Development and performance of a 129-GHz dynamic nuclear polarizer in an ultra-wide bore superconducting magnet

Objective: We sought to build a dynamic nuclear polarization system for operation at 4.6 T (129 GHz) and evaluate its efficiency in terms of ¹³C polarization levels using free radicals that span a range of ESR linewidths.

Materials and methods: A liquid helium cryostat was placed in a 4.6 T superconducting magnet with a 150-mm warm bore diameter. A 129-GHz microwave source was used to irradiate ¹³C enriched samples. Temperatures close to 1 K were achieved using a vacuum pump with a 453-m³/h roots blower. A hyperpolarized ¹³C nuclear magnetic resonance (NMR) signal was detected using a saddle coil and a Varian VNMRS console operating at 49.208 MHz. Samples doped with free radicals BDPA (1,3-bisdiphenylene-2-phenylallyl), trityl OX063 (tris{8-carboxyl-2,2,6,6-benzo(1,2-d:4,5-d)-bis(1,3)dithiole-4-yl}methyl sodium salt), galvinoxyl ((2,6-di-tert-butyl-α-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy), 2,2-diphenylpicrylhydrazyl (DPPH) and 4-oxo-TEMPO (4-Oxo-2,2,6,6-tetramethyl-1-piperidinyloxy) were assayed. Microwave dynamic nuclear polarization (DNP) spectra and solid-state ¹³C polarization levels for these samples were determined.

Results: ¹³C polarization levels close to 50 % were achieved for [1-¹³C]pyruvic acid at 1.15 K using the narrow electron spin resonance (ESR) linewidth free radicals trityl OX063 and BDPA, while 10–20 % ¹³C polarizations were achieved using galvinoxyl, DPPH and 4-oxo-TEMPO.

Conclusion: At this field strength free radicals with smaller ESR linewidths are still superior for DNP of ¹³C as opposed to those with linewidths that exceed that of the ¹H Larmor frequency.