Linear energy transfer
A high LET will slow down the radiation more quickly, generally making shielding more effective and preventing deep penetration.
On the other hand, the higher concentration of deposited energy can cause more severe damage to any microscopic structures near the particle track.
If a microscopic defect can cause larger-scale failure, as is the case in biological cells and microelectronics, the LET helps explain why radiation damage is sometimes disproportionate to the absorbed dose.
Linear energy transfer is closely related to stopping power, since both equal the retarding force.
The secondary electrons produced during the process of ionization by the primary charged particle are conventionally called delta rays, if their energy is large enough so that they themselves can ionize.
This approximation neglects the directional distribution of secondary radiation and the non-linear path of delta rays, but simplifies analytic evaluation.
These particles cause frequent direct ionizations within a narrow diameter around a relatively straight track, thus approximating continuous deceleration.
As they slow down, the changing particle cross section modifies their LET, generally increasing it to a Bragg peak just before achieving thermal equilibrium with the absorber, i.e., before the end of range.
These averages are not widely separated for heavy particles with high LET, but the difference becomes more important in the other type of radiations discussed below.
This can skew results significantly if one is examining the Relative Biological Effectiveness of the alpha particle in the cytoplasm, while ignoring the recoil nucleus contribution, which alpha-parent being one of numerous heavy metals, is typically adhered to chromatic material such as chromosomes.
The relationship varies widely depending on the nature of the biological material, and the choice of endpoint to define effectiveness.
The International Commission on Radiation Protection (ICRP) proposed a simplified model of RBE-LET relationships for use in dosimetry.
They defined a quality factor of radiation as a function of dose-averaged unrestricted LET in water, and intended it as a highly uncertain, but generally conservative, approximation of RBE.
This model was largely replaced in the 1991 recommendations of ICRP 60 by radiation weighting factors that were tied to the particle type and independent of LET.
[8] When used to describe the dosimetry of ionizing radiation in the biological or biomedical setting, the LET (like linear stopping power) is usually expressed in units of keV/μm.
