The new high-intensity surface muon beam μE4 for the generation of low-energy muons at PSI

Abstract At the Paul Scherrer Institute (PSI, Villigen, Switzerland) the new high-intensity muon beam line μ E 4 is currently being commissioned. It replaces the previous μ E 4 decay muon channel. The beam line with a design momentum of 28 MeV / c has been planned to serve the needs of the low-energy, polarized positive muon source ( LE - μ + ) and LE - μ SR spectrometer at PSI. A large acceptance is accomplished by installing two solenoidal magnetic lenses close to the muon production target E that is hit by the 590 MeV PSI proton beam. These normal conducting solenoids limit the maximum beam momentum to 40 MeV / c . The beam is then transported by standard large aperture beam line elements to the experiment. Several slit systems and an electrostatic separator allow the control of the beam shape, momentum spread, and to reduce the background due to positrons or electrons. We present first beam measurements at the position of the LE - μ + moderator target, where the electrostatic separator was replaced by a straight vacuum tube. At 28 MeV / c a maximum μ + flux of 7.0 × 10 8 /s at a proton current of 1.9 mA has been measured. About 2.3 × 10 8 /s hit the moderator target with a size of 3 × 3 cm 2 . This will translate into a LE - μ + rate of up to ∼ 8000 /s being available at the LE - μ SR spectrometer, an improvement by almost a factor of 10 with respect to the LE - μ + rate achieved so far. In order to cope with the increased intensity a major upgrade of the entire LE - μ + apparatus (hardware and software) has been undertaken parallel to this development.