5. TECHNOLOGIES

The breadth of gamma rays in the electromagnetic spectrum - a range of more than 10⁹ in photon energy - demands a wide variety of technologies. Advances in relevant technologies span the full range of opportunities for gamma-ray instruments, giving a high potential for major improvements in many aspects of the field. Section 5.1 describes the imaging techniques needed in the different parts of the gamma-ray spectrum. Section 5.2 illustrates how technologies are currently being developed to take advantage of the full range of gamma-ray possibilities.

5.1 IMAGING TECHNIQUES

The physics of gamma-ray detection is the ultimate driver for all gamma-ray telescopes. In the keV energy range, gamma rays interact primarily through the photoelectric effect; in the MeV range, primarily through Compton scattering; and at energies above a few tens of MeV, almost exclusively by electron-positron pair production. Only at the lowest gamma-ray energies is any form of reflection possible. Most gamma-ray telescopes require substantial detector areas.

Table 5.1 Comparison of gamma-ray imaging techniques

Imaging Technique	Energy Range	Characteristics
Multi-layer mirrors	below 100 keV	high resolution, narrow field-of-view
coded-aperture mask	below 10 MeV	good resolution, wide field-of-view
Compton telescope	~1 MeV - ~ 100 MeV	good resolution, wide field-of-view
Pair telescope	above 10 MeV	good resolution, wide field-of-view
Atmospheric Cerenkov	above 100 GeV	good resolution, narrow field-of-view

5.1.1 MULTILAYER MIRRORS

The familiar technical challenge to extending traditional grazing incidence optics into the hard X-ray band (E /10keV) is the decrease with energy in incident angle (referred to as graze angle) for which significant reflectivity can be achieved. For a Wolter or conical approximation mirror geometry, the graze angle, g, on a given mirror shell is related to the focal ratio by g = 1/4 x (r/f), where r is the shell radius and f is the focal length. Coating the reflective surfaces with multilayer structures, which operate on the principal of Bragg reflection, can substantially increase the maximum graze angle for which significant reflectivity is achieved over a relatively broad energy range, while maintaining realistic focal ratios. Other concentrating techniques and mirror geometries such as polycapillary optics and Kirkpatrick-Baez telescopes can also be extended into the hard X-ray band; however, given the current state of technology and the desire for good imaging performance, systems based on Wolter-I or conical optics are the most attractive.

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