Dynamic nuclear polarization techniques for magnetic resonance imaging and particles targets experiments

Within a collaboration with the Sample Environment and Polarised Targets (SEPT) group at Paul Scherrer Institut (PSI), we developed two Dynamic Nuclear Polarization (DNP) machines operating at   1 K. The first one is working at an electron spin resonance frequency of 94 GHz and the other is an upgrade to 140 GHz, with an improvement of about 50% in polarization. The DNP mechanism we take advantage of relies on the transfer of polarization from the highly thermally polarized electron spin of TEMPO (2,2,6,6-Tetramethylpiperidine-1-oxyl) to the surrounding nuclear spins. The equipment we developed allows to bring the dynamically polarized frozen sample to room temperature and to inject it into a living rodent or in a standard tube standing in the nuclear magnetic resonance (NMR) magnet within   4 – 6 seconds. NMR signal enhancement factors on the order of 10000 have been routinely observed. The solid state DNP data were treated with a set of equations that (semi-quantitatively) describe the stationary state of the DNP spin system under microwave irradiation. The situation we were confronted with had not been discussed before in the literature since the existing theories were restricted to high temperature approximation or strong saturation limit. Taking advantage of the high electron spin polarization of the photo-excited triplet state of pentacene, we developed an X-band "Triplet DNP" polarizer (operating in a field of 0.3 T and at a temperature of   100 K), and performed DNP in pentacene doped naphthalene samples. In the late eighties, W.Th. Wenckebach invented the technique using single crystals, and more recently, M. Takeda showed it is also quite effective on powder ground from a crystal. We show that this approach also works well in samples obtained by rapidly cooling a melt (a "glass") and we describe some features of our apparatus. Glassy samples could eventually host molecules of interest. We thus show that the scope of application of the method can be broadened and that "Triplet DNP" has the potential to become an interesting tool in solid state NMR and chemistry.


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