13)X-ray Spectrometry– HVC Capacitor, HV Ceramic Capacitor to build All kinds of X-ray machine.
PARTICLE-INDUCED X-RAY EMISSION
PIXE, using proton energies between 1 and 4 MeV, serves as
an established method of nondestructive analysis in numerous
application fields. Under these normal conditions, high-Z elements
are usually detected via their L X-ray lines. In addition, the range
of protons at these energies is 10 -100 ím, allowing the method
relatively surface sensitive. Denker et al. (H1) proposed high-
energy PIXE using 30-100-MeV protons allowing an efficient
excitation of K lines of high- Z elements. Because of the large
proton range and the small absorption of high-energy X-ray lines,
high-energy PIXE provides a nondestructive analytical tool allow-
ing the analysis of bulk material under a coating layer. Quanti-
fication using the GUPIX software extended to proton energies
up to 100 MeV was tested on alloy standards with various
thicknesses, giving promising analytical results based on K lines
of heavy elements up to a thickness of2 mm. In contrast, for
biological applications, the accurate information on major, low-Z
elements is critical, which are most of the cases obtained using
RBS at the ion microprobes. The evolution of thin-window EDX
detectors allows the detection of light elements down to boron
also for PIXE. Kerte´szetal.(H2) reported on a PIXE arrangement
using two EDX detectors simultaneously, a large-area Si(Li)
detector with Be window, and a second Si(Li) detector with an
ultrathin polymer window. Quantification of elements from C to
U in tissue samples could be obtained by simultaneous iteration
of the two spectra.
Depth-selective analysis using PIXE based on the different
excitation range for ions having different energies was reported
by a Slovenian group and called “differential PIXE” ( H3). The
variation of the proton energy was achieved in a simple manner,
by variation of the air path length between the exit window and
the target. They developed a procedure for deconvolution of
concentration profiles from a series of measurements by PIXE at
different energies. Secondary fluorescence and projectile energy
straggling was taken into account in the calculation model. The
depth distribution of major elements could be determined, but a
major drawback of the lateral broadening of the beam in the air
path was encountered. The authors suggest a combination of
stopping foils and air path length variation to overcome this
problem.
A number of ion microprobes have been commissioned
recently, allowing study of two-dimensional elemental distributions
by PIXE analysis routinely with a resolution of 1 ím. The “dynamic
analysis” matrix transform developed by Ryan et al. (H4) facilitates
generation of real-time quantitative elemental PIXE images. The
calculation model was extended to take into account matrix and
pile-up effects caused by strong concentration variations in a self-
consistent manner. The authors illustrated the necessity of treating
these effects for each pixel in the elemental images. Simultaneous
scanning transmission ion microscopy (STIM) measurement can
offer normalization for PIXE images, since it provides quantitative
information on the two-dimensional variation of the sample mass
density. Because of the small widening of ion beam diameters in
a few hundreds of micrometers in a light matrix, a French group
( H5)useda1-í m ion beam as a tomographic probe for non-
destructive imaging of the 3D structure of biological samples at
the cell level. The internal structure of cryofixated and freeze-
dried cancer cells was explored using STIM tomography, giving
access to the 3D distribution of mass density within the analyzed
volume. The authors propose the combination of STIM and PIXE
tomography for the 3D distributions of elemental concentrations
at the cell level, which is currently under development. The
feasibility of PIXE tomography for biological objects was studied
by Beasley and Spyrou (H6), taking into account radiation damage,
as well as X-ray attenuation and secondary fluorescence contribu-
tions in 3D in order to reconstruct elemental distributions.
Several applications of PIXE need at least semiquantitative
elemental analysis of thick targets. Preoteasa et al. ( H7) presented
a standardless quantification procedure for PIXE analysis of thick
biomineral targets, aiming at trace elemental analysis. This type
of approach was necessary because of the lack of good reference
materials. To handle matrix effects correctly, the concentration
of major light elements had to be determined by a complementary
method, elastic recoil detection analysis (ERDA), allowing the
calculation of the mean atomic number. The obtained relative
concentrations allowed the classification of dental composites by
multivariate statistics. Changes on the in vitro demineralization
of dental enamel suggested that a dissolution of Ca compounds
in the outermost layer results in the uncovering of deeper layers
containing higher trace element levels.
The combination of PIXE with high-resolution crystal spec-
trometers allows detection of changes in emission line shape and
position changes due to chemical effects. Kave`ie` et al. (H8) applied
a WD spectrometer in Johansson geometry, with an energy
resolution below the natural line width of the S-K R line. Results
on pure S and Fe
2(SO
4
)
3
standards demonstrated a clear depen-
dence of the KR energy shifts on the chemical state sulfur in the
sample. The oxidation state of S could be determined in an aerosol
sample containing 28 íg/cm
2
S, requiring a measuring time of 2
h. The high-resolution PIXE system composed of a curved crystal
and a position-sensitive proportional counter presented by a
Japanese group (H9) allowed rapid chemical-state analysis without
the need of evacuating the samples. Using KR 1,2
spectra for
monitoring the chemical shift, a measuring time below 1 min was
sufficient, allowing in situ experiments. The authors, however,
used the less intense Kâ spectra, because they reflect the nature
of the chemical bonding more prominently. Time-dependent
variation of the fine structure due to oxidation in air was
recognized clearly in the S K â spectra obtained from marine
sediment samples. Reis et al. (H10 ) reported on the possibility of
chemical-state analysis using semiconductor ED detectors that
have 2 orders of magnitude worse energy resolution than crystal
spectrometers. The proposed method is based on the dependence
of L shell relative yields on the chemical composition of the target.
The authors studied the variation of the W L X-ray spectra on
incident ion beam energy for three different W chemical com-
pounds. Although the variation patterns contain information on
the chemical species composition of the target, the method needs
measurements at different proton energies requiring a long time
for speciation elements in unknown samples.
Extraterrestrial expeditions need a simple, reliable, but robust
X-ray spectrometer for on-site characterization of minerals. During
the 2004 NASA expedition to Mars, an R-particle X-ray spectrom-
eter (APXS) based on a
244
Cm radioactive source was used. The
244
Cm source emits bothR particles and the L X-rays of plutonium.
This combined excitation provides a unique advantage, since the
ionization cross section for R particles is higher for light elements,
while Pu-L X-rays excite the K shell of medium- Z elements
efficiently, covering effectively the geologically interesting ele-
ments from Na to Zr. Because of the combined excitation of R
particles and X-rays, as well as the complex excitation geometry,
Omand et al. (H11 ) were inspired to develop a simulation code
combining analytical and Monte Carlo methods, to predict the
X-ray yields from minerals exposed to the APXS that was used
for rock and soil analysis during the 2004 NASA expedition to
Mars. Agreement is found between these simulated X-ray yields
and the yields obtained by earthbound measurement of geochemi-
cal standards using an identical APXS system.