Shower maximum detector for the CDF plug upgrade calorimeter

We describe a detector designed to measure the position of electrons and photons in the new CDF Plug Upgrade Calorimeter. The detector comprises 6400 individual scintillator strips placed at the shower maximum location of the electromagnetic calorimeter section and coupled to wave-length-shifting fibers read-out by multi-anode photomultipliers. We describe the design criteria, the R&D studies that led to the choice of scintillator and fiber, the mechanical assembly, and the quality control tests during production and assembly of the detector elements.