  Nitrogen-vacancy (NV) centers in millimeter-scale diamond samples were
produced by irradiation and subsequent annealing under varied conditions. The
optical and spin relaxation properties of these samples were characterized
using confocal microscopy, visible and infrared absorption, and optically
detected magnetic resonance. The sample with the highest NV- concentration,
approximately 16 ppm = 2.8 x 10^{18} cm^{-3}, was prepared with no observable
traces of neutrally-charged vacancy defects. The effective transverse spin
relaxation time for this sample was T2* = 118(48) ns, predominately limited by
residual paramagnetic nitrogen which was determined to have a concentration of
52(7) ppm. Under ideal conditions, the shot-noise limited sensitivity is
projected to be ~150 fT/\sqrt{Hz} for a 100 micron-scale magnetometer based on
this sample. Other samples with NV- concentrations from .007 to 12 ppm and
effective relaxation times ranging from 27 to 291 ns were prepared and
characterized.
