Shell Atomic Model and Energy Levels

Transcription

1 Shell Atomic Model and Energy Levels

2 (higher energy, deeper excitation) - Radio waves: Not absorbed and pass through tissue un-attenuated - Microwaves : Energies of Photos enough to cause molecular rotation which is experienced as heat - Infrared: Molecular vibrations, higher absorption and less penetration to tissue - In the visible and ultraviolet range: enough energy to excite electrons inside atoms to higher orbits. - In the upper ultra violet range, x-rays, and Gamma rays: energy is large and can only be absorbed by disrupting atoms and whole molecules, causing ionization.

3 X-Ray Production For diagnostic or research purposes. Produced by accelerating electrons with high voltage and allowing them to collide with metal target (anode), e.g, Tungsten. Three Events (Two types of x-ray) a) Heat b) Events 1, 2, and 3 depict incident electrons interacting in the vicinity of the target nucleus, resulting in bremsstrahlung (braking radiation) production caused by the deceleration and change of momentum, with the emission of a continuous energy spectrum of x-ray photons. c) Event 4 demonstrates characteristic radiation emission, where an incident electron with energy greater than the K-shell binding energy collides with and ejects the inner electron creating an unstable vacancy. An outer shell electron transitions to the inner shell and emits an x-ray with energy equal to the difference in binding energies of the outer electron shell and K shell that are characteristic of tungsten. 12/20/16 3

4 Characteristic X-Rays Characteristic X-Ray because of the target element and the energy of the photon produced Discrete energies depending on e-binding energies of the target

5 Bremsstrahlung (Slowed-down Radiation) Intensity Bremsstrahlung is produced by e - interacting with the nucleus of a target atom Bremsstrahlung X- Ray Spectrum 12/20/16 5

6 The Interaction of X-Rays with Matter Associated Processes Interaction with loosely bound or free electron the Compton effect; Interaction with an inner shell or bound electron photoelectric absorption; Pair production conversion of the energy of the photon into the mass of an electron-positron pair

7 The Interaction of X-Rays with Matter Transmitted: pass through unaffected, as primary or direct radiation (the X-ray image is formed by the transmitted photons) Absorbed: transferring to the matter all of their energy (photoelectric effect, electron-pair production); Scattered: diverted in a new direction, with or without loss of energy transferring to the matter (Compton effect)

8 The Interaction of X-Rays with Matter Compton Effect The photon passing through the material bounces off a free electron or outer shell electron, which recoils and takes away some of the energy of the photon as kinetic energy The excited electron is called Compton electron and is ejected or moved into an excited atomic state The photon is scattered, i.e. diverted in a new direction, with reduced energy When high energy x-ray photon (Gamma ray) enter the body the mostly loose their energy through the Compton effect and - It causes fogging of the x-ray film - It represents a health hazard during fluoroscopic examinations.

9 Compton Scattering

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11 Photoelectric Effect (a) photon absorption and electron ejection and (b) fluorescent X-ray emission

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13 The Interaction of X-Rays with Matter Attenuation Attenuation refers to the fact that there are fewer photons in the emerging beam rather than in the beam entering the material; Def.: process by which radiation loses power as it travels through matter by interacting with it. The half-value-layer (HVL) is the thickness of stated material that will reduce the intensity of a narrow beam of X-ray to ½ of its original value; The HVL decreases as: - the density of the material increases - the atomic number of the material increases - the photon energy of the radiation decreases

14 The Interaction of X-Rays with Matter Compton Scattering - Free, outer-shell, and loosely bound electrons. Photoelectric Effect - Inner-shell, tightly bound electrons. - Energy of x-ray is higher than the binding energy of electrons. Attenuation A process by which radiation loses power (reduce intensity) as it travels through matter due to the interaction with it. Deferential Absorption The deference of absorption of radiation (x-ray) by deferent body tissues. e.g., image of the bone is produced because more x-rays are absorbed by bone than by the surrounding soft tissue. 1)Resulted from - Transmitted x-ray through tissue. - Compton scattering by tissue. - Photoelectric effect. 2)Depends on 1) Incident radiation (x-rays) energy. 2) Tissue atomic number. 3) Tissue mass density