Trigger Efficiency of a ZnS:6LiF Scintillation Neutron Detector Readout With a SiPM

The efficiency of detecting signals from neutron absorption events (trigger efficiency) in detectors using ZnS scintillators loaded with neutron absorbers (
⁶
Li,
¹⁰
B) is prone to be limited to values well below 100%: the non-transparency of the scintillator results in a wide dynamic range of the detected signals with some fraction always falling under the detection threshold set to satisfy the requirements on the gamma sensitivity, multi-count ratio, and the background count rate of the detector. The question of the limited trigger efficiency of such detectors is very seldom discussed in publications, yet it is very important as the product of the trigger efficiency and the probability of neutron absorption in the detector defines its overall efficiency. Available data indicate that the trigger efficiency of PMT-based detectors used in neutron diffraction experiments can be as low as ~ 50%. In this work we evaluate the trigger efficiency of a SiPM-based ZnS:
⁶
LiF scintillation neutron detector developed to replace a
³
He-detector of the POLDI time-of-flight diffractometer at the Paul Scherrer Institut in Switzerland. Requiring the gamma sensitivity with a
⁶⁰
Co source ≤10
^-7
, the multi-count ratio ≤10
^-3
with the detector dead time set to 10 μs, and the background count rate 10-3Hz we obtain the maximum possible trigger efficiency of ~85% with the main restriction coming from the gamma sensitivity condition. Considering the neutron absorption probability of ~80% at 1°Å, the detector efficiency at this wavelength amounts to ~ 70%, which is comparable to that of the current
³
He-detector of the POLDI instrument.