Chapter 2: Principles of Radiography, XRay Absorption, and XRay Fluorescence

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Chapter 2 Principles of Radiography, X-Ray
Absorption, and X-Ray Fluorescence

• Radiography is a method
• to study invisible details,
• cracks, joints, in different
• archaeological artifacts

• X-ray fluorescence is a
• method to understand the
• chemical and elemental
• constituency of the artifacts

There is a multitude of applications Analysis of
coins, or metal materials, pottery
techniques, paper paintings.

• Radiography is the first
• survey technique
• X-ray fluorescence is
• on-site analysis technique

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The Value of Art and Paintings
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Investment in Analytical Techniques
X-ray facilities as quick testing tool
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2.1. Physics of X-ray Sources
Characteristic X-rays Ex h? hc/? DEkin
Ekin(i)-Ekin(f) Bremsstrahlung
EmaxEkin(i)
bremsstrahlung
K-lines
EKRH(Z-1)2 (1-1/4)
Emax
characteristic X-rays
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Bremsstrahlung
Bremsstrahlung (from the German bremsen to
brake and Strahlung radiation, i.e. braking
radiation) is electromagnetic radiation produced
by the acceleration of a charged particle, such
as an electron, when deflected by another charged
particle, such as an atomic nucleus (often
metals). Bremsstrahlung has a continuous
spectrum. The phenomenon was discovered by
Nikolas Tesla during high frequency research he
conducted between 1888 and 1897. Accelerated
charges give off electromagnetic radiation, and
when the energy of the bombarding electrons is
high enough, that radiation is in the X-ray
region of the electromagnetic spectrum. It is
characterized by a continuous distribution of
radiation which becomes more intense and shifts
toward higher frequencies when the energy of the
bombarding electrons is increased.
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Characteristic X-Rays
X-ray production typically involves bombarding a
metal target in an x-ray tube with high speed
electrons which have been accelerated by tens to
hundreds of kilovolts of potential (see third
transparency). The bombarding electrons can eject
electrons from the inner shells of the atoms of
the metal target. Those vacancies will be quickly
filled by electrons dropping down from higher
levels, emitting x-rays with sharply defined
frequencies associated with the difference
between the atomic energy levels of the target
atoms. The frequencies of the characteristic
x-rays can be predicted from the Bohr model .
Moseley measured the frequencies of the
characteristic x-rays from a large fraction of
the elements of the periodic table and produces a
plot of them which is now called a "Moseley
plot".
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X-ray beam
The X-ray energy distribution is characterized
by Bremstrahlung and characteristic lines
depending on anode material and electron energy.
The use of filters originates a
quasi-monochromatic x-rays beam.
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X-ray absorption
The effect of X-ray radiography, depends on the
absorption (transmission) probability of X-rays
through sample matter and the photographic plate
to generate an image. The image results from the
difference in X-ray absorption probabilities
which is defined by the absorption coefficient µ
for a particular material!
µ is in units 1/cm Often tabulated as µ/?
cm2/g with ? being the density of the material
g/cm3
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X-ray absorption mechanism
Incoming X-ray photon
X-rays absorption in matter increases with
decreasing energy Absorption µ s
E-3. Absorption edges indicate additional
excitation of electrons from the next inner shell
(M,L,K).
As higher energy as less absorption
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Absorption edges
The absorption edges correspond to the ionization
energies for electrons from the inner shells of
the atom Kedge, L-edge, M-edge ...
Absorption edges for each element can be found
in tabulations e.g. http//www.csrri.iit.edu/mucal
.html
K-edge EK (Z-1)213.6 L-edge EL
(Z-sL)213.61/4 M-edge EM (Z-sM)213.61/9
Data for Zn Z 30 atomic weight 65.3800
density 7.14000 g/cm3 K-edge 9.65900 keV
L-edges 1.19600, 1.04400, 1.02100 keV M-edge
0.139000 keV
Data for Pb Z 82 atomic weight 207.210
density 11.3400 g/cm3 K-edge 88.0060 keV
L-edges 15.8600, 15.1980, 13.0350 keV M-edge
3.85000 keV
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2.2. Radiography with X-rays
Classical technique (photography) only with high
energy photons (X-rays). X-ray sensitive film or
photomultiplier.
X-rays partly absorbed
Image from transmitted x-rays
Material has no (or reduced) opacity for X-rays,
can be used for depth profiling and material
structure analysis.
J. Lang A. Middleton Radiography of Cultural
Material Butterworth Heinemann, Oxford 1997
Library  N 8558 .R33 1997
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The Virgin and the Child
Workshop of Dirck Bouts Netherland, c.
1420-1475 Oil on wood, 30.5 x 21.6 cm
The cradle at the back of the panel appears as
grid structure on the X-radiograph. To improve
the image, the spaces in the cradle were filled
with a resin with an X-ray opacity similar to
that of the wooden cradle.
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X-ray energies
X-rays need energy to pass through the material
to be analyzed and to reach the detector or
photographic plate. X-rays had insufficient
energy to pass through the wood.
More absorption with d more absorption with r
30 reduced transmission through wood gt50
reduced transmission in heavy metal paint content.
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Details of the Face
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Choice of Energy
The optimum energy for X-ray beams depend on
interaction cross section and on the nature
(density) of material. The differences in
attenuation coefficients should be maximized by
choice of energy.
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Portrait of a Man, 1541
Master of the 1540s Netherland, 1540-1551 Oil on
wood, 40.3 x 35 cm
Analysis of past damage repair
The X-radiograph of Portrait of a Man reveals
that the white paint for the collar extends
under the area that is now black.
The join between two boards shows up as a light
area in the X-radiograph because the boards had
come apart and were glued together using an
adhesive mixed with lead white.
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Lead in the Portrait of a Man
Suppose you have an X-ray source with an
intensity of I0. Calculate the difference in
absorption for the dPb 0.1 cm layer of white
lead based paint with an attenuation coefficient
of µPb22.6 cm-1 versus the absorption in the
doil/canvas 0.3 cm thick layers of canvas and
oil paint with an attenuation coefficient
µC0.21cm-1.
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Vermeers Woman with a String of Pearls
1660-1665
Covered wall-hanging tile structure of the
floor details of the chair
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Another Vermeer
Vermeer van Delft The woman with the balance
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LOOKING INSIDE A WORK OF ART
The Thinker by Auguste Rodin, France 1880
The sculpture is extremely frontal, with most of
its weight projecting forward. Such an imbalance
is anchored by a lead counterweight placed in the
interior of the base. In the x-radiograph, the
lead anchor is visible as a white mass at the
back of the base. Also visible in the
x-radiograph are iron armatures inside the
sculpture. Seen as curling gray forms, the
armatures were used to hold internal core
material in place during casting.
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Get the right energy!
To get difference in intensity
high energy x-rays are required to penetrate
massive metal material
For calculation of attenuation coefficients m
see http//www.csrri.iit.edu/mucal.html
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Reconstruction of Art
X-ray radiograph
before
after
Ornate iron cross of unknown origin from around
the mid-17th century. Traces of gold on the
surface indicate that the cross was once gilt.
X-ray radiography reveals original design and
guided the restoration process.
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Back to the value of paintings
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Summary X-ray radiography
Radiography is a powerful tool with a wide range
of applications. Its usefulness is mainly based
on the differences in material densities which
affects the x-ray attenuation coefficients. This
determines the x-ray opacity for heavy metal or
high density material compared to low density
material like paper. The method gives only
qualitative differences on photo-screen, it is a
tool for first investigation, a detailed analysis
requires more sophisticated studies.