Does the muon spin reach thermal equilibrium?

The usual assumption is that the muon's polarisation decays to zero in a relaxation measurement since the initial polarisation of 100% is much greater than the typical Boltzmann population. In high fields and low temperatures this is no longer a good approximation: the Brillouin function for B=5T and T=30mK gives P=49% assuming the muon reaches equilibrium with the spins in the sample which are causing the relaxation, and that these spins are also in thermal equilibrium. Reversing the field will change the sign of the effect and thus change the apparent amplitude of the relaxing signal.