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arXiv · 2608.07466

Characterization of a prototype parallel-plate $^{238}$U fission chamber with DD and DT fusion neutron sources

V. Hagenlocker·R. A. Tinguely·X. Wang·J. L. Ball·B. Buschmann·M. Gatu-Johnson·R. Gocht·P. Raj
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Abstract

The SPARC tokamak will employ $^{238}$U-based fission chambers (FCs) to monitor high-performance deuterium-tritium (DT) plasma operations, spanning neutron yield rates from ${\sim}10^{15}$ to ${>}10^{19}$ n/s. This work validates the $^{238}$U FC design, which utilizes a parallel-plate detector geometry and borated polyethylene collimation to prioritize unscattered DD and DT fusion neutrons. Experimental testing with both DD and DT neutron generators corroborates vendor-specified efficiencies and demonstrates excellent detector linearity, with measured count rates showing good agreement with OpenMC neutronics simulations. Further characterization confirms the $^{238}$U FC's robustness against SPARC-relevant environmental challenges, including stray magnetic fields up to 14 mT and possible signal degradation risks associated with $\sim$30 m long cable runs. These results confirm that the $^{238}$U FC, supported by indirect neutron shielding as well as appropriate pulse height thresholds, provides a reliable solution for fusion power measurements as part of the SPARC neutron diagnostics suite.

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V. Hagenlocker, R. A. Tinguely, X. Wang, J. L. Ball, B. Buschmann, M. Gatu-Johnson, R. Gocht, P. Raj. 2026-08-07. Characterization of a prototype parallel-plate $^{238}$U fission chamber with DD and DT fusion neutron sources. https://arxiv.org/abs/2608.07466

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