= Longitudinal nuclear spin relaxation
{title2=$T_1$}
With no transverse drive, the <Bloch equations> give
$$
M_z(t)=M_0+[M_z(0)-M_0]e^{-t/T_1}.
$$
<Longitudinal nuclear spin relaxation> restores the population imbalance and thus the <magnetization> parallel to the static <magnetic field>. It corresponds to <population relaxation> for the two-level model. Starting from $M_z(0)=0$, waiting $T_1$ recovers $1-e^{-1}$ of $M_0$, whereas waiting $5T_1$ recovers about $99.3\%$. This is why repeated measurements require a recovery-time decision.
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