Semiempirical Models of Organic Scintillator Response to Neutron Interactions on Hydrogen and Carbo

The International Atomic Energy Agency (IAEA) system of international safeguards employs a variety of technical measures to verify compliance with obligations under the Nuclear Non-Proliferation Treaty. Because special nuclear material emits neutrons, neutron detectors comprise a key element of the safeguards system. Currently deployed neutron detection safeguards instruments include coincidence counting systems employing 3He to detect thermalized neutrons.;Fast neutron detection systems that do not require thermalization of neutrons have several potential advantages over thermal systems, and active research is underway to design and characterize such systems. Additionally, the IAEA requires systems that do not rely on the limited supply of 3He. Organic scintillator detectors are promising candidates to address these needs. However, the IAEA has noted the need for robust Monte Carlo simulations of organic scintillator-based systems before they can be authorized for international safeguards deployment.;The response of organic scintillator detectors to neutrons is a complex process; in particular, the scintillation light generated in response to neutron collisions is nonlinear with respect to energy deposited. Accurate models of scintillation response (also called light output) are required for Monte Carlo simulations of organic scintillator-based systems. These models are also used in experiments to convert collected pulse heights to energy depositions for neutron spectroscopy and imaging applications.;The choice among the several available empirical and semiempirical models for neutron light output can profoundly impact the accuracy of simulated pulse height distributions (PHDs). These models diverge significantly from one another in extrapolations to low energy (less than approximately 1 MeV energy deposition). In this work, EJ-309 light output data from neutrons depositing 1.15 MeV to 5.15 MeV on hydrogen are analyzed using empirical models as well as semi-empirical models based on the work of Birks and Voltz. The models are then tested by comparing a measurement and MCNPX-PoliMi simulation of an EJ-309 detector response to fast neutrons from a 252Cf spontaneous fission source. The agreement between the measured and simulated PHDs varies significantly depending on the light output model used. The best agreement between simulated and measured neutron PHDs is achieved by using the Birks model.;The first measurements of energy-dependent light output from carbon recoils in the liquid organic scintillator EJ-309 are presented. For this measurement, neutrons were produced by an associated particle deuterium-tritium generator and scattered by a volume of EJ-309 scintillator into stop detectors positioned at four fixed angles. Carbon recoils in the scintillator were isolated using triple coincidence among the associated particle detector, scatter detector, and stop detectors. The kinematics of elastic and inelastic scatter allowed data collection at eight specific carbon recoil energies between 2.86 and 3.95 MeV. The light output caused by carbon recoils in this energy range is found to be approximately 1.14% of that caused by electrons of the same energy, which is comparable to the values reported for other liquid organic scintillators.;The application of semiempirical proton light output models and accurate carbon light output and resolution functions is shown to substantially improve agreement between simulated and experimental detector response of EJ-309 organic liquid scintillators. This improved agreement, and the methods and models used to characterize the response, will support ongoing efforts to realize deployable IAEA safeguards systems based on organic liquid scintillator.