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

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.