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Fused-Silica Activation Cherenkov Detector for Pulsed D--T Fusion Yields

N. Kaneshige, S. Alawabdeh, W. Hennig, D. Cech, M. Hua, R. Grazioso

Abstract

We demonstrate a compact, non-toxic, low-cost neutron-yield diagnostic for pulsed D--T fusion systems using an undoped fused-silica (SiO$2$) rod as both activation target and Cherenkov radiator. D--T neutrons (14.1 MeV) activate $^{28}$Si and $^{16}$O to produce short-lived $^{28}$Al ($T{1/2}=134,\mathrm{s}$) and $^{16}$N ($T_{1/2}=7.13,\mathrm{s}$). The resulting $β^-$ particles exceed the Cherenkov threshold and generate UV--visible light detected by a fast photomultiplier tube. A SiO$_2$ rod of dimensions $6,\mathrm{in}\times1,\mathrm{in}$ (length $\times$ diameter) is optically coupled and read out with a CAEN DT5730 digitizer operating in list mode with digital pulse processing. The post-pulse count rate is fit using fixed $^{16}$N and $^{28}$Al half-lives together with background terms to infer neutron fluence. Testing at the ZEUS D--T Dense Plasma Focus established a reference calibration and agreement with a praseodymium-calibrated silver activation detector. Measurements near a D--D Dense Plasma Focus show no activation signal, confirming D--T selectivity. The diagnostic enables pulse-to-pulse yield measurements within minutes following a pulse and is being deployed on Helion Energy's seventh fusion prototype, Polaris.

Fused-Silica Activation Cherenkov Detector for Pulsed D--T Fusion Yields

Abstract

We demonstrate a compact, non-toxic, low-cost neutron-yield diagnostic for pulsed D--T fusion systems using an undoped fused-silica (SiO) rod as both activation target and Cherenkov radiator. D--T neutrons (14.1 MeV) activate Si and O to produce short-lived Al () and N (). The resulting particles exceed the Cherenkov threshold and generate UV--visible light detected by a fast photomultiplier tube. A SiO rod of dimensions (length diameter) is optically coupled and read out with a CAEN DT5730 digitizer operating in list mode with digital pulse processing. The post-pulse count rate is fit using fixed N and Al half-lives together with background terms to infer neutron fluence. Testing at the ZEUS D--T Dense Plasma Focus established a reference calibration and agreement with a praseodymium-calibrated silver activation detector. Measurements near a D--D Dense Plasma Focus show no activation signal, confirming D--T selectivity. The diagnostic enables pulse-to-pulse yield measurements within minutes following a pulse and is being deployed on Helion Energy's seventh fusion prototype, Polaris.
Paper Structure (19 sections, 2 equations, 7 figures, 1 table)

This paper contains 19 sections, 2 equations, 7 figures, 1 table.

Table of Contents

  1. Introduction
  2. Background and Related Work
  3. Detector and Readout
  4. Fused Silica Optics
  5. Activation channels
  6. Electronics
  7. Acquisition and Analysis
  8. Binning and model
  9. D--D and D--T Testing
  10. Uncertainties
  11. Results
  12. ZEUS D--T validation and calibration
  13. MJOLNIR D--D insensitivity
  14. Discussion
  15. Operating envelope
  16. ...and 4 more sections

Figures (7)

  • Figure 1: Photo and diagram of the fused-silica activation Cherenkov detector. A SiO2 rod is optically coupled to a Hamamatsu H10580 PMT. D--T neutrons induce $^{28}\mathrm{Si(n,p)}^{28}\mathrm{Al}$ and $^{16}\mathrm{O(n,p)}^{16}\mathrm{N}$; ensuing $\beta^-$ particles generate Cherenkov photons detected by the PMT and are digitized by a CAEN DT5730 digitizer.
  • Figure 2: ENDF/B-VII.1 evaluated cross sections vs incident neutron energy for $^{28}\mathrm{Si(n,p)}^{28}\mathrm{Al}$ and $^{16}\mathrm{O(n,p)}^{16}\mathrm{N}$, plotted using the Curie toolkit CurieLib.
  • Figure 3: (a) ZEUS post-shot count rate $R(t)$; points: 0.1s bins for shot 241022-06; line: best-fit model with individual components (fit starts at $t=10s$ to exclude PMT/digitizer saturation). (b) 2D histogram of per-event integral (energy proxy) vs time.
  • Figure 4: Linearity of fitted activation components vs ZEUS fluence. Calibrated $K_{\mathrm{Si}\to\mathrm{Al}}$ and $K_{\mathrm{O}\to\mathrm{N}}$ against an independent ZEUS yield-to-fluence reference.
  • Figure 5: Comparison of ZEUS (D--T) and MJOLNIR (D--D) shots at comparable fluence. Post-shot $R(t)$ shows minimal signal for D--D neutrons.
  • ...and 2 more figures