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High Power X-ray Generator for XAFS Experiments 1. Introduction
X-ray absorption fine structure (XAFS) provides
information on the atomic-scale structure around a
specific atom, and has become an essential analytical
tool in materials, biology and many other sciences [1,
2]. The availability of a synchrotron source has
facilitated the application of XAFS, because a strong
smooth spectral distribution is important for the
measurements, and synchrotron radiation is emi-
nently suitable. However, as is often the case in
materials study, the difficulty in obtaining quick
feedback ready for the next sample preparation is one
of the big disadvantages. In addition, as the number of
scientists involved in this field increases rapidly,
many problems, including the strictly limited beam
time of the facility become significant.
Instruments for carrying out measurements in an
ordinary laboratory have been continuously devel-
oped and improved since the middle of 1970s [3-6].
Above all, a lot of work has been devoted to
developing a spectrometer [7-12], to produce
monochromatic X-rays efficiently from a continuum
spectrum generated in a tube. However, from a
practical point of view, performance so far has not
always been satisfactory. A long measuring time was
usually required, and sometimes there was a problem
with the quality of the data.
The authors consider that further scope for
improvement comes with the development of an X-
ray generator [13-17] that is suitable for XAFS
experiments. Most X-ray generators for industrial use
have been designed for either X-ray diffraction or X-
ray fluorescence analysis, and no X-ray source has
been specially designed for XAFS so far. This report
describes the necessary specifications for such a
source and show how the present new X-ray generator
has raised the standard of laboratory XAFS
experiments.
2. Design Considerations
Since XAFS is absorption spectra in the X-ray
region, a simple transmission technique is generally
used as an ordinary measurement. One has only to
measure X-ray intensities before and after trans-
mitting the sample as a function of incident X-ray
energy scanned by a monochromator. However, this
is not easy because XAFS is a small oscillation of the
absorption coefficient and therefore precise measure-
ments are needed. The main problems are as follows:
(1) the low intensity of incident X-ray flux at the
sample position, resulting in insufficient count
statistics of the signal, because XAFS uses
monochromatic X-rays from a weak bremsstrahlung
component, (2) the degradation of the spectrum
caused by 2nd or 3rd order harmonic reflections from
a crystal monochromator, and (3) the effect of non-
smooth spectral distribution mainly due to the
characteristic lines of tungsten, which is the filament
material and which evaporates on the anode.
XAFS experiments need high intensity mono-
chromatic X-rays free from high energy components
41
The Rigaku Journal

The Rigaku Journal
Vol. 12/ number 2/ 1995
CONTRIBUTED PAPERS
HIGH POWER X-RAY GENERATOR FOR XAFS EXPERIMENTS
KENJI SAKURAI AND HITOSHI SAKURAI*
National Research Institute for Metals, Tsukuba, lbaraki 305, Japan
*Rigaku Corporation, Akishima, Tokyo 196, Japan
A new X-ray generator has been developed for X-ray absorption fine structure (XAFS) experiments. Over the years, much
research has been performed at synchrotron radiation facilities, and it has been believed that the measurements in an ordinary
laboratory are not of a competitive standard. In this report, we describe the basic idea of a laboratory X-ray source dedicated to
XAFS and show how our generator is a powerful and practical alternative. It realizes a very high tube-current of 1,100 mA at low
tube-voltage of 18 kV, and therefore provides extremely intense monochromatic X-rays, which are completely free from higher
order reflections. It enables the measurement of a spectrum with the same quality as that using a synchrotron in a reasonably short
time, typically 30 min to 2 h. This X-ray generator is now available in a normal laboratory, and a new phase of laboratory XAFS is
now beginning. and contamination lines. The expression 'high
intensity' is ambiguous and may sometimes lead to
misunderstanding. Tube power is obviously impor-
tant for obtaining high intensity, but the size and
divergence of the beam at the sample position should
be considered at the same time. The XAFS equipment
uses a monochromator, often a curved crystal
monochromator, and the energy resolution is mainly
determined by geometrical factors such as focal size
and receiving slit width. Thus, in order to obtain high
intensity with reasonable resolution, one should con-
sider not only the tube-current but also the shape of the
X-ray focus. These requirements are similar to the
conditions for focusing optics in X-ray diffraction.
Therefore the narrow intense line focus is important
[13, 14], although the authors never think X-ray
diffraction tube is suitable for XAFS.
Diffraction experiments use characteristic X-rays
from the anode, and therefore the tube-voltage is
usually set rather high, typically 40-60 kV. Con-
versely, XAFS uses the weak bremsstrahlung and
therefore the typical tube-current of 200-300 mA from
an X-ray diffraction source is not enough for XAFS
experiments. In addition, high energy X-rays are un-
necessary and should be eliminated. Low tube-
voltage operation is a perfect way to suppress higher
order harmonics, as long as sufficient intensity can be
obtained. That is, a completely different design is
necessary compared with a diffraction generator. Fur-
thermore, to be free from the tungsten lines, new
filament materials that contain no tungsten should be
employed [18]. Otherwise, tungsten lines from the
anode would have to be rejected by means of some
other techniques.
As a result of these points, the following
specifications have been considered essential for an
XAFS X-ray generator: (a) an extremely high tube-
current (e.g. more than 1,000 mA), (b) a narrow line
focus (e.g. about 0.1 mm at 6 deg. take off), (c) low
tube-voltage operation (e.g. 10-30 kV), and (d) LaB
6
or other non-tungsten filament. We started with a
tungsten filament because of the ease of design, and
therefore specifications (a), (b) and (c) were taken into
consideration for our X-ray generator then, although
very recently we succeeded in developing a new LaB
6
filament [19]. This article reports on the earlier stage
where a tungsten filament was employed. However,
even when a tungsten filament is used, tungsten lines
can be completely suppressed by low tube-voltage
operation.
3. X-ray generator for XAFS
Figure 1 shows a schematic drawing of the
electron gun in the X-ray generator developed for the
dedicated use of XAFS experiments. The electron
gun is the most important component, as almost all
the specifications discussed in the previous section
come with this new electron gun. It allows an extre-
mely high tube-current and a narrow line focus at a
low tube-voltage. The maximum allowable tube-cu-
rrent between the filament (F) and the anode (A)
depends on the work function of the filament
materials, the size of the filament, the tube-voltage
between F and A, and the distance (d) between them.
In the present generator, the filament employed is a
tungsten coil, and the entire coil length, coil diameter,
and wire diameter are 17.7, 5, and 0.6 mm, respec-
tively. This is much larger than a conventional fila-
ment to ensure a high filament-current which results
in high thermoelectronic emission (i.e., tube-current)
from F to A. Since lowering the tube-voltage works
against an increase in the tube-current, it is necessary
to shorten d to compensate. When d is shortened,
focusing becomes difficult in general, but in the
present case, the geometrical parameters (s, u, t, f
1
, f
2
)
of the Wehneit electrode (W) and the bias voltage
applied between
W
and F are optimized. These condi-
tions were experimentally determined through the
observation of the shape and size of the focal spot,
including the degree to which the sub focal spot was
suppressed.
Another important feature of the present X-ray
generator is the use of a compact rotating anode. A
molybdenum anode is 100 mm in diameter, and
Vol. 12 No. 2 1995
42
Fig. 1
Schematics of the electron gun. A: rotating anode,
W: Wehnelt electrode, F: filament. Parameters for the
present X-ray generator are as follows: d: 10.75 mm, s:
5.25 mm, u: 5.5 mm, t: 5.8 mm, f
1
: 8.0 mm and f
2
: 9.0 mm. rotates at 6,000 rpm for cooling purposes by means of
a motor installed inside the cup. Though such a small
anode has been widely used for X-ray diffraction
equipment, it