Linear energy transfer is the energy lost per unit distance as an ionizing particle travels through a material. Linear energy transfer (LET) is used to quantify the effects of ionizing radiation on biological specimens or electronic devices. LET varies enormously depending on the speed and charge of the particle involved.

In general, alpha particles, which are relatively slow moving, have a much higher LET than beta particles or gamma rays. The biological effect of high LET radiations are in general much higher than those of low LET radiations with the same energy. This is because high LET radiation can deposit most of its energy within the volume of one cell of the body and the chance of damage to the cell DNA is therefore larger.