−
2
−
ME12044A
2.
PRINCIPLE OF MEASUREMENT
In XAFS measurement, an absorption coefficient is measured accurately by varying the energy of X-rays
irradiated onto the sample. Because a highly monochromatic X-ray (
∆
E < 10 eV) is required, it is
necessary to perform crystal spectrometry. However, the ordinary spectrometry using a plane crystal can
utilize only a small divergent angle, and therefore the intensity of the X-rays that can be used remains low,
which would increase the time required for measurement. For this reason, the R-XAS Looper adopts an
optical system that performs spectrometry and collection simultaneously using a Johann-type or
Johannson-type curved crystal. In Figure 2.1, S is an X-ray source, A the center of the monochromator
crystal, and F a receiving slit. These three points are arranged on the Rowland circle with radius R, in such
a manner that the relation SA = AF will be satisfied. The energy of the X-rays is varied by controlling that
distance.
The monochromatic X-rays selected in this way are irradiated onto the sample, thereby measuring the
intensity of the X-rays transmitted through the sample. By determining the ratio between that X-ray
intensity and that of the incident X-rays, the absorption coefficient can be measured.
R
O
θ
θ
L
L
F
S
A
x
y
B
Figure 2.1 Principle of the Focusing Optical System
