Absolute cross sections of ultra-soft x-ray radiation in electron scattering upon atoms and the soft-photons approximation

Highlights: • Soft-photon approximation in quantum electrodynamics. • Long-wavelength region of bremsstrahlung spectrum. • Absolute cross section of bremsstrahlung in electron-atom collision. -- Abstract: We calculated the cross sections of ultra-soft x-ray bremsstrahlung in electron scattering upon Ar, Kr and Xe. The results are consistent with the absolute values of the differential cross sections measured by Gnatchenko et al. (2009) [17] for scattering electrons with an energy of 600 eV on these atoms.