Editing Mean free path (section)

===Radiography===
[[File:Photon Mean Free Path.png|thumb|right|400px|Mean free path for photons in energy range from 1 keV to 10 MeV for elements with [[Atomic number|''Z'']] = 1 to 100.<ref>Based on data from {{cite web |date=1998-03-10 |title=NIST: Note - X-Ray Form Factor and Attenuation Databases |url=https://physics.nist.gov/PhysRefData/XrayNoteB.html |access-date=2011-11-08 |website=Physics.nist.gov |publisher=}}</ref> The discontinuities are due to low density of gas elements. Six bands correspond to neighbourhoods of the [[w:noble gas|noble gases]] (<sub>2</sub>He, <sub>10</sub>Ne, <sub>18</sub>Ar, <sub>36</sub>Kr, <sub>54</sub>Xe, <sub>86</sub>Rn). Also shown are locations of [[absorption edge]]s: K,L,M,N-shell electrons. Logarithmic scale 0.1 μm-1 km]]

In [[gamma-ray]] [[radiography]] the ''mean free path'' of a [[pencil beam]] of mono-energetic [[photon]]s 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:

:<math>\ell = \mu^{-1} = ( (\mu/\rho) \rho)^{-1},</math>

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 [[National Institute of Standards and Technology]] (NIST) databases.<ref name=NIST1>{{cite web
 |last=Hubbell |first=J. H.
 |author1-link=John H. Hubbell
 |last2=Seltzer |first2=S. M.
 |title=Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients
 |publisher=[[National Institute of Standards and Technology]]
 |url=http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html
 |access-date = 19 September 2007}}</ref><ref name=NIST2>{{cite web
 |last=Berger |first=M. J.
 |last2=Hubbell |first2=J. H. |author2-link=John H. Hubbell |first3=S. M. |last3=Seltzer |first4=J. |last4=Chang |first5=J. S. |last5=Coursey |first6=R. |last6=Sukumar |first7=D. S. |last7=Zucker
 |title =XCOM: Photon Cross Sections Database
 |publisher =[[National Institute of Standards and Technology]] (NIST)
 |url =http://physics.nist.gov/PhysRefData/Xcom/Text/XCOM.html
 |access-date = 19 September 2007}}</ref>

In [[X-ray]] [[radiography]] the calculation of the ''mean free path'' is more complicated, because photons are not mono-energetic, but have some [[Frequency distribution|distribution]] of energies called a [[spectrum]]. As photons move through the target material, they are [[attenuation|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 to 37% (1/[[e (mathematical constant)|''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, an image with negative logarithm of its intensities is sometimes called a ''number of mean free paths'' image.