The use of profile analysis techniques for refinement of powder data from the Harwell linac

The technique of powder profile analysis for the refinement of structural and thermal parameters has been applied to time-of-flight powder spectra obtained on the Harwell linac. The time-of-flight spectrum is first converted into an absolute cross-section calibrated using a standard vanadium scatterer in barns per steradian and containing the background and absorption corrections. Expressions are derived for the integrated intensity of the coherent peaks compared with the incoherent scattering which appears as a slowly varying contribution to the background on the normalised spectrum. The computer programme then used in the profile fit is discussed in detail and appropriate expressions are given for describing the background contribution, peak positions, peak shapes and peak intensities. The programme gives an excellent fit to nickel powder data from sintheta/lambda between 0.75 and 2A-1. Extinction corrections will be discussed but evidence shows that statistical errors in the spectral parameters are little changed by restricting the fit to the higher order peaks where the extinction corrections are negligible