Hi Larry
Whilst we are here I have used the FCJ correction to fit to the ray tracing
0.5 degree equitorial divergence pattern. To do this I simply informed the
Full Axial Model that divergence in the primary beam is zero. Without
refining on any axial divergence parameters the fit is very poor as you
would expect.
To convert the Full Axial Model to the FCJ correction you need to set
divergence in the axial plane to zero, constrain the source length and
sample length in the axial plane to the same value; this I think corresponds
to something like the FCJ S parameter. Refining on the receiving slit length
in the axial plane corresponds to the other parameter which I think is
called L. In doing so and refining on the two parameters the fit was good
with a few misfits around the peak maxima but nothing too serious (let me
know if you want the data). The refined S and L values refined to:
S = 11.8680272`
L = 10.428035`
What you find with the FCJ correction however is that at high angles the S
and L refined values are different and at 90 degrees 2Th the FCJ correction
breaks down as it predicts zero braodening.
But then again for high angles the emission profile dominates and axial
divergence is only a problem for accurate work. Having said that it is
surprising how much emphasis is placed on small changes in size/strain
Gaussian and Lorentzian broadening without first considering axial
divergence between the range 60 to 120 degrees 2Th. Divergences of 2.5
degrees and less however reduces the problem consdierably for the FCJ
approach.
cheers
alan
-----Original Message-----
From: Larry Finger [mailto:[EMAIL PROTECTED]
Sent: Wednesday, 31 May 2006 7:25 AM
To: rietveld_l@ill.fr
AlanCoelho wrote:
> I would like to conclude that to be critical of a method such as the
> convolution approach to describing instrument line profiles it would serve
> authors best to first investigate the approach rather than to be critical
of
> it without substance.
>
Amen!
I'm really impressed with how well the convolution method has done.
Without any way to test it, I've always had to accept the ray-tracers
argument that they did better. I all knew is that convolution did well
enough. You have shown that it makes no real difference.
Would it be possible for you to generate the data points for the same
set of parameters with peaks at 5 and 10 degrees? That way the asymmetry
will be really pronounced and the convolution method will be even more
stressed. I don't expect it to make any difference, but I'd like to see
it on the screen.
Thanks,
Larry
