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Parratt32
or The Reflectivity Tool



Overview

Parratt32 enables you to calculate optical reflectivities (neutrons and x-rays) from a given model sample over a specified Q-range or fit model parameters to measured sets of data.

The programme is a Microsoft Windows application that runs on either Windows9x or WindowsNT 4.0.



Reflectivity calculation:

The calculations are carried out by means of Parratt's dynamical approach (L. G. Parratt, Phys. Rev., 95(2), 359-369, 1954). This approach is based on stratified media, hence only the refractive index differences perpendicular to the surface are considered.
Roughnesses are included in the Fresnel coefficient as suggested by Nevot and Croce (L. Nevot, P. Croce, Revue de physique appliquée, 15 (1980), 761). Keep in mind that by this approach only roughnesses that are small compared to the layer thickness are handled correctly!



Profile calculation:

The calculation of the scattering length density includes the roughnesses for the interfaces. According to Nevot and Croce (see above) these are calculated by an error function formalism. The given value can be interpreted as the fwhm of the gaussian in the scattering length density gradient at the given interface.



Fitting algorithm:

The chi squared minimization is implemented as a simplified one dimensional Newton-Raphson method (see for example: W. Press, B. Flannery, S. Teukolsky, and W. Vetterling, 'Numerical Recipes', Cambridge Univ Press, Cambridge 1986). This method converges quadratically into the nearest minima, but by varying the parameter to be optimized, somewhat more global minima can be found.

Have a look at this huge screenshot (22 kB)



Download:

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