The Swiss Muon Source, SμS, is one of PSI’s highly successful user facilities. In 2011, the facility again welcomed users from all over the world; in total, about 160 diff erent scientists came to perform their muon spin resonance experiments at one of the six available SμS instruments. Almost 700 days were of-fered on the instruments and more than 220 experiments could be performed.
In 2011, a large share of beam-time was used by Swiss groups, 35% of beam-time was given to users from EU member coun-tries, and another 15% to user groups from other councoun-tries, including Japan, Russia, Canada and the US. It is remarkable that the second-largest foreign user group (behind the UK (11%)) came from Japan (8%), followed by Germany and Italy (both 7%). In total, in 2011 users from 19 diff erent countries
performed their experiments at SμS, a number that demon-strates the signifi cant role played by the PSI muon facility for the international user community.
Research
The papers which appeared in 2011 reflect the scientifi c rel-evance of the use of muons in condensed matter research.
These include publications in journals with high impact factor, such as Science (1), Nature Journals (3), Journal of the Ameri-can Chemical Society (1), Physical Review Letters (5) and the 28 papers in Physical Review B. SμS continued to be the lead-ing μSR centre in the world for the investigation of iron-based superconductors. This research has been further stimulated by the synthesis of new superconductors in the crystal growth group of the Department of Research with Neutrons and Muons (NUM). A summary of this research is reported in a separate contribution to this report [1]. The unique depth-dependent magnetic information obtained by low-energy muons has also resulted in several scientifi c highlights, including the demonstration of dimensional control of electronic properties in oxide superlattices.
Developments
Besides the continuous development of all instruments, a dedicated spin rotator for the low-energy muon facility has been developed and built. This crucial upgrade, which will allow so-called longitudinal fi eld measurements to be per-formed, is now ready to be installed and put into operation.
Essential progress has been obtained towards the realization Figure 1: During the autumn of 2011, the PiE3 beam area was
extended to deliver a fully polarized muon beam to the High-Field µSR facility. The picture shows the two spin rotators, with a refocusing element in between.
102 User facilities – Swiss Muon Source SµS PSI Scientifi c Highlights 2011
of the new high-fi eld μSR instrument, currently the major project of the Laboratory for Muon Spin Spectroscopy (LMU).
The main components of the new facility were installed to-wards the end of 2011. The high-fi eld μSR instrument, which is the only one of its type in the world, will allow a previously inaccessible range in the B-T phase diagram of condensed matter to be studied, ranging up to 9.5 T and down to ~20 mK.
The facility will make use of a ~28 MeV/c muon beam. For most experiments, the spin of the originally fully longitudinally polarized muon beam must be rotated by 90°. This rotation is achieved by a device called a ‘spin-rotator’, which provides crossed electric and magnetic fi elds, both applied perpen-dicular to each other and to the muon’s momentum. In addi-tion to rotating the spin, it also acts as a velocity fi lter (Wien fi lter) and separates the muons from other particles con-taminating the muon beam (mainly positrons).
The design parameters for the spin-rotator device originate from the preferred properties of the muon beam used to study the properties of the target in the spectrometer. Based on experience with other high-voltage devices at PSI (e.g. for the design of oil-insulated vacuum feedthroughs) and techno-logical standards, the maximum supply voltage for the device was chosen to be ± 200 kV, with an operating voltage of ± 175 kV. The gap between the two electrodes has to be as large as possible for maximum transmission, and the length of the electrodes short. A good compromise was found by choosing distances of 120 mm for the electrode gap and 1800 mm for the eff ective length. The matching magnetic fi eld for the op-erating voltage is then ~38 mT. A single device with these parameters leads to a spin rotation of 45°; therefore, two identical devices have been built, and installed in series, with a refocusing quadrupole triplet in between.
Because of the complexity of the system (high-voltage tech-nology, electric and magnetic fi eld matching, vacuum, control system), and in order to ensure compatibility with PSI stand-ards, the decision was made to use in-house expertise and
design the whole device at the Institute. Commercially avail-able components were used whenever possible. However, critical parts were manufactured at PSI or specially supplied by Swiss companies. The design of the electrostatic compo-nents was checked by means of mathematical simulations using the ANSYS electrostatic module, with an envisaged upper limit of 80 keV/cm for the electric fi eld. The design of the beamline, including the spin rotators and the magnets, was based on simulation of particle transport with the pro-grams Transport and Turtle . Finally spin rotation and transmis-sion effi ciencies were optimized by using TRACK and Geant4 Monte Carlo simulations. First measurements demonstrated a beamline performance as expected, with a rate of ca. 5000 μ+/(mA·s·mm2).
The spectrometer magnet is a custom high-homogeneity split-pair recondensing system from Oxford Instruments. Its maximum fi eld is 9.5 T, with a homogeneity of better than 0.1 mT over a centre volume of 10 mm diameter and 4 mm length. The detector system is based on direct readout of fast plastic scintillators (Eljen EJ 232) by Hamamatsu, Multipixel Photon Counters (MPPCs) and front-end electronics developed in-house. The overall time resolution is better than 80 ps (including full DAQ electronics).
To conclude, 2011 was another very successful year for applied muon physics at PSI. This has also been confi rmed by an in-dependent panel called in to evaluate the past 5 years of re-search activity at the Laboratory for Muon Spin Spectroscopy.
We are also confi dent that the important developments which took place during the past year will soon bear scientifi c fruit and will contribute to maintaining the leading role of the Paul Scherrer Institute in the use of muons for condensed matter research.
References
[1] A. Krzton-Maziopa et al., Alkali metal intercalated FeSe superconductors, This report.
PSI Scientifi c Highlights 2011 User facilities – Swiss Muon Source SµS 103
Figure 2: Left: Magnet with detector system. Right: Fourier transform of the fi rst µSR spectrum obtained from a silver sample at 9.5 T. The narrow line demonstrates its excellent magnetic fi eld homogeneity (better than 10 ppm).
The design and construction of the large research facilities at the Paul Scherrer Institute constantly require new and innovative solu-tions at the cutting-edge of current technologies. Both scientists and engineers at the Institute are successfully pushing the limits in vari-ous technological fi elds, from power electronics to precision machin-ing to nanotechnology. Alongside achievements in the various re-search fi elds being investigated at PSI, these accomplishments off er outstanding opportunities for commercialization by industrial part-ners.
The Technology Transfer offi ce at PSI is ready to assist representatives from industry in their search for opportunities and sources of innova-tion at PSI, or to prepare the way for soluinnova-tions to their own techno-logical challenges.
The following pages present some promising technologies still to be discovered by our industrial partners.