Introductions to high-energy astrophysics traditionally begin by deploring the extreme experimental difficulties this discipline has to deal with. Nuclear astrophysics suffers from a handicap that is ultimately caused by the loss of information during inelastic interaction processes of gamma-ray photons with matter. Unlike the photons at longer wavelengths that mainly undergo coherent scattering in the atmosphere and in the telescopes, gamma-rays interact with matter primarily by incoherent processes. We have become used to accept that it is “impossible to reflect or refract gamma-rays” that have wavelengths two to three orders of magnitude shorter than the distances between atoms in solids.
Consequently, present types of telescopes for nuclear astrophysics make use of inelastic interaction processes : most of the instruments are based on geometrical optics (shadowcasting in modulating aperture systems) or quantum optics (kinetics of Compton scattering). Because the collecting area of such systems is identical to the detector area, nuclear astrophysics has come to a mass-sensitivity impasse where “bigger is not necessarily better”. Improving the sensitivity of an instrument can usually be obtained by a larger  collection area - in the case of classical gamma-ray telescopes this can only be achieved by a larger  detector surface. Yet, since the background noise is roughly proportional to the volume of a detector, a larger photon collection area is synonymous with higher instrumental background. The sensitivity is thus increasing at best as the square root of the detector surface.

While space agencies presently express their wish to launch lighter payloads in the future, the new results of SIGMA and GRO indicate that the next generation of instruments should have not only better sensitivities but also better angular and energy resolution.
A possible way out of this impasse consists in reconsidering to take advantage of the phase information of the photons. So far, no telescope relying on elastic coherent scattering has been employed at gamma-ray energies. However, Rayleigh scattering by tightly bound atomic electrons can be significant in high-Z material when very small angles of incidence on a crystal lattice are implied.

Today,the Laue diffraction lens has demonstrated its potential in laboratory measurements even at several hundred keV : Gamma-rays are focused from a large collecting area onto a small detector volume. As a consequence, the background of the crystal diffraction telescope is extremely low, making possible unprecedented sensitivities.

update : january 1999
questions and comments: Peter von Ballmoos