Mean free path
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In physics, the mean free path is the average distance travelled by a moving particle (such as an atom, a molecule, a photon) between successive impacts (collisions),^{[1]} which modify its direction or energy or other particle properties.
The following table lists some typical values for air at different pressures at room temperature.
Vacuum range  Pressure in hPa (mbar)  Pressure in mmHg (Torr)  Molecules / cm^{3}  Molecules / m^{3}  Mean free path 

Ambient pressure  1013  759.8  2.7 × 10^{19}  2.7 × 10^{25}  68 nm^{[2]} 
Low vacuum  300 – 1  220 – 8×10^{−1}  10^{19} – 10^{16}  10^{25} – 10^{22}  0.1 – 100 μm 
Medium vacuum  1 – 10^{−3}  8×10^{−1} – 8×10^{−4}  10^{16} – 10^{13}  10^{22} – 10^{19}  0.1 – 100 mm 
High vacuum  10^{−3} – 10^{−7}  8×10^{−4} – 8×10^{−8}  10^{13} – 10^{9}  10^{19} – 10^{15}  10 cm – 1 km 
Ultrahigh vacuum  10^{−7} – 10^{−12}  8×10^{−8} – 8×10^{−13}  10^{9} – 10^{4}  10^{15} – 10^{10}  1 km – 10^{5} km 
Extremely high vacuum  <10^{−12}  <8×10^{−13}  <10^{4}  <10^{10}  >10^{5} km 
Contents
Radiography
In gammaray radiography the mean free path of a pencil beam of monoenergetic 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 National Institute of Standards and Technology (NIST) databases^{[4]}^{[5]}
In Xray radiography the calculation of the mean free path is more complicated, because photons are not monoenergetic, 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 Xray 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 halfvalue layer (HVL): a material with a thickness of one HVL will attenuate 50% of photons. A standard xray image is a transmission image, an image with negative logarithm of its intensities is sometimes called a number of mean free paths image.
Electronics
see also Ballistic conduction
Electrical mobility, a value directly related to electrical conductivity, is:
where e is the charge, is the mean free time, m^{*} is the effective mass, d is the mean free path, and v_{F} is the Fermi velocity of the charge carrier. The Fermi velocity can easily be derived from the Fermi energy via the nonrelativistic kinetic energy equation. In thin films, however, the film thickness can be smaller than the predicted mean free path, making surface scattering much more noticeable, effectively increasing the resistivity.
Electron mobility through a medium with dimensions smaller than the mean free path of electrons occurs through ballistic conduction or ballistic transport. In such scenarios electrons alter their motion only in collisions with conductor walls.
Optics
If one takes a suspension of nonlightabsorbing particles of diameter d with a volume fraction Φ, the mean free path of the photons is:^{[6]}
where Q_{s} is the scattering efficiency factor. Q_{s} can be evaluated numerically for spherical particles using Mie theory.
Acoustics
In an otherwise empty cavity, the mean free path of a single particle bouncing off the walls is:
where V is the volume of the cavity, S is the total inside surface area of the cavity, and F is a constant related to the shape of the cavity. For most simple cavity shapes, F is approximately 4.^{[7]}
This relation is used in the derivation of the Sabine equation in acoustics, using a geometrical approximation of sound propagation.^{[8]}
Derivation
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Imagine a beam of particles being shot through a target, and consider an infinitesimally thin slab of the target (see the figure). The atoms (or particles) that might stop a beam particle are shown in red. The magnitude of the mean free path depends on the characteristics of the system. An expression for the MFP is
where ℓ is the mean free path, n is the number of target particles per unit volume, and σ is the effective crosssectional area for collision.
The area of the slab is L^{2}, and its volume is L^{2} dx. The typical number of stopping atoms in the slab is the concentration n times the volume, i.e., n L^{2} dx. The probability that a beam particle will be stopped in that slab is the net area of the stopping atoms divided by the total area of the slab:
where σ is the area (or, more formally, the "scattering crosssection") of one atom.
The drop in beam intensity equals the incoming beam intensity multiplied by the probability of the particle being stopped within the slab:
This is an ordinary differential equation:
whose solution is known as BeerLambert law and has the form , where x is the distance traveled by the beam through the target, and I_{0} is the beam intensity before it entered the target; ℓ is called the mean free path because it equals the mean distance traveled by a beam particle before being stopped. To see this, note that the probability that a particle is absorbed between x and x + dx is given by
Thus the expectation value (or average, or simply mean) of x is
The fraction of particles that are not stopped (attenuated) by the slab is called transmission , where x is equal to the thickness of the slab x = dx.
Kinetic theory
In kinetic theory the mean free path of a particle, such as a molecule, is the average distance the particle travels between collisions with other moving particles. The formula still holds for a particle with a high velocity relative to the velocities of an ensemble of identical particles with random locations. If, on the other hand, the velocities of the identical particles have a Maxwell distribution, the following relationship applies:^{[9]}
and using (ideal gas law) and (effective crosssectional area for spherical particles with radius ), it may be shown that the mean free path is^{[10]}
where k_{B} is the Boltzmann constant.
In practice, the diameter of gas molecules is not well defined. In fact, the kinetic diameter of a molecule is defined in terms of the mean free path. Typically, gas molecules do not behave like hard spheres, but rather attract each other at larger distances and repel each other at shorter distances, as can be described with a LennardJones potential. One way to deal with such "soft" molecules is to use the LennardJones σ parameter as the diameter. Another way is to assume a hardsphere gas that has the same viscosity as the actual gas being considered. This leads to a mean free path
where m is the molecular mass, and μ is the viscosity.
These different definitions of the molecular diameter can lead to slightly different values of the mean free path.
Nuclear and particle physics
In particle physics the concept of the mean free path is not commonly used, being replaced by the similar concept of attenuation length. In particular, for highenergy photons, which mostly interact by electron–positron pair production, the radiation length is used much like the mean free path in radiography.
Independentparticle models in nuclear physics require the undisturbed orbiting of nucleons within the nucleus before they interact with other nucleons.^{[11]}
The effective mean free path of a nucleon in nuclear matter must be somewhat larger than the nuclear dimensions in order to allow the use of the independent particle model. This requirement seems to be in contradiction to the assumptions made in the theory ... We are facing here one of the fundamental problems of nuclear structure physics which has yet to be solved.
— John Markus Blatt and Victor Weisskopf, Theoretical nuclear physics (1952)^{[12]}
See also
References
 ^ Brünglinghaus, Marion. "Mean free path". European Nuclear Society. Retrieved 20111108.
 ^ Jennings, S (1988). "The mean free path in air". Journal of Aerosol Science. 19 (2): 159. Bibcode:1988JAerS..19..159J. doi:10.1016/00218502(88)902194.
 ^ Based on data from "NIST: Note  XRay Form Factor and Attenuation Databases". Physics.nist.gov. 19980310. Retrieved 20111108.
 ^ Hubbell, J. H.; Seltzer, S. M. "Tables of XRay Mass Attenuation Coefficients and Mass EnergyAbsorption Coefficients". National Institute of Standards and Technology. Retrieved 19 September 2007.
 ^ Berger, M. J.; Hubbell, J. H.; Seltzer, S. M.; Chang, J.; Coursey, J. S.; Sukumar, R.; Zucker, D. S. "XCOM: Photon Cross Sections Database". National Institute of Standards and Technology (NIST). Retrieved 19 September 2007.
 ^ Mengual, O.; Meunier, G.; Cayré, I.; Puech, K.; Snabre, P. (1999). "TURBISCAN MA 2000: multiple light scattering measurement for concentrated emulsion and suspension instability analysis". Talanta. 50 (2): 445–56. doi:10.1016/S00399140(99)001290. PMID 18967735.
 ^ Young, Robert W. (July 1959). "Sabine Reverberation Equation and Sound Power Calculations". The Journal of the Acoustical Society of America. 31 (7): 918. Bibcode:1959ASAJ...31..912Y. doi:10.1121/1.1907816.
 ^ Davis, D. and Patronis, E. "Sound System Engineering" (1997) Focal Press, ISBN 0240803051 p. 173.
 ^ S. Chapman and T. G. Cowling, The mathematical theory of nonuniform gases, 3rd. edition, Cambridge University Press, 1990, ISBN 052140844X, p. 88.
 ^ "Mean Free Path, Molecular Collisions". Hyperphysics.phyastr.gsu.edu. Retrieved 20111108.
 ^ Cook, Norman D. (2010). "The Mean Free Path of Nucleons in Nuclei". Models of the Atomic Nucleus (2 ed.). Heidelberg: Springer. p. 324. ISBN 9783642147364.
 ^ Blatt, John M.; Weisskopf, Victor F. (1979). Theoretical Nuclear Physics. doi:10.1007/9781461299592. ISBN 9781461299615.
External links
 Gas Dynamics Toolbox: Calculate mean free path for mixtures of gases using VHS model