3.2 - Properties Of Electron Beams

Electron beams can be described in several ways.

Energy Spectra

The energy spectra of an electron beam refers to the distribution of kinetic energies possessed by electrons in a beam.

  • When a beam hits the flattening filter, it is essentially monoenergetic with most electrons possessing a single beam energy
  • By passing through the treatment head and applicator, the distribution of electron energies begins to spread out.
  • At the phantom surface, there is a distribution of electron energies, with a maximum energy of (Emax)0. The mean energy, $\bar{E_0}$, is the average energy of electrons at the surface. The most probable energy, (Ep)0, is the position of the spectral peak - different to the mean energy.
  • As electrons penetrate the phantom, they will lose energy in a stochastic way, meaning that at a particular depth there will be a much broader spectrum of energies than at the surface. The most probable energy at a depth of z cm, (Ep)z, is related to the probable range of the electrons Rp.


Range, as discussed in electron-interactions, is the distance traveled by an individual electron. Range is used to describe several properties of electron beams:

  • The Rx value refers to the depth in centimetres that x% of electrons travel.
  • The Rp value is the practical range, the point on the depth axis that is crossed by a line that continues the linear descent seen in electron depth dose curves.