The backward region of the H1 detector has been upgraded in order to provide improved measurement of the scattered electron in deep inelastic scattering events. The centerpiece of the upgrade is a high{resolution lead/scintillating{ bre calorimeter.The main design goals of the calorimeter are: good coverage of the region close to the beam pipe, high angular resolution and energy resolution of better than 2% for 30 GeV electrons. The calorimeter should be capable of providing coarse hadronic energy measurement and precise time information to suppress out{of{time background events at the rst trigger level. It must be compact due to space restrictions.These requirements were ful lled by constructing two separate calorimeter sections. The inner electromagnetic section is made of 0.5 mm scintillating plastic bres embedded in a lead matrix. Its lead{to{ bre ratio is 2.3:1 by volume. The outer hadronic section consists of 1.0 mm diameter bres with a lead{to{ bre ratio of 3.4:1.The mechanical construction of the new calorimeter and its assembly in the H1 detector are described.

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Abstract:The electronic system developed for the SpaCal lead/scintillating-fibre calorimeters of the H1 detector in operation at the HERA ep collider is described in detail and the performance achieved during H1 data-taking is presented. The 10 MHz bunch crossing rate of HERA puts severe constraints on the requirements of the electronics. The energy and time readout are performed respectively with a 14-bit dynamic range and with a resolution of ∼0.4 ns. The trigger branch consists of a nanosecond-resolution calorimetric time-of-flight for background rejection and an electron trigger based on analog 'sliding windows'. The on-line background rejection currently achieved is ∼10 6 . The electron trigger allows a low energy trigger threshold to be set at ∼0.50 ± 0.08 (RMS) GeV with an efficiency ≥ 99.9%. The energy and time performance of the readout and trigger electronics is based on a newlydeveloped low noise (σ noise ∼0.4 MeV) wideband (f ≤200 MHz) preamplifier located at the output of the photomultipliers which are used for the fibre light readout in the ∼ 1 Tesla magnetic field of H1.