Hyperpolarizing Gases via Dynamic Nuclear Polarization and Sublimation

A high throughput method was designed to produce hyperpolarized gases by combining low-temperature dynamic nuclear polarization with a sublimation procedure. It is illustrated by applications to ${}^{129}\mathrm{Xe}$ nuclear magnetic resonance in xenon gas, leading to a signal enhancement of 3 to 4 orders of magnitude compared to the room-temperature thermal equilibrium signal at 7.05 T.

Figure 1

${}^{129}\mathrm{Xe}$ polarization buildup measured at 1.2 K in $B_0=5\mathrm{T}$. The gray dashed curve corresponds to the function $P(t)=P_{\mathrm{thermal}}+P(t=\infty )[1-\exp (-t/{\tau }_{\mathrm{DNP}})]$ fitted to the data. The observed characteristic time constant is ${\tau }_{\mathrm{DNP}}=141\pm 7\mathrm{s}$. A sketch representing the thermodynamical description of DNP is shown: the microwave irradiation pumps entropy ($S$) from the nuclear spins to the electron spins from where it is dumped to the liquid helium bath. The overall result is a “cooling” of the nuclear spins, i.e., an increase of their polarization above the value expected from the bath temperature.

Figure 2

Sketch representing the six-step procedure to hyperpolarize ${}^{129}\mathrm{Xe}$ via DNP and sublimation. The temperature, the pressure, as well as the external magnetic field $B_0$ are specified for each step.

Figure 3

Light gray (red): Fourier transform of a hyperpolarized ${}^{129}\mathrm{Xe}$ free induction decay following a single 90° pulse. The pressure of the hyperpolarized gas was 5 kPa above atmospheric pressure. Dark gray (blue): Fourier transform (multiplied by a factor 128) of the free induction decay from a fully relaxed xenon gas sample pressurized to 1.1 MPa averaged over 128 experiments with 90° pulses separated by relaxation intervals of 100 s. The acquisitions were performed at 7.05 T and $25°\mathrm{C}$.