Experimental results obtained with the positron-annihilation-radiation telescope of the Toulouse-Argonne collaboration

We present laboratory measurements obtained with a ground-based prototype of the focusing positron-annihilation-radiation telescope developed by the Toulouse-Argonne collaboration. This instrument has been designed to collect 511-keV photons from astrophysical sources when operating as a balloon borne observatory. ound-based prototype consists of a crystal lens holding small cubes of diffracting germanium crystals and a 3×3 germanium array that detects the concentrated beam in the focal plane. Measured performances of the instrument at different line energies (511 and 662 keV) are presented and compared with Monte Carlo simulations; also the advantages of combining the lens with a detector array are discussed. sults obtained in the laboratory have strengthened interest in a crystal-diffraction telescope: the balloon instrument will provide a combination of high spatial and energy resolution (15 arc sec and 2 keV, respectively) with an extremely low instrumental background resulting in a sensitivity of ∼3 × 10−5 photons cm−2s−1. These features will allow us to resolve a possible narrow 511-keV line both energetically and spatially within a Galactic center “microquasar” or in other broad-class annihilators.