Prashant Sharma
Post-Doctoral Fellow,
Weizmann Institute of Science
Israel
RECENT PUBLICATIONS
P. Sharma and T. Nandi, Phys. Rev. Lett. 119, 203401 (2017).
Abstract
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.
Shakeoff Ionization near the Coulomb Barrier Energy
Disentangling the bulk and exit surface contribution on projectile-charge-state evolution
P. Sharma and T. Nandi, Phys. Rev. Accel. Beams 22, 034501 (2019).
Abstract
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 Kα 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.
Theoretical studies on shaking
processes in nuclear transfer reactions
P. Sharma and T. Nandi, Nucl. Phys. A 941, 265 (2015). Abstract
The probabilities of shaking processes during nuclear transfer reactions have been studied using the Mukoyama formalism after the re-examination of formalism for β-decay processes. Electron shakeoff prob-abilities have been calculated for the α-transfer reaction in the range of Z = 10–50. The Z-dependence on the shakeoff probabilities so obtained has been represented by an analytical equation with two parameters. The formalism has been applied on a typical nuclear transfer reaction 56 26 Fe + 12 6 C → 60 28 Ni + 8 4 Be and it is found that electron shakeup, shakedown and shakeoff probabilities dominate for low l quantum number of the respective shells of the projectile-like fragment ion. However, for a particular value of l these processes show high probabilities for low values of n quantum number.
X-Ray Spectroscopy: An Experimental Method To Measure Charge State Distribution during the Ion-Solid Interaction
P. Sharma and T. Nandi, Phys. Lett. A (2015).
Abstract
Charge state distributions of Fe and Ni projectile ions passing through thin car- bon foils have been studied in the energy range of 1.44 - 2.69 MeV/u using newly developed x-ray spectroscopy technique. It is found that the x-ray measurement technique is appropriate to determine mean charge state right at the interac- tion zone; meaning by at t=0. Interestingly the charge state distribution in the bulk show Lorentzian behavior instead of usual Gaussian distribution. Further, different parameters of charge state distribution like mean charge state, distri- bution width and asymmetric parameter are determined and compared with the empirical calculations and ETACHA predictions. It is found that the empirical formalism is not suitable for mean charge state calculations at t=0, whereas ETACHA predictions give satisfactory results for energies ≥ 2 MeV/u.
Experimental Evidence of Beam-Foil Plasma Creation during Ion-Solid Interaction
P. Sharma and T. Nandi, Phys. of Plasmas (2016).
Abstract
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.
Shaking during Ion-Atom Collisions
P. Sharma and T. Nandi, Acta Phys. Polonica A (2016).
Abstract
The shaking (shakeup and shakeoff ) processes accompanying ion-atom collisions are studied using non-relativistic hydrogenic wavefunctions for the K−, L− and M−shell electrons in the sudden approximation limit. The role of recoil amplitude in the shaking processes is discussed. It is found that the suddenness of collision between projectile and target nuclei plays a definitive factor in the shaking of the respective atomic system than the recoil of nuclei.
Role of nuclear charge change and nuclear recoil on the shaking process and its possible implication on physical processes