The modulation effect on the dynamic polarization of nuclear spins

We report on the discovery of a large gain in the dynamic nuclear polarization (DNP) of the 1.5 l target of SMC after application of microwave (MW) frequency modulation (FM). In the glassy deuterated butanol material doped with a paramagnetic EDBA-chromium (V) complex the maximum polarization increases by 65% with FM while in the case of proton material the increase was found to be 10%. Also, the polarization build-up time decreased for the deuterated material by a factor of almost two. The effect appears at temperatures around 0.3 K with a modulation frequency 10–103 Hz and an amplitude of about 30 MHz. Bolometric measurements of the MW energy inside the cavity show that FM gives rise to additional absorption of MW energy which depends on the modulation amplitude and frequency and which is more pronounced in the edges of the EPR absorption line.