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interaction of electrons with matter

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interaction of electrons with matter

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    1. Interaction of Electrons with Matter NE162 – Lecture 8 Chapter 6 of Text book JASMINA VUJIC

    3. Interaction electrons with matter

    4. Interaction electrons with matter Inelastic interactions produce diverse effect including: phonon excitation (heating) cathodoluminescence (visible light fluorescence) continuum radiation (bremsstrahlung radiation) characteristic x-ray radiation plasmon production (secondary electrons) Auger electron production (ejection of outer shell electrons) Most of the energy of an electron beam will eventually end up heating the sample (phonon excitation of the atomic lattice)

    5. Interaction electrons with matter Generalized illustration of interaction volumes for various electron-specimen interactions. Auger electrons (not shown) emerge from a very thin region of the sample surface (maximum depth about 50 Å) than do secondary electrons (50-500 Å).

    6. Secondary Electrons Energy distribution of secondary electrons (after Goldstein et al. 1981).

    7. Auger Electrons Auger-electron emission: The hole in the K shell is filled by an electron from an outer shell (here: L1). The superfluous energy is transferred to another electron (here: L3) which is subsequently ejected as Auger electron.

    8. Interaction electrons with matter Due to a small mass of an electron or positron: They can transfer large fraction of their energy in a single collision Can rapidly change their direction after a collision Rather than Range (difficult to define), keep in mind their pathway After loosing their kinetic energy, positrons will annihilate with electrons and produce 2 gamma rays.

    9. Emission of continuous X-rays (Bremsstrahlung) In addition to loosing its energy in collisions with the atomic electrons, causing ionization or excitation of the atoms along its path, a charged particle (in our case an electron) gives up its kinetic energy by a photon emission as it is deflected (or accelerated) in the EM field of nuclei. The emitted EM radiation has a continuous energy spectrum from 0 to Ek, where Ek is the kinetic energy of a charged particle. For Ek < 100 keV, radiation is emitted at 90° to the direction of the charged particle. For higher Ek the direction of the emitted radiation becomes forward-peaked.

    10. Electron Interactions Comparison of electron paths (top) and sites of X-ray excitation (bottom) in targets of aluminum, copper, and gold at 20 keV, simulated in a Monte Carlo procedure (after Heinrich, 1981)

    11. STOPPING POWER The stopping power is defined as the kinetic energy loss by an electron or positron per unit path length due to collisions or emitted radiation:

    12. STOPPING POWER For monoenergetic electrons with the kinetic energy Ek,o incident on a thick target, the energy emitted as radiation per electron is:

    13. RADIATION YIELD The yield is defined as and represents only 1-2% of total energy deposited for low electron energies.

    15. Electron Range A theoretical expression for the "range" of an electron, the straight line distance between where an electron enters and its final resting place, for a given Eo is (Kanaya & Okayama, 1972):

    16. Electron Tracks Track (2D) 5 keV e- in H20

    17. Cross Sections Total cross sections of interaction (e-)/H2O :

    18. RANGE The following are empirical equations for electrons in low-Z materials:

    19. Interaction of low-energy electrons with water The end product of any form of ionizing radiation is a spatial distribution of low-energy secondary electrons In average, it takes only about 22 eV to produce a secondary electron in liquid water, thus creating a large number of secondary electrons At low energies, radiative loss of energy is negligible Low-energy electrons loose energy mostly by ionization of atoms.

    20. Interaction of higher-energy electrons with materials What fraction of the energy of a 2-MeV beta ray is converted into bremsstrahlung when the particle is absorbed in aluminum and in lead For Al, ZT = 13 x 2 = 26, Y (Al) = 0.016 (or 1.6% will be converted in EM) For Pb, (Z=82), Y = 0.09 (or 9% will be converted in EM)

    21. Restricted Stopping Power (-dE/dx) is the energy lost by a charged particle per unit path length (-dE/dx)delta – includes only those collisions in the energy transfer is less then delta E. This restricts the range of secondary electrons. Linear energy transfer – LET = (-dE/dx)delta

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