Measuring deadtime and double-counts in a non-paralyzable scintillating neutron detector using arrival time statistics

A ⁶LiF:ZnS(Ag) based cold neutron detector with wavelength shifting (WLS) fibers and SiPM photodetector was developed at the NIST Center for Neutron Research for the CANDoR instrument (Chromatic Analysis Neutron Diffractometer or Reflectometer). The scintillation response of ZnS(Ag) to ⁶Li fission products is a long and irregular delayed fluorescence lasting more than 50 μs. To wait as long as 60 μs for complete fluorescence decay would impose an unreasonably long deadtime to the system and would severely limit the count rate of this detector. A real-time algorithm was implemented to compensate for the long fluorescence decay, reducing but not eliminating sensitivity for a 60 μs period. This greatly reduces the deadtime to approximately 3 μs compared to imposing a 60 μs hold-off time, while also achieving a double-count fraction (counting the same event multiple times) below 1E−4. Timestamping capabilities of the detector system were used to compile arrival time statistics. Comparisons of the measured arrival time statistics with Poisson arrival statistics illustrates features in the fluorescence compensation algorithm and enables the calculation of deadtime and double-count fraction.