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