Single-photoelectron noise reduction in scintillation detectors

The 1994-95 search at SLAC for millicharged particles used four 21/spl times/21/spl times/130-cm/sup 3/ Bicron 408 scintillation counters to detect a signal at the single-photoelectron level. The competing noise requiring minimization was due to combination of PM tube (8-inch Thorne EMI 9353KA) afterpulsing and ambient radiation-induced scintillator luminescence. A very slow decay (>30 /spl mu/s) component was observed and received particular attention. Efforts to reduce the SPE noise included photomultiplier tube base modifications, detector shielding and cooling, signal amplification, and veto procedures.