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A. Zoglauer*, R. Andritschke, and G. Kanbach,

MEGA: Imaging low- to medium energy g -rays. 20°. 49 MeV. 2 MeV. 5 MeV. 8 MeV. 12MeV. 25 MeV.

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A. Zoglauer*, R. Andritschke, and G. Kanbach,

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  1. MEGA: Imaging low- to medium energy g-rays 20° 49 MeV 2 MeV 5 MeV 8 MeV 12MeV 25 MeV Abstract: Combined Compton scattering and pair creation telescopes like MEGA measure a large amount of parameters with considerable accuracy for each recorded interaction. Using a list-mode likelihood algorithm (LM-MLEM) in combination with a basic detector response, it is possible to accurately image sources, resolve extended sources as well as sources on high background as it would be expected in a space environment. The new imaging procedures were verified using laboratory tests with radioactive sources, a beam test calibration, and simulations. Gamma radiation in the range from ~400 keV up to 50 MeV interacts in the MEGA telescope via several, energy dependent and partially overlapping, reaction channels: Compton scattering without and with a visible recoil electron (untracked / tracked (>2MeV)/ Comptons) and pair creation events (>8 MeV). Advanced image reconstruction procedures like the List-Mode Maximum-Likelihood Expectation-Maximization (LM-MLEM), where all events are stored in a list instead of a binned, multi-dimensional data space and the response is calculated for each event according to its characteristics were developed. The method, originally based on work in medical imaging, was adopted to the needs of the MEGA telescope. Several changes where necessary: the response has to cover all event types and the algorithm must be able to take into account uniform and structured background. The flexibility of the implementation allows to easily exchange the detector geometry as well as to switch between near and far field and can therefore work on calibration measurements as well as on astrophysical simulations. Point sources: calibration beams from 2 to 49 MeV;event types range from untracked Compton interactions to pair creation events. Extended source: a “radioactive“ ring, produced by 88Y (~0.9 MeV) on a rotating propeller with radius 7.5 cm in the near-field was imaged with the list-mode algorithm Source in high background (simulation): in space a S/N ratio of ~1:100 must be expected. Image reconstruction for 1-5 MeV, ~E-2, photons recorded with a MEGA satellite instrument clearly reveals such a source in a uniform background. • References:Zoglauer A. et al., 2004, NewAR 48, 231-235 • Zoglauer A. et al., 2004, IEEE Nucl. Sci. Symp., to be published • now at Univ. of California, Berkeley, USA A. Zoglauer*, R. Andritschke, and G. Kanbach,

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