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Linear energy transfer (LET) is a term used in dosimetry. It describes the action of radiation upon matter. It is identical to the retarding force acting on a charged ionizing particle travelling through the matter. It describes how much energy an ionising particle transfers to the material transversed per unit distance. 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 attenuate 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. Dosimetry attempts to factor in this effect with radiation weighting factors. 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, and this component does not cause electronic excitations. Hence nuclear stopping power is not contained in LET. Values in the international system are in N but are typically given in units of or . 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.〔("Delta ray" in Encyclopedia britannica online, retrieved 22 Dec. 2012 )〕 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.〔 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.〔 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''.〔 Here, the use of the term "infinity" is not to be taken literally; it simply means that no energy transfers, however large, are excluded. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Linear energy transfer」の詳細全文を読む スポンサード リンク
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