HADES is a versatile magnetic spectrometer aimed at studying dielectron production in pion, proton and heavy-ion induced collisions. Its main features include a ring imaging gas Cherenkov detector for electron-hadron discrimination, a tracking system consisting of a set of 6 superconducting coils producing a toroidal field and drift chambers and a multiplicity and electron trigger array for additional electron-hadron discrimination and event characterization. A two-stage trigger system enhances events containing electrons. The physics program is focused on the investigation of hadron properties in nuclei and in the hot and dense hadronic matter. The detector system is characterized by an 85 % azimuthal coverage over a polar angle interval from 18• to 85• , a single electron efficiency of 50 % and a vector meson mass resolution of 2.5 %. Identification of pions, kaons and protons is achieved combining time-of-flight and energy loss measurements over a large momentum range. This paper describes the main features and the performance of the detector system.

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The cross sections for elastic scattering of the weakly bound 9 Be on 209 Bi around the Coulomb barrier have been measured with ϳ5% absolute accuracy from 40 to 48 MeV. The potential obtained from an optical model analysis has an unusual behavior. At the strong absorption radius the imaginary ͑absorptive͒ potential is increasing ͑rather than decreasing͒ with decreasing energy, as would be consistent with a long range polarization potential arising mainly from couplings to breakup channels. The real part, on the other hand, displays a strong attractive polarization contribution with the maximum at the barrier, as would be normally expected from a polarization contribution arising from strong couplings. The inelastic multiplet in 209 Bi of collective nature around 2.6 MeV, originating from the coupling ͓ 208 Pb(3 Ϫ ) h 9/2 Ϫ ͔ J , was seen only at 48 MeV. The total multiplet cross section is well reproduced by coupled channel calculations with the potential obtained from the optical analysis and the experimental B(E3) strengths of the 209 Bi multiplet levels.

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