  We present an experimental study of the longitudinal electron-spin relaxation
of ensembles of negatively charged nitrogen-vacancy (NV ) centers in diamond.
The measurements were performed with samples having different NV-
concentrations and at different temperatures and magnetic fields. We found that
the relaxation rate T1-1 increases when transition frequencies in NV- centers
with different orientations become degenerate and interpret this as
cross-relaxation caused by dipole-dipole interaction.
