Physics Definitions


The progressive loss of intensity of a beam as it traverses a volume. Can be due to:

  • Inverse square law
  • Absorption of radiation within a medium
  • Scattering of radiation away from the beam direction


Material applied to the surface of a volume. Reasons include:

  • Compensating for irregular contour
    • Either to even isodose lines at depth (photon, electron beams), or
    • Prevent hot spots from forming at sharp contours (electron beams)
  • Increasing the dose to the surface (build up bolus) - used for low energy electron beams and megavoltage photon beams if desired

Effective Energy

The effective energy is a method of describing a polyenergetic photon beam, by assigning it the energy of a monoenergetic photon beam that is attenuated by the same thickness of material. This is difficult to apply in practice, as subsequent HVL layers of a polyenergetic beam increase due to beam hardening.


Fluence is the number of particles (particle fluence) or amount of energy (energy fluence) entering an imaginary sphere with a cross-sectional area of A cm2.
The units of particle fluence are m-2.
The units of energy fluence are J.m-2.

Linear Attenuation Coefficient

The linear attenuation coefficient, denoted by the symbol µ, is a constant that describes the rate of energy loss by a photon beam per centimetre within a medium. It has units of cm-1, and is present in the equation:

\begin{align} I(x)=I_0.e^{-\mu x} \end{align}

Where I(x) is the intensity at depth of x cm, I0 is the original intensity, and µ is the linear attenuation coefficient. Note that the linear energy transfer coefficient and linear energy absorption coefficient share the same units.

Mass Attenuation Coefficient

The mass attenuation coefficient, denoted by µ/ρ, is a constant that describes the rate of energy loss by a photon beam as it traverses a medium, independent of the density of that medium. It is found by dividing the linear attenuation coefficient by the density of the medium. By removing the density, the amount of attenuation that occurs is entirely dependent on the atomic structure of the material - in general, the atomic number and the energy of the photons in the beam. It can be substituted into the attenuation equation:

\begin{align} I(x)=I_0.e^{- \frac{\mu}{\rho}.\rho. x} \end{align}

Density has SI Units of grams per cubic centimetre, or When density divides the linear attenuation coefficient (units of cm-1) the final result is cm2.g-1. Therefore, the units of the mass attenuation coefficient are cm2.g-1.
Importantly, the mass energy transfer coefficient and mass energy absorption coefficient have the same units.

Scattering Power

The angle of deflection of charged particles by a material per unit distance, for a particular quality of radiation and within a particular medium.
For electrons, the scattering power increases proportionally with the square of the atomic number, and inversely with the square of the electron energy. Therefore, higher energy electrons passing through low atomic number materials are scattered the least.
Units: I think the would be cm-1.g-3 to fit with the definition.

Stopping Power

The loss of kinetic energy by charged particles per unit distance, for a particular type of radiation and within a particular medium.
For electrons, stopping power is divided into collisional stopping power and radiative stopping power. Collisional stopping power falls to a constant level with kinetic energies over 1 MeV, and is higher in low Z materials. Radiative stopping power increases proportionally to the square of the atomic number, and linearly in relation to beam energies.
The units of stopping power vary depending on the energy involved; commonly used units are MeV.cm2.g-1.


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