Radiation length
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radiation length
(′rād·ē′ā·shən ′leŋkth)
(nucleonics) The mean path length required to reduce the energy of relativistic charged particles by the factor 1/e, or 0.368, as they pass through matter. Also known as cascade unit; radiation unit.
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Radiation length
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cascade unit
radiation length, radiation unit
astrophysics Applicable to cosmic rays, the mean path length required in a given medium to reduce the energy of a charged particle by 50%; examples are 50 mm in lead, 433 mm in water, and 332 m in air at s.t.p. Cascade unit = (ln 2)-1 times the shower length = 1.442~ shower length.
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Radiation length
In physics, the radiation length is a characteristic of a material, related to the energy loss of high energy, electromagnetic-interacting particles with it.
Definition
High-energy electrons (>~10 MeV) predominantly lose energy in matter by bremsstrahlung, and high-energy photons by e+
e−
pair production. The characteristic amount of matter traversed for these related interactions is called the radiation length X0, usually measured in g·cm−2. It is both the mean distance over which a high-energy electron loses all but 1⁄e of its energy by bremsstrahlung, and 7⁄9 of the mean free path for pair production by a high-energy photon. It is also the appropriate scale length for describing high-energy electromagnetic cascades.
The radiation length for a given material consisting of a single type of nuclei can be approximated by the following expression:[1]
,
where Z is the atomic number and A is mass number of the nucleus.
For electrons at lower energies (below few tens of MeVs), the energy loss by ionization is predominant.
While this definition may also be used for other electromagnetic interacting particles beyond leptons and photons, the presence of the stronger hadronic and nuclear interaction makes it a far less interesting characterisation of the material; the nuclear collision length and nuclear interaction length are more relevant.
Comprehensive tables for radiation lengths and other properties of materials are available from http://pdg.lbl.gov/AtomicNuclearProperties
See also
- Mean free path
- Attenuation length
- Attenuation coefficient
- Attenuation
- Range (particle radiation)
- Stopping power (particle radiation)
- Electron energy loss spectroscopy
References
- ^ Eidelman, S.. Review of Particle Physics.
- S. Eidelman et al. Particle Data Group, Review of particle physics, Phys. Lett. B 592 (2004) (http://pdg.lbl.gov/)
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