The requirements for the multilayer materials are that the K-shell absorption edges not lie in the energy range of interest, and that the two materials employed be chemically compatible for forming stable thin films. In the hard X-ray band, this is satisfied by several material combinations that have been used to fabricate X-ray multilayers with the appropriate dimensions. The most promising combinations for operation above ~5 keV include W/Si (W K-edge at 69.5 keV), Ni/C (both edges below 10 keV, and therefore effective up to ~100 keV), and Pt/C (Pt K-edge at 78.4 keV). Technical limits restrict the operational energy band to below ~ 120 keV. The graze angles for multilayer hard X-ray telescopes are still smaller than those typically employed at low energies. Therefore thin, light-weight, highly-nested mirror substrates are required. Development of such optics is also critical for future missions operating in the soft X-ray band (the spectroscopy telescopes on HTXS, for example), and these efforts are directly applicable to future hard X-ray focusing missions.

5.1.2 CODED APERTURE IMAGING

Although multilayer coatings and small graze angles allow imaging up to perhaps 100 keV (see above), this is restricted to narrow fields-of-view (typically less than 10 arcmin). Although Bragg reflection can be incorporated into Laue lenses at still higher energies (e.g., up to several MeV), these focusing techniques are restricted to narrow energy bands (typically less than 1-2% of the incident energy). Therefore alternative concepts must be used to achieve the important advantages that imaging, with moderate to wide fields-of-view, can provide: simultaneous measurements of source(s) and background without the need to chop on and off source; measurements of source locations with resolution typically much higher than in non-imaging (e.g., collimated) detectors; and resolving source structure for true imaging of extended sources. All of these can be achieved by using coded aperture imaging, whereby images are constructed from shadows of a coded aperture mask cast on a position-sensitive detector located at focal length, below the mask. Coded aperture imaging is particularly well suited for the hard X-ray/soft gamma-ray band (10 keV - 1 MeV) since it depends on source photons being either absorbed (photoelectric) or scattered (Compton) if they strike a closed cell of the coded mask. However the images become increasingly blurred, with consequent loss of sensitivity, as Compton scattering dominates and the coded mask becomes (eventually) optically thin; thus it is not optimum for energies above ~1 MeV.  For a coded mask of open and closed holes with usual open fraction 0.5, images are derived simply by correlating the detected pattern of source counts on the detector with the (known) pattern of the mask. This may be understood simply as measuring the x- and y- shift of the detected shadow on the detector and thus the angular position (in the orthogonal angles giving rise to x- and y- offsets) of the source relative to the optical axis of the telescope. The technique has now been well developed and a variety of successful imaging telescopes have been flown from balloons and in space. The premier space mission to date has been the French/ Russian SIGMA telescope which imaged selected regions of the sky (primarily the galactic center region) down to sensitivities of (typically) 30-50 mCrab in the 35-150 keV band. Future missions are now planned (INTEGRAL) or proposed (e.g., EXIST, BASIS, BLAST) which will be based on coded aperture imaging. The proposed missions are all survey missions and thus require very large fields-of-view for maximum exposure time, temporal coverage and sensitivity. These requirements effectively point to coded aperture imaging as the imaging technique of choice. The technique requires position sensitive detectors of large area and high spatial resolution. New CZT detectors (cf. section 5.2.1) are particularly promising since they provide fixed pixels (which can be very small) and yet high-energy resolution.

5.1.3 COMPTON SCATTER TELESCOPES

Compton telescopes have been used since the early 1970's for making astronomical observations and measurements above about 1 MeV. The first orbiting Compton telescope is COMPTEL on the CGRO and it has performed the first all-sky survey at MeV energies with a resolution of about one degree. The principle behind the instrument is that photons scatter in a low-Z material in a forward detector. The scattered photon is then detected by a second, or rearward detector, typically made of a high-Z material to fully absorb the remaining energy. By requiring that the photon scatter twice, three advantages become apparent. The first is that the telescope has a natural directionality. If the time-of-flight is measured between the triggered detectors, then only photons traveling in the proper direction (forward or backward) need be accepted for further analysis.