A strong peak of the X-ray spectra observed in collisions of 6.6 MeV/u \(^{238}U\) on a \(^{64}Ni\) Frégeau et al. in Phys Rev Lett 108:122701, 2012) was attributed to be originating from the compound nucleus \(^{302}_{120}Ubn\) based ions. Though extremely small evaporation residue cross section \((10^{-9} fb)\) and nuclear decay properties favor such X-ray emission from \(^{302}_{120}Ubn\) , but such a strong X-ray peak from a small fusion cross section is not possible. Furthermore, X-ray emission from the ion formed with the compound nucleus must be affected by the Doppler effects. Thereby, the origin of this X-ray peak ought to be something else. Interestingly, this particular X-ray peak not affected by the Doppler effect is found to be belonging to the two-electron one-photon (TEOP) process of thorium that is produced from the \(\alpha\) -breakup reaction of the uranium projectiles. We show that the TEOP process emits the Doppler-free radiations and we can analyse the entire observed spectra by accounting this process in the atomic structure calculations. Remarkably, the intensity of this peak is nearly equal to that of thorium \({K}_\alpha\) peak. Such an unusual feature is possible from uranium X-ray absorption to the thorium excited states having a vacancy in the K-shell and and then a doubly excited state is formed with the fully vacant K-shell. Thus, we reveal that the particular peak does not at all originate from the compound nucleus rather it comes from the thorium TEOP process. Now this Doppler-free radiation phenomenon can help us to analyse correctly the spectra observed in the X-ray region where both the atomic and nuclear events prevail.
