8.13 Mechanisms for Energy Loss
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The distance of penetration of a moving charged particle until its energy is
depleted is called its ‘range’, provided its forward motion is not significantly
deflected by multiple scattering. Angular deviations which cause the penetration
to vary from one particle to the next is called ‘straggling’, measured by the variation
in the range. The ability of a material to slow a moving charge, measured in
the charge’s energy loss per unit distance per unit material density, is called the
material’s ‘stopping power’ for the given radiation: (1/ρ)dE/dx.
8.13.2 Energy Loss in Materials from Ionizing Beams
The mechanisms by which an ionizing beam interacts with a material can be
categorized in the following terms:
• Excitation of bound electrons by light: The interaction of a passing ionizing
particle with an atom may transfer sufficient energy to excite an electron or a
nuclear particle to a higher energy bound state.
• Ionization of bound electrons by light: The interaction of a passing ionizing
particle with an atom may transfer sufficient energy to knock out an electron
out of the atom.
• Photoelectric effect (ejection of non-localized electrons by light): A photon may
have sufficient energy to eject an electron held by a material in a non-localized
state, such as the conduction electrons in metal. 11
• Thomson scattering (of light): Electromagnetic waves can be elastically scattered
from the charges in a material when a group of those charges respond collectively.
Electrons can act together as a larger mass, coherently re-radiating any absorbed
energy. This process is predicted by classical electrodynamics for such a collection of charges reacting to a passing electromagnetic wave. Thomson scattering
involves no energy loss.
• Compton scattering (of photons by electrons): If an individual energetic photon
hits an individual electron, with the photon energy significantly more than the
binding energy that the electron might have, then the ejected electron will
carry off kinetic energy, making the scattering process inelastic. The outgoing
photon will be reduced in energy (and have a lower frequency and a longer
wavelength). 12
• Møller scattering (of electrons by atomic electrons): A relativistic electron will be
scattered while passing through a material through electron-electron interactions.
The material electron is likely to be ejected from any bound state and will take
up a fraction of the incoming electron’s energy.
11 Einstein, in 1905, was the first to properly describe this process, extending to absorption the new
idea of Planck that light is emitted in bundles of energy called quanta.
12 Compton, in 1923, used the photon idea and energy-momentum conservation to explain how that
part of X-ray scattering which ejected atomic electrons also produced longer wavelength X-rays.
311
The distance of penetration of a moving charged particle until its energy is
depleted is called its ‘range’, provided its forward motion is not significantly
deflected by multiple scattering. Angular deviations which cause the penetration
to vary from one particle to the next is called ‘straggling’, measured by the variation
in the range. The ability of a material to slow a moving charge, measured in
the charge’s energy loss per unit distance per unit material density, is called the
material’s ‘stopping power’ for the given radiation: (1/ρ)dE/dx.
8.13.2 Energy Loss in Materials from Ionizing Beams
The mechanisms by which an ionizing beam interacts with a material can be
categorized in the following terms:
• Excitation of bound electrons by light: The interaction of a passing ionizing
particle with an atom may transfer sufficient energy to excite an electron or a
nuclear particle to a higher energy bound state.
• Ionization of bound electrons by light: The interaction of a passing ionizing
particle with an atom may transfer sufficient energy to knock out an electron
out of the atom.
• Photoelectric effect (ejection of non-localized electrons by light): A photon may
have sufficient energy to eject an electron held by a material in a non-localized
state, such as the conduction electrons in metal. 11
• Thomson scattering (of light): Electromagnetic waves can be elastically scattered
from the charges in a material when a group of those charges respond collectively.
Electrons can act together as a larger mass, coherently re-radiating any absorbed
energy. This process is predicted by classical electrodynamics for such a collection of charges reacting to a passing electromagnetic wave. Thomson scattering
involves no energy loss.
• Compton scattering (of photons by electrons): If an individual energetic photon
hits an individual electron, with the photon energy significantly more than the
binding energy that the electron might have, then the ejected electron will
carry off kinetic energy, making the scattering process inelastic. The outgoing
photon will be reduced in energy (and have a lower frequency and a longer
wavelength). 12
• Møller scattering (of electrons by atomic electrons): A relativistic electron will be
scattered while passing through a material through electron-electron interactions.
The material electron is likely to be ejected from any bound state and will take
up a fraction of the incoming electron’s energy.
11 Einstein, in 1905, was the first to properly describe this process, extending to absorption the new
idea of Planck that light is emitted in bundles of energy called quanta.
12 Compton, in 1923, used the photon idea and energy-momentum conservation to explain how that
part of X-ray scattering which ejected atomic electrons also produced longer wavelength X-rays.