Chapter 2: Principles of X-Ray Fluorescencensl/Lectures/phys178/pdf/chap2_1.pdf · Chapter 2:...
Transcript of Chapter 2: Principles of X-Ray Fluorescencensl/Lectures/phys178/pdf/chap2_1.pdf · Chapter 2:...
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Chapter 2: Principles of Radiography, X-Ray Absorption, and X-Ray Fluorescence
• X-ray fluorescence is amethod to understand thechemical and elementalconstituency of the artifacts
There is a multitude of applications:Analysis of coins, or metal materials,pottery techniques,paper & paintings.
• Radiography is a method to study invisible details, cracks, joints, in different archaeological artifacts
• Radiography is the firstsurvey technique;
• X-ray fluorescence ison-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
Electron Beam
Tungsten ormolybdenum
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Nature & Origin of X-Rays – a Reminder
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2.1. Physics of X-ray Sources
bremsstrahlung
Char. X-rays K-lines
ionization λm
Char. X-Rays: Ex = h·ν = h·c/λ = ∆Ekin = Ekin(i) - Ekin(f)
Bremsstrahlung: electrons being stopped; minimum wavelength: λm= h·c/Ekin(i)
EK=RH·(Z-1)2 ·(1-1/4)
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X-ray beamThe X-ray energy distribution is characterized byBremstrahlung and characteristic lines dependingon anode material and electron energy. The use of filters originates a quasi-monochromatic x-rays beam.
Quasi monochromatic x-rays
Low energy x-rays are absorbed in filter material
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X-ray absorption mechanism
λ~1/EX-rays are increasingly absorbed in matter with decreasing energy
Absorption: µ ~ σ ~ E-3.Absorption edges indicate the
additional excitation of electronfrom an inner shell (M,L,K), which requires more energy.
M-edge
L-edge
K-edge
K-edge
IncomingX-ray photon
As higher energy as less absorption
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Absorption edgesThe absorption edges correspond to the ionization energies for electrons from the inner shells of the atom: K=edge, L-edge, M-edge ...
K-edge: EK= (Z-1)2·13.6L-edge: EL= (Z-σL)2·13.6·1/4M-edge: EM= (Z-σM)2·13.6·1/9
Absorption edges for each element can be found in tabulations e.g.http://www.csrri.iit.edu/mucal.html
Data for Zn; Z = 30 atomic weight = 65.3800; density = 7.14000 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 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 energyphotons (X-rays). X-ray sensitive film or photomultiplier.
Material has no (or reduced) opacity for X-rays, can beused for depth profiling and material structure analysis.
X-rays partly absorbed
Image fromtransmittedx-rays
J. Lang & A. Middleton; Radiography of Cultural Material; Butterworth Heinemann, Oxford 1997; Library # N 8558 .R33 1997
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X-ray tube
painting
photo-plate
The Virgin and the ChildWorkshop of Dirck BoutsNetherland, c. 1420-1475Oil on wood, 30.5 x 21.6 cm
The cradle at the back of the panelappears as grid 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 energiesX-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.
I d I e ndi
i( ) = ⋅ = ⋅− ⋅ ∑0 0
µ µ σ More absorption with dmore absorption with ρ
energy [keV]
0.1
1
10
100
1000
10000
0 100 200 300 400 500 600
µ c
oeffi
cien
t
carboncadmiumlead
X-ray absorption coefficient
0
0.2
0.4
0.6
0.8
1
1.2
0 100 200 300 400 500 600
energy [keV]
Tran
smis
sion
I(d
)/I0
Cadmium (0.1cm)wood (2cm)canvas (0.2cm)Lead (0.1cm)
X-ray transmission 30% reducedtransmission through wood>50% reducedtransmissionin heavy metalpaint content.
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Details of the FaceSome pigments, including lead white and lead-tin yellow, contain elements of highatomic weight. These colors will absorbX-rays, will locally block the blackening of the film, and will appear as light areas in the X-radiograph. Here, lead white was locally applied in the under-modeling of the Virgin's face, on the ridge of the nose, on the upper lip, and in the eye sockets. Other pigments and cracks absorbed the X-rays to a lesser extent, and thus appear dark on the X-radiograph.
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Choice of EnergyZ=48 CadmiumZ=51 Antimony
Z=79 GoldZ=29 Copper
Co-Au,Cd,Sb
Optimum energy for X-ray beam depends on interaction cross section and on 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
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.
Master of the 1540sNetherland, 1540-1551Oil on wood, 40.3 x 35 cm
Analysis of pastdamage & repair
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Lead in the Portrait of a Man Suppose you have an X-ray source with an intensityof I0. Calculate the difference in absorption for the dPb= 0.1 cm layer of white lead based paint with an attenuation coefficient of µPb=22.6 cm-1 versus the absorption in the doil/canvas= 0.3 cm thick layers of canvas and oil paint with an attenuation coefficient µC=0.21cm-1.
I d I e I e I
I d I e I e I
Pbd
Cd
Pb Pb
C C
( ) .
( ) .
.
.
= ⋅ = ⋅ = ⋅−
= ⋅ = ⋅ = ⋅−
− ⋅ −
− ⋅ −
0 02 26
0
0 00 063
0
0104
0 94
µ
µ
only 10% of the x rays pass through the lead - paint layer!
94% of the x rays pass through the canvas- oil paint layer!
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Vermeer’s Woman with a String of Pearls
Covered wall-hangingtile structure of the floordetails of the chair
1660-1665
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Another Vermeer
Vermeer van Delft; The woman with the balance
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LOOKING INSIDE A WORK OF ARTThe 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:
I dI d
eCu
Pb
dPb Cu( )( )
( )= − ⋅µ µ
high energy x-rays are required to penetrate massive metal material
For calculation of attenuation coefficients µ see:http://www.csrri.iit.edu/mucal.html
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Reconstruction of Art
before
X-ray radiograph
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 rangeof applications. Its usefulness is mainly based onthe 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.