Performance of a prototype detector for use as the sweeping magnet photon-veto in the KOPIO experiment

A detector system for use as a gamma (γ) veto in the volume of the high field of the sweeping magnet of the KOPIO experiment at BNL is presented here. A laminar design of alternating 5 mm plastic scintillator tiles and 2.54 mm Pb sheets was used. The scintillator signals were readout on both ends by embedded Kuraray Y11-200 multi-clad wave-shifting 1 mm x 19 fibers/tile (WSF). The calibration of the detectors through the single photo-electron (spe) peak of the photomultiplier tubes (PMTs) and the light attenuation measurements of the fibers were performed using a commercial light emitting diode (LED, blue 465 nm) pulsed with 30 ns pulses at 1kHz. Absolute light yield of a single tile averaged 60 pe per minimum ionizing particle (mip) from cosmic rays. An assembly of 10 scintillator layers combined with nine Pb layers yielded ∼ 400 pe/mip. Light intensity attenuation of about 1 dB/m allows the embedded fibers to be extended to lengths of more than 6 m in order to carry the detector signal to PMTs outside the magnetic field, thus simplifying the readout