Hi, I sent this to Jon this afternoon and thought that I'd pass the e-mail on.
Here goes ... > You'll probably find a pre-PRODD CCSL subroutine that I wrote that > goes along the lines of > > width^2 = u^2 * ( tan(theta)-tan(theta_m) )^2 + w^2 > > Two neat things about this equation is that it doesn't go negative and > also theta_m should refine to close to half the monochromator take-off > angle. Bill P.S. I also sent him a slightly longer note later ... Because of the quadrature properties of Gaussians, we get strain terms like width^2 = e^2 * tan(theta)^2 size terms go like width^2 = s^2 / cos(theta)^2 = s^2 * (1 + tan(theta)^2) so if the monochromator half angle is theta_m so that the pure instrument term looks like width^2 = u^2 * ( tan(theta)-tan(theta_m) )^2 + w^2 then instrument & size & strain go like width^2 = u^2 * ( tan(theta)-tan(theta_m) )^2 + w^2 + e^2 * tan(theta)^2 + s^2 * (1 + tan(theta)^2) Obviously you get u, tan(theta_m) and w from a calibration like LaB6 - you can then plug in (certainly into TOPAS) - then with u0, v0 and w0 fixed at the LaB6 values we get width^2 = (u0 + f^2) * tan(theta)^2 + v0 *tan(theta) + (w0 + s^2) where e^2 = f^2-s^2 - and we've got the Gaussian component of the size and strain directly from the Cagliotti relationship. -----Original Message----- From: Jon Wright [mailto:wri...@esrf.fr] Sent: 19 March 2009 20:49 To: alan.he...@neutronoptics.com Cc: Rietveld Method Subject: Re: UVW - how to avoid negative widths? Alan Hewat wrote: > Jon Wright said: >> Quick question - does anyone have a trick to stop the Cagliotti formula >> going negative? > > This can happen if the resolution is relatively flat, so that there is no > well defined minimum. Seems to be the problem - also rather close to zero anyway. > .... if you have access to the refinement code. Here I am lucky! As suggested off list - simply return the function to the form it was in before I edited the code :-) Thanks a lot for the help Jon -- Scanned by iCritical.
