The M20 channel is both a (low momentum) surface muon and a (high momentum) decay muon beamline with a long history; and its present form reflects that history. The original (1975) M20 was rebuilt in 1982 as a split beamline (M20A&M20B) with the capacity to run `forward' (173 MeV/c) and `backward' (86.5 MeV/c) beams simultaneously into separate experiments, or to run `surface' muons (29 MeV/c) with spin-rotation into the B-leg alone. After M9B was built the M20A leg was dismantled to make M20(B) a dedicated surface-muon channel, albeit with a long `decay' section of 10 quadrupole magnets. It is, in principle, still usable for high-momentum backward-decay muons ( 80 MeV/c) and even higher-momentum forward muons ( 170 MeV/c), but it has never been used that way; M20 is, de facto, a dedicated surface-muon channel.
As with all surface muon beams, the flux of µ- is disappointing.
M20 takes muons from the 1AT2 target, typically 10 cm beryllium, and delivers a high flux of surface muons, but with a large final focus imaging the large production target. The beam flux is nearly doubled if a bare carbon target is used, but there have been problems with durability. In the table of muon beamlines, fluxes for a 10 cm Be production target are listed.
The spin rotator can provide clean beam with longitudinal or transverse (spin-rotated) polarization by operating at 50 or 140 kV/plate, respectively. Slits limit the final spot size and the momentum range transmitted. The spin rotator also selects momentum, and therefore reduces total flux, at full 90° rotation.