BMD Monochromator

The 16-BM-C’s double crystal monochromator is configured in an artificial channel-cut geometry, and has a unique design adapting a torque-free sine bar rotation mechanism for changing the monochromator angle. The Bragg rotation center is located at the surface of the second crystal instead of the first crystal to maintain a nearly fixed exit within 100 µm offset over 6-60 keV range of energy variation [1]. The sine bar is pushed by a high-precision (20 nm resolution) stepper motor stage at the bottom of the chamber. The energy calibration is made as a function of the translator position (x in the equation below). The energy resolutions at selected energies are summarized in Table 1.

Table 1. The energy resolution at different incident energies resulting from the convolution of intrinsic Darwin width of Si 111 and the geometric beam divergence defined by the slits in front of monochromator, ΔθS = 25 µrad. The last column represents the practical energy resolutions as measured.

 

Energy (keV)

mono. angle

θm (degrees)

Darwin width

Δθm (µrad)

ΔE/E theoretical

cot θm∙√(Δ2θS+ Δ2θm)

ΔE/E as-measured

24

4.73

10.7

3.29×10-4

5.6×10-4

30

3.78

8.54

4.00×10-4

6.8×10-4

42

2.70

6.09

5.46×10-4

9.3×10-4

 

References:

1.         Narayanan, S., et al., Design and performance of an ultra-high-vacuum-compatible artificial channel-cut monochromator. Journal of Synchrotron Radiation, 2008. 15: p. 12-18.