Hard X-ray and Gamma-Ray Detectors for
the NEXT mission

Tadayuki Takahashi a,b , Kazuo Makishima b ,Yasushi Fukazawa c , Motohide Kokubun b ,Kazuhiro Nakazawa a , Masaharu Nomachi b , Hiroyasu Tajima d ,Makoto Tashiro e , Yukikatsu Terada fa Institute of Space and Astronautical Science, Yoshinodai 3-1-1, Sagamihara,Kanagawa, Japanb Department of Physics, University of Tokyo, Hongo, Bunkyoku, Tokyoc Department of Physics, Hiroshima University, Higashi-Hiroshima, 739-8526,

Japand Stanford Linear Accelerator Center, Stanford, CA 94309-4349
e Department of Physics, Saitama University,Saitama
f Riken, Wako, Saitama

Abstract
When compared with X-ray astronomy, the gamma-ray astronomy, especially inthe energy band from 10 keV to several MeV, is still immature and significant
improvements should be done to obtain sensitivity comparable to that achieved in
the energy band below 10 keV. In order to fill this “sensitivity gap”, the NeXT(New X-ray Telescope) mission has been proposed as a successor of the Astro-E2mission. The high-energy response of the super mirror will enable us to perform firstsensitive imaging observation up to 80 keV. One idea for the focal plane detector
is to combine a fully depleted X-ray imaging device (soft X-ray detector) and apixelated CdTe (Cadmium Telluride) detector. 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 has been proposed to obtain much higher
sensitivity than present instruments.
Key words: gamma-rays: instruments, Compton telescope, CZT, CdTe
PACS: 07.85 sep 95.55

Preprint submitted to New Astronomy

2 October 2003

1Introduction

The hard X-ray and gamma-ray bands are important windows for exploring
the energetic universe. It is in these energy bands that non-thermal emission,
mostly due to accelerated high energy particles, becomes dominant. In fact,
the results from recent cosmic X-ray observatories have shown that sensitive
observations in these high energy bands are necessary to constrain the continuum spectra of many classes of X-ray sources. However, the lack of sensitive
instruments in these energy ranges prevents us from further studying the nonthermal spectra. Therefore, hard X-ray and gamma-ray instruments in the21st century should provide much improved sensitivity for both point andextended sources over the instruments in use today. The NeXT (New X-rayTelescope) mission planned at ISAS (the Institute of Space and AstronauticalScience) is a successor to the Astro-E2 mission, with much higher sensitivity
in the energy from 0.5 keV to ∼ 1 MeV. The first imaging observation up
to 80 keV with a focusing telescope will open up a new window to explore
the non-thermal universe. Here we describe current ideas about hard X-raydetector and soft gamma-ray detectors proposed for the NeXT mission.

2Hard X-ray Observation with a super mirror and CdTe pixels

In the 10-100 keV band, the most promising approach to improve the sensitivity is to extend the energy coverage of the focussing telescope to higher
energies. This could be achieved by a multi-layer, grazing incidence hard X-ray
mirror (“super mirror”) (Yamashita et al., 1998) in conjunction with a hard Xray imaging detector. In addition to the imaging capability, the small volumeof the focal-plane hard X-ray detector significantly reduces the background,since the background flux generally scales with the size of the detector. Inorder to match the energy range covered by the super mirror (0.5 - 80 keV),
the focal plane detector is required to cover a very wide energy band. One idea
is to stack a fully depleted solid-state imaging detector in soft X-rays such asan X-ray CCD above a pixelated CdTe detector (Takahashi et al. 2000). For
this, we have been working on research and development of the high performance CdTe detectors (Takahashi et al. 2001). An energy resolution of 1 keV(FWHM) is obtained with the CdTe diode with an area of 2 mm × 2 mm anda thickness of 0.5 mm, operated at 5 ◦ C . The technology for gold-stud bumpbonding suitable for CdTe/CdZnTe semiconductor has been developed andused to connect each pixel to the fanout board consisting of bump pads andpatterns to route the signal from the pad or the ASIC itself. With this technology, we have developed prototype pixel detectors with an array of 16×16pixels using a with pixel size of 2×2 mm2 and 64×64 pixel detector with pixel
size of 200×200 µm2 .
2

Fig. 1. Schematic of the hybrid detector proposed for the NeXT. The CdTe pixel
detector is placed underneath the X-ray CCD. The Si strip detector is to shieldfluorescence lines from CdTe.

Figure 1 shows a schematic design of the Hard X-ray Imager (HXI), proposedas the focal-plane detector for the super mirrors. Soft X-rays will be absorbed
in the CCD, while hard X-rays will penetrate the CCD and be absorbed in theCdTe pixelated array. In the latter device, pixel electrodes are formed on thesurface of CdTe and each electrode is bump-bonded to the readout electronics
chip. In order to obtain the ultimate sensitivity for hard X-ray enegy, weincorporate active shielding by BGO crystal. The detectors will have a fast
timing resolution of 10 to 100 µs, so that we can veto the signal when there isa hit in the active shield. With NeXT, we expect to achieve an area of about
500 cm2 at 30 keV with a typical angular resolution of a spot of 30”. For apower-law spectrum with photon index of 1.7, the source detection limit in 100
ksec (assuming a negligible background in the source region) would roughly be
10−14 erg cm2 s−1 in terms of the 10-40 keV flux, i.e. two orders of magnitude
lower than the present instrumentation.

3Narrow FOV Compton telescope

In the energy range above hundred keV (sub MeV region), shielding against
background photons becomes important yet difficult. The phoswich configuration and a tight and active “well-type” shield, which we developed for
the Astro-E Hard X-ray Detector (HXD) (Tashiro et al. 2002, Makishima etal. 2002) is a solution to achieve a very low background rate. The well-typeshield made of BGO scintillators, which surrounds the detection part (GSOscintillator) is expected to reduce the background to the limit achieved by theconfiguration of active shield and collimator.
In order to further reduce the background and thus to improve the sensitivity, we have proposed a new detector called the Soft Gamma-ray Detector(SGD) for the NeXT mission, which utilizes the idea of a narrow FOV Compton telescope (Takahashi et al. 2002) for the NeXT mission as illustrated in3

Fig.2, In the SGD, we combine a stack of Si strip detectors and CdTe pixel
detectors, to form a Compton telescope, which is mounted inside the bottomof the well-type active shield. Firstly, the stack configuration and individual
readout provide informations of the interaction depth. This depth informationis very useful to reduce the background, because we can expect that low energy gamma-rays interact in the upper layers and, therefore, we can reject low
energy events detected in lower layers. Moreover, since the background rate
is proportional to the detector volume, low energy events collected from thefirst few layers in the stacked detector have a high signal to background ratio,in comparison with events obtained from a monolithic detector with the same
thickness as the sum of all layers.
Important feature of the SGD is that we can require each SGD event to interact
twice in the stacked detector, once by Compton scattering in the Si part,and then by photo-absorption in the CdTe part (Compton mode). Once thelocations and energies of the two interactions are measured, the Compton
kinematics allow us to calculate the angle between the telescope axis and theincident direction of the event. The high energy resolutions of the Si and CdTe
devices help reducing the width of these ”Compton rings”. We can determine
the location of point sources as intersections of multiple rings, although theangular resolution is limited to ∼ 1deg at 300 keV due to the finite momentumof the Compton-scattering electrons (Zoglauer and Kanbach 2002). Regarding
to the reconstruction of the order of the scattering, we can use the relation thatthe energy deposition by Compton scattering is always smaller than that of thephoto absorption for energies below Eγ = 255 keV (Eγ = me /2). This relation
eholds above this energy, if the scattering angle θ is smaller than cos−1 (1− 21 m).
EγThe major advantage of employing the Compton kinematics, however, is toreduce the background. By having a narrow FOV, and by requiring the Compton ring of a valid aperture gamma-ray event to intersect with the FOV, wecan reject most of the background events. Particularly, this is expected todrastically reduce the background from radio-activation of the detector materials, which becomes a dominant background in the case of the Astro-E HXD.
Furthermore, we can eliminate Compton rings produced by bright sources located outside the FOV, which would produce significant background without
the narrow FOV. The narrow-FOV design of the SGD is matched to the main
objective of the NeXT mission, i.e., the use of the super mirror coupled to theHXI.
We are studying the optimum configuration of the geometry to be used inthe SGD based on the currently available Si and/or CdTe imaging devices(Takahashi et al. 2003, Tajima et al., 2003). The efficiency of the SGD whenemploying 24 layers of 0.5 mm thick Si double strip detectors and a 6 mmthick CdTe pixel detector is shown in Fig. 3 (b). The line and continuum
sensitivities of the SGD and the HXI for 100 ks observation are shown in4Fig. 2. A conceptual design of the narrow FOV Compton telescope, composed of astack of 24 silicon double strip detectors and a CdTe pixel detector. Both are buriedin a deep BGO well (left). That of the SGD employing 16 units of the Compton
telescope (right). The geometrical area of 400 cm2 is currently assumed for the sumof 16 units.

Fig. 3. Efficiency of the SGD shown in Fig. 2Fig. 4, together with those of the HXI and other missions. With the SGD,
we can also measure polarization of incident gamma-rays from the azimuthal
distribution of Compton scattered photons. We may improve the lower-energy(below ∼ 150 keV) angular resolution by incorporating coded-mask or Fouriersynthesis (Kotoku et al. 2003) optics.
5Fig. 4. The expected HXI and SGD senstivities for line (left) and continuum (right)
emissions, assuming an observation time of 100 ks.

4Conclusion

Based on our recent achievements of high performance CdTe and Si imaging
detectors, we have proposed the HXI and the SGD for the NeXT mission.
By combining these detectors, we are expected to achieve the highest sensitivity ever in the hard X-ray and sub-MeV gamma-ray region for both line
and continuum emission. The SGD will provide the capability of polarization
measurements above 100 keV.

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