A Millikelvin Thermometer

Phantom IRM

(a) Hyperpolarized experimental 2H spectrum of a sample consisting of 300 μL of 60% ethanol-d6, 30% D2O, 10% H2O, and 40 mM TEMPOL, obtained with exorcycled β−τ−β−τ quadrupolar echoes with β =18° in a field B0 = 6.7 T where ν0(1H) = 285.3 MHz and ν0(2H) = 43.8 MHz. The asymmetry between the “horns” of the quadrupolar Pake pattern is highlighted by a blue arrow. (b) Numerical simulations assuming different spin temperatures Tspin. The case that agrees best with the experiment in (a) is highlighted in blue, corresponding to Tspin = 3.5 ± 0.5 mK or P(Iz) = 29 ± 4%.

Dynamic nuclear polarization at low temperatures, typically 1.2 < Tsample < 4.2 K, allows one to achieve spin temperatures of as low as Tspin = 2 mK. This leads to characteristic asymmetries in powder spectra. The line shapes due to the quadrupolar couplings of deuterium spins present in virtually all solvents used for such experiments (“DNP juice”) allow the quick yet accurate determination of the deuterium spin temperature or, equivalently, the deuterium polarization. The observation of quadrupolar echoes excited by small flip-angle pulses allows one to monitor the build-up and decay of the positive or negative deuterium polarization.

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