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

At the Paul Scherrer Institute (PSI, Villigen, Switzerland) the new high-intensity muon beam line μE4 is currently being commissioned. It replaces the previous μE4 decay muon channel. The beam line with a design momentum of 28MeV/c has been planned to serve the needs of the low-energy, polarized positive muon source (LE-μ+) and LE-μSR 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 590MeV PSI proton beam. These normal conducting solenoids limit the maximum beam momentum to 40MeV/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 28MeV/c a maximum μ+ flux of 7.0x108/s at a proton current of 1.9mA has been measured. About 2.3x108/s hit the moderator target with a size of 3x3cm2. This will translate into a LE-μ+ rate of up to ∼8000/s being available at the LE-μSR 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