Linear energy transfer (or LET), often a non-linear function, is defined as the instantaneous energy deposited into the local environment by a given charged particle per unit of distance (dE/dx).
LET spectra are an important area of study. They tell scientists what sort of energy distribution exists above the Earth's atmosphere and in the interplanetary medium. This information is necessary in order to establish what sort of radiation astronauts will be exposed to during space missions. More importantly, scientists must evaluate the possible biological effects (short- and long-term) of this radiation.