"Outer bremsstrahlung" is the term applied in cases where the energy loss by radiation greatly exceeds that by ionization as a stopping mechanism in matter. This is seen clearly for electrons with energies above 50 Mev.
"Inner bremsstrahlung" is the term applied to the more infrequent case of radiation emission during beta decay, resulting in the emission of a photon of energy less than or equal to the maximum energy available in the nuclear transition. Inner bremsstrahlung is caused by the abrupt change in the electric field in the region of the nucleus of the atom undergoing decay, in a manner similar to that which causes outer bremsstrahlung. In electron and positron emission the photon's energy comes from the electron/neutron pair, with the spectrum of the bremsstrahlung decreasing continuously with increasing energy of the beta particle. In electron capture the energy comes at the expense of the neutrino, and the spectrum is greatest at about one third of the normal neutrino energy, reaching zero at zero energy and at normal neutrino energy.
Betatropic substances sometimes exhibit a weak radiation with continuous spectrum that is due to both outer and inner bremsstrahlung, or to one of them alone.
Bremsstrahlung is a type of "secondary radiation," in that it is produced as a reaction in shielding material by the primary radiation (beta particles). In some cases, the bremsstrahlung produced by some sources of radiation interacting with some types of radiation shielding can be more harmful than the original beta particles would have been.