An accurate model of the nonlinear detector response of organic scintillators to neutrons is required to correctly simulate fast neutron detection, as well as interpret measured pulse height data. Several empirical and semi-empirical models are available to fit measured scintillator light output data. In this work, EJ-309 light output data from neutrons depositing 1.15 MeV to 5.15 MeV on hydrogen wereanalyzed using empirical models as well as semi-empirical models based on the work of Birks and Voltz. Although all tested models fit the experimental light output data well in the measured range, the models were observed to diverge in low-energy extrapolation. The modelswerethen tested by comparing a measurement and MCNPX-PoliMi simulation of an EJ-309 detector response to fast neutrons from a 252 Cf spontaneous fission source. The agreement between the measured and simulated pulse height distributionsvaried significantly depending on the light output model used. The best agreement between simulated and measured neutron pulse height distributionswas achieved by using the Birks model. The bin-by-bin agreement was better than 5% over the range 0.08 to 2.18 MeVee, and better than 10% from 2.18 to 3.13 MeVee. The integral count rate over the range 0.08 to 3.14 MeVee differed by less than 1% in absolute units. 1.0 Introduction The IAEA is interested in high-fidelity Monte Carlo modeling of detector technologies for international safeguards applications[1]. Several ongoingsafeguards projects employ organic scintillators as fast neutron detectors, such as theLiquid-Scintillator Neutron Coincidence Collar (LS-NCC) [1],the Fast Neutron Multiplicity Counter (UM-FNMC) [2,3],radiation portal monitors (RPMs)[4,5], and the Dual Particle Imager (DPI) [6-8]. Organic scintillators are also frequently employed in a wide variety of applications including, but not limited to, nuclear physics [9], material characterization [6,3,10,2], imaging [6-8], and nuclear medicine[11,12].
