2.3 Particle Induced X-Ray Emission PIXE

The previous section concentrated on X-ray fluorescence. Thissection discusses a different X-ray production technique thatcan lead to the development of 2-D/3-D elemental imaging.

Technique is based on the use of 2-6 MeV particle accelerators

The PIXE arrangements

Faraday Cup

Si(Li) detector

Proton beam

sample

X-rays

Typically proton beams; protons transmit energy to the inner-shell electrons, ionize atoms with subsequent X-ray de-excitation

PIXE SpectrumM-shell

L-shell

K-shellKα1 Kβ1Kα2

Lα1 Lα2

Ex=E (n,l)i-E (n,l)f

Same energies as in theprevious section, onlyexcitation mechanismis different!

X-Ray Energies as before!Characteristic X-Ray Energies

0

10

20

30

40

0 20 40 60 80 100Atomic Number A

X-ra

y en

ergy

[ke

V]

Ka

Kb

La

Lb

L series−

= − ⋅ ⋅ −

E Z eV

nxi

( ) . [ ]σ 22 2136

12

1

K series−

= − ⋅ ⋅ −

E Z eV

nxi

( ) . [ ]1 136 112

2

Sensitivity of PIXE

coun

ts

energy

PIXE shows superior peak-background sensitivity than electron beam induced or X-ray fluorescence methods. This reduces the samples size and improves sensitivity to 0.5 - 5 ppm value range.

Micro-beam techniquesProton beam isfocused onto aµm spot-size andthen scanned inx-y directionacross the sample

micro-beam line applicationsparticle beam

sample steerer

quadrupol

Mapping of gold globules grown on crystal surfaces

Mapping of Deuteriumconcentration inhuman hair.

External PIXE beam applications

Micro beam penetrates thin Ni exitwindow. Distance in air dependson initial energy. The range for 2.5 MeV protons in air is 10 cm.

10 cm

Si detector

Applications in Art & ArchaeologyGold jewelry artifacts, • pottery, • ancient record books, • painting material• writing material in Medieval & Renaissance manuscripts, • Dinosaur bone and eggshell fossil,• Human and Neanderthal man bone• obsidian tools, • human bones, • rock varnish.

Because of limited depth-profile of method mostly applications in surface studies!

Tracing Material OriginsThe red stone eyes of the statue of the Parthian goddess of love Ishtarwere originally thought by Louvrecurators to be made of colored glass

PIXE analysis showed that the inlays were rubies. AL2 SIO4 (F,OH)2+(Cr,Fe rich)

Where did the rubies come from?

Comparison of Fe versus Crcontent in Ishtar rubies withrubies from various provenances shows strong indication thatrubies did originate from Burma.Ancient trade connections between near and far east empires!

Systematic study of Ruby impurity concentrations with PIXE

Thailand and Burma are the mainsupplier of natural rubies. The color of Thailand ruby is violet to deep crimson, of Burma ruby from pinkish to blood red. PIXE analysis of 60 rubies indicated that lighter color of Burma rubies results from higher impurity concentration on Chromium or Vanadium.

0.01

0.02

0.05

0.06

0.08

0.13

0.25

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

Vanadium C

oncentration

Iron Concentration

Thailand

Burma

Native American and European CopperCopper made objects havebeen found in 1400-1500 AD‘pre-colonization’ settlements along the Ohio River valley.PIXE analysis of the relativeAg, Sb and Pb content revealedorigin from native sources (A)or European (trade) sources (B)

Depth-profiling with PIXEPIXE 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.

Range of protons in matter

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 1 2 3 4 5

proton energy [MeV]

rang

e [

mm

]

carbonirongold

Simple Approximation Formula for Range Calculations

R cmA

R cm

[g/cm ]A

AA

Aair

A

A

[ ] . [ ]≈ ⋅ ⋅ ⋅

≡≡

−32 10 4

3

ρ

ρ density in atomic number of absorber

Example: range R208 of 3 MeV protons in Lead (A=208, ρ=11.35 g/cm3)

R

R

208

208

≈ ⋅ ⋅ ⋅

≈ ⋅ =

−

−

32 10208

113513

529 10 53

4

3

..

[ ]

.

cm

cm mµ

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140 160 180

proton range in air Rair [cm]

prot

on e

nerg

y [M

eV]

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

R

Robserved is about

58

58

≈ ⋅ ⋅ ⋅

≈ ⋅ =

−

−

32 1058

8 913

35 10 3532

4

3

..

[ ]

.

cm

cm mmµµ

Beam Stopping in Reality

7 MeV beam energyR≈60 cm

0.3 MeV beam energyR≈1cm