Forward-angle electron spectroscopy

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].

As a next step, an experimental study of ELC for U89+ colliding with different gaseous target is envisaged, at a projectile energy just above the threshold for electron impact ionization. For these collision systems, relativistic CDW calculations predict a deviation of the electron energy distribution from first-oder perturbation due to the effect, that the electron emitted by the projectile is attracted by the target nucleus.

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 [6].

 

References / Selected Publications

References

#

Title

Author

Reference

1

Forward-angle electron spectroscopy in heavy-ion atom collisions studied at the ESR

P-M. Hillenbrand et. al.

J. Phys. Conf. Ser. V635 (2015)

2

Electron-loss-to-continuum cusp in U88++N2 collisions

P-M. Hillenbrand et. al.

Phys. Rev. A V90 (2014) 042713

3

Electron-capture-to-continuum cusp in U88++N2 collisions

P-M. Hillenbrand et. al.

Phys. Rev. A 91 (2015) 022705

4

Radiative-electron-capture-to-continuum cusp in U88++N2 collisions and the high-energy endpoint of electron-nucleus bremsstrahlung

P-M. Hillenbrand et. al.

Phys. Rev. A 90 (2014) 022707

5

Strong asymmetry of the electron-loss-to-continuum cusp of multielectron U28+ projectiles in near-relativistic collisions with gaseous targets

P-M. Hillenbrand et. al.

Phys. Rev. A 93 (2016) 042709

6

Experimental concepts of positron spectroscopy at HESR

P-M. Hillenbrand et. al.

Phys. Scripta T166 (2015)


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