linear energy transfer (LET)

The amount of energy per unit length lost by a particle or photon traversing a material.

NOTE 1 The energy carried away by the secondary electrons that are produced must be taken into account when calculating the energy loss.

NOTE 2 The charge transfer mechanism will depend on the type and energy of the radiation, e.g., pair-production, Compton scattering, Bremsstrahlung, collisions, photoelectric effect, and radiative capture.

NOTE 3 LET is strictly defined in terms of energy divided by distance, e.g., MeV/cm, eV/nm, keV/nm. However, since the energy lost is directly proportional to the density of the material traversed, it is useful to divide the LET by the density of the material. For the purposes of JEP133B and JESD57, this derived quantity, whose units are typically expressed as MeV·cm²/mg (i.e., MeV/cm divided by mg/cm²), is also referred to as linear energy transfer (LET).

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