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Calculating scatteing lengths for D₂O

Calculating SLD values for D₂O in a few steps:

1. Open NIST tabular values
   
2. Press on H and find the coherent scattering lenght (Coh b) for deuterium (D = ²H), i.e. 6.671 fm (fm = femtometer).
   
3. Go back to the periodic table and find the coherent scattering lenght (Coh b) for oxygen (O). This time, use the value for O (5.803 fm), which is for the average mix of isotopes of O. In this case, Coh b for O is the same as for ¹⁶O, as O is on average 99.8% ¹⁶O (see the conc column).
4. The coherent scattering lenght for D₂O is the sum: b = 2$\cdot$6.671 fm + 5.803 fm = 19.145 fm.