2.3. x-ray absorption & fluorescencensl/lectures/phys10262_2014/art...2.3. x-ray absorption...
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2.3. X-ray absorption & fluorescence
X-ray absorption is classical spectroscopy technique originating from
absorption line analysis of solar chromo-sphere spectrum
Technique has been utilized for a wide range from visible, UV and
X-ray light by absorption of light or X-ray spectrum in vapor.
Disadvantage: probe
must be chemically
prepared & vaporized
X-Ray Fluorescence X-ray fluorescence is based
on the excitation of material
& subsequent X-ray emission
- fluorescence -
bremsstrahlung
Excitation or
Ionization
by X-ray
bombardment
Ka
Ka
Kg
Kb
Only characteristic lines for material are emitted
Analysis of X-ray energies Advantage of X-ray fluorescence is the unique determination of
element characteristic X-ray radiation, Ka , Kb , or La , Lb, Lg lines
Critical component for X-ray fluorescence studies is the necessity
for high energy resolution in the X-ray spectrum to separate
energetically the characteristic X-ray lines of different elements.
Characteristic x-Ray Energies The characteristic X-ray energies
correspond to the energies of the
K or L transitions in the atom:
Ex=h·=h·c/=ΔE=En(i)-EK/L(f)
or it corresponds to the difference
of the absorption edge energies:
Ex =EK,L -En
Data for Pb; Z = 82
atomic weight = 207.210;
density = 11.3400
Absorption edges K-edge: 88.0060 keV
L-edges: 15.8600, 15.1980, 13.0350 keV
M-edge: 3.85000 keV
Characteristic energies Ka1,Kb1: 74.9570 84.9220 keV
La1,Lb1: 10.54900 12.6110 keV
Analyzing the strength of characteristic x-ray transitions gives the
abundance of element in sample: good energy resolution is required
The Lithium drifted Silicon
Si(Li) detector When an X-ray strikes the detector,
it will generate a photoelectron
within the body of the Si. As this
electron travels through the Si, it
generates electron-hole pairs. The
electrons and holes are attracted to
opposite ends of the detector with
the aid of a strong electric field.
The size of the current pulse thus
generated depends on the number
of electron-hole pairs created,
which in turn depends on the
energy of the incoming X-ray. Thus,
an X-ray spectrum can be acquired
giving information on the elemental
composition of the material under
examination.
Applications for XRF techniques
Non-destructive applications
Analysis of small amounts <100 mg of material
Determination of chemical content and structure
• ancient coin material - metal composition
• ancient oil and varnish material
• pigment in ancient paint
• medieval scripts and miniatures
Lucretius Trio
136 BC
Publius Clodius
42 BC
Marcus Antonius
31 BC
Roman Economy
0
20
40
60
80
100
120
-150 -100 -50 0 50 100 150 200 250
Per
cen
t C
om
po
siti
om
Year [AD]
Copper
Silver
The average of the measured Silver and Copper
content in the Roman Denarii as a function of time.
Clearly visible are the debasement ordered by
Octavianus Augustus and Nero, as well as the
continuing debasement of the Denarii during the
Severan period of the empire in the first half of the
third century.
The Andrea Guarneri Violin
The secret of the varnish of the violins made
around 1660 by Andrea Guarneri in Cremona
has been solved by XRF analysis of the varnish
contents - oil, resin, wax. Additional inorganic
substances influence color, hardness etc. The
varnishing technique of the Cremona violin
builders has been lost! The detailed XRF analysis
of all the contents:
• Provides the recipe for the varnish.
• Detects forgery
What kind of varnish does he use?
Violin varnish is applied with a brush in two or more coats. It is composed of sun-thickened oils such as
linseed or walnut, oxidized resins and added coloring matter. Good historic Italian varnish is highly
translucent and soft, yet friable. The surface has texture and a delicate patina, not a gloss. The colors should
be warm and rich.
20 different elements detected:
Fe, As, Pb, pigments in varnish
Zn,Cu,Pb drying substrate in oil
Total x-Ray Fluorescence
Reflector
Si(Li) detector
Sample Holder
X-ray beam
X-ray tube
slits
Alex von Bohlen, Historical Violin Varnishes Spectroscopic Studies and Characterization – Actes de la journee d`etude les
vernis de violin – Cite de la Musique -2006, pg. 60
Alex von Bohlen Friedrich Müller, Microanalysis of old violin varnishes by total-reflection X-ray fluorescence;
Spectrochimica Acta Part B: Atomic Spectroscopy 52, 1053–1056 (1997)
Characteristic X-ray energies
for varnish element K-transitionss
E Z eV
E eV eV
E eV eV
E eV eV
E
x
x
x
x
x
( ) . [ ]
( ) . [ ] . [ ]
( ) . [ ] . [ ]
( ) . [ ] . [ ]
1 136 11
2
15 136 11
22 29
19 136 11
2368
21 136 11
24 50
2
2
2
2
2
2
2
2
for S: Z = 16;
for Ca: Z = 20;
for Ti: Z = 22;
for Fe: Z = 26;
( ) . [ ] . [ ]
( ) . [ ] . [ ]
25 136 11
26 37
29 136 11
2858
2
2
2
2
eV eV
E eV eVxfor Zn: Z = 30; keV
keV
keV
keV
keV
Hair & Hemoglobin:
C3032H4816O872N780S8Fe4
For additional color, lakes are ground fine and stirred into the liquid varnish or applied as a
glaze between coats of varnish. Lakes are organic pigments made by precipitating natural
dyestuffs usually onto alum. Three common lakes are "robbia" made from madder root,
"verzino" made from brazilwood shavings and "carmina" made from cochineal. Grinding is
done with a glass muller over a sheet of roughened glass. The microscopic colored crystal lake
particles remain isolated and suspended within the varnish layer allowing light to freely
penetrate between the crystals, maintaining the translucency of the varnish. Yet, as light passes
through the varnish layer on its course to and from the wood, it is refracted in multiple
directions by the colored crystals, lighting up the varnish as if it were one
Varnish
Stradivari violin
varnish, ground layer and wood
A. Von Bohlen, F. Meyer; Spectrochimica Acta B 52 (1997) 1053
Heavy element components vary from violin
to violin, indicating that different secret
recipes have been used by violin makers!
Modern versus old varnish
Bogdan Constantinescu, Catalina Pauna, Daniela Stan, Authentication of ancient stringed musical instruments – the
contribution of X-Ray Fluorescence analysis 2011
K, Ca, Fe, Cu, Zn
Pb
Comparison between Violins Recipes
Fe2O3 in umber or ochre; As2S3 golden auripigmentum,
As4S4 red realgar; PbO yellow massicot, Pb3O4 red lead