  A detailed analysis of the bremsstrahlung spectrum at nonrelativistic
electron scattering on argon and xenon is carried out. It is shown that the
approximate formulas widely used for the description of bremsstrahlung spectra
lead to predictions that significantly differ from the exact results. In the
limit when the photon frequency tends to zero, a rigorous proof of the
relationship between the spectrum of the bremsstrahlung with a transport cross
section of electron scattering on an atom is given. This proof does not require
any assumptions about the dependence of the scattering phases on energy. For
electron energies lower than the luminescence threshold, it is shown that the
predictions for a number of radiated photons obtained by the exact formula are
in good agreement with the available experimental data.
