Extinction within the limit of validity of the Darwin transfer equations. II. Refinement of extinction in spherical crystals of SrF₂ and LiF

The extinction-correction formalisms derived in a previous article have been applied to two sets of diffraction data on spherical crystals. Application to the neutron data on SrF₂, collected by Cooper & Rouse at three different wavelengths, shows that the theory gives a slightly better fit than a special empirical formula used previously to allow for extinction in this data set. A simultaneous refinement on all the data varying both particle size and mosaic spread and allowing for primary extinction leads to physically reasonable results. It is found that Lorentzian or Fresnellian mosaic-spread distributions fit the data considerably better than a Gaussian model. A similar refinement on the two-wavelength data on LiF confirms Lawrence's original conclusions that the extinction is mainly of the primary type. However, the results are in sharp disagreement with a treatment of the same data by Killean and co-workers, in which only secondary extinction was considered. The physical upper limit for the particle size in the sample is found to be 1.9 × 10^-4 cm.