Simulation studies for upgrading a high-intensity surface muon beamline at Paul Scherrer Institute

Abstract

TheμE4-LEM beamline at Paul Scherrer Institute (PSI, Switzerland) is a special muon beamline combining the hybrid-type surface muon beamlineμE4with the low-energy muon (LEM) facility and deliversμ+with tunable energy up to 30 keV for low-energy muon spin rotation (LE-μSR) experiments. We investigate a possible upgrade scenario for the surface muon beamlineμE4by replacing the last set of quadrupole triplets with a special solenoid to obtain 1.4 times the original beam intensity on the LEM muon moderator target. In order to avoid the muon beam intensity loss at the LEM spectrometer due to the stray magnetic field of the solenoid, three kinds of solenoid models have been explored and the stray field of the solenoid at the LEM facility is finally reduced to the magnitude of the geomagnetic field. A more radical design, “Super-μE4,” has also been investigated for further increasing the brightness of the low-energy muon beam, where we make use of the currentμE4channel and all sets of quadrupole triplets are replaced by large aperture solenoids. Together with the new slanted muon target E, at least 2.9 times the original muon beam intensity can be expected in the Super-μE4beamline. Our work demonstrates the feasibility of upgrading surface muon beamlines by replacing quadrupole magnets with normal-conducting solenoids, resulting in higher muon rates and smaller beam spot sizes.