In the high energy part of the GDH experiments at ELSA from 500 £ n £ 3000 MeV,
the new Bonn GDH detector [Hel93],[Hel97],[Sau98] is employed in the measurements.
To test the suitability of the detector for the GDH experiments,
we have performed a unpolarized
pilot experiment
with a tagged photon beam in the PHOENICS area at ELSA employing the photohadronic method.
The setup of this pilot experiment is sketched in the next figure (not to scale). Its very similar to the GDH setup of
the polarized measurements which will take place in the ELAN area at ELSA.
The Bonn GDH detector covers almost 4 p solid angle.
It allows the detection of charged and uncharged particles,
particularly of decay photons with approximately 100% coverage in solid angle
(1.5° £ J £ 173° , 0° £ F £ 360°).
As indicated in the topmost sketch, it consists of a central and of a forward part. The central part, which is
build out of the modules named BW, CEN and FW, is centered around the target (black square).
The forward part is identical to the one of the DAPHNE setup.
It is composed of a gas threshold-Cerenkov detector (CER), the STAR - and
the Farforward-detector FFW.
Each of the modules BW,CEN,FW and FFW consits of a plastic scintillator plate (green) of 1.0 cm thickness
for the detection
of charged particles and of a Pb - plastic scintillator sandwich (red) of 7.5 cm thickness for the detection
of uncharged particles.
As in the DAPHNE setup at MAMI a gas threshold Cerenkov detektor is
is employed to identify electrons of the strong
electromagnetic background ,
caused by compton scattering and pair production at the atom.
To establish an efficient supression of this electron background events,
the Cerenkov signal is used as trigger veto.
STAR
(Scintillators from Tuebingen 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 followed by nine plastic
scintillator rings with decreasing diameter, mounted concentrically around the beamline.
References
- [Hel93]
- K. Helbing, diploma thesis, Physikalisches Institut Universität Bonn, 1993
- [Hel97]
- K. Helbing, PHD thesis, Physikalisches Institut Universität Bonn, 1997
- [Sau96]
- M. Sauer et al., Nucl. Inst. Meth. A378(1996)143
- [Sau98]
- M. Sauer, PhD thesis, Physikalisches Institut Universität Tübingen, 1998
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