1From International Tables of Crystallography (1992BB12), Vol. C, §6.4.8, one has E = 1 - x/2..., with x = (Nc[lambda]Dr0Fhkl)2, yielding E = 0.999 for the (1,1,1) reflection, Nc being the inverse of the unit-cell volume and r0 the Thomson scattering length.

2Evaluated using the elastic constants measured by McSkimin & Andreatch (1972BB17) and [Delta][theta] equal to less than 100 µrad (contributions from incident collimation, finite crystallite size and incident wavelength band).