An APD for the Detection of the Fast Scintillation Component of BaF_2

Barium fluoride crystals are the baseline choice for the calorimeter of the Mu2e experiment at Fermilab. By utilizing the fast 220 nm scintillation component (0.9 ns decay time) and discriminating against the larger slow component at 300 nm (650 ns decay time), it is possible to build a radiation hard calorimeter with good energy and time resolution and high rate capability. This requires a solid state photosensor with high quantum efficiency at 220 nm, discrimination against the 300 nm component, and fast rise and decay times. Progress on the development of such a sensor will be discussed.