The M9 beamline has two legs coming from the 1AT2 target:
Leg A is being rebuilt as a surface-muon beamline for µSR experiments.
The B leg, built by the University of Tokyo, is a 20.3 m long backward µ+/µ- channel with a 6.5 m superconducting solenoid for the decay section. It delivers a high flux beam over a momentum range approximately 40-105 MeV/c; both ends are limited by dropping intensity: an intrinsic lack of low energy pions at the low end, and the current limits on some beam elements, particularly the B2 magnet, at the high end. It has the unique ability to deliver useful intensities of transversely-polarized muons, especially at the high-momentum end.
See also notes on Setting and tuning M9b.