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A secondary beam facility is in operation at the relativistic heavy ion synchrotron SIS at GSI, Darmstadt, since September 1998 providing secondary pion, electron and nuclear fragment beams to be used for physics experiments as well as for complex detector tests and calibrations.
For production of pion and electron beams primary Carbon beams of up to 2 A GeV and proton beams of up to 4.6 GeV are applied. The production targets are water-cooled using high-density low-Z materials in cylindrical pencil-like shapes. The transport beam line has a large geometrical and momentum acceptance of (((p/p = 25 msr%, providing a momentum band of about 8 %. The beam optics has been optimized using first and second order transport calculations. The target shape and material and the transport geometry including the hodoscopes have been studied in simulation calculations based on the GEANT code.
The pion yield follows a smooth spectrum ranging from 0.5 GeV/c to 2.5 GeV/c. The intensity is largest at the HADES target site, the beams are less intense by an order of magnitude at the other experimental sites at larger distances. The magnets can be switched for both polarities of the secondary beams. With primary Carbon beams the intensity typically peaks at 107 per spill (duration > 2 sec) at pion momenta around 1 GeV/c.
By tracking the secondary beam particles a momentum resolution of (p/p = 0.5 % including multiple scattering effects is achieved. The tracking is performed using hodoscopes based on stripes of plastic scintillators coupled to photomultiplier tubes. Contaminations of other secondary beam particles can be discriminated by a measurement of the time of flight of the particles. At a resolution of 100 ps (() the electron contamination can be totally discriminated. The electron yield on the order of 3 % mostly stems from pion decays in flight. For detector test experiments the fast electrons can be selected from pions having lower velocities by means of a transmission gas Cherenkov detector. A LH2 target is available with its cold head presently adapted to the HADES di-lepton spectrometer geometry.

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