The measurement of scintillation emission spectra by a coincident photon   counting technique
  In the evaluation of novel scintillators, it is important to ensure that the spectrum of the light emitted by the scintillator is well matched to the response of the photomultiplier. In attempting to measure this spectrum using radioactive sources, it is found that so few photons are emitted per scintillation event that conventional spectroscopic techniques cannot easily be used. A simple photon counting technique is presented, using two photomultipliers operated in coincidence, the one viewing the scintillator directly, while the other views it through a monochromator. This system allows the spectrum to be measured without using specially cooled photomultipliers, intense radioactive sources or particle beams. 
J E McMillan, C J Martoff