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SAXS beamline

Calibrants

  At the SAXS beamline various standards are used for the angular (s-scale) calibration of the different detectors:
  • Rat tail tendon for the SAXS detector - high resolution (rtt*.dat)
  • Silver behenate for the SAXS detector – medium and low resolution (agbeh*.dat)
  • Para-bromo benzoic acid for the WAXS detector – WAXS range 1 and 2 (pbromo*.dat)
  • Combination of Cu, Al foils and Si powder for the WAXS detector – WAXS range 2 and higher
 In Figure 1 a typical diffraction pattern of rat tail tendon is shown, depicting the diffraction orders (from the first to the 14thorder) measured with a "high" resolution set-up (2.3 m) and the delay-line gas detector. The d-spacing is assumed to be 650 Å, but this value can vary depending on humidity up to 3%. Thus, the rat tail tendon is often used only to determine the position of the direct beam (zero order), while the absolute calibration is performed using the diffraction pattern of Silver behenate powder. Fig. 2 depicts a diffraction pattern of Silver behenate measured with "medium" resolution set-up (1.0 m) from the first to the 4thorder (repeat spacing 58.4 Å) [1].
Figure 1. SAXS diffraction pattern of the collagen structure of rat tail tendon fibre at a distance of 2.3 m
Figure 2. SAXS diffraction pattern of Ag behenate powder at a distance of 1.0 m

In Figure 3 a typical WAXS pattern of p-bromo benzoic acid is shown. The diffraction peaks are indexed according to the values given in Table 2, taken from [2].
 
 Table 2. d-spacings and relative intensities of p-bromo benzoic acid according to [2].
d-spacing/Å rel. intensity d-spacing/Å rel. intensity
14.72 18000 4.25 490
7.36 1200 3.96 2380
6.02 330 3.84 10300
5.67 980 3.74 26530
5.21 6550 3.68 1740
4.72 26000 3.47 760
 
 
 

Figure 3. Calculated diffraction pattern of p-bromo benzoic acid. d-spacings are given in Å.


 Table 3. d-spacings and 2 theta values for several calibration standards

Calibration Standard
d-spacing/Å
2theta @ 5.4 keV
2theta @ 8 keV
2theta @ 16 keV
   Aluminium foil (Al)
 
2.338
58.4
38.5
19.0
2.024
68.6
44.7
21.9
1.431
105.6
65.1
31.2
1.221
138.0
78.2
36.8
1.169
154.4
82.4
38.5
1.012
-
99.0
44.7
   Copper foil (Cu)
 
3.615
37.0
24.8
12.3
2.556
53.4
35.3
17.4
2.087
66.7
43.6
21.4
1.808
78.9
50.8
24.8
1.617
90.5
57.3
27.7
1.476
102.1
63.3
30.4
1.278
127.8
74.6
35.3
1.205
144.6
80.0
37.5
   Silicon powder (Si)
 
3.134
42.7
28.4
14.1
1.919
72.9
47.3
23.1
1.637
88.3
56.1
27.2
1.357
114.3
69.1
33.0
1.245
132.5
76.4
36.0
   Boron Lanthanum (LaB6)
 
4.15785
 
21.353
 
2.93927
 
30.386
 
2.39986
 
37.444
 
2.07803
 
43.516
 
1.85865
 
48.986
 
1.69690
 
53.994
 
1.46974
 
63.216
 
1.38538
 
67.562
 
1.31438
 
71.755
 
1.25332
 
75.847
 
1.20003
 
79.867
 
1.15290
 
83.847
 
1.11100
 
87.790
 
   Sodium Chloride (NaCl)
 
3.258
 
27.3504
 
2.821
 
31.6907
 
1.994
 
45.4470
 
1.701
 
53.8498
 
1.628
 
56.4749
 
1.410
 
66.2239
 
1.294
 
73.0605
 
1.261
 
75.2983
 
1.1515
 
83.9660
 



The s-scale for both, the SAXS and the WAXS range, can be obtained by linear regression, i.e., the linear relation between the known s-values of the calibrant versus the measured peak positions has to be found. A further correction is regarding the flat field response (efficiency) of the detectors. For this correction, the fluorescence light of various foils are used to illuminate the detectors rather homogeneously:

At 8 keV: iron foil (100 mm thick), fluorescence energy: 6.4 keV Ka, 7.1 keV Kb (effic*.dat)
At 16 keV: copper foil (> 100 mm thick), fluorescence energy: 8.028 keV Ka2, 8.048 keV Ka1, 8.905 keV Kb (effic*.dat)

The measured scattering pattern are corrected for the detector efficiency simply by dividing them by the fluorescence pattern. Note: The average of the detector efficiency data should be set to unity and a small threshold should be applied to avoid any division by zero.
 
[1] T.N. Blanton et. al., Powder Diffraction 10, (1995), 91
[2] K. Ohura, S. Kashino, M. Haisa, J. Bull. Chem. Soc. Jpn. 45, (1972), 2651

Last Updated on Tuesday, 11 February 2020 17:20