  The nitrogen-vacancy center in diamond attracts a lot of attention in sensing
applications, mainly for temperature, magnetic field, and rotation
measurements. Nuclear spins of carbon-13 surrounding the nitrogen-vacancy
center can be used as a memory or sensing element. In the current work, a
diamond plate with a relatively large concentration of carbon-13 was
synthesized and examined. The spectrum of optically detected magnetic resonance
was recorded and analyzed in a magnetic field range of 5-200 G. A
strain-independent measurement technique of carbon-13 isotope concentration
based on the analysis of magnetic resonance spectra was developed.
Additionally, narrow features in the spectrum were detected and understood.
