Abstract
AbstractThis study describes the charge‐state evolution of relativistic lead ions passing through a thin aluminum stripper foil. It is motivated by the Gamma Factory project at CERN, where optical laser pulses will be converted into intense gamma‐ray beams with energies up to a few hundred MeV, achieved via excitation of atomic transitions in few‐electron heavy‐ions at highly relativistic velocities. The recently developed BREIT code is employed together with theoretical cross‐sections for single‐electron loss and capture of the projectile ions. All charge‐states starting from Pb54 + up to bare ions are considered at kinetic projectile energies of 4.2 and 5.9 GeV u−1. During recent Gamma Factory beam tests, the calculated preparatory predictions allow to effectively produce Pb80 + and Pb81 + ions from Pb54 + in the transfer beam line between the PS and SPS synchrotron accelerators at CERN and consequently, to store partially stripped ions in the LHC for the very first time. Reasonable agreement is found between the calculations and the very few experimental data available. The study lays the groundwork to optimize the yields of charge states of interest for experiments within the future Gamma Factory project, including the upcoming Gamma Factory Proof‐of‐Principle experiment for which predictions for the production of Pb79 + are presented.
Highlights
Charge-changing processes, i.e. loss or capture of electrons, occurring in ion-atom and ion-ion collisions belong to the most basic interactions being present in all types of plasmas as well as at accelerator facilities
As they are typical for the CERN facility, the cross section for electron loss is much larger as compared to all capture processes, the equilibrium charge-state distribution after the passage through a thick stripper target is clearly dominated by the bare charge state
As the Gamma Factory project will employ x-ray transitions of few-electron ions for converting optical laser pulses into intense gamma-ray beams, the efficient production of ions in the desired charge states requires the use of thin stripper-foils and dealing with non-equilibrium charge-state distributions of heavy ions [2,3,4]
Summary
Charge-changing processes, i.e. loss or capture of electrons, occurring in ion-atom and ion-ion collisions belong to the most basic interactions being present in all types of plasmas as well as at accelerator facilities. 2 identify the best material and thickness of the stripper target to produce a specific charge state, it is necessary to model the charge-state evolution as a function of the foil thickness In this context recent Gamma Factory beam tests aimed for the efficient production of hydrogenlike (Pb81+) and helium-like (Pb80+) lead ions. Compared to the aforementioned charge-state distribution codes which have built-in charge-exchange cross-sections covering a specific parameter range, the BREIT code provides a plain solver for the balance rate equations between the various charge states while the underlying cross-sections for capture and loss of projectile electrons need to be supplied by the user This allows to apply dedicated cross-section predictions tailored to the specific needs of the experiment and to overcome the limitations of the commonly used codes. We compare the results of the BREIT code to the very limited experimental data that are available for highly-relativistic lead beams, namely to the findings of the aforementioned Gamma Factory beam tests as well as to an older measurement of the bare ion (Pb82+) yield of aluminum foils of various thicknesses [9]
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