= Linear attenuation coefficient
{title2=$\mu$}
The <linear attenuation coefficient> is the infinitesimal probability of removal from the uncollided beam per unit path length. Its units are inverse <length>. In the homogeneous monoenergetic model, $\mu=-d\ln I/dx$; $1/\mu$ is the thickness leaving fraction $e^{-1}$ uncollided. The <mass attenuation coefficient> is $\mu/\rho$ for material <mass density> $\rho$. Removing a <photon> by <scattering of light> does not imply deposition of all its <energy> at that point; attenuation and <energy> absorption are different quantities.<Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients by John H. Hubbell and Stephen M. Seltzer/2. The Mass Attenuation Coefficient>[Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients by John H. Hubbell and Stephen M. Seltzer; NIST §2, mass thickness, Eq. (1), and total interaction cross section; distinction from the mass energy-absorption coefficient]{ref}
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