Bremsstrahlung

  Radiation emitted by a charged particle under acceleration. In particular, the term is used for radiation caused by decelerations (the word is German for braking radiation) when passing through the field of atomic nuclei (external bremsstrahlung). Radiation emitted by a charged particle moving in a magnetic field is called synchrotron radiation.

The energy emitted by an accelerated particle is proportional to 1/m², with m the rest mass of the particle; bremsstrahlung therefore plays a particularly important role for light particles; up to energies of 100 GeV, bremsstrahlung contributes substantially to energy loss in matter for electrons only. At the critical energy , for electrons approximately given by  MeV/Z, the average energy loss by radiation and by ionization is the same (Z is the atomic number of the traversed material).

The energy spectrum of rays due to bremsstrahlung of electrons decelerated in the field of atomic nuclei depends on the energy levels of the atomic electrons, due to the screening effect they have on the moving particle, and on the particle velocity. The spectrum extends up to quanta of the energy of the moving particle. In the high-energy limit the probability density is given by

where k = radiated energy, x = path length, X₀ = radiation length, and F is a slowly varying function not very different from unity, that can be approximated by

with R=1-k/E [Lohrmann81]. To a reasonable approximation, the amount of energy radiated per energy interval is constant.

Integration of the above formula results in the average energy loss per unit length which comes out to be

(this is more or less the definition of the radiation length X₀).

In the relativistic limit, the radiated energy is contained in a narrow cone of average half-angle

independent of radiated energy. For more details, see [Jauch80],[Rossi65].

The term internal bremsstrahlung is used to describe the radiation of non-virtual quanta, i.e. photons or gluons, by particles participating in an interaction. The formulae given for internal bremsstrah lung in electron scattering in the relativistic limit are [Bjorken64]

where is the square of the four-momentum transfer, m the particle mass and F(E,k) has been given above.

In high-energy physics, bremsstrahlung has been put to use in constructing photon beams. Coherent bremsstrahlung on crystals with incident energetic electron beams has produced photon beams with energies > 200 GeV/c ([Bilokou83], [Jackson75]).