A fast scintillation detector for analytical X-ray instruments

A detector was constructed which consisted of a relatively fast plastic scintillator coupled to two photomultipliers operated in coincidence. The coincidence gate reduced noise from random photomultiplier dark current pulses. The scintillator was shaped so that total internal reflection tended to collimate scintillation light, and it was shown that this increase quantum efficiency. Fast counting electronics were built using ECL (emitter coupled logic) gates; this included a coincidence gate, a single-channel analyzer, and afterpulse rejection circuitry. System dead time was 37 ns. When adjusted to observe 8.1-keV X-rays, the noise pulse rate was 2.0 s^-1 and the quantum efficiency was 86%. This noise rate was significantly lower than that reported by others using organic scintillators and was attributed to the use of coincidence gating and afterpulse rejection