LEM Developments

At the low-energy muon facility various upgrades are planned to significantly extend the current experimental capabilities, and to open new research direction in LE-µSR.

In the LEM instrument we plan to install a new sample-transfer system. So far, samples are mounted on a sample plate on air before transferring them to the ultra-high vacuum system of LEM, severely limiting, for example, the study of magnetic surfaces, due to surface contamination. In order to address this issue, a sample-preparation chamber is under development that will make it possible to mount and transfer samples in vacuum to the LEM instrument. This will also significantly reduce the time for sample change, which takes about three to four hours at the moment.

A further focus is to enable the study of relatively small samples of 5×5 mm² by muon-beam collimation. The accompanying rate loss will be compensated by improving the focusing of the µE4 beamline on the LEM moderator target using a solenoid at the end of the beamline (see below), thereby increasing the rate of low-energy muons by 40 %. The ability of working with such small samples will be in particular important for novel quantum materials, which can be often grown with high quality only on small substrates. Investigating them with LE-µSR is currently only possible if a mosaic of at least four of such samples can be grown with high and similar quality within a reasonable time, which is not always possible. Also, applying an electric field or passing a high electrical current through samples can be controlled significantly better for small sample sizes. Finally, with the ability to generate smaller beam spots by collimation at a still acceptable beam rate, one can mount up to five samples at once, thus significantly reducing the time needed for sample changes.

As a first step towards a higher beam rate on the existing 30x30 mm² moderator target, a special, normal conducting solenoid with compensated fringe fields is under development. By replacing the last quadrupole triplet of the current µE4 beamline, a 40% increase of surface muon beam rate on moderator target can be envisaged, while keeping the total beam rate constant. In collaboration with the magnet group of the GFA Division at PSI, the solenoid is planned to be ready for installation in 2025.

As a final, ultimate upgrade, we continue a simulation and design study of the “Super µE4” beamline, where all quadrupole triplets are replaced by normal conducting solenoids. A ~50% rate increase of the total beam rate appears to be feasible, while a gain on moderator of 65% seems to be possible.