Single-sided μ-strip detector with backplane readout for fast trigger applications

Motivated by a project to trigger on the charged decay of neutral strange particles, a single-sided Si-μ-strip vertex detector (SVX) has been placed in vacuum close to the liquid hydrogen target of the Crystal Barrel spectrometer at LEAR (CERN). An event is accepted if the multiplicity of charged tracks outside the SVX increases by 2 or more. For this purpose the signals from the detector backplane are read out by electronics specially adapted to the problems of speed, high detector capacitance and high leakage current. This provides a fast first level trigger (0.5 μs), gives an additional energy deposit measurement and helps in pile up recognition. The exact event multiplicity for a second level trigger can be determined from discriminators in the strip signal ADCs during the serial readout (26 μs), using a CAMAC based multiplicity encoder. If no digital control signals are applied to the SVX, the signal-to-noise ratio for minimum ionising particles (MIPs) on the backplane has been measured to be around 12, in good agreement with noise calculations, where noise cross-talk between the two preamp systems is neglected. The backplane signal provides, moreover, an intrinsic trigger for the readout of the strip side, allowing for instance the detection of X-rays absorbed in the detector.