Linear energy transfer

Linear energy transfer (LET) is a measure of the energy transferred to material as an ionizing particle travels through it. Typically, this measure is used to quantify the effects of ionizing radiation on biological specimens or electronic devices.

Linear energy transfer is closely related to stopping power. Whereas stopping power, the energy loss per unit distance, dE/dx, focusses upon the energy loss of the particle, linear energy transfer focuses upon the energy transferred to the material surrounding the particle track, by means of secondary electrons. Since one is usually interested in energy transferred to the material in the vicinity of the particle track, one excludes secondary electrons with energies larger than a certain value Δ[1] (Since electrons of high energy have a large range, this energy limit effectively excludes electrons that travel far from the primary particle).

Hence, linear energy transfer (also called "restricted linear electronic stopping power"[2]) is defined by

L_\Delta = \frac{dE_\Delta} {dx}

where dE_\Delta refers to the energy loss due to electronic collisions minus the kinetic energies of all secondary electrons with energy larger than Δ. When Δ approaches infinity, there can be no electrons with higher energy, and linear energy transfer becomes identical to the linear electronic stopping power.

Application fields

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.

In space applications, electronic devices can be disturbed by the passage of energetic electrons, protons or heavier ions that may alter the state of a circuit, producing "single event effects"[3]. The effect of the radiation is described by the LET (which is here taken as synonymous with stopping power), typically expressed in units of MeV·cm²/mg of material, the units used for mass stopping power (The material in question is usually Si for MOS devices). The units of measurement arise from a combination of the energy lost by the particle to the material per unit path length (MeV/cm) divided by the density of the material (mg/cm³). [4].

"Soft errors" of electronic devices due to cosmic rays on earth are, however, mostly due to neutrons which do not directly interact with the material and whose passage can therefore not be described by LET. Rather, one measures their effect in terms of neutrons per cm2 per hour, see Soft error.

References

  1. ^ ICRU Report 60. International Commission on Radiation Units and Measurements, Bethesda, MD, USA (1998)
  2. ^ ICRU Report 37. International Commission on Radiation Units and Measurements, Bethesda, MD, USA (1984)
  3. ^ V. Zajic and P. Thieberger, "Heavy Ion Linear Energy Transfer Measurements during Single Event Upset Testing of Electronic Devices," IEEE Transactions on Nuclear Science 46, pp. 59-69, (1999)
  4. ^ Radiation Effects & Analysis Home Page of NASA