Raphael: 165+ Renaissance Paintings

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EDX enables compositional analysis of all elements from boron to uranium to a spatial resolution of approximately one micron [1]. SEM SEM images are formed by scanning an electron beam across a surface and collecting signals, either secondary electrons or back scattered electrons, from the beam-surface interaction. SEM analysis has been used extensively for textile investigations from fibre identification to damage assessment [2, 3]. A typical SEM image of a metal thread surface is shown in Figure 2.

Figure 2. SEM image of a metal thread from the Abraham and Melchizedek tapestry An accelerated electron beam, generated by an electron gun e. A current applied to two sets of scan coils deflects the electron beam through an aperture and controls the raster scanning. Magnification is achieved through the ratio of the raster scanned area on the specimen to the area on the cathode ray tube CRT screen.

The following variable machine settings control the resolution, depth of focus and signal to noise ratio of the final 19 Chapter 2. As the primary electron beam strikes the specimen, backscattered primary electrons, secondary electrons, Auger electrons, X-rays and light photons are emitted. Some electrons are absorbed into the specimen where they generate a current. Non-conducting samples must therefore be typically gold or carbon coated to dissipate the generated heat and charge.

The SEM instrument used in this study is a Hitachi S N and is equipped with two detectors; a scintillator-photomultiplier type Everhart-Thornley secondary electron detector and a retractable four quadrant solid state backscattered electron detector. Secondary electrons are fewer and of lower energy than backscattered electrons but are emitted from a small region near the surface of the specimen close to the primary electron beam, while back scattered electrons are emitted from a wider and deeper region, thus a secondary electron image will offer higher resolution but less contrast.

The number of backscattered electrons which escape the specimen depends on the atomic number of the backscattering atoms; i. Therefore backscattered electron images offer not only topographical information but also chemical information through atomic number contrast.


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An added benefit of the four quadrant solid state detector is that it works in variable pressure. A lower vacuum in the analysis chamber allows charge dissipation without the need of gold or carbon coating for non-conducting samples. FEG-SEM Opposed to the commonly used tungsten filament thermal emission gun, the field emission gun possesses a much smaller tip radius and higher emission current density resulting in a higher resolution image and enabling the use of very low accelerating voltages to obtain a true surface image.

XPS 2. EDX X-rays are emitted when a primary electron hits the surface of the specimen and causes the ejection of a secondary electron from one of the orbital shells of an atom. In order for the atom to come back to its stabilised state, an electron from a higher energy orbit fills the resulting gap. During that transition an x-ray photon with characteristic energy and wavelength is emitted.

Energy dispersive X-ray microanalysis EDX allows the measurement of the characteristic energy and thereby provides information about the chemical elements present on the surface of the specimen. The majority of X-rays are emitted from a relatively wide area from the lower two thirds of the interaction volume of the electron beam, the spatial resolution of X-ray microanalysis is, therefore, relatively poor compared to secondary electron images. A typical EDX spectrum of a metal thread surface is shown in Figure 2. When incoming X-rays strike the lithium-drifted silicon semi-conductor photoelectrons are generated which in turn generate electron-hole pairs.

A strong electric field attracts the electrons and holes to opposite ends of the detector generating a current pulse, the magnitude of which is directly proportional to the number of electron-hole pairs and therefore proportional to the energy of the incoming X-rays. X-ray photons cause the ejection of photoelectrons upon collision with atoms.

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The basis of XPS is the analysis of the energy of such 21 Chapter 2. XPS photoelectrons. The binding energy EB of electrons is specific for each element and its chemical state. The electrons from each atomic orbital have certain probabilities of being ejected on collision with X-ray photons.

Consequently the spectra of most elements will have a unique set of peaks of approximately constant relative intensities at certain binding energies, which allow unambiguous identification of the elements present. A typical spectrum of gold is shown in Figure 2. Table 2. However, these effects contribute little information to the XPS spectra.

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  • Therefore the inelastic mean free path controls the sampling depth, which, in general, is in the order of a few nm. Information about chemical state of the elements can be gained from peak shifts; in general an increase in positive charge of an atom leads to increased binding energy of its electrons.

    Peak shifts due to electrical charging of the sample during analysis will affect every peak in the same manner; therefore reference is usually made to the C 1s peak which, under neutral conditions, occurs at an EB of eV. Quantification in terms of atomic percentage of the elements present on the sample surface is achieved by dividing the peak area by the relative sensitivity factor. Peak areas are determined after appropriate background correction of the spectrum using the linear, Shirley or Tougaard methods [8].

    Relative sensitivity factors for all elements can be calculated [9] or determined empirically usually by the instruments manufacturer, e. Kratos sensitivity factors.

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    Bremsstrahlung, satellites, ghost lines and weaker emission of X-rays from additional transitions result in a broadening of the incident X-ray linewidth 0. Back-diffraction from a quartz crystal is employed to monochromate the Xrays and thereby reduce the linewidth to Chapter 2. XPS The X-rays are focused either directly from the anode face dual anode or by the monochromator onto the sample. Ejected photoelectrons from the sample enter the electron energy analyser system, the concentric hemispherical analyser. Photoelectrons are directed by scan plates through an adjustable aperture and electron-optical lenses and are retarded before entering the hemispherical electron energy analyser.

    Constant pass energy potentials are applied to the two concentric hemispheres so that injected electrons of a particular retarded energy are brought to a focus at the opposite end of the hemisphere. By progressively altering the retardation ratio, electrons of the complete energy range will be scanned and the photoelectron spectrum produced. On leaving the hemispherical analyser the photoelectrons are detected by a multichannel electron system, consisting of eight independent detectors each of which provides single electron counting.

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    In SIMS a surface usually a solid surface is bombarded with energetic primary particles; usually primary ions but may also be electrons, neutrons or photons. This causes sputtering, i. The mechanism of sputtering and ionisation of the ejected secondary particles is not yet fully understood, but there are various kinetic, electronic and thermodynamic models that try to explain the process [10]. Widely recognized is that the quantity of secondary ion emission and the secondary ion current depends upon the number of primary ions, the primary ion current, the sputtering coefficient, the ionisation probability and the concentration of particles to be studied on the sample surface.

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    With the aid of focusing lenses the primary ion beam can be raster scanned across the surface and by collecting secondary ions of a certain mass to charge ratio at each point of the area a chemical image of varying elemental or molecular intensities can be generated. If the primary ion impact rate is below the accepted threshold level of 1x ions cm —2, the technique examines a stable surface and the method is known as Static SIMS.

    If the primary ion beam is repeatedly scanned across an area the surface will erode over time as the solid lattice structure is disrupted and secondary ions are ejected off the surface layers. Changes of elemental composition with depth can thus be monitored and represented as depth profile graphs of Time versus Intensity for certain elemental or molecular isotopes; this is known as Dynamic SIMS [].

    Each pulse of the primary ion beam causes the emission of secondary ions which are accelerated to the same kinetic energy.

    This results in ions of different velocities since heavier ions with the same kinetic energy will travel faster than lighter ions. A high 25 Chapter 2. XRD transmission two-stage grid reflectron time-of-flight mass analyser with a pair of chevron-type multichannel plates with a 10 kV post-acceleration gap is used for ion detection at the end of the flight tube. The PHI instrument also comprises a pulsed charge neutraliser for insulating samples. An electron beam 0 - 70 eV is pulsed out of phase with the primary ion beam to allow the electrons to reach the sample.

    The primary ion beam can be successively scanned across a well defined surface area, thereby sputterremoving surface layers. The emitted positive and negative secondary ions are analysed in the triple focussing mass spectrometer, generating a depth profile. Most X-rays, however, are scattered in random directions and will be out of phase, partially cancelling one another, Figure 2.