Application of CdTe for the NeXT Mission
Tadayuki Takahashi a,b Kazuhiro Nakazawa a Shin Watanabe a
Goro Sato a,b Takefumi Mitani a,b Takaaki Tanaka a,b
Kousuke Oonuki a,b Ken’ichi Tamura a,b Hiroyasu Tajima c
Tuneyoshi Kamae c Greg Madejski c Masaharu Nomachi dYasushi Fukazawa e Kazuo Makishima b Motohide Kokubun b
Yukikatsu Terada f Jun Kataoka g Makoto Tashiro ha The Institute of Space and Astronautical Science

3-1-1 Yoshinodai, Sagamihara, Kanagawa, 229-8510, Japanb University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo 113-0033, Japanc Stanford Linear Accelerator Center, Stanford, CA 94309-4349, USA
d Osaka University, Toyonaka, Osaka, 560-0043, Japane Hiroshima University, Higashi-Hiroshima, Hiroshima, 739-8526, Japanf RIKEN, Wako, Saitama, 351-0198, Japang Tokyo Institute of Technology, Meguro-ku, Tokyo, 152-8551, Japanh Saitama University, Saitama, 338-8570, Japan

Abstract
Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and γ-ray detection.
The high atomic number of the materials (ZCd =48, ZT e =52) gives a high quantum
efficiency in comparison with Si. The large band-gap energy (Eg = 1.5 eV) allows tooperate the detector at room temperature. Based on recent achievements in highresolution CdTe detectors, in the technology of ASICs and in bump-bonding, wehave proposed the novel hard X-ray and γ-ray detectors for the NeXT mission in
Japan. The high-energy response of the super mirror onboard NeXT 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 up to ∼ 1 MeV, a narrow field-of-view Compton γ-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.
Key words: gamma-ray astronomy, Compton telescope, CZT, CdTe

Preprint submitted to Elsevier Science

6 November 2004

PACS:

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, primarily due to accelerated high-energy particles,
becomes dominant. The Compton Gamma-ray Observatory (CGRO) satellite,
which operated from 1991 to 2000, revealed that there are a variety of γ-ray
sources. With its four onboard instruments, CGRO covered a wide energy
range, from 20 keV to 30 GeV. This wide energy coverage, six decades in photon energy, played an important role in studying the realm of non-thermal
astrophysics.
We can expect a further revolution brought by the INTEGRAL, Astro-E2,
GLAST and other near-future missions. However, when compared with X-ray
astronomy, hard X-ray and γ-ray astronomy are still immature. As clearlyshown in Fig. 1, the sensitivity of instruments is far from the level achievedby the current X-ray missions employing focusing telescopes in the energy
band below 10 keV. The situation is particularly unsatisfactory in the region
between ∼10 keV and ∼100 MeV. In this “sensitivity gap”, the jump at thelow energy end comes from the fact that current X-ray mirrors cannot focus
hard X-rays. At energies above ∼100 MeV, sensitivity is recovered by utilizing
pair-production telescopes.
In order to study the non-thermal emission and to draw a more complete picture of the non-thermal universe, highly sensitive observations by high energy
missions are important. In particular, hard X-ray and γ-ray observations tomatch observations at other wavelengths are indispensable. To this end, futurehigh-energy instruments should provide much improved angular and spectral
resolutions over the instruments in use today. Cadmium telluride (CdTe) andcadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials to realize such instruments for the next generation of astronomical observations in the hard X-ray and γ-ray regions [1–3]. Here, we report
recent achievements of high-resolution CdTe detectors and their application
to the Japanese future high-energy mission.
2

Fig. 1. Sensitivities of past and future X-ray and γ-ray missions together with thesensitivity of ground based imaging atmospheric Cherenkov telescopes (IACTs).
The spectrum of 0.1 % of the Crab Nebula is shown.

2

High resolution CdTe detector

Among the range of semiconductor detectors available for γ-ray detection,
CdTe and CdZnTe have a privileged position because of their high density
and the high atomic number of their components, as well as a wide bandgap.
In the 1990’s, the remarkable progress in the technology of producing a highquality single crystal of CdTe and the emergence of CdZnTe have dramaticallychanged the situations of high resolution room temperature detectors based onthese materials. A review of the recent progress of CdTe and CdZnTe detectors
is available in Takahashi & Watanabe [4]. The performance of CdTe for the
IBIS instrument onboard the INTEGRAL mission and CdZnTe for the BAT
instrument on the Swift mission are described in Lebrun et al. [5] and Sato etal. [6], respectively.
For an imaging detector, the good energy resolution and ability to fabricatecompact arrays expected from semiconductor detectors are very attractive
features in comparison with inorganic scintillation detectors coupled to either
photodiodes or photomultiplier tubes. The uniform charge transport properties of the wafer are very important, not only for fabricating large area strip or
pixel detectors but also for constructing a large scale γ-ray camera consistingof many individual detectors. Recently, based on single crystal growth by theTravelling Heater Method (THM-CdTe) by ACRORAD [7], the crystal produced is large enough to obtain (1 1 1)- oriented single-crystal wafers with anarea as large as 40×40 mm2 (Fig. 2 (a)) [8]. Although the energy information
can not be obtained, the fine-pitch CdTe γ-ray camera recently developed by
ACRORAD already demonstrates the potential of a γ-ray imager made of3

Fig. 2. (a) (left) photo of THM-CdTe wafer and (b) (right) γ-ray photo taken with
the CdTe γ-ray camera with a pixel size of 200 µm (courtesy of ACRORAD Co.).

highly uniform CdTe semiconductor (Fig. 2 (b)).
Based on the THM-CdTe wafers, we have developed CdTe detectors with highenergy resolution for both planar and pixel configurations [3,9–13,15]. We havedemonstrated that the wafer shows good uniformity when we make a large area
CdTe detector with planar electrodes [9,11]. According to measurements using
a collimated γ-ray beam from an 241 Am source, the location of the peak in thepulse height distribution is obtained to within 0.1 % and the variation of thearea of the 60 keV peak is less than 0.9 %, regardless of the position in the
21 mm × 21 mm surface. In addition to the highly uniform wafer, we haveachieved a significant improvement in the spectral properties by forming the
Schottky junction on the Te side of the CdTe wafer [3]. For the detector withan area of 2 mm × 2 mm and a thickness of 0.5 mm, the leakage current of
the new CdTe diode is as low as 10 pA at 20 ◦ C at a bias voltage of 800
V. As demonstrated in Fig. 3 (a) and (b), an energy resolution of 1.1 keV(FWHM) for the 122 keV line from 57 Co is obtained [13]. When we cool down
the detector, the energy resolution is improved. A CdTe diode with an area of
3 mm × 3 mm and a thickness of 1 mm installed in the system manufactured
by Amptek [14] has an energy resolution of 260 eV (FWHM) at 6.4 keV when
operated at −40 ◦ C .

3

NeXT Mission

The NeXT (Non-thermal Energy eXploration Telescope) mission proposed inJapan is a successor to the Astro-E2 mission, and is optimised to study thehigh-energy non-thermal universe [16,17]. NeXT will carry three hard X-ray
telescopes (HXTs) for the wide band X-ray imager (WXI), one soft X-ray
telescope for the soft X-ray spectrometer, and a soft γ-ray detector (SGD).
The HXT is based on a depth-graded multi-layer mirror, referred to as a super4

Fig. 3. (a) 57 Co spectrum obtained with the CdTe diode operated at 20 ◦ C .The
active area of the detector is 2×2×0.5 mm3 . The applied voltage is 800 V. (b) 57 Co
spectrum obtained with the CdTe diode system manufactured by Amptek [14] . The
detector size is 3×3×1 mm3 and the operating temperature is −40 ◦ C .

mirror, to enhance the hard X-ray reflectivity. 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 [19]. 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 telescope [18,20] to improve the sensitivity in the soft γ-ray region formatching 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 up to 1 MeV. Furthermore, as described in a separate paper [23],the SGD will be sensitive to the polarization of the incident γ-rays, and will
potentially be the first hard X-ray polarimeter in orbit.

4

Hard X-ray Imager (HXI)

In order to match the energy range covered by the super mirror (0.5 − 80 keV),
the focal plane detector for the HXT is required to cover a very wide energy
band. To this end, we have proposed a new focal plane detector based on
an idea of combining an X-ray CCD and a CdTe (Cadmium Telluride) pixel
detector as the Wide-band X-ray Imager (WXI) [3,20,21]. As schematicallyshown in Figure 4, the WXI is composed of two sub-instruments; the hard
X-ray Imager (HXI) and the soft X-ray Imager (SXI). In the WXI, soft Xrays will be absorbed in the CCD of the SXI, while hard X-rays will penetrate
through the CCD and be absorbed in the CdTe pixel detector in the HXI. For
the detection of soft X-rays below 10−20 keV with the high position resolution,a CCD with very thin dead layer will be used in the SXI [22].
Semiconductor detectors such as CdTe with a high mass absorption coefficientare crucial for the detection of hard X-ray photons. For CdTe, even a detector
5Table 1
Specification for the HXI [16]HXI

Energy Range

8 – 80 keVEnergy Resolution

0.5 – 1 keV (FWHM)

Trigger Threshold∼ 8 keV

Pixel Size

200 ∼ 500 µm

Detector Size

20 mm × (20 – 30) mm

Timing Resolution< 100 µs

Operating Temperature−50 – 0 degreewith a thickness of 0.5 mm provides a good detection efficiency for the hard
X-ray region up to 80 keV. 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 ahigh energy resolution, better than 1 keV (FWHM), in the energy range from
5 keV to 80 keV. Signals from the individual pixel electrodes formed on thesurface 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. Specifications of the HXI are listed in Table 1.
As shown in Fig. 4, 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 background in the energy range ofthe 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 thatreach the CdTe detector and give rise to activation.
With NeXT, we expect to achieve an area of about 750 cm2 at 30 keV witha typical angular resolution of 30”. Fig. 5 shows the efficiency of the WXIwith 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 be 10−14 erg cm−2 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 ordersof magnitude better than present instrumentation.

5

Soft Gamma-ray Detector

Highly sensitive observations in the energy range above the bandpass of thehard X-ray telescope are crucial to study the spectrum of accelerated particles.
6

Fig. 4. 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 beneath the 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 fluorescence lines from
CdTe [18].

Fig. 5. Efficiency of the SXI with a 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.

The SGD outperforms previous soft-γ-ray instruments in background rejection capability by adopting a new concept of narrow-FOV Compton telescope[21]. In the energy range above one hundred keV (sub-MeV region), shielding
against background photons becomes important yet difficult. The well-type
phoswich counter, which we developed for the WELCOME balloon experiment [24] and also for the Astro-E Hard X-ray Detector (HXD) [25–27] is a7

Fig. 6. Conceptual design of the SGD module [21]. A stack of 24 Si DSSDs and
CdTe pixel detectors are assembled to form 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 view of the telescope to 30’ for low energy photons (<100keV). With this narrow FOV, events are rejected as background if the reconstructed
Compton ring does not intercept the FOV.

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 the background to <10−4 counts/cm2 /s/keV, almost thelimit achieved by the configuration of active shield and collimator.
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 telescope is thenmounted inside the bottom of a well-type active shield. As shown schematically in Fig.6, the telescope consists of 24 layers of DSSDs (double-sided silicon
strip detectors) and 2 layers of thin CdTe pixellated detectors surrounded by 5mm thick CdTe pixellated detectors. The opening angle provided by the BGO
shield is 4 degrees at 500 keV. As compared to the HXD, the shield part ismade compact by adopting the newly developed avalanche photodiode [30,31].
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 the FOV. These modules are then arrayed to provide
the required area (Figure 9).
An important feature of the SGD is that we can require each event to interact
8Fig. 7. Effective area of the SGD. Solid line shows the area calculated for photoabsorption. Dashed line shows the area when the detector is operated as a Compton
cameratwice in the stacked detector, once by Compton scattering in a stack of Si
strip detectors, and then by photo-absorption in the CdTe part (Comptonmode). Once the locations and energies of the two interactions are measured,
the Compton kinematics allows us to calculate the energy and direction (as acone in the sky) of the incident γ-ray by following the Compton equation,
Ein = E1 + E2 ,cos θ = 1 − me c2 (

(1)
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[28,29]. As shown in the equations,
incident direction of a γ-ray is calculated from the position and the energy
information of interactions. The high spectral and spatial resolution of Si and
CdTe semiconductor detectors allow the instrument to achieve high angularresolution. Since the major interaction of photons above 60 keV in Si is Compton scattering, the stack of DSSDs acts as an efficient scatterer especially forthe low energy region below 300 keV. Regarding the uncertainty of the orderof 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 forenergies below Eγ = 255 keV (Eγ = me /2). As shown in Fig. 8, this relation
holds above this energy and there is only one solution, if the scattering anglee 2) ).
θ is smaller than cos−1 (1 − 21 ( m
Eγ
The major advantage of employing a narrow FOV is that the direction ofincident γ-rays is constrained inside the FOV. If the Compton ring does notintercept the FOV, we can reject the event as background. Most backgrounds
can be rejected by requiring this condition (albeit with an corresponding re9

Fig. 8. The ordering of Compton scattering and photo-absorption is uniquely identified, if the scattering angle is smaller than the value shown by the solid curve.
With this condition, the energy deposited by Compton scattering (first scattering)is always smaller than that of the photo-absorption.

duction in instrument effective area). Background photons from the BGO andcopper 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 source of background contributes only within a limited range ofscattering angle. Combining background suppression techniques available in
the SGD, we expect to achieve background levels of 5×10−7 counts/s/cm2 /keVat ∼ 100 keV and 2×10−7 counts/s/cm2 /keV at ∼ 500 keV. As shown in Fig.
7, the effective area of the SGD in the Compton mode is 120 cm2 at 200 keV
and 50 cm2 at 400 keV, if we use 25 units (total geometrical area of 625 cm2 ).
The concept of a narrow FOV Compton telescope is expected to reduce drastically the background from radio-activation of the detector materials, which
is a dominant background in the case of the Astro-E2 HXD [27]. The 30’ FOVof the fine collimator is required to improve the sensitivity limited by source
confusion below a few hundred keV. With the SGD, we can also measure polarization of incident γ-rays from the azimuthal distribution of the Compton
scattered photons [12,23].
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. We have successfully obtained clear images and spectra in
the energy range from 80 keV up to 700 keV. The performance of the prototype
and results of polarization measurements are described in other publications[12,13,33,34,36]. In order to realize the Compton Camera that fits in the BGO
well, we need to optimize the mechanical support structure and the readout
scheme. Efforts to improve the performance of CdTe pixel detectors as thickas 5 mm are under way (Fig. 12).
10

Fig. 9. 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. A geometricalarea of 525 cm2 is currently assumed for the sum of 25 units. If we select an eventthat 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 efficiency.
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]

Fig. 10. (left) Photo of three layers of DSSD stack developed for the prototype
Si/CdTe Compton Camera [33,34] (right) DSSD Energy spectrum from an 241 Am
source. The signal from each strip is processed by a newly developed analog front-end
ASIC developed with Ideas ASA [32,35]. An energy resolution of 1.3 keV (FWHM)
for 60 keV can be achieved at 0 ◦ C .

6

Conclusion

The continuum sensitivities of the NeXT mission could reach several × 10−8
photons/s/keV/cm2 in the hard X-ray region and a few × 10−7 photons/s/keV/cm2
11

Fig. 11. (left) Photo of large area CdTe pixel detectors developed for the prototype
Si/CdTe Compton Camera [12,13]. The detector has an area of 3.2 cm × 3.2 cm anda thickness of 0.5 mm. Pixel size is 2 mm × 2 mm. (right) Energy spectrum from
CdTe (one pixel) with an 241 Am source. The energy resolution is 1.6 keV (FWHM)
for 60 keV at 0 ◦ C .

Fig. 12. (left) Photo of the mechanical model of the DSSD stack for the SGD. The
signals from each strip will be extracted from the side wall of the stack and processedby front-end ASICs. (right) 5-mm thick CdTe Pixel detector with a pixel size of 2
mm × 2 mm (64 pixels).

in the soft γ-ray region [16,21]. The high-energy response of the super mirrorof 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 detector of the super mirror, providing fineimaging 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 andallows us to study the high energy end of the particle spectrum through thesensitive observation of the γ-ray spectrum up to 1 MeV. By combining thesedetectors, we expect to achieve an unprecedented level of sensitivity in thehard X-ray and sub-MeV γ-ray region for both line and continuum emission.
12

Acknowledgement
This research was partially supported by the Ministry of Education, Science,Sports and Culture, Grant-in-Aid in Japan (13304014, 14079207).

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