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Physical Methods for Cultural Heritages Radiography and XRF Spectroscopy

Physical Methods for Cultural Heritages Radiography and XRF Spectroscopy G. Valentini - Tel. 6071 - gianluca.valentini@polimi.it. Electromagnetic spectrum and X-Ray band. Range of interest in material analysis 1 – 30 keV.

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Physical Methods for Cultural Heritages Radiography and XRF Spectroscopy

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  1. Physical Methods for Cultural Heritages Radiography and XRF Spectroscopy G. Valentini - Tel. 6071 - gianluca.valentini@polimi.it

  2. Electromagnetic spectrum and X-Ray band Range of interestin material analysis1 – 30 keV A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  3. Radiography • X radiation propagates in solid materials with low propability of undergoing scattering events (Rayleigh scattering) • This property allows one to “see” through objects with X radiation • In standard radiography the object projects a “shadow” on the plate • The plate undergoes a chemical activation of Ag halide which is inversely proportional to the “density” of the material crossed by X Ray (negative image) • In conservation of works of art radiography is applied: • To study the technique used by the artist to make the artwork • To assess the conservation status of the works of art • To find and map previous restorations • The fields of application include almost all the movable works of art • Paintings on canvas or woods • Sculptures made of wood, bronze or stones (e.g marble) • Archaeological specimens: amphoras, pottery and clay

  4. Electron kicked out from the atom X ray photon Exciting electron 2 1 1 X ray production • X ray are generated by the bombardment of a target with charged particles (electrons or ions) or with high energy radiation • The spectrum of the emitted X rays contains a wideband component and peaks characteristic of the material the target is made of Characteristic radiation Bremsstrahlung radiation

  5. nucleus Kicked out electron + Emitted X ray 2 Exciting electron 1 1 X photon electron X ray production • La production of X rays take place because of: • Bremsstrahlung • Photoelectric effect

  6. Example of spectrum of X radiation • The low energy photons are absorbed by the window of the X tube • The maximum photon energy (80 keV) is given by the acceleration potential of electrons

  7. Emission spectrum of a X ray tube with Tungsten (W) anod

  8. Principles of radiography • The sample is put between the X tubeand the radiographic plate • The spatial resolution of the radiographydepends on the size of the X emitter and onthe distance between the object an the plate • The capability of an image to record the finedetails of an object is given by the Modulation Transfer Function (MTF) • The MTF quantify the image contrast as afunction of the size of details (lines/mm)

  9. The radiographic plate • The radiographic plate is made of a polyester film covered by a gelatine emulsion containing argent salt (AgBr) • The X radiation creates a latent image due to activation of Ag halide grains that are made visible through the development of the plate • The development reduces the AgBr grains to silver (colloidal state) that gives the characteristic black “colour” to the plate • The fixer (Sodium thiosulfate) dissolves AgBr grains non reduced and allows the light exposure of the plate • The density of a radiographic plate is given by the equation: • The sensitometric curve gives the density of the plate as a function of the exposure to X radiation • The slope of the curve is the gparameterof the plate B = light emerging from the developed plate whenilluminated with intensity B0

  10. Example of application to a painting Betsabea at her bath, Rembrandt (Musee du Louvre)

  11. Interaction mechanisms of X rays with matter X photon interacts with an electon of an internal shell The electron is kicked out A fluorescenza X photon can be emitted Fast electron Fast electron Photon 1 j Photon Photon Photon 2 1 Photoelectric effect Compton effect • X photon interacts with an electron of an external shell • The electron is kicked out • The X photon loses energy and is deflected

  12. X ray attenuation in materials dx S A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  13. X ray attenuation in materials A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  14. X ray attenuation in materials I x μ = Mass attenuation coefficientρ = Material density L Photoelectrict absorption Rayleigh scattering Compton scattering A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  15. Attenuation of X rays in silicon Silicon (A = 28) 104 http://physics.nist.gov/PhysRefData/Xcom/html/xcom1.html r = 2.33 g/cm3 102 Rayleigh (coherent) scattering Compton scattering Photoelectric absorption 100 10-2 10-2 10-3 10-1 A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  16. Attenuation of X rays in silicon 104 Silicon 102 34 cm2/g 100 10 keV 10-2 10-2 10-3 10-1 A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  17. Attenuation of X rays in silicon 104 Silicon 102 34 cm2/g 100 10 keV 10-2 10-2 10-3 10-1 A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  18. Silicon absorption length (UV X-ray Soft-Gamma range) High-resolution imaging X-ray spectrometers, L. Struder,Nuclear Instruments and Methods in Physics Research A 454 (2000) 73}113 A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  19. Probability of the photoelettric effect Interaction of X photons with materials • The transmission of X radiation through a slab of thickness s is given by: • The absorption coefficient a dependson material and on X photon energy • The coefficient a/r (mass attenuationcoefficient m) is independent on material density but depends on atomic number Z m = absorption coefficient

  20. Mass attenuation coefficient • Low energy (< 25 keV) • Photoelectric region • High energy (> 50 keV) • Compton region • To achieve an optimal image thefollowing condistion must be satisfied: • The previous equation gives insight on the choice of the X photons energy • For dense materials or large thicknessg rays (high energy) must be used a (cm-1) m Z3 Regione Compton (m cost)

  21. Linear attenuation coefficients (a) • The X photon energy should be selected according to the criterion as 2

  22. Application of the radiography • Paintings on canvas and woods • Pigments can be distinguished on the basis different opacity to X rays, e.g. white lead (basic lead carbonate) • Hidden paintings beneath the visible painting can be revealed • The artistic “style” of a painter can be studied by analysing the changes performed before the final painting • The conservation status of paintings can beassessed (e.g. damage produced by xylophagous insect in wood can be detected)

  23. Tomography • In tomographic recording the radiographic plate is replaced by a X detector or by an array of X detectors • During the measurement the sample is rotated in such a way to explore all the possible oblique paths through the object • By means of a proper mathematical operation (Radon transform) the internal 3D structure of the sample is recovered

  24. Applications of tomography • Taking advantage of different material attenuations the internal structure of an object can be recovered • Application limits for the different materials: • Wood: up to > 1 meter of diameter • Marble and stones: max diameter  50 cm • Pottery and clay: density lower than that ofstones  greater upper diameters • Metals (bronze) max diameter of some centim.Greater diameters for empty sculptures

  25. XRF mesurement technique • A low power X ray beam isdelivered to the sample • Every chemical element in the sample emits characteristic X rays that give a fingerprint of the element • An energy sensitive detector measures the energy spectrum of the emitted X rays • The spectrum is measured by counting the photons that have the same energy by means of a device called multichannel analyzer • The analysis of the peaks of the X fluorescence spectrum allows one to identify and quantify the elements contained in the sample • Almost all the elements having an atomic number above a certain threshold can be detected • In portable detectors, especially useful for the analysis of cultural heritages, the lighter detectable atom is usually sodium 11Na • The limit to detectable elements is given by the presence of air and by the entrance window of the detector, which is usually made of a low atomic number element (beryllium)

  26. Photoelectric effect Fast electron Photon Photon 2 1 1 We are interested to this X-ray: X-ray fluorescence X ray fluorescence emission mechanism A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  27. Typical emission spectrum of a modern pigment

  28. K L M X fluorescenza spectral lines • The X fluorescence is given by photons emitted as a consequence of photoelectric effect

  29. X emission lines of elements

  30. 99.4 149.7 M L3 L2 (1839 -99.42) eV L1 1839 eV EKa1=EbK1-EbL3 K1 X emission lines of silicon http://xdb.lbl.gov/Section1/Table_1-2.pdf A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  31. A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  32. X ray detection methods • Methods that uses the wavelength dispersion • The X rays exiting the sample are collimated and delivered to a crystal • The crystal “reflects” only the X rays that satisfy the Bragg condition • The reflected radiation is further collimated and measured with an X ray detector (e.g. proportional counter or scintillation counter) • The X ray spectrum is measured by rotating the crystal and moving the detector

  33. 10 +4 S i l i c o n 10 /g] +3 2 10 +2 P h o t o e l e c t r i c 10 +1 Mass attenuation coefficient [cm R a y l e i g h 10 0 T o t a l 10 -1 C o m p t o n 10 -2 10 -3 1 1 0 1 0 0 E n e r g y [ k e V ] X ray detection methods • Methods that uses the energy dispersion • A solid state detector (Si) is used to measure the energy of X photons • The X rays interact with silicon manly through photoelectric effect • Each X photon generates in silicon a number of free electrons proportional to its energy • The electrons are collected and“counted” by a system that includesa p-n junction and an amplifier

  34. Electrons Holes vout X ray A Eph N+ anode Amplification circuit N type Si P+ cathode Reverse Bias vPeak The amplitude of theoutput signal is proportional to the energy of the incoming X ray A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  35. Conductionband Free electronlevel 99.4 eV 99.8 eV 149.7 eV Valenceband K L M L3 L2 L1 1839 eV K1 *Approximate scales Elettronic levels of silicon A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  36. K L M Population in elettronic levels of silicon Conductionband Free electronlevel 99.4 eV 149.7 eV Valenceband L3 L2 L1 1839 eV K1 A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  37. Elettronic levels of silicon A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  38. 99.4 eV 149.7 eV L3 L2 L1 1839 eV K1 Interaction of UV-VIS-NIR radiation with silicon Conduction and valence bands Free e- h+ couples A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  39. Ee=Eph0-EbK1 Conductionband Free electronlevel 99.4 eV 149.7 eV Valenceband Eph0 Fast electron L3 free e- h+ couples L2 L1 EbK1=1839 eV K1 Interaction of X rays with silicon by photoelectric effect A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  40. Conductionband Free electronlevel 99.4 eV 149.7 eV Valenceband L3 EKa1=EbK1-EbL3 L2 L1 1839 eV An X ray is emitted K1 The hole is filled by an electron from external levels A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  41. Ee= EKa1–EbL Ee=Eph f-EbK1 Ee= EbL Si Si Si Si EKa1+ELa=EbK1 Ee=ELa Eph f Eph= EbL Eph1=EKa1 Eph2=ELa Energy conversion mechanism of a X photon in e-h couples Through a cascade of events the energy of the incoming photon is transferred to the fast electrons, which generates e- h+ couples A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  42. The fast electrons generate electron-hole couples The e-h couple generation process is a stochastic process The incoming photon energy is transferred to the fast electrons The average numberof e-h couples generated in the detector is: Thestatistical spreadingof thenumber of couples e-h: εcoup ≈ 3.6 eV F ≅ 0.12 in Silicon (Fano factor) εcoup ≈ 30 eV in gas A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  43. The Fano factor The generation of e- h+ couples is a statistical process characterized by a sub-Poissonian spreading: F ≈ 0.125 for Silicon at ∼ room temperature. F. Perotti, C. FioriniNucl. Instr. and Meth. in Phys. Res. A 423 (1999) 356-63 A. LONGONI Rivelatori Microsensori e Microsistemi Politecnico di Milano AA 2008-09

  44. Anode Anode The p-i-n diode • The diode is reversely biased in order to fully deplete from free carriers the semiconductor bulk. • The electrons generated by the X-ray interactionare collected at the anode, the holes at the cathode. The detector capacitance CD is proportional to the active area A. Longoni Rivelatori Microsensori e Microsistemi Politecnico di Milano A.A. 2008-09

  45. Eph E The energetic resolution The resolution in the measurement of the charge delivered by the interaction depends on the detector properties and on the noise of the electronic circuit Incident photon. monoenergetic Statistical spreading of thenumber of couples e-h Electronic noise. contribution We must try to keep this term as low as possible A. Longoni Rivelatori Microsensori e Microsistemi Politecnico di Milano A.A. 2008-09

  46. Anode The SDD detector for X radiation The SDD for X-ray spectroscopy: for the same active area the SDD has much smaller CD than the classical PIN diode Anode The electrons are collected by the small anode,characterised by a low output capacitance, whose value is independent on the active area of the detector. A. Longoni Rivelatori Microsensori e Microsistemi Politecnico di Milano A.A. 2008-09

  47. A new type of X detector with high resolution • The “Silicon Drift Detector” (SDD) has been developed at Politecnico di Milano • It has high energetic resolution and can work at temperature close to room temperature (around 0 °C) • It is very suitable to set up portable XRF instruments since it does not require liquid nitrogen cooling Prof. Longoni, FioriniDipartimento di ElettronicaPolitecnico di Milano

  48. Performances of SDD vs. P-I-N detectors A. Longoni Rivelatori Microsensori e Microsistemi Politecnico di Milano A.A. 2008-09

  49. Application fields of XRF spectroscopy in archeometry • Analysis of paintings • Identification of pigments • Verification of authenticity • Determination of the composition of metallic alloys • Analysis of ancient coins • Study of bronze archaeological specimens • Determination of the composition of glasses and pottery • The precise measurements of impurities in pottery can allow the identification of the site where the clay was taken from • Analysis of other materials: obsidian, ivory, inks, etc. Chart showing the fields of use of XRF spectroscopy in Cultural Heritages

  50. Analysis of paintings (Prof. Longoni, Fiorini) Lorenzo Lotto (S.Michele al Pozzo Bianco, Bergamo) Analysis of the blue blanket by means of XRF with with SDD detector The presence of Si, K, Fe e Co allows the identification of the ‘smaltino veneto’. Ni, As e Bi are specific components of “cobaltite”

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