New Century of X-ray Astronomy
ASP Conference Series, Vol. 251, 2001
H. Inoue and H. Kunieda, eds.

Future Hard X-ray and Gamma-ray Observations
Tadayuki Takahashi
Institute of Space and Astronautical Science, Sagamihara, Kanagawa,229-8510, Japan
Tuneyoshi Kamae
Department of Physics, Stanford University, Stanford, CA 94305-4060
Kazuo Makishima
Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo, 113-0033,
Japan
1. Introduction
Hard X-rays and gamma-rays are important frequency windows for expoloring the energetic universe. It is in these energy bands that the non-thermal emission, mostlyduehighparticles, becomes dominant. In fact, the results fromto accelerated energyand 
showed that sensitive observations in these bands are necessary to constrain the continuum spectra of many classes of X-ray sources. However, thelack of sensitive instruments prevents us from further studying the non-thermal spectra. As clearly shown in Fig.1, the situation is much worse in the hardX-raysoft andgamma-ray bands, where the reduction of background is difficult. Thesatellite
revealed the fact that a variety of gamma-ray sources
exist in the sky and the improvedsatellite will be a good advance,
angular and spectral resolution by the 
but the sensitivity of instruments are far from the level achieved by the current X-raymissions employing focusing telescopes in the energy band below 10 keV.
In order to fully explore the dynamic universe, hard X-ray and gamma-ray instruments in the 21st century should provide much improved sensitivity for both point andextended sources over the instrumens in use today.
2. Hard X-ray Imaging Observations
In the 10–100 keV band, the most promising approach is to extend the energy coverageof the focusing telescope to higher energy. This could be achieved by a multi-layer,grazing incidence hard X-ray mirror (“super mirror”, Kunieda et al. 2001). In addition
to the focusing capability, the small volume of a hard X-ray imaging detector as a
focal plane instrument significantly reduces the background, since the background flux
generally scales with the size of the detector. In order to cover the field of view of
the telescope, the detector should have an area of several cm . Fast timing of several
hundred ns will be required for active shielding, which provides a low backgroundenvironment in space. Detectors should have a fine position resolution of a few 100  mand a high energy resolution better than 1 keV (FWHM) in this energy range. Research210

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Figure 1.
The sensitivity for point-sources with a continuum spectrum forvarious past and future X-ray and gamma-ray missions.

and development toward these goals are under way by several groups by utilizing highZ semiconductor materials such as CdTe (e.g. Takahashi et al. 2001)
Since the super mirror is able to cover the energy range from  0.5 keV up to
40–60 keV, the focal plane detector is required to cover a very wide energy band. For
this, a combination of a back-illuminated X-ray CCD and a pixelated CdTe detector has
been proposed as a new concept (Takahashi et al. 1999). In this “hybrid detector” (Fig.
2), soft X-rays will be absorbed in the X-ray CCD, and hard X-rays will penetrate theCCD and be absorbed in the CdTe pixelated array (see also Tsuru et al. 2001).

3. Applications of Compton Scattering
The sensitivity of observations with hard X-ray and gamma-ray detectors are limited by
the background rate. The phoswich configuration and the narrow field of view obtainedby a tight active “well-type” shield developed for the Astro-E Hard X-ray Detector(HXD) is the solution to achieve a very low background rate in the energy range up
to several hundred keV (Makishima et al. 2001). In the well-type configuration, thedetection part is surrounded in almost all directions by the active shield. Therefore,
background photons which deposit some of their energy in the shield through Comptonscattering are rejected efficiently.
In order to further reduce the background, we have proposed a new detector basedon thin CdTe pixel or strip device and the well-type shield (Fig. 3). The key concept is
the requirement of Compton kinematics for events which interact twice (one by Compton scattering and the other by photo-absorption) in the stacked detector. Since werequire that gamma-rays should come from the narrow FOV of the well-type collimatorand do not escape from the CdTe part, we can constrain the energy of two gamma-ray
interactions from the measured scattering angle from the formula of Compton scattering. The energy resolution, again, is an important issue for this kind of configuration.
Twenty four layers of a 0.5 mm thick CdTe detector give the sufficient detection probabilty of  20 % via single Compton scattering and subsequent photo absorption. One

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Future Hard X-ray and Gamma-ray Observations
Backside X-ray CCD (several 10 microns thick)!

SIDE VIEW

CdTe pixel arrays"Bump Bonding to ASIC

BGOActive Shield &CollimatorPhotomultiplier#TOP VIEW
Readout contoller

Figure 2. A schematic diagram of a wide band imaging spectrometer detector to be used at the focal plane of a super mirror. The detector is a combination of a back-illuminated X-ray CCD and a pixelated CdTe detector covering 0.1 - 60 keV (from Takahashi et al. 1999).

important aspect of this concept is that we can measure polarization from the azimuthal
distribution of Compton scattered photons.

4. Next generation Compton telescope
As gamma-ray energy increases to the MeV region, the detection becomes very difficult. By utilizing the technique of the Compton telescope, the $%&(')+*, instrument
on-board the $.-/% satellite has achieved pioneering results in this energy region. Inorder to go beyond the level achieved by the $%&(')*, and fill the gap, secondgeneration Compton telescopes have been proposed by several groups (e.g. Kanbachet al. 2000). Among them, a concept called the Multiple Compton Method, which
was originally proposed by Kamae et al. (1987), is very attractive. In this method, astack of several tens ( 0 50) of layers of thin semiconductor imaging detectors is used
to follow multiple Compton scatterings. If at least the first three or four Compton scatterings take place in the layers and the energies and positions are recorded, we can findthe correct order of the first two scattering points and then calculate the directions andthe energies of the incident gamma-rays. The key points of this method is that, if theincident gamma-rays undergo multiple interactions which are sufficient to reconstruct
the correct order, gamma-rays do not need to be fully absorbed by the detector and areallowed to escape. The conceptual Advanced Compton Telescope (Kurfess et al. 2000)
uses the multi-Compton kinematics and has a very large area and intends to improve
the sensitivity in the MeV region for two orders of magnitude in comparison with thatof $.%1&2')*., .
Since we have to handle many channels from many layers of imaging devices, weneed advanced technology for both the materials and front-end electronics. In order toestablish the concept of the multi-Compton camera, we are now working on a small

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Figure 3.
The conceptual design of the next generation well-type gammaray detector. A stack of CdTe pixel or strip detectors are buried in the deep
BGO well.
detector for the detection of the polarization of gamma-rays from gamma-ray bursts, by
combining high resolution Si and CdTe imaging detectors.
5. Conclusion
Our knowledge about the universe is very limited in the energy region above several 10
keV where the non-thermal emission, produced by energetic particles becomes dominant. However, when compared with X-ray Astronomy, gamma-ray astronomy is still
immature and significant improvements should be done to obtain sensitivity comparable to that achieved in the energy band below 10 keV.
References
Kamae, T. et al. 1987, Nucl. Instr. Meth., A260, 254
Kanbach, G. et al. 2000, MEGA proposal
Kunieda, H. et al. 2001, in this proceedings
Kurfess, J. et al. 2000, in the Fifth Compton Symposium, (New York : AIP) 510, 789
Makishima, K. et al. 2001, in this proceedings
Takahashi, T. et al. 1999, Nucl. Instr. Meth., A436, 111Takahashi, T. et al. 2001, IEEE N.S. in press
Tsuru, T. et al. 2001, in this proceedings