Central Particle Tracking Detectors in the PEN Experiment

The PEN Collaboration is conducting a new measurement of the $\pi^+\to e^+\nu(\gamma)$ ($\pi_{e2}$ decay) branching ratio at the Paul Scherrer Institute, with the goal uncertainty of $\Delta B/B \simeq 5\times 10^{-4}$ or lower. At present, the combined accuracy of all published $\pi_{e2}$ decay measurements lags behind the theoretical calculation by a factor of 40. In this contribution we describe the redesigned central region tracking detectors of the PEN detector. The design and performance of a pair of two-piece wedge degraders for simultaneous horizontal and vertical tracking of the $\pi^+$ beam particles are presented in detail. After passing through the tracking degrader, the $\pi^+$ beam is stopped in the center of an active target scintillator. The positrons from $\pi^+$ and $\mu^+$ decays are tracked in a pair of cylindrical MWPC's, and detected in a thin plastic scintillator hodoscope and a pure CsI electromagnetic calorimeter.