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.