Muon Beams at the Paul Scherrer Institute
Intense beams of spin-polarised muons are available at the PSI Accelerator Facilities in the experimental areas µE1, µE4, piM3, piE1 and piE3. Areas with "M" in their name are connected to a thin target M ("Mince" in french, 5 mm of graphite), those with "E" to the thick E target ("Epaisse" in french, 60 mm or 40 mm of graphite). The muons originate from the decay of pions produced by the interaction of the 2 mA, 590 MeV proton beam with the nuclei of the target material.
The µE1 beam line is dedicated to the µSR User Facilities and is equipped with a superconducting decay channel, providing beams of positive or negative muons with useful momentum ranges 40-125 MeV/c.
The new µE4 beam is a large acceptance beam line for muons with low momentum (< 40 MeV/c). It was specially designed to serve the needs of the low-energy muon (LEM) beam facility at PSI, i.e. maximum positive muons intensity at 28 MeV/c and a beam spot fitting to the size of the muon moderator target of the LEM apparatus.
The piM3 beam line is entirely dedicated to the µSR User Facilities. It is equipped with a spin rotator and a fast-switching electrostatic "kicker". The surface-muon beam can be shared or switched ("muons on request", MORE) between the GPS and LTF Facility Instruments.
Depending on the beamline, one can obtain:
- Positive muons emerging from the production targets: "surface" (1) and "sub-surface muons" (2) (momentum range 5-30 MeV/c)
- Positive or negative "cloud muons" (3) (momentum 10-280 MeV/c).
- Low energy positive muons are created in µE4 by converting with a cryogenic moderator surface muons to epithermal muons. A system of electrostatic elements for reacceleration and low-energy muon transport can provide muons energies up to 30 keV.
- Surface muons originate from pions stopped close to the surface of the target. They have kinetic energies around 4.1 MeV (momentum 28 MeV/c).
- Sub-surface muons originate from pions stopped in the bulk of the target, thus losing some energy on their way to the surface.
- Cloud muons originate from pions decaying outside the target.