Spin Analysis

  • 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).
  • 𝜔3 can be measured directly via SQUID magnetometry, or we can use a technique called spin dressing to set 𝜔3n = 0 in the absence of non-zero nEDM.
  • 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↑↓↑↑ > 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