A. Jarmola, A. Berzins, J. Smits, K. Smits, J. Prikulis, F. Gahbauer, R. Ferber, D. Erts, M. Auzinsh, and D.
We present systematic measurements of longitudinal relaxation rates (1=T1) 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  axis with 200 keV electrons from a transmission electron
microscope with varying doses to achieve spots of different NV– center concentrations. Values of
(1=T1) were measured for each spot as a function of B
Applied Physics Letters 107, 242403 (2015);