Our collaboration has built the world's strongest low-energy negative muon beam at the piE5 beam line of the Paul-Scherrer-Institute in Villigen, Switzerland. It delivers about 1000 muons per second with kinetic energies of only a few keV.
This energy is just low enough to stop the muons in a few cm3 of hydrogen or helium target gas at a pressure of a few mbar.
The low target gas pressure is one key to the success of the experiment: If the pressure was higher, the 2S state would be quickly deexcited in collisions with molecules of the surrounding target gas.
Here is the piE5 beam line seen from the top
In the lower left corner you can just not see the cyclotron trap (CT). The bent thing is the muon extraction channel (MEC). And at the end of the MEC you can see the 5 Tesla solenoid (SOL) where the experiment takes place.
The long vacuum tube starting in the upper left corner is the laser light beam line. You see a some power supplies, air conditioners for the laser hut, and electronics racks.
The cyclotron trap, during installation:
The cyclotron trap is a cool device made from two 4 Tesla superconducting coils, with a 2 Tesla magnetic field in the center. We shoot pions into the CT, decelerate them a bit to trap them in the magnetic bottle, and let the pions decay into muons.
The muons are further decelerated in a 200 nm thin Ni-coated plastic foil which is mounted in the center of the CT. A high voltage of -20 keV is applied to the plastic foil. Once the muons are slow enough to overcome the magnetic confinement, the HV of the foil pushes the negative muons gently out of the CT, along its axis. The muons are then entering the
muon extraction channel (MEC):
The MEC is the curved beam line with the black coils, mounted on top of the golden posts. The magnetic field inside the MEC guides only the muons, all unwanted background particles (electrons, neutrons, gammas) are separated from the muons.
Finally the muons enter our 5 Tesla solenoid (SOL).
That's the shiny tube behind me.