Investigation of Compact Liquid Organic Scintillators for Neutron Spectrum Measurements in Tokamaks

Abstract

Neutron diagnostics play a key role in both magnetic confinement and inertial confinement fusion platforms and will be increasingly more important as those systems continue to improve with more devices achieving burning plasma performance. Many fuel cycles, such as deuterium-tritium, produce neutrons as a reaction product. Since neutrons are electrically charge-neutral, they leave the plasma carrying important information about ion temperature, macroscopic flow velocity, fuel ratios, and more. These neutrons are detected and analyzed with a variety of different methods including flux monitors, emission profile monitors, and spectrometers. Neutron spectrometers work based on different neutron interactions and come in different shapes and sizes. The liquid organic scintillator is one type of neutron spectrometer that has been around for many years and has the benefits of being cost effective and well-characterized. One downside to these detectors is the need for post-processing scripts, commonly referred to as unfolding algorithms, that take the raw energy spectrum comprised solely of Compton and recoil edges to convert it to an energy spectrum containing peaks, which is more convenient for analysis. There are many unfolding schemes being developed and in use, each with its own set of built-in assumptions and sources of error. A new method for liquid organic scintillator spectral analysis that eliminates the need for spectral unfolding is presented and applied to fusion-relevant measurements. It incorporates the AIMS Data AcQuisition (ADAQ) libraries with associated data acquisition and analysis user interfaces and the GEANT4 Monte Carlo particle transport simulation toolkit to produce synthetic detector spectra for EJ-301/NE-213 liquid organic scintillators. Changes to plasma properties cause variations in the synthetic spectra, which can be quantified to develop an analysis workflow that can be used with experimental spectra. The workflow is applied to measure neutron rate, ion temperature, and toroidal rotation velocity. Future work including applying the workflow to other types of organic scintillators and other potential plasma measurements are discussed.