Modifying the Influence Method for Scintillators using Time-of-Flight tagged Neutrons

Sammanfattning

The Influence Method [1] provides a novel and simple approach to determine detection efficiencies without prior knowledge of the particle flux, but has been shown to produce wrong predictions for scintillators [2]. In this work, the Influence Method is applied to plastic scintillators using fast neutrons with energies between 1.5 MeV and 6.5 MeV from an Am-Be source. The neutrons are time-of-flight tagged so that an energy-dependent efficiency prediction canbe performed. It is investigated if setting a lower detection threshold to simulate neutron removal can improvethe Influence Method predictions. Additionally, an alternative method (the No Influence Method) is developed where the efficiency estimator uses assumptions more appropriate for the interactions of fast neutrons with scintillators. The obtained results for the total detection efficiency, both from the unaltered Influence Method, as well as the new No Influence Method are close to the prediction derived from the hydrogen scattering cross section. The energy-dependent efficiency prediction of the No Influence Method decreases with higher neutron energies, as expected for hydrogen scattering. The experimental part is accompanied by a theoretical model which tries to recreate the Influence Method predictions as closely as possible so that the predictions can be corrected for effects that are not accounted for in the original Influence Method. The model predicts an optimal lower detection threshold of 40% of the incident neutron energy to correct for systematic errors of the Influence Method prediction for plastic scintillators over the Am-Be energy range. This threshold could not yet be verified experimentally because the detectors used are not equivalent enough in detection efficiency and gain. [1] I.J. Rios and R.E. Mayer. Absolute particle flux determination in absence of known detector efficiency. The “Influence Method”. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 775:99–104, 2015. [2] A. Sperduti and M. Cecconello. On the limits of the Influence Method in the determination of the intrinsic efficiency of liquid scintillators. Journal of Instrumentation, 14(06):P06004, jun 2019.