Experimental Characterization and Modeling of Lithium-Based Scintillating Neutron Flux Monitors for use in the SPARC Tokamak*

Neutron flux monitors (NFMs) are the primary tools used to measure the fusion power of the SPARC tokamak.[1] NFMs measure neutron flux in the tokamak hall and deduce neutron yield rates through a calibration process that directly relates to fusion power. We present the characterization and modeling of a lithium-based scintillator NFM coupled with a photomultiplier tube. This detector’s several parallel plates of scintillation materials and Li-6-containing coatings use the lithium-neutron reaction, a unique approach compared to other NFMs. Li-6 absorbs neutrons, and its absorption products enter scintillators to emit measurable photons. We will measure the detector response to deuterium-deuterium and deuterium-tritium neutron generators as well as sensitivity to detector position and orientation. Further, Li-6 is intrinsically sensitive to thermal neutrons,[2] so we will test different moderation techniques, intended to enhance the NFM’s performance, including varying thickness and boron concentration. All experiments will be validated in OpenMC.[3] Finally, this detector’s performance in a SPARC environment will be simulated in OpenMC to test its effectiveness. These findings help us better understand this NFM and consequently aid the fusion power measurement of SPARC.

This work was supported in part by Commonwealth Fusion Systems.

[1] Creely, A. J., M. J. Greenwald, et al. “Overview of the SPARC Tokamak.” Journal of Plasma Physics 86, no. 5 (2020): 865860502. 

[2] Sei-ichi KOMODA & Sin-iti IGARASI (1978) Neutron Cross Sections of Li-6,

Journal of Nuclear Science and Technology, 15:1, 79-81, DOI: 10.1080/18811248.1978.9733203

[3] Paul K. Romano, Nicholas E. Horelik, Bryan R. Herman, Adam G. Nelson, Benoit Forget, Kord Smith,

OpenMC: A state-of-the-art Monte Carlo code for research and development,

Annals of Nuclear Energy, Volume 82, 2015, Pages 90-97, ISSN 0306-4549,