Internal bremsstrahlung: Exact relativistic independent-particle-approximation calculations

We have reformulated the relativistic theory of internal bremsstrahlung in electron capture (IBEC) and developed a numerical code for calculating a transition amplitude for any initial atomic state. All relativistic as well as screening effects are included within the independent-particle approximation. We have shown the advantage of the theory even in the pole-dominance region of the IBEC spectra, thus making it suitable for comparison with the observed spectra in experiments that search for light- and/or heavy-neutrino mass components. In addition, we have applied our code to study the interference effects in the IBEC transition amplitude of ¹⁶³Ho and found, in agreement with experimental data, a smaller interference suppression than predicted by several recent models.