Atomic collisions with relativistic heavy ions. IX. Ultrarelativistic collisions

The behavior of projectile electron capture and ionization cross sections at ultrarelativistic energies (>10 GeV/amu) is discussed. Three mechanisms contribute to electron capture at these energies: radiative and nonradiative capture and the capture of the electron following electron-positron pair production. The radiative and nonradiative cross sections both fall off as ${\ensuremath{\gamma}}^{\mathrm{\ensuremath{-}}1}$ at high energies, (\ensuremath{\gamma}-1)${\mathrm{Mc}}^{2}$ being the ion energy. The vacuum capture cross sections increase as the ln\ensuremath{\gamma}. Projectile 1s ionization cross sections also increase as ln\ensuremath{\gamma} at high energies if target screening is negligible. However, when the projectile atomic number is smaller than the target atomic number, the transverse interaction giving the ln\ensuremath{\gamma} term is screened and reduced. Therefore, the total 1s ionization cross sections are dominated by Coulomb ionization, and are nearly constant with increasing projectile energy. Tables of reduced capture and ionization cross sections are given, and are applied to calculating the charge states of ions in matter and the lifetime of ions in storage rings.

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