  A thermal neutron detector based on ZnS(Ag)/6LiF scintillator,
wavelength-shifting fibers (WLS) and silicon photomultipliers (SiPM) is under
development at the Paul Scherrer Institute (PSI) for upgrading the POLDI
instrument, a pulse-overlap diffractometer. The design of the detector is
outlined, and the measurements performed on a single channel prototype are
presented. An innovative signal processing system based on a photon counting
approach is under development. Its principle of operation is described and its
performances are evaluated on the basis of a Monte Carlo simulation.

Instrumentation and Detectors