The art of slit setting | Stefan Krumm | Rev. 1 | May 1997 |
The length of the sample that is touched by X-rays
depends on the slit settings and on the diffraction angle.
At lower 2 theta a larger area on the sample is iluminated compared to
higher 2 theta if the slit is left unchanged. Care must be taken
not to irradiate the sample support as this leads to unwanted background
effects and to changes in relative intensities of the peaks.
If automatic divergence slits are used, the irradiated
sample length is kept equal over a certain range of 2 theta. However, some
slits can not be closed to values lower than 0.1° angular opening (SIEMENS
D5000). That means, that a lower limit does exist beyond that this
equipment can no longer be used (see below).
Minimum sample length ("illuminated area") needed to give correct intensities for various slit apertures and diffraction angles:
Slits [mm] | 0.1 | 0.2 | 0.4 | 1 | 2 | 4 | 6 | 12 |
Slits [°] | 0.05 | 0.1 | 0.2 | 0.5 | 1 | 2 | 3 | 6 |
2theta = 2° | 10 mm | 20 mm | 40 mm | 100 mm | 200 mm | 400 mm | 601 mm | 1204 mm |
2theta = 4° | 5 mm | 10 mm | 20 mm | 50 mm | 100 mm | 200 mm | 300 mm | 602 mm |
2theta = 6° | 3 mm | 7 mm | 13 mm | 33 mm | 67 mm | 133 mm | 200 mm | 402 mm |
2theta = 8° | 3 mm | 5 mm | 10 mm | 25 mm | 50 mm | 100 mm | 150 mm | 301 mm |
2theta = 10° | 2 mm | 4 mm | 8 mm | 20 mm | 40 mm | 80 mm | 120 mm | 241 mm |
2theta = 12° | 2 mm | 3 mm | 7 mm | 17 mm | 33 mm | 67 mm | 100 mm | 201 mm |
2theta = 20° | 1 mm | 2 mm | 4 mm | 10 mm | 20 mm | 40 mm | 60 mm | 121 mm |
2theta = 30° | 1 mm | 1 mm | 3 mm | 7 mm | 13 mm | 27 mm | 40 mm | 81 mm |
2theta = 60° | 0 mm | 1 mm | 1 mm | 3 mm | 7 mm | 14 mm | 21 mm | 42 mm |
2theta = 90° | 0 mm | 0 mm | 1 mm | 2 mm | 5 mm | 10 mm | 15 mm | 30 mm |
Calculations are made for a goniometer radius of 20 cm. For
others, the formula:
L = R * tan(alpha) / sin(theta)
should be used.
L = iradiated sample length [cm]
R = diffractometer radius [cm]
alpha = angular opening of the slits
The "Virtual crystallographic calculators"
allow an online calculation of "L",
"alpha", and "theta"!
Check them out!
The SIEMENS D5000 with automatic slits:
The SIEMENS D5000 can be equipped with automatic theta-compensating slits, that keep the same sample area iluminated over a wide angular range. However, the slits can be closed to an opening of 0.1°. That means, that at 2° 2theta and at maximum closing of the slit, a length of 20 mm is touched by the X-ray beam. At 12° 2theta and 0.1° slit aperture, this length is 3 mm. The following conclusion can be drawn from that (assuming standard sample holders):
The 1° "universal" slit and clay mineralogy:
The 1° divergence slit is the choice for alround diffraction work and many users of diffractometers may never have thought about slit settings or changing slits. However, for clay mineral analysis, especially if 00l intensity data is to be comparable, slit setting is crucial. Even for the large c. 5 cm slides a 0.5° slit must be used if the intensity data are to be obtained from about 6° 2theta (eg. the chlorite 001). If the 2 cm standard sample holders are used, the slit must be as narrow as 0.2°!