  We present systematic measurements of longitudinal relaxation rates ($1/T_1$)
of spin polarization in the ground state of the nitrogen-vacancy (NV$^-$) color
center in synthetic diamond as a function of NV$^-$ concentration and magnetic
field $B$. NV$^-$ centers were created by irradiating a Type 1b single-crystal
diamond along the [100] axis with 200 keV electrons from a transmission
electron microscope with varying doses to achieve spots of different NV$^-$
center concentrations. Values of ($1/T_1$) were measured for each spot as a
function of $B$.
