We measure the projectile K x-ray spectra as a function of the beam energies around the Coulomb barrier in different collision systems. The energy is scanned in small steps around the barrier aiming to explore the nuclear effects on the elastically scattered projectile ions. The variation of the projectile x-ray energy with the ion-beam energies exhibits an unusual increase in between the interaction barrier and fusion barrier energies. This additional contribution to the projectile ionization can be attributed to the shakeoff of outer-shell electrons of the projectile ions due to the sudden nuclear recoil (∼10^{-21} sec) caused by the attractive nuclear potential, which gets switched on near the interaction barrier energy. In the sudden approximation limit, the theoretical shakeoff probability calculation due to the nuclear recoil explains the observed data well. In addition to its fundamental interest, such processes can play a significant role in dark matter detection through the possible mechanism of x-ray emissions, where the weakly interacting massive particle-nucleus elastic scattering can lead to the nuclear-recoil-induced inner-shell vacancy creations. Furthermore, the present work may provide new prospects for atomic physics research at barrier energies as well as provide a novel technique to perform barrier distribution studies for two-body systems.
Charge state evolution of the energetic projectile ions during the passage through thin carbon foils has been revisited using the X-ray spectroscopy technique. Contributions from the bulk and the solid surface in the charge changing processes have been segregated by measuring the charge state distribution of the projectile ions in the bulk of the target during the ion–solid interaction. Interestingly, the charge state distribution measured in the bulk exhibits Lorentzian profile in contrast to the well-known Gaussian structure observed using the electromagnetic methods and the theoretical predictions. The occurrence of such behavior is a direct consequence of the imbalance between charge changing processes, which has been seen in various cases of the laboratory plasma. It suggests that the ion-solid collisions constitute high-density, localized plasma in the bulk of the solid target, called the beam-foil plasma. This condensed beam-foil plasma is similar to the high-density solar and stellar plasma which may have practical implementations in various fields, in particular, plasma physics and nuclear astrophysics. The present work suggests further modification in the theoretical charge state distribution calculations by incorporating the plasma coupling effects during the ion–solid interactions. Moreover, the multi-electron capture from the target exit surface has been confirmed through comparison between experimentally measured and theoretically predicted values of the mean charge state of the projectile ions.
Charge state evolution of the projectile ions while traversing through the solid target medium has been explored using the radiative electron capture process. The measured centroid energies of the convoluted radiative electron capture peak structures have been used to determine the mean K-shell binding energies and mean charge state of the projectile ions. It has been observed that the mean charge states of present measurements are lower than the earlier measurements done using the characteristic K α x-ray transitions. The difference is due to the capture of target electrons in the inner-shell vacancies, created during the collision process, of projectile ions. Further, the measured mean charge states are compared with the empirical predictions. A significant discrepancy between experimental and theoretical values has been observed, which is attributed to the multielectron capture by projectile ions due to nonradiative electron capture process from the exit surface while exiting from the foil. The significant variation between mean charge state values obtained from different tools provides a clear indication of the dynamic nature of the charge-changing mechanism at different regions (entrance surface, bulk, and exit surface) of the ion-solid interaction. The present results can be used to validate the departure between the theory and experiment on the charge state dependent stopping powers.
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