  Pulsed ESR experiments are reported for ensembles of negatively-charged
nitrogen-vacancy centers (NV$^-$) in diamonds at X-band magnetic fields
(280-400 mT) and low temperatures (2-70 K). The NV$^-$ centers in synthetic
type IIb diamonds (nitrogen impurity concentration $<1$~ppm) are prepared with
bulk concentrations of $2\cdot 10^{13}$ cm$^{-3}$ to $4\cdot 10^{14}$ cm$^{-3}$
by high-energy electron irradiation and subsequent annealing. We find that a
proper post-radiation anneal (1000$^\circ$C for 60 mins) is critically
important to repair the radiation damage and to recover long electron spin
coherence times for NV$^-$s. After the annealing, spin coherence times of T$_2
= 0.74$~ms at 5~K are achieved, being only limited by $^{13}$C nuclear spectral
diffusion in natural abundance diamonds. At X-band magnetic fields, strong
electron spin echo envelope modulation (ESEEM) is observed originating from the
central $^{14}$N nucleus. The ESEEM spectral analysis allows for accurate
determination of the $^{14}$N nuclear hypefine and quadrupole tensors. In
addition, the ESEEM effects from two proximal $^{13}$C sites (second-nearest
neighbor and fourth-nearest neighbor) are resolved and the respective $^{13}$C
hyperfine coupling constants are extracted.
