関: LET

WordNet

mark with lines; "sorrow had lined his face"
a length (straight or curved) without breadth or thickness; the trace of a moving point
a single frequency (or very narrow band) of radiation in a spectrum
reinforce with fabric; "lined books are more enduring"
a particular kind of product or merchandise; "a nice line of shoes" (同)product line, line of products, line of merchandise, business line, line of business
the road consisting of railroad track and roadbed (同)railway line, rail line
a commercial organization serving as a common carrier
acting in conformity; "in line with"; "he got out of line"; "toe the line"
something (as a cord or rope) that is long and thin and flexible; "a washing line"
a conceptual separation or distinction; "there is a narrow line between sanity and insanity" (同)dividing_line, demarcation, contrast
a formation of people or things one behind another; "the line stretched clear around the corner"; "you must wait in a long line at the checkout counter"
a formation of people or things one beside another; "the line of soldiers advanced with their bayonets fixed"; "they were arrayed in line of battle"; "the cast stood in line for the curtain call"
a fortified position (especially one marking the most forward position of troops); "they attacked the enemys line"
a mark that is long relative to its width; "He drew a line on the chart"
a spatial location defined by a real or imaginary unidimensional extent
in games or sports; a mark indicating positions or bounds of the playing area
persuasive but insincere talk that is usually intended to deceive or impress; "`let me show you my etchings is a rather worn line"; "he has a smooth line but I didnt fall for it"; "that salesman must have practiced his fast line of talk"
text consisting of a row of words written across a page or computer screen; "the letter consisted of three short lines"; "there are six lines in every stanza"
be in line with; form a line along; "trees line the riverbank" (同)run along
cover the interior of; "line the gloves"; "line a chimney"
fill plentifully; "line ones pockets"
move from one place to another; "transfer the data"; "transmit the news"; "transfer the patient to another hospital"
the act of transfering something from one form to another; "the transfer of the music from record to tape suppressed much of the background noise" (同)transference
transferring ownership (同)transference
application of a skill learned in one situation to a different but similar situation (同)transfer of training, carry-over
someone who transfers or is transferred from one position to another; "the best student was a transfer from LSU" (同)transferee
a ticket that allows a passenger to change conveyances
change from one vehicle or transportation line to another; "She changed in Chicago on her way to the East coast" (同)change
transfer somebody to a different position or location of work (同)reassign
move around; "transfer the packet from his trouser pockets to a pocket in his jacket" (同)shift
transfer from one place or period to another; "The ancient Greek story was transplanted into Modern America" (同)transpose, transplant
cause to change ownership; "I transferred my stock holdings to my children"
(physics) a thermodynamic quantity equivalent to the capacity of a physical system to do work; the units of energy are joules or ergs; "energy can take a wide variety of forms" (同)free_energy
a healthy capacity for vigorous activity; "jogging works off my excess energy"; "he seemed full of vim and vigor" (同)vim, vitality
any source of usable power; "the DOE is responsible for maintaining the energy policy"
an imaginative lively style (especially style of writing); "his writing conveys great energy"; "a remarkable muscularity of style" (同)muscularity, vigor, vigour, vim
enterprising or ambitious drive; "Europeans often laugh at American energy" (同)push, get-up-and-go
forceful exertion; "he plays tennis with great energy"; "hes full of zip" (同)vigor, vigour, zip
measured lengthwise; "cost of lumber per running foot" (同)running
designating or involving an equation whose terms are of the first degree (同)additive
(of a leaf shape) long and narrow (同)elongate
of or in or along or relating to a line; involving a single dimension; "a linear measurement" (同)one-dimensional

PrepTutorEJDIC

『線』,筋,けい / (色・縫い目などの)『線』,しま;(顔・手などの)しわ(wrinkle) / (物・人の)『列』,行列(row) / 『綱』,糸,針金 / 電線,電話線,電信線 / (文章の)『行』;(詩の)『行』 / 《しばしばa~》《話》『短い手紙』(note) / 《複数形で》輪郭,外形(outline) / 境界線(boundary);限界,限度(limit) / (鉄道・バスなどの)路線;航路,空路 / 《複数形で》せりふ / 進路,道順(course);(考えなどの)筋道,方針 / 《しばしばone's~》(活動の)分野,方面;特意 / 家系,血統,血筋 / 在庫商品,仕入れ品 / 導管,パイプライン / 《しばしば複数形で》戦線,防御線 / …‘に'『線を引く』,筋(しま,しわ)をつける / …‘に'『沿って並ぶ;』(…を)…‘に'沿って並べる《+『名』+『with』+『名』》
(材料で)〈着物など〉‘に'裏を付ける《+『名』『with』+『名』》 / (…で)…‘を'満たす,(…を)…‘に'詰め込む《+『名』+『with』+『名』》
(…から…へ)〈人・物〉‘を'『移す』,移転させる,乗り換えさせる《+名〈人・物〉+from+名+to+名》 / (…から…へ)〈権利・財産など〉‘を'譲り渡す,移転する《+名〈権利・財産〉+from+名+to+名》 / 〈絵画・デザインなど〉‘を'写す,転写する / 〈人が〉(ある人・場所から他の人・場所などへ)『移る』,移される;転任する,転任させられる《+from+名+to+名》 / (…から…へ)列車(バスなど)を乗り換える《+from+名+to+名》 / 〈U〉〈C〉(…を)『移すこと』,(…が)移転されること《+of+名》;転任,転校 / 〈C〉移る人(物),移される人(物);転校生,転任者 / 〈C〉乗り換え切符 / 〈C〉乗り換え場所 / 〈C〉写し絵
(一般的な力としての)『精力』,気力,元気 / (個人の)精力,活動力 / エネルギー
線の,直線の / 線から成る,線を使った;線状の / (方程式が)一次の

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/08/28 09:17:22」(JST)

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Diffusion cloud chamber with visible tracks of ionizing radiation (alpha particles)

Linear energy transfer (LET) is a measure of the conservative force acting on a charged ionizing particle travelling through matter, generally measured using particles with an energy over a certain threshold.^[1] Since the energy loss is part of the 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 beam 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 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.

Values in the international system are in N but are typically given in units of keV/μm or 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 exclude interactions that produce delta rays with energies larger than a certain value.^[1] This energy limit is meant to exclude secondary electrons that carry energy far from the primary particle track. 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 symbolized by

where is the energy loss of the charged particle due to electronic collisions while traversing a distance dx, excluding all secondary electrons with kinetic energies larger than from the count. If , the result is called the unrestricted linear energy transfer and is identical to the electronic stopping power.^[1]

Here, the use of the infinity symbol is not to be taken literally; it simply means that no energy transfers, however large, are excluded. , on the other hand, symbolizes the LET from direct ionizations and excitations by the primary particle only. This is not a commonly used quantity, since it excludes the bulk of energy deposition.

Application to Radiation Types

During his investigations of radioactivity, Ernest Rutherford coined the terms alpha rays, beta rays and gamma rays for the three types of emissions that occur during radioactive decay.

Alpha particles and other positive ions

Bragg curve of 5.49 MeV alpha particles in air. This radiation is produced from the initial decay of radon (²²²Rn). Stopping power (which is essentially identical to LET) is plotted here versus particle range

Linear energy transfer is best defined for monoenergetic ions, i.e. protons, alpha particles, and the heavier nuclei called HZE ions found in cosmic rays or produced by particle accelerators. 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. At equilibrium, the incident particle essentially comes to rest or is absorbed, at which point LET is undefined.

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 later 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]

Beta

Linear energy transfer is less meaningful for electrons, because of their low mass relative to the atoms they ionize. Their track is often a crooked series of curves, along which they lose more energy to secondary electrons and bremsstrahlung photons than to direct ionizations. The photons and some of the higher energy electrons, called delta rays, carry energy far from the particle track before depositing it as ionization or excitation. Hence the range over which the primary electron loses energy is not so tightly related to the final density of energy deposition in the absorbing material.

Gamma

Gamma rays are photons, whose absorption cannot be described by LET. When a gamma quantum passes through matter, it may be absorbed in a single process (photoelectric effect, Compton effect or pair production), or it continues unchanged on its path. (Only in the case of the Compton effect, another gamma quantum of lower energy proceeds). Gamma ray absorption therefore obeys an exponential law (see Gamma rays); the absorption is described by the absorption coefficient or by the half-value thickness.

LET has therefore no meaning when applied to photons. However, many authors speak of "gamma LET" anyway,^[5] where they are actually referring to the LET of the secondary electrons, i.e., mainly Compton electrons, produced by the gamma radiation.^[6] The secondary electrons will ionize far more atoms than the primary photon. This gamma LET has little relation to the attenuation rate of the beam, but it may have some correlation to the microscopic defects produced in the absorber. It must be noted that even a monoenergetic gamma beam will produce a spectrum of electrons, and each secondary electron will have a variable LET as it slows down, as discussed above. The "gamma LET" is therefore an average.

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

The ICRP used to recommend quality factors as a generalized approximation of RBE based on LET.

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 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. Different iterations of their model are shown in the graph to the right. The 1966 model was integrated into their 1977 recommendations for radiation protection in ICRP 26. 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. ICRP 60 revised the quality factor function and reserved it for use with unusual radiation types that did not have radiation weighting factors assigned to them.^[7]

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".^[8] 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).^[9]

"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

^ ^a ^b ^c ^d International Commission on Radiation Units and Measurements (October 2011). "Fundamental Quantities and Units for Ionizing Radiation". In Seltzer, Stephen M. Journal of the ICRU (Revised ed.) 11 (1). doi:10.1093/jicru/ndr012. ICRU report 85a. Retrieved 18 December 2012.
^ 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
^ ^a ^b ^c ^d International Commission on Radiation Units and Measurements (1970). Linear Energy Transfer. Washington D.C. ISBN 978-0913394090. ICRU report 16. Retrieved 1 December 2012.
^ 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."
^ Sinclair, Dr. W. K. et al (January 2003). "Relative biological effectiveness (RBE), quality factor (Q) and radiation weighting factor (Wr)". Annals of the ICRP 33 (4). 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

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