We studied the relaxation rate constants for coherences (1/T 2 ), and populations (1/T 1 ) . The results are presented in Fig. . The relaxation rate constants are shown on the right as a function of TEMPOL concentration, while the pulse sequence applied in each experiment is depicted on the left. The samples were prepolarized with a 2 T Halbach magnet and pneumatically shuttled into the µ-metal shield, where a Helmholtz coil provided a field of 100 µT along the sensitive axis. This field was high enough so that the spin states remain eigenstates of the Zeeman basis. The (1/T 1 ) rate constant is determined by indirect sampling of a variable delay time τ at different fields. In Fig. the sample was kept at 100 µT for a variable duration before the field was rapidly switched off prior to data acquisition.