9)X-ray Spectrometry– HVC Capacitor, HV Tụ gốm to build All kinds of X-ray machine.
The properties of the primary X-ray photons, produced deep
in the irradiated sample, are modified significantly by the inter-
actions (scattering, secondary excitations, etc.) before leaving
the specimen. For a better understanding of these processes,
Ferna´ ndez et al. (D23 ) published a detailed 3D mathematical
model to describe how the emission spectra are influenced by
interactions with matter based on the transport theory. On the
basis of the calculations, the authors found that not only the
sample composition but the surrounding geometry system has
great importance in the determination of the radiation field. They
used their code to reconstruct the detected X-ray radiation
generated by a narrow monochromatic ( E ) 15 keV) beam
focused on a homogeneous Fe target. In the calculation model,
several processing chains were considered as photoelectric-
photoelectric, photoelectric-Rayleigh, Rayleigh-photoelectric,
photoelectric -Compton, and Compton-photoelectric.
A new theoretical approach was published by Willis and
Lachance ( D24 ) for replacing the usual additive corrections for
absorption and interelement enhancement effects by introducing
a multiplicative factor. The authors demonstrated by a mathemati-
cal description that the absorption for primary and secondary
radiations can be modeled by a more simple calculation than the
generally used classical formalism in XRF spectrometry. To use
FPM for quantification of the material composition with EDXRF
analysis, the availability of the detector efficiency function is a
basic requirement. Unfortunately, in the low-energy region, the
efficiency is strongly influenced by absorption of the emerging
X-rays in the entrance window of the detector. Scholze and Procop
published new results about this fundamental problem ( D25 ), for
a polymer window that contains C, N, and O, has 200-400-nm
thickness, is supported by a Si grid to improve the mechanical
stability, and is coated by a very thin Al layer. The absorption of
these structural components determines the total absorption of
the detector window. This parameter depends on the near-edge
structure of the attenuation function versus the energy. To disclose
this effect, the authors proposed a new model for the calculation
of the detector efficiency for thin-window EDXRF detectors. The
model was based on the calculation of absorption from the front
end contact and any other possible contamination and on
measurements of window transmittance. Some examples demon-
strate the reliability of the proposed model for a Si(Li) detector
equipped with an AP3.3 window and 8-nm Ni contact on the
detector crystal contaminated with 2 íg/cm
2
O. Applying this new
efficiency model, they found a drastic reduction of the number of
unknown parameters used in the characterization of the detector
efficiency calculation.
Finally we review the publication of O’Meara and Campbell
(D26 ) about corrections of the efficiency function of a conventional
Si(Li) detector by Monte Carlo simulation of the creation of the
full-peak intensity and of the contribution of the coherent scatter
interactions after photoelectric absorption events up to 30-keV
X-ray energy. They measured the efficiency curve and fitted it by
a least-squares procedure, using a conventional model. They
applied this model correction to estimate its contribution to the
efficiency function versus energy. For the experiments,
55
Fe,
65
Zn,
57
đồng,
241
Am,
109
Đĩa CD,
133
Ba, and
210
Pb radioactive point sources were
used with a Si(Li) detector having nominal thickness of 5 mm
and an active area of 80 mm
2
, with 8- ím Be DuraBeryllium
cửa sổ.
TOMOGRAPHY, HOLOGRAPHY, AND X-RAY
SCATTERING
During the last two decades, the X-ray fluorescence holography
(XRFH) and X-ray fluorescence tomography methods were rapidly
developed, both experimentally and in terms of evaluation
algorithms and theoretical models. X-ray holography is capable
of investigating atomic localizations in solid materials using the
X-ray intensity and phase information obtained from the interfer-
ence of different X-ray beams that are emitted by atoms from
different parts of the solid material. Tegze et al. published ( E1)a
general overview about the principles of the X-ray holography and
described the necessary experimental techniques and measuring
methods and the evaluation procedures for data analysis; they
demonstrated with some examples how an atomic-sized resolution
can be reached. The X-ray holographic measurement is simple;
the atoms are excited by an external X-ray beam and emit X-ray
fluorescence or scattered radiation, and the angular distribution
of this radiation has to be measured. The basic difficulty is the
extremely low signal-to-background ratio, 10
-3
, which may result
in a 2-month-duration measurement using a conventional X-ray
tube. The authors showed some examples about a 3D model of
solid crystals determined by X-ray holography: e.g., determination
of the 3D atomic structure of Ni, SrTiO
3
, CoO, and NiO crystals
and determination of the local position of Mn atoms in an AlPdMn
quasi-crystal. They emphasized that intensive development is
necessary in experimental and data evaluation techniques before
this excellent method will become widely used for routine analysis.
A Japanese research group reviewed (E2) a new torodial-shaped
bent graphite analyzer installed in an X-ray holography measuring
system at the SPRING8 synchrotron facility, at the BL37XU
beamline, to determine the 3D structure of atoms in a local
structure around Cu atoms in 300-nm thin films of EuBa
2Cu3O7
,
which were epitaxially grown on a MgO substrate. The Cu-KR
characteristic intensity was focused by a graphite mirror into the
front of the detector; this experimental setup increased drastically
the detectable X-ray intensity. The whole measurement time was
2.5 h, when measuring in an azimuthal angle range of 0-360°
and 0-70°; the angular steps were between 0.3 ° and 1°, and the
measuring time for one individual pixel was 0.1 s with a 2 10
6
counts/s count rate. Faigel and co-workers published (E3)a
second paper about X-ray holography methods, and they gave a
systematic description of two basic arrangements: inside source
holography and inside detector holography. What can this method
give us that other techniques cannot? The authors answered that
XRFH gives local information on the atoms such as EXAFS, but
XRFH yields a direct 3D order of the atoms in a selected sample.
The authors emphasized that the method is capable of imaging a
nonperiodic sample structure; that analytical feature makes XRFH
one of the important experimental procedures for 3D imaging of
atomic-sized microobject and it will fit excellently into the high-
brilliance fourth-generation X-ray sources such as free electron
lasers. The holography technique is a very promising tool for
determination of atom localizations in semiconductor crystals as
Takahashi et al. ( E4) published with their experimental results
of XRF holography measurements in a GaAs crystal. They
performed first an X-ray absorption analysis on the As K absorption
edge and then recorded three different XRF holograms at three
different excitation energies around the As K edge. Comparison
of these identical holograms yielded information on the position
of the surrounding Ga localized in the neighborhood of the As
atoms, which information increases the accuracy of the calculated
atomic distances. Inverse Fourier analysis was applied in X-ray
fluorescence holographic experiments for estimation of the
distances between neighboring atoms in a publication of Hayashi
(E5); the method was based on the measurement of the Au
fluorescence LR (9.712 keV) line into a wide solid angle at different
azimuthal and polar angles. The author determined the interatomic
distances on the basis of 16 different holograms recorded at
different excitation energies in the range of 22.5-30 keV in 0.6-
keV steps, and the detector setup consisted of a LiF analyzing
crystal and an avalanche photodiode. The obtained distances of
the Au atoms in different direction were compared to the data
calculated from EXAFS measurements, and a good agreement was
found; the typical accuracy for determination of atomic position
based on the XRFH measurements was within 0.3% The X-ray
radiography- and tomography-based imaging methods suffer from
the poor contrast; this can be improved by increasing the
irradiating X-ray flux that yields higher radiation damage in the
sample. Finally, in this subchapter of X-ray holography, we review
of a new holography setup published Suzuki et al. ( E6). In this
paper, they described an X-ray holography microscope consisting
of a Fresnel zone plate lens (FZP) placed between the sample
and the detector. The FZP acted as an objective lens and a wave
front divider interferometer because the holographic information
was obtained in the interference of two identical X-ray beams. The
two beams were produced by the FZP from the original coherent
beam; half of the beam propagated through the object sample
and the other half without interaction with the sample (reference
wave). The reference beam propagated though an X-ray prism;
this optical element modified the beam direction in such a way
that both beams met on the surface of the detector and produced
interferences that contained the holographic information. Con-
sidering the optical capability of the FZP and refractive prism lens
in their holographic microscope, they estimated the achievable
theoretical spatial resolution as 10 nm. An American research
group published (E7) their results in Fourier transform X-ray
holography, using soft X-rays originating from clusters of Au
nanoballs located on the surface. A resolution of 30 nm was
obtained when large clusters of nanoballs were used as reference.
The method is a lensless procedure that can be applied very easily
because magnifying imaging optics are not necessary and only
an off-axis reference beam and a Fourier transformation are
necessary, with digital reconstruction. The method could be
applied to any kind of complex object, and it was capable of 3D
tomography imaging if the analyzed object and the reference
objects were both on the rotation axis. Peele et al. ( E8) developed
a measuring and data evaluation method using a phase-contrast
X-ray tomography procedure applied for absorption tomography.
The authors demonstrated their proposal in practice and showed
that it can be applied in the case of a microfocus laboratory X-ray
source or bending magnet insertion device at synchrotron facili-
ties. The projections on the samples were taken at angles from 0
to180 ° in 0.25 ° steps. The size of the monochromatized beam was
102 35ím
2
with 60 2.1 írad
2
divergence; the sample was
mounted in a goniometer head that was rotated and moved in
two identical and orthogonal directions in front of the X-ray beam.
The imaging detector was a 300- ím-thick CdWO4 scintillation
screen, coupled to a CCD camera with 1024 1024 resolution,
that was 4.65 ím in size. A combined tomography measuring setup
was designed and built ( E9) at a 10-MW research reactor using
mixed radiation consisting of neutron,ç- and X-rays, for industrial
applications in case of macroscopically sized samples. For the
evaluation procedure, the authors used the commonly applied the
back-projection procedure, using 180 identical projected pictures
recorded by a CCD camera and a connected scintillator screen.
To improve the tomography image, the authors introduced some
corrections in the reconstruction procedure: (i) extraction of the
relevant area of the projection containing the range of interest of
the object, (ii) intensity correction to arrange constant background
intensity, (iii) elimination of the nonuniform sensitivity of the
detector plate, and (iv) filtering the highest intensive pixels. Wang
et al. ( E10 ) published about a new high-pressure chamber that
was constructed for high-pressure X-ray tomography (HPXTM)
measurements. A spatial resolution of10ím was achieved. In
comparison with the conventional tomography method, most of
the resolution loss occurred in the high-pressure chamber is due
to the hard scattering and absorption in the irradiated sample.
The scope of the new method is to study the inner structure of
both crystalline and amorphous materials with phase-contrast and
diffraction-enhanced tomography imaging procedures. The main
motivation of the authors for developing HPXTM was to study
Fe-rich melt segregation mechanisms from silicate matrixes. The
original high-pressure chamber was modified by aluminum rings
that allow penetration of hard X-rays into the sample material that
is compressed up to 8 GPa. A new device for X-ray diffraction
microscopy and tomography was developed by the research group
of Beetz ( E11 ) for complex analysis of cryogenically cooled
biological samples, to obtain the maximum information on the
microstructure of noncrystalline light matrix samples. The sample
can be placed into a cryogenically cooled specimen holder that
can be tilted in the range of 0°-80° for 3D imaging with a four-
axis goniometer. For the purpose of direct imaging, a FZP can
be additionally inserted into the measuring setup. Cooling of the
samples having light matrixes, such as most biological samples,
is necessary due to the damage effect in the sample bulk caused
by the intensive heat load by the X-ray beam transmitted through
the matrix. The CCD camera used for recording the diffraction
patterns had a resolution of 1340 1300 pixels with a 20-ím pixel
size. That device was kept at -45 °C to reduce unwanted
electronic noise.
The quantification procedures of XRF tomography are a central
problem in data evaluation due to the huge amount of data.
Chukalina et al. ( E12 ) developed a special reconstruction algo-