•   At ILL in Grenoble by Krüger, et al

•   {220} Si lattice spacing: 543 pm,
  "invariant quantity of nature"

•   Helps determine:
  1. atomic mass of neutron (A_r(n)=m_n/u),
  2. Avogadro constant (N_A), and
  3. h/m_n.

•   Uses Si Crystal (331) lattice planes and 1600 m/s spin polarized/modulated (750 kHz) monochromatic neutron beam (wavelength about 0.25mm) backreflected in vacuum, whose time-of-flight/total modulation over a 10 m distance is analyzed.

•   24 ppb: 137.036 0084(33).
  Resulted in smaller alpha in 1998 CODATA determination.

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