The cylindric detector DAPHNE is about 2 m long with a diameter of about 1.5 m. It covers a large anglular range (20^° £ J £ 160^° , 0^° £ F £ 360^°) and therefore has a large angular acceptance.

DAPHNE consists of three different detector components. The target is surrounded by three layers of coaxial multiwire proportional chambers (MWPC). The MWPC signals can be used to determine the trajectories of charged particles and to reconstruct the reaction vertex. The second component is a three layer plastic scintillator hodoskop, which allows for particle identification. The last component is a Al/Pb - plastic scintillator calorimeter with high efficiency for the detection of decay photons. Both, the cylinder of the hodoskop and of the calorimeter are composed of 16 segments.

The GDH detector setup at Mainz, in which DAPHNE plays the central role is sketched here:

To enlarge the solid angle acceptance, forward angles smaller than J = 20^° are covered by additional detectors.

• MIDAS, a Si microstrip detector, which is mounted inside DAPHNE covers the forward angle region of 7^° £ F £ 20^°. It consists of three ringshaped wavers with a central apperture. The first two wavers allow one to determine the particle trajectories, the third MIDAS component is a sandwich calorimeter which can be used for particle identification and energy determination.

• One of the most important components is the gas threshold Cerenkov detector, which is employed to identify electrons of the strong electromagnetic background , caused by compton scattering and pair production in the atom field. To establish an effective supression of these electron background events, Cerenkov signal is used as trigger veto.

  

• STAR
  (Scintillators from Tübingen for Angle Reconstuction ) [Sau96]
  The purpose of STAR is the extrapolation of the cross sections to the experimentally not accessible region around J = 0^° Therefore, it consists of a plastic scintillator plate of an front area of » 1 m² followed by nine plastic scintillator rings with decreasing diameter, mounted concentrically around the beamline.
  
• FARFORWARD-Detector
  The Farforward detector is built out of two components, a 1cm thick ``triggerplate'' for the detection of charged particles and a 7.5 cm thick Pb - plastic scintillator sandwich detector for an efficient detection of decay photons. Its size is about 1 m², its weight about 600 kg.

The usage of a detector like DAPHNE, which allows track reconstruction and particle identification is practicable, because at lower photon energies atmost only 4 particles appear in the final state. The detection of a larger number would require a even more sophisticated and also much more expensive detector system.

Beside a track reconstruction and a very effective particle identification, DAPHNE allows the determinination of the energy-momentum-fourvector of the detected particle. This makes the seperation of the different final states and the measurement of partial cross sections possible. Therfore, DAPHNE ist particularly well suited for the measurement of the helicity-dependent partial channels of one and two pion photoproduction. In a number of unpolarized experiments DAPHNE has proven its fitness for such tasks. Some results are summarized in [A2-96]:

References

[Aud91]

G. Audit et al., Nucl. Instr. Meth. A301(1991)473

[A2-96]

A2-Kollaboration at MAMI B Mainz, Annual Report 1996
WWW-A2-Report 1996

[Sau96]

M. Sauer et al., Nucl. Inst. Meth. A378(1996)143

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• Table 1 (2 kByte) ,Table 2 (2 kByte)