  We present an experimental study of the longitudinal electron-spin relaxation
time (T1) of negatively charged nitrogen-vacancy (NV) ensembles in diamond. T1
was studied as a function of temperature from 5 to 475 K and magnetic field
from 0 to 630 G for several samples with various NV and nitrogen
concentrations. Our studies reveal three processes responsible for T1
relaxation. Above room temperature, a two-phonon Raman process dominates, and
below, we observe an Orbach-type process with an activation energy, 73(4) meV,
which closely matches the local vibrational modes of the NV center. At yet
lower temperatures, sample dependent cross relaxation processes dominate,
resulting in temperature independent values of T1, from ms to minutes. The
value of T1 in this limit depends sensitively on magnetic field and can be
tuned by more than an order of magnitude.
