Chemistry Reference
In-Depth Information
Excitation is possible by a plane grating monochromator in the range of
about 100 eV up to 1860 eV so that light elements, such as C, N, O, Na, Mg, and
Al, can be excited by their K peaks, and transition elements, such as Fe, Cu, and
Zn, by their L peaks. The instrumental assembly allows the direct TXRF of
light element contaminations with the lowest detection limits that reach down
to 100 fg or 10
9
atoms/cm
2
. Nanolayers can be characterized by GI-XRF and
composition, thickness, and density can be determined by a reference-free
quantification. Finally, NEXAFS can reveal information about the chemical
speciation of light elements by tuning the excitation energy.
7.1.2NewVariantsofX-RaySources
New types of X-ray sources were constructed and used for excitation in X-ray
spectral analysis aiming at a high X-ray flux.
The first variant was constructed that represents a special X-ray tube with a
spiral orbit of electrons [33]. It is a tabletop instrument of lab-size based on a
microtron injector for high-energetic electrons, which are guided on an orbit of
8 cm radius in a betatron-like motion. With energies up to 1 MeV, the electrons
are focused on a target and decelerated. Because of multipulse resonance
injection, the X-radiation is pulsed with a repetition rate of 1 kHz and a pulse
width of 10
μ
s. White radiation is produced and emitted from a focus of 10
μ
m
diameter at a wide divergence of some 100 mrad (5.7
°
). K
α
peaks of heavy
elements, such as tungsten, can be excited (59.3 keV). A high X-ray flux of 10
14
photons/s/mm
2
/mrad
2
/0.1% BW is achieved so that NEXAFS measurements
can be carried out within 30 min.
This device is a sophisticated X-ray source where highly accelerated electrons
of a microtron are decelerated by collisions with a target (bremsstrahlung). It is
not a synchrotron where highly accelerated electrons are forced to run in a
circular orbit and are decelerated by a magnetic deflection (synchrotron radia-
tion). The new MIRRORCLE X-ray source was applied to two examples, but
both examples did not make use of total reflection geometry. First, X-ray
absorption near-edge structure (XANES) measurements were carried out in
absorption mode [34]. A flat Si(111) crystal was used for an energy scan of
18-22 keV, an ellipsoidally curved Si crystal focused the X-ray beam onto a
sample consisting of a Mo foil or a K
2
MoO
4
powder. The transmitted X-rays were
recorded by an X-ray charge-coupled device (CCD) camera and the XANES
spectra were Fourier-transformed. They showed radial distributions with four
oscillations and main maxima at 0.20 and 0.33 nm.
The second application of the MIRRORCLE X-ray source demonstrated
residual stress by XRD measurements [35]. A gear made of chromium-
molybdenum steel after carbonizing was analyzed. At an energy of 57 and
97 keV, XRD spectra of Fe (110) with
d
=
0.205 nm were recorded at 2
θ
angles
of 5.9
°
and 3.7
°
, respectively. Compressive stress of some 100 MPa (negative
values) could be determined in a depth up to 0.5 mm.
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