LET (Linear Energy Transfer)
LET (Linear Energy Transfer) refers to the energy lost by a particle per unit path length as it passes through a material. The standard unit of LET is MeV·cm2/mg, which normalizes the energy loss (in MeV/cm) by the target material’s density (mg/cm3), allowing the value to be independent of the specific material.
Ionization loss is a critical process in semiconductor devices, and LET is used to quantify the energy deposited per unit path length during this ionization process. This deposited energy represents the particle’s energy being transferred to the material as it slows down and interacts with atomic electrons.
In general, LET can also be expressed in other units such as MeV·cm-1, eV·nm-1, or keV·nm-1. However, since energy loss scales with the density of the material, it is often more useful to normalize LET by that density. According to several JEDEC standards, LET is commonly reported in [MeV·cm2/mg], derived by dividing the energy loss (MeV/cm) by material density (mg/cm3), making it easier to compare across different environments.
The following graph shows the depth-wise distribution of LET. It illustrates how a 5.49 MeV alpha particle traveling through air gradually increases its energy deposition until near the end of its path, where the rate of energy transfer rises sharply before dropping to zero — a characteristic known as the Bragg Peak. This peak represents the point where LET reaches its maximum.
Bragg curve of 5.49 MeV alpha particles in air.
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