Rearrangement of the A4 calorimeter for the backward angle measurement

Abstract

The A4 collaboration investigates the contribution of strange quarks to the vector form factors of the nucleon at the electron accelerator facility MAMI in Mainz by measuring the parity violating asymmetry in elastic electron proton scattering at forward and backward angles. A 1022 channel PbF2-Calorimeter with a weight of 5 tons has been designed for detecting the scattered elastic electrons under forward angles. For the backward angle measurements the detector support had been redesigned and had been put on a rotatable platform. Changing from forward to backward kinematics is possible with little effort. The PbF2-detector had been operational after the reconstruction in April 2005. First measurements at backward angles showed, that photons from π0 decay in the energy range of the elastic scattered electrons dominate the counting rate. An additional system of 72 scintillation counters has been developed and installed in order to separate elastic scattered electrons from background. The system is operational and has been used with electron beam on target for the parity violation experiment during more that 1500 h. The new experimental setup and first measurements will be presented.