Ultrahigh nitrogen-vacancy center concentration in diamond

High concentration of negatively charged nitrogen-vacancy ($\text{NV}^{-}$) centers was created in diamond single crystals containing approximately 100 ppm nitrogen using electron and neutron irradiation subsequent thermal annealing a stepwise manner. Continuous wave paramagnetic resonance (EPR) used to determine the transformation efficiency from isolated N atoms $\text{NV}^{-}$ each production step its highest value as high 17.5 %. Charged vacancies are formed after shown by EPR spectra, but restores sample quality defect signal diminishes. We find that about 25 % form NVs during process. The large allows observe orientation dependent spin-relaxation times also determination hyperfine quadrupole coupling constants with precision spin echo (ESE) electron-nuclear double (ENDOR). observed associated so-called W16 centers, whose spectroscopic properties might imply dimer-vacancy center for origin.