Chemistry Reference
In-Depth Information
15
where
h
is the Planck's constant and
c
is the speed of light (
h
≈
4.136
×
10
s;
c
≈
2.998
×
10
8
m/s).
The conversion of energy and wavelength of Equation 1.1 can be made by
the simple relationship
eV
1
:
23984
λ
nm
E
keV
(1.2)
Frequently used physical constants are listed in Table 1.3 with the latest
numerical values from NIST (National Institute of Standards and Technology,
Gaithersburg, MD). They are given in SI units, mostly with 9 to 11 digits and with
a relative uncertainty of some 10
8
[47]. The values with SI units can be
transformed into atomic units by the relationship 1 J
=
6.241 509 34
×
10
18
eV.
In the text, physical constants will be given with only 3 to 5 digits and atomic units.
1.3.2X-RayTubesasX-RaySources
X-rays are originally produced by the bombardment of matter with accelerated
electrons. Usually, such a
primary
radiation is produced by an X-ray tube of the
Coolidge-type as mentioned earlier and shown in Figure 1.4. It consists of a
vacuum-sealed tube with a metal-glass cylinder. A tungsten filament serves as
hot cathode, and a pure-metal target, such as chromium, copper, molybdenum,
or tungsten, serves as the anode. Electrons are emitted from the heated
filament and accelerated by an applied high voltage in the direction of the
anode. The high-energy bombardment of the target produces heat above all
while the electrons are absorbed, retarded, or scattered. Finally, X-rays and
Auger electrons can be produced. The heat is dissipated by water-cooling of the
anode while the X-rays emerge from a thin exit window as an intense X-ray
beam. Mostly, a 0.2-1 mm thick beryllium window is used. Reflected electrons,
including Auger electrons, cannot escape from this window.
The X-ray tube is supplied by a stabilized high-voltage generator. High
voltage and current applied to the tube determine the intensity of the X-ray
beam. The voltage can usually be chosen between 10 and 60 or even 100 kV, the
current between 10 and 50 mA, so that an electric power of several kilowatts
can be supplied. However, only about 0.1% of the electric input power is
converted into radiation and most of it is dissipated as heat. For that reason,
such X-ray tubes have to be cooled intensively by water. A flow rate of 3 to 5
l/min is commonly needed.
The primary X-ray beam is normally used to irradiate a sample for analysis.
By this
primary
irradiation, the atoms in the sample are generally excited to
produce
secondary
X-rays by themselves. This effect is called X-ray fluores-
cence. The secondary radiation can be used as a color pattern of the sample as
its
chromatic
composition changes with the
element
composition. The spectral
pattern can be recorded like a barcode by means of an X-ray detector and
constitutes the basis of XRF analysis.
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