Absorption coefficient

Japanese: 吸収係数 - きゅうしゅうけいすう

This is the attenuation constant for the absorption of light, radiation, particle beams, etc. in a material. For example, when light _with intensity I0 travels a distance x through a uniform material, the intensity I is expressed as an exponential function of x _, I = I0e -αx . This is called Lambert's law. Rewriting this using logarithms ^, log( I0 / I ₎ = αx , we can see that the logarithm of the intensity ratio of the incident light to the transmitted light is proportional to the thickness of the absorbing layer. This proportionality constant α is the absorption coefficient, but it is also called the extinction coefficient when light is absorbed by a gas or liquid. There are various absorption coefficients and related terms, and their definitions vary, so care should be taken (extinction coefficient, mass absorption coefficient, linear mass coefficient, absorbance, etc.). In general, α includes the scattering coefficient (turbidity) that does not involve energy absorption due to scattering, etc., and the true absorption coefficient due to energy absorption, and α is the sum of these, which is called the total absorption coefficient. This relationship also holds for other radiation and particle beams. In the case of radiation, the absorption coefficient is defined by introducing the concept of the cross-sectional area per atom.

[Atsunosuke Nakajima]

[Reference] | Absorption

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

光および放射線や粒子線などの物質中での吸収に関する減衰定数のこと。たとえば強度I₀の光が一様な物質中を距離xだけ進むと、その強さIはxの指数関数I＝I₀e^-αxで表される関係が成り立つ。これをランバートの法則という。対数を用いて書き直せばlog（I₀/I）＝αxとなり、入射光と透過光の強度比の対数が吸収層の厚さに比例することがわかる。この比例定数αが吸収係数であるが、光の気体または液体による吸収については吸光係数ともいう。吸収係数および関連用語はいろいろあって、その定義もまちまちであるので注意を要する（消衰係数、質量吸収係数、線質量係数、吸光度など）。一般にαには、散乱などによるエネルギー吸収を伴わない散乱係数（混濁度）と、エネルギー吸収による真吸収係数とが含まれ、αはその和となり全吸収係数とよばれる。このような関係は他の放射線や粒子線に対しても成り立つ。放射線などの場合には原子1個当りの断面積という概念を導入して吸収係数が定義される。