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Giuseppe Dalba, La Fisica dei Raggi X, Dipartimento di Fisica, Università di Trento, a.a. 1999-2000 Transmission MATTER Scattering Compton Thomson Photoelectric.

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Presentation on theme: "Giuseppe Dalba, La Fisica dei Raggi X, Dipartimento di Fisica, Università di Trento, a.a. 1999-2000 Transmission MATTER Scattering Compton Thomson Photoelectric."— Presentation transcript:

1 Giuseppe Dalba, La Fisica dei Raggi X, Dipartimento di Fisica, Università di Trento, a.a Transmission MATTER Scattering Compton Thomson Photoelectric absorption Pair production > 1M eV X-rays Interaction X-rays - Matter Decay processes Fluorescence Auger electrons Primary competing processes and some radiative and non-radiative decay processes

2 Giuseppe Dalba, La Fisica dei Raggi X, Dipartimento di Fisica, Università di Trento, a.a Thomson Observed data Electron positron pairs Compton Photoelectric absorption Photonuclear absorption Cross section (barns/atom) eV1 KeV1 GeV1 MeV Cu Z=29 Energy Li Z=3Ge Z=32Gd Z=64 Energy (KeV) (Barns/atom) X-ray attenuation: atomic cross section h h sample

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4 Absorption coefficient

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13 k is a constant which changes with the element and the shell. Between two absorption edges decreases with the photon energy approximately following Bragg-Pierce law At the edge energy the photoelectric absorption coefficient presents a discontinuity. Such discontinuity is described through the jump ratio r The edge discontinuity is described through the jump ratio r

14 Giuseppe Dalba, La Fisica dei Raggi X, Dipartimento di Fisica, Università di Trento, a.a De-excitation mechanisms Radiative transition: Fluorescence Non radiative transition: Auger effect Cu K Cu 29 N M K L De excitation Cu 29 N M K L Cu K photon Auger electron Primary X-ray photon Excited system Cu 29 N M K L e - photoelectron Competing decay processes following the presence of a core hole

15 Giuseppe Dalba, La Fisica dei Raggi X, Dipartimento di Fisica, Università di Trento, a.a Fluorescent Yield Atomic Number Z K L 2 p 3/2 2 p 1/2 2 s 1 s K fluorescent x-ray Fluorescence 2 p 3/2 2 p 1/2 2 s 1 s Auger electron Auger effect Prevails for heavvy atoms Prevails for light atoms X s = probability of emission of a fluorescence photon; s denotes the edge. A s = probability of emission of a fluorescence photon from an s edge. Fluorescence yield: Auger yield:

16 Giuseppe Dalba, La Fisica dei Raggi X, Dipartimento di Fisica, Università di Trento, a.a Because of Auger effect the lines of a givel series (for instance the K lines: K 1, K 2,, K 1, …) are not as intense as the would be predicted from the number of vacancies which are present at the associated energy level (tke k level for the K lines). The K fluorescence yield is defined also as the number of photons of all K lines emitted in the unit time divide by the number of K vacancies formed in the same time, i.e.; Represets the number of photons of the spectral line i emipptted in unit time is the rate of production of vacancies in the K shell The fluorescence yields characteristic of the the shell L, L or the shell M, M,… are defined similarly. The Auger yield is defined as the ratio of the Auger electrons prosuced in the unit time and the vacancies created in the same time interval. Were it not for the Auger effect the yield fluorescence of a given line series would always be 1. The fluorescent yield depends on the atomic number Z and the line series; an empirical approximation is given by: Where A is a constant having value 10 4 for K lines and 10 8 for L lines.

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18 The characteristic spectrum

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22 It can then be shown that the refractive index for a material with N atoms per unit volume is given by

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31 where n is the order diffraction, d is the interplanar spacing and is the angle between the reflecting planes and the incident beam.


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