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In physics, the mean free path is the average distance traveled by a moving particle (such as an atom, a molecule, a photon) between successive impacts (collisions), which modify its direction or energy or other particle properties. The following table lists some typical values for air at different pressures and at room temperature. ==Mean free path in radiography== In gamma-ray radiography the ''mean free path'' of a pencil beam of mono-energetic photons is the average distance a photon travels between collisions with atoms of the target material. It depends on the material and the energy of the photons: : where μ is the linear attenuation coefficient, μ/ρ is the mass attenuation coefficient and ρ is the density of the material. The Mass attenuation coefficient can be looked up or calculated for any material and energy combination using the NIST databases In X-ray radiography the calculation of the ''mean free path'' is more complicated, because photons are not mono-energetic, but have some distribution of energies called a spectrum. As photons move through the target material they are attenuated with probabilities depending on their energy, as a result their distribution changes in process called Spectrum Hardening. Because of Spectrum Hardening the ''mean free path'' of the X-ray spectrum changes with distance. Sometimes one measures the thickness of a material in the ''number of mean free paths''. Material with the thickness of one ''mean free path'' will attenuate 37% (1/e) of photons. This concept is closely related to Half-Value Layer (HVL); a material with a thickness of one HVL will attenuate 50% of photons. A standard x-ray image is a transmission image, a minus log of it is sometimes referred as ''number of mean free paths'' image. 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「Mean free path」の詳細全文を読む スポンサード リンク
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