Raytracing

Site map

Up Raytracing Learnt Profiles
In the example an URD-65 (manufacturer: Rich. Seifert & Co. Freiberger Präzisionsmechanik GmbH) equipped with a curved secondary graphite monochromator was used as diffractometer. The following control file (urdneu11.sav) was written for the calculation of the instrumental function (all values in mm):
% instrumental function (1st step)
VERZERR=urdneu11.ger
% interpolated instrumental function (2nd step)
GEQ=urdneu11.geq
% Device geometry (REFLEXION/TRANSMISSION/CAPILLARY)
GEOMETRY=REFLEXION
% Measuring circle radius
R=200
% Focus dimensions of the X-ray tube
FocusH=12
FocusW=0.04
% Dimensions of the divergence slit
HSlitW=0.5
% Distance of divergence slit to goniometer axis
HSlitR=R-72
% Dimensions of the axial divergence slit (if present)
VSlitH=15
VSlitR=R-90
% Aperture of the primary collimator (if present)
PColl=0.5/14
% Sample diameter
SamplD=25
% Aperture of the secondary collimator (if present)
SColl=0.5/14
% Dimensions of the receiving slit
DetH=15
DetW=0.25
% Distance of a secondary monochromator from the sample
MonR=R+50
% 2-theta angles for the raytracing calculation (1st step)
zweiTheta[1]=6
zweiTheta[2]=10
zweiTheta[3]=16
zweiTheta[4]=24
zweiTheta[5]=34
zweiTheta[6]=48
zweiTheta[7]=60
zweiTheta[8]=90
zweiTheta[9]=120
zweiTheta[10]=150
% Intensity correction for small samples
GSUM=Y
% Angular raster for the interpolation (with makegeq)
WMIN=6
WMAX=150
pi=2*acos(0)
WSTEP=3*sin(pi*zweiTheta/180)

A more extended explanation of the variables used is available.

The calculation is carried out in two steps. First, a raytracing algorithm models the distorted profiles at discrete 2-theta-angles (zweiTheta[..]). The command is:

geomet urdneu11

A raytracing algorithm needs to model millions of events. Don't wonder if the calculation is time-intensive. After the raytracing calculation an interpolation is necessary to construct a file for the Rietveld calculation with BGMN:

makegeq urdneu11
Now the instrumental function is ready to use. You can display its shape at each 2-theta position with the command:
showger urdneu11 2-theta-value

As a brandnew option, you may include the switch

Extension=Y
into the file gertest.cfg. In this case, the extension of the calculated profile to squared Lorentzians is plotted on the screen, too. (New feature of version 2.2.8).

In the figure the shape of the instrumental function for 2-theta=20 degree is shown.

Please note the distortion and the shift of the maximum to lower angles.These effects are later automatically considered in the Rietveld calculation. You can see some more

and explanations to this problem.

Tube tails

Since the time of our first usage of fundamental parameters profile model, our measurements are disturbed by so-called "tube tails". Tube tails mean: A small part of the X-rays emitted by the tube originates not inside the focus, but up to 1cm outside the focus. This means 1mm equatorial broadening towards both sides of the optical focus. If not corrected, this tube tails may totally disturb the profile shapes calculated by fundamental parameters. Starting from Version 2.4.0, BGMN includes an optional Tube Tails correction.