2.5. Depth-profiling with PIXE
PIXE opens with the opportunity for depth
profiling an additional dimension in art and
artifact analysis. Sequences of layers can
be analyzed to probe the preparation
procedure and the preparation techniques.
0.14
Range of protons in matter
0.12
carbon
iron
gold
range [mm]
0.1
0.08
0.06
0.04
0.02
0
0
1
2
3
proton energy [MeV]
4
5
Depth scanning with protons
Numerous applications,
particularly in radiation
treatment applications.
Simple Approximation Formula
for Range Calculations
AA
RA [cm] ≈ 32
. ⋅ 10 ⋅
⋅ Rair [cm]
ρA
14
−4
Example: range R208 of
3 MeV protons in Lead
(A=208, ρ=11.35 g/cm3)
R 208
R 208
208
≈ 3.2 ⋅ 10 ⋅
⋅ 13 [cm]
1135
.
≈ 5.29 ⋅ 10−3 cm = 53μm
−4
10
proton energy [MeV]
ρ A ≡ density in [g/cm3 ]
AA ≡ atomic number of absorber
12
8
6
4
2
0
0
20
40
60
80
100
120
proton range in air Rair [cm]
140
160
180
3 MeV protons on Nickel/Iron layer
Confirm the observed
range of the 3 MeV
proton beam in Nickel!
A=58, ρ=8.9 g/cm3
58
R 58 ≈ 3.2 ⋅ 10 ⋅
⋅ 13 [cm]
8.9
R 58 ≈ 35
. ⋅ 10−3 cm = 35 μm
observed is about 32 μm
−4
Beam Stopping in Reality
7 MeV beam energy
R≈60 cm
0.3 MeV beam energy
R≈1cm
PIXE: experimental set-up
protons from accelerator
• beam exit: thin Kapton foil
• moveable mirror inflects laser
on beam axis
laser beam
• two detectors at 135°
• shielding against radiation
from exit foil
HPGe
• xy table
shielding
• camcorder surveys and
documents beam spot
object
moveable
mirror
foil
Si(Li)
xy table
Depth profiling of the
“14 Nothelfer” by Lucas Cranach the Elder
PIXE analysis
at two different
proton energies
probes material
at different depth
C. Neelmeijer et al., Nucl. Instr. Method. B 118 (1996) 338-345
The red robe of the holy Christopherus
70
60
Hg
Pb
50
40
content [%]
30
20
10
0
Hg
Pb
4
3.7
3
2.5
2.1
1.5
energy [keV]
element
The spectrum shows the
three L-transitions for Hg &
Pb at two different energies.
Depth distribution of Hg and Pb
indicates that the deeper layer
of the coat was painted with
minium Pb3O4 and the surface
layer with vermilion HgS.
HgS
IHg
HgS depth profile
Calculate the range of the 2 MeV and the 4 MeV
proton beam in HgS; assume pure Hg!
(A=200, ρ=13.6 g/cm3). The thickness of the HgS
layer corresponds to the 50% Hg X-ray intensity.
Rair(2 MeV)=8 cm, Rair(4 MeV)=22 cm
Pb3O4 R Hg (2 MeV ) ≈ 3.2 ⋅ 10−4 ⋅ 200 ⋅ 8 [cm] = 2.7 ⋅ 10−3 cm = 27 μm
13.6
200
R Hg (4 MeV ) ≈ 3.2 ⋅ 10−4 ⋅
⋅ 22 [cm] = 7.3 ⋅ 10−3 cm = 73 μm
13.6
HgS thickness corresponds to about 50 μm
Art History Motivation:
19th century painting
Judith with the head of Holofernes
• many copies of
Allori, even by
himself
• this copy:
unknown artist
19th century??
• high quality
of painting:
(brushstroke...)
⇒ much older?
Christofano Allori (1615)
Pitti Palace, Firenze
trust collection, Berlin
A. Denker & J. OpitzCoutureau, Nucl. Instr. Meth. B213 (2004) 677-682
copy of Allori:
indirect dating
• analytical task:
identification of pigments used
(indirect dating by pigment
chronology)
• paint layers > 100 µm thick
⇒ high proton energies necessary
• 15 different spots analysed
on the painting
• on all but one spot:
only lead, iron, calcium
19th century or much older?
• stylistic evaluation not sufficient
• more “solid” arguments: chronology of pigments
• collaboration with restaurateurs/art historians
copy of Allori:
yellow colour
• Fe, Pb, Ca
• from Fe Kα/Kβ:
iron on top of lead
⇒ most probably yellow ochre
• no Cd, Cr ... ⇒ modern pigments can be excluded
• No Sb, As ⇒ Naples Yellow and Auripigmentum can be
excluded
copy of Allori:
blue colour
• the only spot
without Fe
⇒ Prussian Blue
excluded (after 1735)
• no Co, Cu ⇒ Smalt,
Cobalt Blue, and Azurite excluded
• ⇒ either ultramarine (Na, Al) or indigo (organic)
both “invisible” for PIXE (in air), both used since antiquity
PIXE in the analysis of metal
surface structure
Because of the large stopping power of high Z material
(metals) for charged ion beams, protons cannot penetrate
deeply into metal material. Nevertheless PIXE emerged
as a very successful method for studying metal surface
layers to learn about ancient metal treatment techniques.
e.g. surface analysis of
gilded metal artifacts
like this drinking cup.
The thickness of gold gilding
Gold has the atomic number A=198
and a density of ρ=19.3 g/cm3.
198
R Au (4 MeV ) ≈ 3.2 ⋅ 10 ⋅
⋅ 22 [cm] = 51
. ⋅ 10−3 cm = 51 μm
19.3
50 μm thickness
A
gold gilding
gildingoflayer
tothickness
can be investigated,
A gold
up toof50up
μm
can be investigated.
Topenetrate
penetratethrough
through thicker
thicker layers
is required
required!
to
layers higher
higherbeam
beamenergy
enery is
−4
(example E p = 10 MeV, R air = 115 cm):
198
R Au (10 MeV ) ≈ 3.2 ⋅ 10 ⋅
⋅ 115 [cm] = 2.7 ⋅ 10−2 cm = 270 μm
19.3
−4
Penetration through surface layers
(lacquer, patina, …)
• range of 4 MeV protons in plastic:
protons
~ 0.25 mm
ƒ energy loss in 1 mm plastic
(ρ » 1 g/cm3): ~ 1 MeV
X-rays
plastic
copper
⇒ use of 68 MeV protons:
ƒ small lateral straggling
0 1 2 3 4 5 6 7 8
depth (mm)