# Linear energy transfer (LET)

Source: https://nuclide.cc/terms/linear-energy-transfer/  
Nuclide record `linear-energy-transfer` (Term). Data CC BY-NC 4.0, attribute "Data from Nuclide (nuclide.cc)"; commercial use needs a licence.

## TL;DR

A measure of how densely a particle of radiation deposits its energy as it travels through tissue. Alpha particles dump their energy in a short, dense burst; beta particles and gamma rays spread it thinly over a much longer track.

## Summary

Linear energy transfer (L or LET) is the average energy that a charged ionising particle transfers to the medium per unit length of its track, L = dE/dl, expressed in keV per micrometre (keV/µm). It describes the spatial density of energy deposition (and therefore of ionisation events) along a particle's path, which is what ultimately determines the biological severity of the damage caused for a given absorbed dose.

Representative values illustrate the range: cobalt-60 gamma rays and diagnostic x-rays are low-LET (roughly 0.2-2 keV/µm), beta particles from radionuclides such as lutetium-177 are also low-LET (a 1 MeV beta particle deposits roughly 0.2 keV/µm and travels several millimetres), while alpha particles are high-LET, depositing on the order of 80-150 keV/µm over a track of only tens to about a hundred micrometres (a handful of cell diameters). High-LET radiation causes densely clustered ionisations that produce more complex, harder-to-repair DNA double-strand breaks than the sparse ionisation from low-LET beta or gamma radiation, which is the physical basis for targeted alpha therapy's distinct toxicity and efficacy profile compared with beta-emitting radioligand therapy.

LET is the mechanistic quantity underlying both the regulatory radiation weighting factor used in equivalent dose (wR = 20 for alpha versus 1 for beta/gamma) and the empirically measured relative biological effectiveness used to compare therapeutic effect between alpha- and beta-emitting radiopharmaceuticals.

## Fields

- Kind: Term
- Last checked: 2026-09-22

## Sources

- NRC training materials: interaction of charged particles with matter, LET and stopping power: https://www.nrc.gov/docs/ml1122/ml11229a666.pdf
- ScienceDirect Topics: linear energy transfer, representative values: https://www.sciencedirect.com/topics/immunology-and-microbiology/linear-energy-transfer

## Connected records

- terms: [Absorbed dose and the gray (Gy)](https://nuclide.cc/terms/absorbed-dose-term/), [Alpha vs beta emitters](https://nuclide.cc/terms/alpha-vs-beta/), [Equivalent dose and effective dose (sievert)](https://nuclide.cc/terms/equivalent-dose/), [Relative biological effectiveness (RBE)](https://nuclide.cc/terms/relative-biological-effectiveness/)
- isotopes: [Lutetium-177](https://nuclide.cc/isotopes/lu-177/)
- technologies: [Auger-electron therapy](https://nuclide.cc/technologies/auger-electron-therapy/), [Radioligand therapy (beta emitters)](https://nuclide.cc/technologies/radioligand-therapy/), [Targeted alpha therapy](https://nuclide.cc/technologies/targeted-alpha-therapy/)

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