Pigment Response to Xrays Emission

Pigments have been characterized by their color and their relative transparency to infrared and x-radiation. The induced emission of x-rays also provides a means for pigment identification. An incident x-ray photon is absorbed and an electron is ejected from the atom, leaving an unfilled state. There is a specific set of energy states occupied by the electrons. No two electrons can occupy the same state. An electron in the atom makes a transition to fill the empty state, and an x-ray is emitted. The energy of the emitted x-ray is given by the energy lost by the electron that has made the transition between the two states, as shown in Figure 7.4. The emitted x-rays are detected and their energies measured. The measured energy values identify the specific elements contained in the pigment, and the number of x-rays emitted indicates the amount of the element. Pigments are composed of specific elements, each of which has a specific number of electrons and electron energy levels. For example, vermilion is composed of atoms of mercury with 80 electrons and sulphur with 16 electrons. Electron binding energy levels are those required by an incident photon to remove an electron from an atom.

An electron in mercury requires

Particle Induced Ray Emission Pixe

Fig. 7.5. Examination by proton-induced x-ray emission (PIXE) of the elemental composition in an oil paint film. The round stainless-steel tube directed toward the shotgun image in the painting is the end of a proton beamline that allows energetic protons to induce x-ray emission. The detector for measurement of x-ray energies is not shown. Courtesy of W.S. Taft and P. Revesz, Cornell University.

Fig. 7.5. Examination by proton-induced x-ray emission (PIXE) of the elemental composition in an oil paint film. The round stainless-steel tube directed toward the shotgun image in the painting is the end of a proton beamline that allows energetic protons to induce x-ray emission. The detector for measurement of x-ray energies is not shown. Courtesy of W.S. Taft and P. Revesz, Cornell University.

83,000 eV to be removed from the atom, whereas sulphur electrons require a much lower energy of 2,500 eV (see Appendix F, Table F.3, Electron Binding Energies). Each atom has a unique set of electron energy levels. The laws of quantum mechanics, established in the first half of the twentieth century, have led to a rigorous and exact description of the energy levels within the atom. The procedure shown in Figure 7.5, proton induced x-ray emission (PIXE), identifies the elements present (and therefore pigments) in localized areas of the painting of about 1 mm in diameter. Other methods of identifying pigments such as x-ray diffraction are discussed in Appendix E.

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Freehand Sketching An Introduction

Freehand Sketching An Introduction

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