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 (E
_{max})_{0}. The mean energy, $\bar{E_0}$, is the average energy of electrons at the surface. The most probable energy, (E_{p})_{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, (E
_{p})_{z}, is related to the**probable range**of the electrons R_{p}.

## 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 R
_{x}value refers to the depth in centimetres that x% of electrons travel. - The R
_{p}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.