Senza titolo - Università degli Studi di Ferrara

104 Pages·2011·26.9 MB·English

by  Fauzia Albertin

Università degli Studi di Ferrara

DOTTORATO DI RICERCA IN
FISICA

CICLO XXIII

COORDINATORE Prof. Filippo Frontera

K-edge Radiography
and
applications to Cultural Heritage

Settore Scientifico Disciplinare FIS/07

Dottorando Tutori
Dott. Albertin Fauzia Prof. Gambaccini Mauro
Prof. Petrucci Ferruccio

Anni 2008/2010

Contents
Introduction 7
1 K-edge Radiography 9
1.1 Elemental analysis on painting layers: K-edge technique . . . . . . . . . . 9
1.2 X-ray absorption in matter . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3 Quasi-monochromatic beams . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.4 Image Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.4.1 Image corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.4.2 Lehmann algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.5 Pigments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2 Experimental apparatus 17
2.1 X-Ray tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2 Mosaic crystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3 Optical bench . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.4 Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3 Radiation damage on CCDs 23
3.1 Charge Coupled Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.1.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.2 Optimization of the system . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.2.1 Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.2.2 Optimization of the gate voltage . . . . . . . . . . . . . . . . . . . 25
3.3 Radiation Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4 Characterization of the X-ray source 31
4.1 Measurement setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3
4 CONTENTS
4.2 CZT energy resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.3 Iron K-edge: 7.11 KeV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.4 Cobalt K-edge: 7.71 KeV . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.5 Copper K-edge: 8.98 KeV . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.6 Zinc K-edge: 9.66 KeV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.7 Arsenic K-edge: 11.87 KeV . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.8 Bromine K-edge: 13.79 KeV . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.9 Strontium K-edge: 16.20 KeV . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.10 Molybdenum K-edge: 19.99 KeV . . . . . . . . . . . . . . . . . . . . . . . 45
4.11 Silver K-edge: 25.51 KeV . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.12 Cadmium K-edge: 26.71 KeV . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.13 Tin K-edge: 29.20 KeV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.14 Barium K-edge: 37.44 KeV . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.15 Air attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5 K-edge imaging 57
5.1 Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5.2 Cobalt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.3 Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.4 Zinc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.5 Arsenic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
5.6 Cadmium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.7 Tin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.8 More complex samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
5.9 Paintings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.9.1 Test Painting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.9.2 La moisson a Montfoucault . . . . . . . . . . . . . . . . . . . . . . 82
5.9.3 “Landscape” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.9.4 “Sea landscape” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
6 Other X-rays applications 87
6.1 X-ray Scanner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.2 Some application of X-ray digital
radiography of paintings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.2.1 Landscape, wooden panel, XX cent. . . . . . . . . . . . . . . . . . 89
CONTENTS 5
6.2.2 Wooden Crucifix . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
6.2.3 “Maddalena penitente”, oil on canvas, XVIII cent. . . . . . . . . . . 94
7 Conclusions 97

Introduction
The present work of thesis is focused on application of X-ray K-edge technique to
paintings.
This technique allows one to achieve a topographic map of a pigment on the whole surface
of the painting.
The digital acquisition of radiographic images by using monochromatic X-ray beams
allows to take advantage of the sharp rise of X-ray absorption coefficient of the elements,
the K-edge discontinuity.
Working at different energies, bracketing the K-edge peak, allows recognition of the target
element.
The K-edge radiography facility installed at Larix Laboratory, at Department of Physics
in Ferrara, consists of a quasi-monochromatic X-ray beam obtained via Bragg diffraction
on a mosaic crystal from standard X-ray source.
In the first 3 chapters a description of the K-edge technique and the experimental appara-
tus are presented.
In chapter 4 the characterization of the monochromatic beams in the 7-40 KeV range is
presented.
K-edge elemental mapping of Iron, Cobalt, Copper, Zinc, Arsenic, Cadmium and Tin, on
test object and test painting, have been carried out and result is shown in chapter 5.
In the end, a transportable facility for digital radiography is presented and some radio-
graphic analysis of works of art performed are shown.
7

Chapter 1
K-edge Radiography
1.1 Elemental analysis on painting layers: K-edge technique
The traditional X-ray radiography plays an important role in scientific diagnostics of
cultural heritage. It can reveal execution techniques and underpaintings information. But
it is based on X-ray attenuation of materials and it cannot provide analytical composition
[1].
Widely used, non-invasive techniques to perform elemental analysis on painting layers
are X-ray fluorescence (XRF) [2] and Particle-Induced X-ray Emission (PIXE) [3]. These
techniques allow for simultaneous detection of elements on a painting but the inspection
is intrinsically local and it is not designed to perform exhaustive analyses in large areas.
A topographic map of one pigment on the whole surface of a painting can be obtained
with K-edge technique, originally proposed by Lehmann [4] for medical applications [7, 8]
and in recent years applied to cultural heritage [9, 10].
This technique takes advantage of the sharp rise of X-ray absorption coefficient of the
elements, the K-edge discontinuity.
Working at different energies, below and above the K-edge peak, allows to make recogni-
tion of the target element. Realizing two radiographies with this energy choice means
maximizing the signal variation of target element while maintaining almost unchanged
the response from the background.
The images are processed by Lehmann algorithm to obtain two new images: the first one
giving the mass density distribution (g/cm2) of the K-edge element while the second one
giving the distribution of all other materials in the sample.
An elemental map by dual energy radiography can be obtained in a reasonable time
with monochromatic X-ray beams, as produced by a synchrotron source [10]. The aim
of this work is to investigate this technique using a device transportable in a museum
that consists of a quasi-monochromatic X-ray source obtained via Bragg diffraction on a
mosaic crystal and a standard X-ray tube.
9
10 K-edge Radiography
Figure 1.1: Photoelectric effect, Compton effect and pair production and their dominance at
different energies and Z of absorber.
1.2 X-ray absorption in matter
In X-ray interaction with matter three process can occur, depending on X-ray energy
and atomic number of the target: photoelectric absorption, Compton effect and pair
production.
X-rays trasmission through a target of thickness x is expressed by
I = I0e−µx (1.1)
where I is the number of photons emerging from the target, I the number of photons
0
impinging on the target and µ is the total linear attenuation coefficient.
Linear attenuation coefficient µ represents the sum of the coefficients related to the three
different process: τ for photoelectric, σ for Compton and κ for pair production.
µ = τ +σ+κ (1.2)
The three different process and their dominance are shown in Fig.1.1 as a function of
photon energy and Z of the absorber.
As shown at low energy (1-100 KeV) and relatively low atomic numbers (Z 20 50)
÷
photoelectric effect dominates and it is possible to consider total linear attenuation
coefficient only related to τ.
Due to the independence of X-ray interaction from physical and chemical state of the
target, photon mass attenuation coefficient µ/ρ (cm2/g) is usually used, whose values are
tabulated in the NIST database [16].
(µ)ρx
I = I0e− ρ (1.3)
If a compound and not a single element is used as a target the total absorption
depends on the absorption of different elements and on their relatives weights
µ µ µ µ
( ) = ω ( ) +ω ( ) +ω ( ) (1.4)
ρ tot A ρ A B ρ B C ρ C