Time profile of fluorescence in organic scintillator out to 10 microseconds

A technique to identify energetic cosmic-ray electrons in the face of large nuclei backgrounds is to use a calorimeter followed by a shower tail-catcher boron-doped scintillator on a balloon or satellite instrument. Thermalized neutrons produced in interactions of the cosmic ray within the material of the instrument can be detected at late times (several microseconds) following the triggering event. To this end, for example, the DAMPE satellite instrument includes a Neutron Detector at the bottom of its instrument stack, and the ISS-CREAM space station experiment similarly includes a Boronated Scintillator Detector. One difficulty of interpreting the thermal neutron capture signal in the scintillator is that a late fluorescence signal is also present, with time scales similar to those of the 2.7 microsecond exponentially falling neutron capture time distribution. Thus the two effects must be carefully disentangled. We have measured the response of a non-boronated thick scintillator to electron and pion showers in CERN beam tests, and find the presence of a significant late fluorescence component in the light yield that must be accounted for. The measured scintillator response is well represented by a three component yield model, with time constants from ~10 ns to ~2.4 microseconds and appropriate relative strengths. GEANT4 was modified to accommodate this three-component scintillation model, yielding excellent agreement with the CERN beam test data. We describe the measurements and the model in this work.