IReNA Online Seminar featuring Esther Menz (University of Cologne / GSI Laboratory)

Hosted by: Jordan Marsh (University of Edinburgh)

Abstract: The GSI accelerator facility currently hosts two storage rings, the Experimental Storage Ring (ESR) and the low-energy storage ring CRYRING@ESR, which together cover a wide range ion species and beam energies and are used mainly for experiments on atomic and nuclear physics. This talk will focus mainly on dielectronic recombination experiments performed at CRYRING@ESR but will also present a proposed experiment on nuclear excitation by electron capture (NEEC).

Dielectronic recombination (DR) is a recombination process whereby an electron is captured from the continuum while a second bound electron is excited, resulting in a doubly-excited state of the recombined ion which is vulnerable to auto-ionisation but can also relax via photon emission. Previous experiments have found notable discrepancies with theoretical rates for low transition energies around 1 eV, meaning that experimental data from storage rings are important for the validation of underlying rate equation networks in astrophysical modeling [1].

As a resonant process DR is also a useful probe of the electronic structure of the atom capturing the electron and can be used as a test for QED calculations which in contrast to optical methods allows easy access to non-dipole transitions that are “E1-forbidden” [2].

The CRYRING storage ring has long been used for DR experiments, taking advantage of the extremely low electron beam temperatures that can be achieved at the electron cooler. Since the ring was moved to GSI we have endeavored to continue this work and to also use the possibilities provided by CRYRING’s new home at GSI to perform DR experiments on heavy highly-charged ions. The talk will illustrate the DR setup at CRYRING@ESR and show some results, obtained over the last few years, for both heavy and light ions.

The process of nuclear excitation by electron capture (NEEC) is similar to DR in that an electron is resonantly captured into an atomic bound state. However instead of a bound electron, the nucleus of the atom is excited. NEEC is a fundamental process that is important for plasma and astrophysics and might have useful applications such as producing desirable nuclear isomeric states. However, despite significant efforts, no uncontroversial observation has yet been reported. A claimed observation by Chiara et al [3] deviated from theoretical predictions by nine orders of magnitude [4] and could not be independently confirmed in an experiment by Guo et al [5].

The talk will describe a proposed storage ring experiment using similar methodology as the highly-established DR measurement method. If successful this should provide an unambiguous observation of the NEEC process.

[1] H. Netzer, Astrophys. J. 604, 551 (2004)

[2] C. Brandau et al. Physica Scripta T166, 014022 (2015)

[3] C. J. Chiara et al., Nature 554, 216 (2018)

[4] Y. Wu et al., Phys. Rev. Lett. 122, 212501 (2019)

[5] S. Guo et al., Phys. Rev. Lett. 128, 242502 (2022)