Low-temperature EPR spectra of synthetic diamond and type Ib nanodiamond

Low-frequency resonance studies of single crystal diamond and type Ib nanodiamond (powder) unirradiated and irradiated by protons at temperatures of 300 and 500 K are reported. EPR spectra typical of paramagnetic nitrogen centers are observed. It is shown that when synthetic diamond crystals are irradiated by protons, additional lines appear in the EPR spectra which can be attributed to radiation induced nitrogen-vacancy centers (NV defects).