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.

通常的假设是，在弛豫测量中μ子的极化度会衰减至零，因为100%的初始极化度远大于典型的玻尔兹曼布居。在高场和低温条件下，这不再是一个良好的近似：假设μ子与样品中引起弛豫的自旋达到平衡，且这些自旋也处于热平衡状态，那么对于磁场强度B=5特斯拉（T）、温度T=30毫开尔文（mK）的情况，布里渊函数（Brillouin function）给出的极化度P=49%。反转磁场会改变效应的符号，从而改变弛豫信号的表观幅度。