This leads to an additional requirement of small material budget in terms of radiation length.

The characteristic amount of matter traversed for these related interactions is called the radiation length .

The window has provided vacuum tightness combined with reasonably small amount of material (only 0.3% to 0.4% of 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.

The radiation length for a given material consisting of a single type of nuclei can be approximated by the following expression:

Perkins also includes some useful material for experimental physicists such as stopping power and multiple scattering formulae and definition of radiation lengths.

In particular, for high-energy photons, which mostly interact by electron-positron pair production, the radiation length is used much like the mean free path in radiography.

The muon detectors were between 4 and 10 radiation lengths deep and were sensitive to muons with energies of at least 1-2 GeV.

Comprehensive tables for radiation lengths and other properties of materials are available from the Particle Data Group site: http://pdg.lbl.gov/AtomicNuclearProperties.

The spherical bessel function can also be simplified by assuming that the radiation length scale is much larger than the source length scale, which is true for most antennas.

The RICH detector was required to be less than 20 cm in the radial direction, between the drift chamber and the calorimeter, and less than 12% of a radiation length.

The shielding of photon beam is therefore exponential (with an attenuation length being close to the radiation length of the material); doubling the thickness of shielding will square the shielding effect.