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].

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].

 

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

Radiative electron capture to the continuum in U89++N2 collisions: Experiment and theory

P-M. Hillenbrand et. al.

Phys. Rev. A, 101, 022708 (2020)

7

Electron-loss-to-continuum cusp in collisions of U89+ with N2 and Xe

P-M. Hillenbrand et. al.

Phys. Rev. A 104, 012809

8

Experimental concepts of positron spectroscopy at HESR

P-M. Hillenbrand et. al.

Phys. Scripta T166 (2015)


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