Infoscience

Thesis

Methods to hyperpolarize nuclear spins via dissolution and sublimation DNP at high magnetic field

Dynamic Nuclear Polarization (DNP) is, nowadays, the most well-established and widespread Nuclear Magnetic Resonance (NMR) hyperpolarization technique. With respect to other methods, the main advantage of DNP is, with no doubt, its versatility. In theory each NMR active nuclear spin species can be polarized via DNP, allowing to obtain enhanced NMR signals from samples in the solid state, liquid state and gas state. Thus its utility ranges from material science to biomedicine. The DNP process efficiency is strongly influenced by experimental conditions (temperature and magnetic field) as well as samples preparation. The fine tuning of these features is often asserted as the key point for practical purposes, where it is desirable to obtain a nuclear polarization as high as possible in a relatively short amount of time. The aim of the present thesis work is to look into some of these aspects, trying to understand up to which point the technique can be improved. Big emphasis is given to the study of the phenomenon at high magnetic field, one of the current hot topics within the DNP community. Moreover the feasibility for the production of hyperpolarized Xe-129 gas is studied, showing that sublimation-DNP represents a promising alternative to the well-established Spin Exchange Optical Pumping (SEOP), when the sample preparation is carefully optimized. The most interesting and innovative part of the thesis is centered around the investigation of a new kind of photo-induced non-persistent radicals. Their use as DNP polarizing agents was tested on several nuclei (C-13, Xe-129, Li-6, H-1) in unfavorable hyperpolarized (HP) solution transfer conditions. It was indeed demonstrated that, the natural recombination of the paramagnetic centers, when the temperature is increased, helps in preserving the highly out-of-equilibrium nuclear spin order of the dissolved sample. Moreover, we believe the UV-radicals unique property of self-quenching could represent the first step towards the development of a simple way to handle and store nuclear polarization, by means of the generation of highly polarized radical-free solid samples.

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