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Hyperpolarized Xenon for NMR and MRI Applications
Published on: September 6, 2012
Cluster formation restricts dynamic nuclear polarization of xenon in solid mixtures
N N Kuzma1, M Pourfathi, H Kara
1Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA. kuzma@upenn.edu
The Journal of Chemical Physics
|September 18, 2012
Summary
Dynamic nuclear polarization (DNP) using xenon-129 NMR reveals spontaneously formed xenon clusters. This method identifies overheating and spin-diffusion bottlenecks, improving polarization by four-fold.
Area of Science:
- Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy
- Dynamic Nuclear Polarization (DNP) techniques
- Quantum spin dynamics and relaxation
Background:
- Solid xenon-129 NMR spectra show a single peak broadened by neighboring protons under DNP conditions.
- Annealing induces a second peak, indicative of pure xenon clusters formed spontaneously.
- Microwave irradiation enables control over both positive and negative polarization states.
Purpose of the Study:
- To investigate the formation and properties of xenon clusters during DNP.
- To model the time evolution of xenon-129 polarization within these clusters.
- To identify and mitigate factors limiting DNP efficiency, such as sample overheating and spin diffusion.
Main Methods:
- Solid-state dynamic nuclear polarization (DNP) experiments at 1.5 K and 5 T.
- Nuclear Magnetic Resonance (NMR) spectroscopy of xenon-129.
- Spherical-cluster modeling to analyze spin diffusion and relaxation dynamics.
- In situ evaluation of DNP parameters and sample temperature.
Main Results:
- Observation of pure xenon clusters via a distinct NMR peak after annealing.
- Demonstration of tunable polarization (positive/negative) for both homogeneous mixture and clusters.
- Identification of a spin-diffusion bottleneck at cluster boundaries and significant microwave-induced sample overheating.
- Quantification of a four-fold increase in xenon-129 polarization (5.3% to 21%) after system optimization.
Conclusions:
- The spherical-cluster model serves as a sensitive tool for in situ DNP parameter evaluation.
- Microwave irradiation can cause significant sample overheating, impacting DNP efficiency.
- Optimizing DNP systems to reduce overheating dramatically enhances polarization.
- Spontaneous xenon cluster formation provides insights into spin dynamics and DNP limitations.
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