Radiation length
Encyclopedia
In physics, the radiation length is a characteristic of a material, related to the energy loss of high energy, electromagnetic-interacting particles
with it.
by bremsstrahlung
, and high-energy photons by pair production. The characteristic
amount of matter traversed for these related interactions is called the radiation length ,
usually measured in g·cm−2. It is both the mean distance over which a high-energy
electron loses all but of its energy by bremsstrahlung, and 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:
,
where is the atomic number
and is mass number
of the nucleus.
For electrons at lower energies (below few tens of MeV
s), the energy loss by ionization
is predominant.
While this definition may also be used for other electromagnetic interacting particles beyond lepton
s 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
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
with it.
Definition
High-energy electrons (>~10 MeV) predominantly lose energy in matterby bremsstrahlung
Bremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
, and high-energy photons by pair production. The characteristic
amount of matter traversed for these related interactions is called the radiation length ,
usually measured in g·cm−2. It is both the mean distance over which a high-energy
electron loses all but of its energy by bremsstrahlung, and 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:
,where is the atomic number
Atomic number
In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...
and is mass number
Mass number
The mass number , also called atomic mass number or nucleon number, is the total number of protons and neutrons in an atomic nucleus. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion...
of the nucleus.
For electrons at lower energies (below few tens of MeV
MEV
MeV and meV are multiples and submultiples of the electron volt unit referring to 1,000,000 eV and 0.001 eV, respectively.Mev or MEV may refer to:In entertainment:* Musica Elettronica Viva, an Italian musical group...
s), the energy loss by ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
is predominant.
While this definition may also be used for other electromagnetic interacting particles beyond lepton
Lepton
A lepton is an elementary particle and a fundamental constituent of matter. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons , and neutral...
s and photons, the presence of the stronger hadronic
Strong interaction
In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...
and nuclear interaction
Nuclear force
The nuclear force is the force between two or more nucleons. It is responsible for binding of protons and neutrons into atomic nuclei. The energy released causes the masses of nuclei to be less than the total mass of the protons and neutrons which form them...
makes it a far less interesting characterisation of the material; the nuclear collision length
Nuclear collision length
Nuclear collision length is the mean free path of a particle before undergoing a nuclear reaction, for a given particle in a given medium. The collision length is smaller than the nuclear interaction length because the elastic and quasi-elastic reactions are excluded from its definition....
and nuclear interaction length
Nuclear interaction length
Nuclear interaction length is the mean path length required to reduce the numbers of relativistic charged particles by the factor 1/e, or 0.368, as they pass through matter....
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 pathMean free pathIn physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...
- Attenuation lengthAttenuation lengthIn physics, the attenuation length or absorption length is the distance \lambda into a material when the probability has dropped to 1/e that a particle has not been absorbed...
- Attenuation coefficientAttenuation coefficientThe attenuation coefficient is a quantity that characterizes how easily a material or medium can be penetrated by a beam of light, sound, particles, or other energy or matter. A large attenuation coefficient means that the beam is quickly "attenuated" as it passes through the medium, and a small...
- AttenuationAttenuationIn physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...
- Range (particle radiation)Range (particle radiation)In passing through matter, charged particles ionize and thus lose energy in many steps, until their energy is zero. The distance to this point is called the range of the particle...
- Stopping power (particle radiation)Stopping power (particle radiation)In passing through matter, fast charged particles ionize the atoms or molecules which they encounter. Thus, the fast particles gradually lose energy in many small steps. Stopping power is defined as the average energy loss of the particle per unit path length, measured for example in MeV/cm...
- Electron energy loss spectroscopyElectron energy loss spectroscopyIn electron energy loss spectroscopy a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. Some of the electrons will undergo inelastic scattering, which means that they lose energy and have their paths slightly and randomly deflected...