  We reveal an interplay between temperature and radical concentration
necessary to establish thermal mixing (TM) as an efficient dynamic nuclear
polarization (DNP) mechanism. We conducted DNP experiments by hyperpolarizing
widely used DNP samples, i.e., sodium pyruvate-1-$^{13}$C in water/glycerol
mixtures at varying nitroxide radical (TEMPOL) concentrations and microwave
irradiation frequencies, measuring proton and carbon-13 spin temperatures.
Using a cryogen consumption-free prototype-DNP apparatus, we could probe
cryogenic temperatures between 1.5 and 6.5 K, i.e., below and above the boiling
point of liquid helium. We identify two mechanisms for the breakdown of TM: (i)
Anderson type of quantum localization for low radical concentration, or (ii)
quantum Zeno localization occurring at high temperature. This observation
allowed us to reconcile the recent diverging observations regarding the
relevance of TM as a DNP mechanism by proposing a unifying picture and,
consequently, to find a trade-off between radical concentration and electron
relaxation times, which offers a pathway to improve experimental DNP
performance based on TM.
