On the Hunt for the Most Efficient Paramagnetic Center for the Dnp Process

Achieving high dynamic polarizations using the Dynamical Nuclear Polarization (DNP) process in a paramagnetically doped solid is a much more difficult task for deuterons in comparison to protons, a fact which is mainly attributed to the small magnetic moment of the deuteron. But from the concept of spin temperature¹, which is the most successful theoretical approach to the DNP process, it may be seen that this problem can be circumvented by the use of a suitable paramagnetic center. If the centers are produced by irradiating the solid with ionizing particles, the DNP process even benefits from the weak magnetic environment of the free electron. Amorphously frozen beads of perdeuterated butanol has been irradiated with 20 MeV electrons at liquid argon temperature and subsequently polarized in the Bochum dilution refrigerator. Deuteron polarizations up to -54% at 2.5 T and up to -71% at 5.0 T have been reached. A nuclear relaxation time of about 200 hours have been observed under low field and low temperature conditions showing the suitability of the material for frozen spin experiments.

The radiation induced center could be identified by EPR studies of different partially deuterated alcohols to be a hydrogen vacancy at a regular position in the molecule. These studies have been also extended to other corbonhydrate compounds like polyethylene as well as to alkanes of shorter molecular lengths. In particular in the case of polyethylene a paramegntic center could be produced, which is not only DNP active, but also possess a long term stability at room temperature.