Performance of a shower maximum detector with avalanche photodiode readout

We have constructed a shower maximum detector (SMD) using 1-cm wide scintillating strips. The SMD measures the position of electrons and photons in the electromagnetic calorimeter, and improves the e/π separation capability. We use avalanche photodiodes (APDs) to read out the scintillation light; they are directly attached to each end of a strip so as to construct a compact detector. We operate the SMD at room temperature. This report describes the structure of the SMD and its performance as measured in beam tests for future linear collider experiments.