2.2.1 Production of a muon beam

The heart of TRIUMF (TRI-Universities Meson Facility) is a 520 MeV H^- cyclotron with a high beam-intensity (140$\mu$A). An accelerated proton beam is lead to a production target made of Graphite or Beryllium. Proton hit the target-nuclei and create pions via nuclear reactions.

To obtain muons, one must wait for the pions to decay, which typically takes 26 ns (pion lifetime). To obtain positive muons ($\mu^+$), one can stop positive pions ($\pi^+$) in the production target and wait for their decay, because the positive charge of the pions prevent them from being absorbed into the surrounding nuclei. These stopped positive pions produce positive muons which are fully polarized anti-parallel to their momenta (see Fig.6). These muons are called the `surface muons', because they are emitted from the surface of the production target. Surface muons have (1) high spin-polarization ($\sim$100%), (2) low momentum (29.8 MeV/c$\sim$170 mg/cm² as the stopping range), and (3) small beam image (a few centimeters in diameter). These features are all favorable for $\mu$SR measurements. The muon channels M13, M15 and M20 at TRIUMF are designed to deliver surface muons.

  

Figure 10: A schematic view of the M15 beam line at TRIUMF.

Fig.10 shows a schematic view of the M15 muon channel at TRIUMF. Four dipole magnets (B1-B4) on the beamline bend the beam and select the momentum of the muons. Quadrupole magnets (QAQB, Q1-Q17) focus the muon beam. The DC-separators (SEPARATOR1 and 2) provide crossed electric and magnetic fields perpendicular to the beamline (see the inset of Fig.10). This apparatus eliminates positrons from the muon beam by setting the field ratio (E/B) to the muon velocity ($v_{\mu}$). Secondly, this apparatus can rotate muon spins away from the momentum direction, by applying higher E and B fields. All of the surface muon channels at TRIUMF are equipped with at least one DC-separator; M20 and M15 have the capability to rotate the muon spins by 90$^{\circ}$.