Linear energy transfer
Encyclopedia
Linear energy transfer is a measure of the energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

transferred
Energy transfer
Energy transfer is the transfer of energy from one body to another.There are a few main ways that energy transfer occurs:*Radiant energy *Heat conduction*Convection*Electrical power transmission*Mechanical work...

to material as an ionizing particle travels through it. Typically, this measure is used to quantify the effects of ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

on biological specimens or electronic devices.

Linear energy transfer is closely related to stopping power
In passing through matter, fast charged particles ionize the atoms or molecules which they encounter. Thus, the fast particles gradually lose energy in many small steps. Stopping power is defined as the average energy loss of the particle per unit path length, measured for example in MeV/cm...

. Whereas stopping power, the energy loss per unit distance, , 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
Secondary electrons
Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation . This radiation can be in the form of ions, electrons, or photons with sufficiently high energy, i.e. exceeding the ionization potential...

. 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 (Since electrons of high energy have a large range
In passing through matter, charged particles ionize and thus lose energy in many steps, until their energy is zero. The distance to this point is called the range of the particle...

, this energy limit effectively excludes electrons that travel far from the primary particle).

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

where 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
In passing through matter, fast charged particles ionize the atoms or molecules which they encounter. Thus, the fast particles gradually lose energy in many small steps. Stopping power is defined as the average energy loss of the particle per unit path length, measured for example in MeV/cm...

.

Application fields

When used to describe the dosimetry
Dosimetry
Radiation dosimetry is the measurement and calculation of the absorbed dose in matter and tissue resulting from the exposure to indirect and direct ionizing radiation...

of ionizing radiation in the biological or biomedical setting, the LET (like linear stopping power
In passing through matter, fast charged particles ionize the atoms or molecules which they encounter. Thus, the fast particles gradually lose energy in many small steps. Stopping power is defined as the average energy loss of the particle per unit path length, measured for example in MeV/cm...

) is usually expressed in units of keV/µm
Micrometer
A micrometer , sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier,...

.

In space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

applications
, electronic devices can be disturbed by the passage of energetic electrons, protons or heavier ions that may alter the state of a circuit
Electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...

, producing "single event effects
Single event upset
A single event upset is a change of state caused by ions or electro-magnetic radiation striking a sensitive node in a micro-electronic device, such as in a microprocessor, semiconductor memory, or power transistors. The state change is a result of the free charge created by ionization in or close...

". 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
In passing through matter, fast charged particles ionize the atoms or molecules which they encounter. Thus, the fast particles gradually lose energy in many small steps. Stopping power is defined as the average energy loss of the particle per unit path length, measured for example in MeV/cm...

(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). .

"Soft errors" of electronic devices due to cosmic rays on earth are, however, mostly due to neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s 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
Soft error
In electronics and computing, a soft error is an error in a signal or datum which is wrong. Errors may be caused by a defect, usually understood either to be a mistake in design or construction, or a broken component. A soft error is also a signal or datum which is wrong, but is not assumed to...

.