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SEC5-InteractionsofRadiationwithMatter - Section 5...

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1 Section 5: Interactions of Radiation with Matter All radiation is detected through its interaction with matter. This section will focus on what happens to a particle and its environment when radiation passes through matter, schematically particle medium The environment that the particle sees as it encounters the sea of electrons presented by matter is analogous to a space ship travelling through an asteroid belt (e.g. the Millenium Falcon in The Empire Strikes Back). The success of both the particle and the space ship will depend on the thickness and density of the electron sea/asteroid belt (without Han Solo to mavigate). This subject has many practical applications, e.g. detection of radiation, radiation safety, environmental and biological hazards of radiation, and risk assessment. Four types of radiation are relevant to these discussions: (1) Positive Ions: X +q e.g. α particles, fission fragments, cosmic rays, particle beams (2) Electrons: β± , internal conversion and Auger electrons, cosmic rays (3) Photons: γ x-ray uv visible (4) Neutrons: nuclear reactors, nuclear weapons, accelerators Positive Ions Positive ions are defined as cations, A X +q , where q is the atomic ionization state. Fig. 5.1 shows a calculations of the energy loss process as an ion stops in matter. Fig. 5.1 Calculation of multiple collisions between positive ions (red) and atomic electrons of the medium as the ions pass through silicon.
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2 The possible interactions of an ion in passing through matter are with either atomic electrons or nuclei. When one compares the ratio of cross sections σ ( σ = probability) for such interactions, it is immediately apparent that particle-electron collisions dominate: Possible interactions: Nuclei σ ~ 10 24 cm 2 Orbital e s σ ~ 10 16 cm 2 The qualitative properties of charged particle-electron collisions are (1) the particle velocity is less than the velocity of light (v I < < c , usually ~ 0.01 0.1 c); (2) the mass of the cation is much greater than that of the electron, thus requiring multiple collisions to stop the ion (the bowling ball-ping pong ball effect), and (3) the cation trajectory is a nearly straight line up until the very end of its path. The average path length of the ion in the medium is defined as the range . Three primary stages of energy-loss characterize the stopping process: Electronic Stopping (v I >> v e -) – Electronic stopping accounts for about 95% of the distance traversed by the ion (its range) as it passes through matter. This process dominates as long as the velocity of the ion is much greater than the velocity of the atomic electrons of the medium. As it initially enters the electron sea presented by the medium, the atomic electrons of the ion are rapidly removed via electron-electron collisions, a process called stripping , as illustrated below Z A q X + Z A Z X + ; e.g., 8 16 2 O + 8 16 8 O + ion medium (electron sea) For the general case, the charge of an energetic ion after stripping is equal to the atomic number of the ion. During electronic stopping the ion follows a nearly straight-line trajectory as it loses
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SEC5-InteractionsofRadiationwithMatter - Section 5...

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