Shifting scintillator prototype large pixel wavelength-shifting fiber detector for the POWGEN3 powder diffractometer

A prototype neutron scintillation detector has been developed, specifically aimed at the needs of the powder diffractometer POWGEN3 at the Spallation Neutron Source at Oak Ridge National Laboratory. This instrument requires a detector array with large (6 mm×40 mm) pixels and an area of several square meters. The prototype uses a two-dimensional grid of wavelength shifting fibers, with the fiber axes parallel to the scintillator screen, to collect the scintillation photons. The fiber ends for each pixel go to a specific set of four photomultiplier tubes, so that the position of each event can be determined by a 4-tube 2Cn×2Cn coded coincidence. The observed maximum light yield with a 6LiF/ZnS:Ag neutron scintillation screen, summed over four tubes, is greater than 200 photons/neutron. This is about 0.13% of the ∼150,000 photons/neutron produced in the scintillator. The light yield is sufficient to allow pulse discrimination between neutron signals and gamma-ray background. Further light collection gains should be achievable using double-clad fiber and green-enhanced photomultiplier tubes. Currently, the shape, structure, and specific composition of the scintillator are being investigated, on the assumption that 6LiF/ZnS:Ag will be the chosen scintillator material.