We assume that the electron spin-lattice relaxation time is sufficiently short, so the electron spin polarization is maintained at its thermal equilibrium value P0 at all times. We also assume that the nuclear spin transfers its polarization through flip-flop transitions to its neighboring nuclear spins at a rate of the order of nuclear transverse relaxation rate 𝑇 "' . In this case the nuclear spin can be considered to be polarized according to the rate equation .