Use of IMD for neutron multilayers calculations
===============================================


The theory of multilayers is the same for neutrons and photons (x-rays 
included). 

Therefore, it is possible to use IMD for neutron calculations, provided the 
neutron refraction indices and optical constants. 

Remarks: 
========

i) The polarization factor for neutrons is one (corresponding to the 
   s-polarized x-ray beam).  Therefore, for working with neutrons, the 
   reflectance and other variables should use s-polarization (however, in 
   most cases the error introducing when using unpolarized beams is 
   negligeable).

ii) Energy/wavelength units. For neutron calculations, WAVELENGTH units should 
    always be used. The "energy" unit in IMD is for photons only 
    (energy[eV][A]=12398.4/wavelength[A]). For neutrons, the "energy" is 
    calculates in another way [...].

iii) All "labels" x-ray-oriented should be ignored. 


Optical constants
=================

The files in the nk.dir and f1f2.dor directories have been replaced for new 
files containing the neutron optical constants.

i) We have created .nk files with the refraction index staring from the 
   scattering lengths available in [1] and placed in the nk.dir (which 
   previously was empied from all photon data). 

   Notes:
	a) Density values used for calculating the refraction index for 
	   the different isotopes are those of the natural elements (density 
	   data for the isotopes are not available)
        b) An empty file Au.nk was placed, because IMD checks for the 
	   existence of this file at startup. 

ii) The files in the f1f2.dir directory have been replaced, in order to be 
    able to create new .nk file using 
    IMD->Materials/Optical Constants->Create new X-ray optical constant data.
    The information in the .ff files is just the neutron scattering length (b) 
    (which is a constant over the wavelength) written in some "odd" units in 
    order to be used directly by IMD to build the .nk file. 


Suggestion for improvents of IMD for neutron use
================================================

i) implement the "neutron energy" and "neutron speed" units in addition to 
   the existing wavelengths.

ii) Change x-ray labels for neutron ones:
    Electric Fields -> Amplitud ???
    Materials/Optical Constants->Create new X-ray optical constants... 
    (add a new one for neutrons with file comments changed?)



References:
===========
 [1] http://www.ati.ac.at/~neutropt/scattering/table.html
     Also avalable at "Neutron Scattering Lengths"
     by H. Rauch and W. Waschkowski, Chapter 1.1 in "Neutron Data Booklet"
     Edited by the Institute Laue-Langevin, October 2001. 


M. Sanchez del Rio (srio@esrf.fr) and L. Alianelli (alianell@ill.fr) 2003-04-04

