection capability
by adopting a new concept of narrow-FOV Compton telescope.3, 4, 11 In the energy range above one hundredkeV (sub-MeV region), shielding against background photons becomes important yet difficult. The well-type
phoswich counter, which we developed for the WELCOME balloon experiment20, 21 and also for the Astro-E
Hard X-ray Detector (HXD)22–26 is a possible solution to achieve a very low background rate. The phoswich
configuration and a tight and active “well-type” shield made of BGO scintillators is expected to reduce thebackground to <10−4 counts/cm2 /s/keV, almost the limit achieved by the configuration of active shield andcollimator.
In order to lower the background dramatically and thus to improve the sensitivity as compared to the HXD,
we combine a stack of Si strip detectors and CdTe pixel detectors to form a Compton telescope. The telescopeis then mounted inside the bottom of a well-type active shield. As shown schematically in Fig.6, the telescopeconsists of 24 layers of DSSDs (double-sided silicon strip detectors) and 2 layers of thin CdTe pixellated detectors
surrounded by 5 mm thick CdTe pixellated detectors. Specifications for the SGD and for each component for
the Compton telescope are listed in Table 2 and Table 3, respectively. The opening angle provided by the BGO
shield is 4 degree at 500 keV. As compared to the HXD, the shield part is made compact by adopting the newly
developed avalanche photodiode.27, 28 An additional copper collimator restricts the field of view of the telescope
to 30’ for low energy photons (<100 keV) to minimize the flux due to the Cosmic X-ray Background from theFOV. These modules are then arrayed to provide the required area (Figure 7). Figure 8 shows a drawing of thedesign goal of the SGD instrument which consists of a 5 × 5 array of identical detector modules.
An important feature of the SGD is that we can require each event to interact twice in the stacked detector,
once by Compton scattering in a stack of Si strip detectors, and then by photo-absorption in the CdTe part

Figure 6. Conceptual design of the SGD module. A stack of 24 Si DSSDs and CdTe pixel detectors are assembled toform a semiconductor Compton Telescope. The telescope is mounted inside the bottom of a well-type active shield. The
opening angle provided by the BGO shield is 4 deg at 500 keV. An additional copper collimator restricts the field of viewof the telescope to 30’ for low energy photons (<100 keV). With this narrow FOV, events are rejected as background, if
the reconstructed Compton ring does not intercept the FOV.

(Compton mode). Once the locations and energies of the two interactions are measured, the Compton kinematics
allows us to calculate the energy and direction (as a cone in the sky) of the incident γ-ray by following theCompton equation,
Ein = E1 + E2 ,(1)
cos θ = 1 − me c2 (

1
1
−
),
E2
E1 + E2(2)

where E1 is the energy of the recoil electron, E2 is the energy of the scattered photon and θ is the scattering
angle. The high spectral and spatial resolution of Si and CdTe semiconductor detectors allow the instrument toachieve high angular resolution. Since the major interaction of photon above 60 keV in Si is Compton scattering,
the stack of DSSDs acts as an efficient scatterer especially for the low energy region below 300 keV. Regarding theuncertainty of the order of scattering in an event, we can use the relation that the energy deposition by Compton
scattering is always smaller than that of the photo-absorption for energies below Eγ = 255 keV (Eγ = me /2).
e 2 11This relation holds above this energy, if the scattering angle θ is smaller than cos−1 (1 − 21 ( mEγ ) ).
The major advantage of employing a narrow FOV is that the direction of incident γ-rays is constrained
inside the FOV. If the Compton ring does not intercept the FOV, we can reject the event as background. Most
backgrounds can be rejected by requiring this condition (albeit with an corresponding reduction in instrument
effective area). Background photons from the BGO and copper collimator for which the reconstructed Compton
ring intersects the FOV, cannot be eliminated if there is no signal detected in the active shield, however this sourceof background contributes only within a limited range of scattering angle. Combining background suppression
techniques available in the SGD, we expect to achieve background levels of 5×10−7 counts/s/cm2 /keV at ∼ 100
keV and 2×10−7 counts/s/cm2 /keV at ∼ 500 keV. As shown in Fig. 2, the effective area of the SGD in theCompton mode is 120 cm2 at 200 keV and 50 cm2 at 400 keV, if we use 25 units (total geometrical area is 625
cm2 ).

Figure 7. A schematic drawing of the SGD. It consists of 25 units of the narrow FOV Compton telescope surrounded by
24 shield counters made of BGO. The geometrical area of 525 cm2 is currently assumed for the sum of 25 units. If we selectan event that has two hits in the detector (and no hit in the BGO shields) and if we require the proper reconstruction
for the Compton kinematics, the effective area at 200 keV becomes ∼ 100 cm2 , including the reconstruction efficiecy.

Figure 8. Artist drawing of the SGD for one unit (left) and 25 units (right). The side BGO shield is not shown.

Figure 9. (left) Expected energy spectrum for a 100 ks observation of 0.1 − 100 mCrab sources with the Compton mode.
The background level of 5 × 10−7 counts/s/cm2 /keV is assumed. (right) Comparison of the energy spectrum for a 100 ks
observation of 1 mCrab source expected from the SGD operated in the Compton mode (bottom plot) and the instrumentwith the effective area of 3300 cm2 and the background level of 5 × 10−4 counts/s/cm2 /keV (top plot). A systematic
error of 5 % is included in the background estimation for both cases.

The concept of a narrow FOV Compton telescope is expected to reduce drastically the background fromradio-activation of the detector materials, which is a dominant background in the case of the Astro-E2 HXD.25
Figure 9 (left) shows the expected energy spectrum for a 100 ks observation of 0.1 − 100 mCrab sources by
the SGD with Compton mode, under an assumption of the background level of 5 x 10−7 counts/s/cm2 /keV.
Fig. 9 (right) compares the energy spectrum for a 100 ks observation of 1 mCrab source (photon index 1.7)
expected from the SGD Compton mode (bottom plot) and that for an instrument with an effective area of 3300
cm2 (50 times the SGD effective area) and a background level of 5 x 10−4 counts/s/cm2 /keV . These results,
derived from simple simulations, show that a high signal-to-background ratio is important to achieve the highsensitivity. In addition to the reduction of background, it should be noted that the 30’ FOV of the fine collimatoris required to improve the sensitivity limited by source confusion below a few hundred keV. With the SGD, we
can also measure polarization of incident gamma-rays from the azimuthal distribution of the Compton scatteredphotons.5, 31
Based on our CdTe and Si detector technologies, we are working on a Si/CdTe semiconductor Compton
telescope. The photo of DSSD and CdTe pixel detectors used in the prototype and spectra taken from those detectors are shown in Fig. 10 and Fig. 11. Performance of the prototype and results of polarization measurements
are described in other publications.18, 30–32

4. SUMMARY
The line and continuum sensitivities of the NeXT mission for 100 ks observation are shown in Fig.12 and Fig.13,
together with those of other missions. The continuum sensitivity could reach several × 10−8 photons/s/keV/cm2
in the hard X-ray region and a few × 10−7 photons/s/keV/cm2 in the soft γ-ray region. The high-energy responseof the super mirror of NeXT will enable us to perform the first sensitive imaging observations up to 80 keV. By
combining an X-ray CCD and a CdTe pixel detector, the WXI is an ideal solution for the focal plane detectorof the super mirror, providing fine imaging capability and high spectral resolution with almost full detection
efficiency. The narrow field-of-view Compton γ-ray telescope realized by the SGD extends the bandpass to well
above the cutoff for the hard X-ray telescope and allows us to study the high energy end of the particle spectrum
through the sensitive observation of the γ-ray spectrum up to 1 MeV. By combining these detectors, we expect
to achieve an unprecedented level of sensitivity in the hard X-ray and sub-MeV γ-ray region for both line andcontinuum emission.

13.9 keV

Entry [Counts/0.2 keV]

100000241

Am17.6 keV

80000

21.0 keV
60000

26.3 keV
40000

59.5 keV
20000

0
0

20

40

60

Energy [keV]

Figure 10. (left) Photo of three layers of DSSD stack developed for the prototype Si/CdTe Compton Camera30, 32 (right)
Energy spectrum from DSSD for 241 Am source. The signal from each strip is processed by a newly developed analog
front-end ASIC.29 An energy resolution of 1.3 keV (FWHM) for 60 keV can be achieved at 0 ◦ C .

Figure 11. (left) Photo of large area CdTe pixel detectors developed for the prototype Si/CdTe Compton Camera.18, 31
The detector has an area of 3.2 cm × 3.2 cm and a thickness of 0.5 mm. Pixel size is 2 mm × 2 mm. (right) Energy
spectrum from CdTe (one pixel) for 241 Am source. An energy resolution is 1.6 keV (FWHM) for 60 keV at 0 ◦ C .

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