Ultra-low energy ion implantation of radioactive isotopes for PAC studies of surfaces and interfaces

After years of anticipation, the Apparatus for Surface Physics and Interfaces at CERN (ASPIC) will make a comeback to the forefront of materials science at ISOLDE [1]! During the 1990’s, the experimental setup saw many successful perturbed angular correlation (PAC) experiments, leading to a number of publications on the magnetic properties of single crystal and thin film surfaces and interfaces [2]. Since that time,however, the ASPIC has not been active anymore, due to a lack of financial support and dedicated manpower. Soon, the ASPIC makes a comeback, turning its attention to low-energy PAC isotope implantation (111mCd,77Se, 204mPb, 68mCu...) into nanostructures at the cutting edge of solid state physics, such as 2D materials (graphene, MoS2...),(multi)ferroic materials and topological insulators.

The setup is currently being upgraded at the university of Göttingen, after which it is moved back to the ISOLDE experimental hall. As before, ASPIC will offer a variety of surface and thin film modification techniques(deposition, ion sputtering, annealing) and characterization methods (Auger electron spectroscopy, low-energy electron diffraction (LEED)), allowing the preparation of clean surfaces without breaking the ultra-high vacuum (< 10-9 mbar).

The ASPIC will be complimented by a brand-new chamber: the Apparatus for Surface physics and interfaces at CERN’s Ion Implantation chamber (ASCII).In this extension, a custom-made deceleration unit will be installed, which slows the ISOLDE ion beams down from 60 keV to energies on the order of 10 eV (Fig.1). These PAC ions are focused onto the surface of the sample, allowing fast incorporation of PAC probes into the studied materials. This new ion beam setup will replace the slower and more cumbersome catcher system that was used in ASPIC before. Faster implantation and quick transfer allow for more samples to be measured during beamtimes, as well as increase the signal-to-noise ratio during PAC measurements. The ASCII, combined with the wide range of thin film configurations the ASPIC offers, will provide users precise control of the location of PAC probes in nanofilm structures.Commissioning and testing of these chambers will continue throughout 2021. Their return to ISOLDE is planned for late 2021, early 2022.

Figure 1: Using the deceleration stage of the ASCII, PAC isotopes will be implanted unto sample surfaces with energies as low as 10 eV.

[1] H. Hofsäss, K. van Stiphout, ‘Letter of Intent: Upgrade of the UHV-system ASPIC for the investigation of surfaces and two-dimensional materials by ultra-low energy implantation and deposition of radioactive probe atoms (CERN-INTC-2020-001; INTC-I-208)’, Tech. rep. (2020).
[2] H. Bertschat, et al., Hyperfine Interactions 129(1),475 (2000).