Neutronics Simulation and Experimental Analysis of U238 Parallel-Plate Fission Chambers for Neutron Diagnostics in the SPARC Tokamak*

This study presents the U238 parallel-plate fission chamber (FC) as an effective neutron flux monitor (NFM) for advanced neutron diagnostics in high-field compact tokamaks like SPARC.[1] The U238 FC detects neutron-induced fission events in the U238 coating, generating electrical pulses that correspond to neutron flux, from which measurements like fusion power and reaction rates can be derived.[2] By leveraging U238's fission threshold of ~1 MeV, the device’s parallel-plate structure, and its insensitivity to gamma radiation, we can maximize the detection of collimated fast neutrons.[3] We have modeled the U238 FC using OpenMC neutronics simulations to support its experimental application within the relevant energy spectrum of SPARC.[4] Our experiments measure neutron flux through various configurations of the FC placed within DT and DD generator beam lines. The experimental design includes assessing the detector's linearization over a range of flux values, angular dependence, different shielding materials, responses to neutron sources from DD (2.45 MeV) and DT (14.1 MeV) generators, and evaluating its performance against vendor-proposed pulse neutron and gamma sensitivities. These methods will be used to simulate and calibrate the U238 FC within the actual SPARC environment. 

[1] Creely 2020 J. Plasma Phys

[2] Bielecki 2018 J. Fusion Energy 

[3] Esposito 2022 J. Fusion Energy

[4] Romano 2015 Ann. Nucl. Energy