You calculate the cross-section for absorption (for each element) by making
the sum of the cross-sections of all the processes that reduce the neutron
beam :
- coherent scattering (i.e. diffraction)
- incoherent scattering
- magnetic scattering
- true absorption
The reference to be taken for cross sections is V.F. Sears, Neutron News,
3(3) (1992) 26-37.
The cross-section for true absorption is given in tables for lambda=1.798
(v=2200m/s). You must correct it depending on the wavelength you have used
following :
sigma true abs=sigma true abs (at 2200m/s).lambda/1.798
We suppose that true absorption depends on 1/v where v is the velocity of
neutrons. This is true only in the absence of any resonant effect. Resonant
absorption does exist also for neutrons : it can be dangerous to neglect it.
However absorption resonance peaks are difficult to find in literature. If
someone has a reference for that, it would be very interesting for me.
You add the different cross-sections :
sigma abs (in barn)=sigma coh+sigma incoh+sigma magn+sigma true abs
Then you can calculate the absorption coefficient for each element:
mu/rho=sigma abs*1E-24*Avogadro number/molar weight (in cm2/g)
You can easily now calculate the value of mu in cm-1 depending of the
concentrations of each element in your compound.
The value of muR is then calculated (R is radius not diameter, in cm) but
taking into account the packing of your sample in the capillary (generally
not more than 30-50%) (you can measure it) and you can enter it in the
proper place in FULLPROF.
I have written an EXCEL sheet making those calculations and I can send it to
you if interested.
If the value of muR is more than one, you have to face a great problem
because the correction is no more valid. However you can still use some
tricks...
It is strange that you didn't succeed in refining your data because normally
absorption correlates completely with overall thermal factors. Therefore you
should have fitted well with low (even negatives) B factors.
Hope this helps. Any comments appreciated.
Jean-Marc Joubert
---------------------------------------------------------------
Jean-Marc Joubert
Laboratoire de Chimie Metallurgique des Terres Rares
1, Place A. Briand
F-92195 MEUDON CEDEX
phone : (33)(0)1 45 07 52 80
fax : (33)(0)1 45 07 58 44
E-mail : joubert@cnrs-bellevue.fr