Focusing gamma rays for sub-MeV astrophysics : State of the art of Laue lenses

Nicolas Barrière (IASF, Rome, Italie) — au CESR

Séminaires d’Astrophysique

de l’Observatoire Midi-Pyrénées

Nicolas BARRIERE

(IASF, Rome, Italie)

Focusing gamma rays for sub-MeV astrophysics : State of the art of Laue lenses

Despite the soft gamma-ray sky is very rich in physics, the development of nuclear astrophysics is hampered by technical difficulties. Observing celestial gamma-ray emission is first and foremost a matter of extracting a weak signal from an intense and complex instrumental background. This is partly due to the fact that all existing instruments are based on concepts where the collecting area itself is also the sensitive area. In the soft gamma ray domain, instrumental background in a detector is roughly proportional to its volume. Hence focusing the celestial signal from a large collecting area onto a small detector volume would lead to a dramatic improvement in sensitivity. Focusing gamma rays has long been thought impossible, but it is now becoming a reality in the 100 keV ? 1 MeV range band, thanks to Laue lenses.

A Laue lens focuses gamma rays by using Bragg diffraction in the volume of crystals (i.e. in the Laue geometry). The typical focal length of such lens ranges between 10 and 100 m, which calls either for two formation flying satellites or an extensible boom.

In this talk, I will present the state of the art of Laue lenses, and what we can expect from this technology. Performance of Laue lenses relies entirely on crystals that compose it, thus the search for efficient crystals and their characterization in high-energy beams is of prime importance. I will present our activities for crystals development, and our latest results obtained at energies up to 816 keV. Crystals measured reflectivities are then input in lens models, allowing realistic estimates of effective area. In a second part, I will show reasonably achievable performance (sensitivity, field of view, imaging) with a telescope featuring a Laue lens of 20m focal length.