A highly compact electromagnetic calorimeter for the VLQ spectrometer in H1 at HERA

A highly compact electromagnetic calorimeter for the VLQ (very low momentum transfer) spectrometer within the H1 experiment at HERA is being presented. Due to severe space constraints in its position close to the beampipe the complete device has a length of 16 cm including the active volume as well as the entire readout system consisting of custom made photodiode arrays and very low noise analog front-end ASICs. The calorimeter stack is composed of tungsten absorber sheets interleaved with narrow scintillator bars providing a projective readout of the energy deposition. An energy histogramming method for shower reconstruction yields a spatial resolution of better than 1 mm. This number depends crucially on the perfomance of the custom made readout chip. The chip has been developed in the ASIC laboratory at the University of Heidelberg and realised in the AMS (Austria Micro Systems) 1.2 μm CMOS technology. The charge sensitive amplifier features a measured noise performance of 226+19×C(pF) electrons rms for a shaping time of 200 ns.