The UCN density is too low to allow a direction measurement of their precession frequency. Instead, we measure the difference between the neutron and Helium-3 the precession frequencies (𝜔3n = 𝜔n − 𝜔3).
𝜔3n is determined from the rate of neutron + Helium-3 capture reactions: 𝑛 + 3He → 𝑝 + 𝑡 + 764 keV. The cross section is highly spin-dependent: σ↑↓/σ↑↑ > 6,000, so the capture rate varies sinusoidally at the frequency 𝜔3n.
The capture reaction products (a proton and a triton) will stop in the surrounding Helium, producing a bright burst of extreme ultraviolet (EUV) light (𝜆 ≈ 80 nm, ~4,600 photons). (View image)
The EUV light must be converted to visible wavelengths so it can be transported to remote sensors. To do this the measurement cells must be coated with a wavelength-shifting chemical. Tetraphenyl butadiene works well, but we must use a deuterated version (dTPB) because hydrogen is incompatible with UCN storage
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