Linear energy transfer
In dosimetry, linear energy transfer (LET) is the amount of energy that an ionizing particle transfers to the material traversed per unit distance. It describes the action of radiation into matter.
It is identical to the retarding force acting on a charged ionizing particle travelling through the matter.[1] By definition, LET is a positive quantity. LET depends on the nature of the radiation as well as on the material traversed.
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
Linear energy transfer is closely related to stopping power, since both equal the retarding force. The unrestricted linear energy transfer is identical to linear electronic stopping power, as discussed below. But the stopping power and LET concepts are different in the respect that total stopping power has the nuclear stopping power component,[2] and this component does not cause electronic excitations. Hence nuclear stopping power is not contained in LET.
The appropriate SI unit for LET is the newton, but it is most typically expressed in units of kiloelectronvolts per micrometre (keV/μm) or megaelectronvolts per centimetre (MeV/cm). While medical physicists and radiobiologists usually speak of linear energy transfer, most non-medical physicists talk about stopping power.
Restricted and unrestricted LET
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.[3] Many studies focus upon the energy transferred in the vicinity of the primary particle track and therefore exclude interactions that produce delta rays with energies larger than a certain value Δ.[1] This energy limit is meant to exclude secondary electrons that carry energy far from the primary particle track, since a larger energy implies a larger range. This approximation neglects the directional distribution of secondary radiation and the non-linear path of delta rays, but simplifies analytic evaluation.[4]
In mathematical terms, Restricted linear energy transfer is defined by
where is the energy loss of the charged particle due to electronic collisions while traversing a distance , excluding all secondary electrons with kinetic energies larger than Δ. If Δ tends toward infinity, then there are no electrons with larger energy, and the linear energy transfer becomes the unrestricted linear energy transfer which is identical to the linear electronic stopping power.[1] Here, the use of the term "infinity" is not to be taken literally; it simply means that no energy transfers, however large, are excluded.
Application to radiation types
During his investigations of radioactivity,
Alpha particles and other positive ions
Linear energy transfer is best defined for monoenergetic ions, i.e.
Since the LET varies over the particle track, an average value is often used to represent the spread. Averages weighted by track length or weighted by absorbed dose are present in the literature, with the latter being more common in dosimetry. 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.[4]
Often overlooked for alpha particles is the recoil-nucleus of the alpha emitter, which has significant ionization energy of roughly 5% of the alpha particle, but because of its high electric charge and large mass, has an ultra-short range of only a few
Beta particles
Electrons produced in nuclear decay are called beta particles. Because of their low mass relative to atoms, they are strongly scattered by nuclei (Coulomb or Rutherford scattering), much more so than heavier particles. Beta particle tracks are therefore crooked. In addition to producing secondary electrons (delta rays) while ionizing atoms, they also produce bremsstrahlung photons. A maximum range of beta radiation can be defined experimentally[5] which is smaller than the range that would be measured along the particle path.
Gamma rays
LET has therefore no meaning when applied to photons. However, many authors speak of "gamma LET" anyway,
The transfer of energy from an uncharged primary particle to charged secondary particles can also be described by using the mass energy-transfer coefficient.[1]
Biological effects
Many studies have attempted to relate linear energy transfer to the relative biological effectiveness (RBE) of radiation, with inconsistent results. The relationship varies widely depending on the nature of the biological material, and the choice of endpoint to define effectiveness. Even when these are held constant, different radiation spectra that shared the same LET have significantly different RBE.[4]
Despite these variations, some overall trends are commonly seen. The RBE is generally independent of LET for any LET less than 10 keV/µm, so a low LET is normally chosen as the reference condition where RBE is set to unity. Above 10 keV/µm, some systems show a decline in RBE with increasing LET, while others show an initial increase to a peak before declining. Mammalian cells usually experience a peak RBE for LET's around 100 keV/µm.[4] These are very rough numbers; for example, one set of experiments found a peak at 30 keV/µm.
The International Commission on Radiation Protection (
Application fields
When used to describe the
In
"Soft errors" of electronic devices due to
References
- ^ PMID 24174259. ICRU report 85a.
- ^ Smith, Roger (1997). Atomic & ion collisions in solids and at surfaces: theory, simulation and applications. Cambridge, UK: Cambridge University Press.
- ^ "Delta ray" in Encyclopedia britannica online, retrieved 22 Dec. 2012
- ^ ISBN 978-0913394090. ICRU report 16.)
{{cite book}}
: CS1 maint: location missing publisher (link - ^ G. Knop and W. Paul: Interaction of electrons in Alpha- Beta- and Gamma-Ray Spectroscopy edited by K. Siegbahn, North-Holland, Amsterdam, 1966
- ^ ICRP (International Commission on Radiation Protection) publication 103, ICRP 37 (2-4) (2007): "(116) Photons, electrons, and muons are radiations with LET values of less than 10 keV/microm."
- ^ Chabot, George. "Radiation Basics — Radiation Quantities and Units". Ask the Experts FAQ. Health Physics Society. Retrieved 12 December 2012.
When the term "stopping power" is used in reference to photons, as seems to be the case for the example you give, it is not really being used for the photons themselves, but for the electrons set free by the photon interactions.
- ISBN 978-0-08-044311-9. ICRP Publication 92.
- ^ 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)
- ^ Radiation Effects & Analysis Home Page of NASA