Chemistry Reference
In-Depth Information
K
α
K
α
K
β
K
α1
K
α2
λ
SWL
Wavelength (
λ
)
Wavelength (
λ
)
(
a
)
(
b
)
FIGURE 3.1 X-ray spectrum generated by a sealed X-ray tube or rotating anode generator.
(a) The spectrum consists of continues (white) radiation and characteristic radiation lines K
a
and K
b
; (b) K
a
line is a combination of two lines K
a1
and K
a2
.
where V is the voltage applied to the X-ray generator. With increasing generator
voltage, the intensity of the whole spectrum increases and the white radiation also
extends to shorter wavelengths. For X-ray diffraction, the three most important
characteristic lines are K
a1
,K
a2
, and K
b
.TheK
a1
and K
a2
lines are so close in
wavelength that they are also called the K
a
doublet. The K
a1
line is about twice the
intensity of K
a2
. When the K
a1
and K
a2
lines cannot be resolved, they are simply
referred to as the K
a
line. The wavelengths of characteristic lines are determined by
the target (anode) material of the X-ray generator. Table 3.1 gives a list of common
anode materials, their average photon energy corresponding to the K
a
line, and their
wavelengths. Since the K
a
line is the most intense characteristic line, the tube anode is
typically chosen based on thewavelength of the K
a
line. The longer thewavelength is,
the larger the separation between the diffraction peaks. Therefore, fewer diffraction
peaks can be measured within the limited 2u range. The contrary is true with shorter
wavelengths. The Cu-K
a
radiation with a wavelength of 1.54 A
is the most frequently
used in X-ray diffraction since the wavelength is suitable for applications of most
TABLE 3.1 Wavelengths of Characteristic Lines of Common Anode Elements
Wavelength (A
¼10
1
nm)
Energy (K
a
)
Anode
keV
K
a
K
a1
K
a2
K
b
Ag
22.11
0.560868
0.5594075
0.563789
0.497069
Mo
17.44
0.710730
0.709300
0.713590
0.632288
Cu
8.04
1.541838
1.540562
1.544390
1.392218
Co
6.93
1.790260
1.788965
1.792850
1.62079
Fe
6.40
1.937355
1.936042
1.939980
1.75661
Cr
5.41
2.29100
2.28970
2.293606
2.08487
