This model, introduced in ref. , will be referred to as model (A). In the experiments discussed in this work, however, only 1 H are present, and once polarized, 1 H can only be repolarized from direct interactions with the electrons, under µw irradiation. Therefore, it is assumed that the nuclear spin magnetization density at point r, m z (r, t), builds up exponentially towards its stationary value m st 0 (r), taken in this case as the stationary polarization level obtained by DNP, with the time constant γ n,ee :