In collisions of heavy highly-charged projectile ions with atomic targets, the energy distribution of the emitted electrons is a characteristic observable for the underlying elementary charge-transfer processes. At the ESR, a dedicated magnetic electron spectrometer was installed downstream from the gas-jet target, which enables the measurement of high-energetic electrons emitted in ion-atom collisions with velocities similar to the projectile velocity within a small cone in the forward direction. This provides the ability to extend the well known study of cusp electrons towards heavy-ion atom collisions at near-relativistic projectile energies. Through the electron-loss-to-continuum cusp, double-differential cross sections of projectile ionization can be studied even for the heaviest few-electron projectiles. But also a new channel opens up, the radiative electron capture to continuum, which can be directly compared to its non-radiative counterpart. Using the electron spectrometer in combination with detectors for emitted x rays and charge-exchanged projectiles, the study of the collision system U⁸⁸⁺ + N₂ @ 90 MeV/u revealed three processes, each characterized by a unique shape of the electron cusp [1]:

• The process of electron loss to continuum (ELC) corresponds to the ionization of an electron from the projectile into the projectile continuum during the collision with the target, U⁸⁸⁺ + N₂ → U⁸⁹⁺ + [N₂]^* + e^-. For the ELC, the measured spectrum has been compared to first-order perturbation theory using fully-relativistic Dirac wavefunctions [2].
• The process of electron capture to continuum (ECC) corresponds to the capture of a target electron into the projectile continuum, while the excess energy is carried away by the recoil of the generated target ion: U⁸⁸⁺ + N₂ → U⁸⁸⁺ + [N₂⁺]^* + e^-. For the ECC, the measured spectrum has been compared to calculations in the impulse approximation using semi-relativistic Sommerfeld-Maue wavefunctions, and to calculations in the continuum-distorted-wave (CDW) approach [3].
• The process of radiative electron capture to continuum (RECC) corresponds to the capture of a target electron into the projectile continuum, while the excess energy is carried away by a photon: U⁸⁸⁺ + N₂ → U⁸⁸⁺ + [N₂⁺]^* + e^-+ γ. This process can be seen as the high-energy endpoint of bremsstrahlung studied in inverse kinematics. For the RECC, the measured spectra have been compared to calculations using fully-relativistic Dirac wavefunctions, and to calculations in the impulse approximation using semi-relativistic Sommerfeld-Maue wavefunctions [4].

Furthermore, the process of ELC was investigated for multi-electron projectiles in the collision systems

U²⁸⁺ +H₂ → U²⁹⁺ +[H₂]^* +e⁻, U²⁸⁺ +N₂ → U²⁹⁺ +[N₂]^* +e⁻ and U²⁸⁺ + Xe → U²⁹⁺ + Xe^* +e⁻.

The experimental data revealed a significant electron cusp asymmetry, which increases towards heavier targets. This behavior is not yet consistent with presently available theories based on first-order perturbation using fully-relativistic wavefunctions [5].

In a more recent study, the RECC was measured for the collision system U⁸⁹⁺ + N₂ @ 76 MeV/u, and an im- proved agreement of the experimental data and theory was achieved [6]. Within the same experimental campaign, the ELC for U⁸⁹⁺ colliding with N₂ and Xe was studied, showing a deviation of the electron energy distribution from first-oder perturbation for the Xe target due to the effect that the electron emitted by the projectile is attracted by the target nucleus [7].

Future concepts of applying the same technique to positron spectroscopy in relativistic heavy-ion atom collisions at the HESR of FAIR are currently under consideration [8].