(color online) Plots of NV concentration versus electron irradiation dose. In samples S9–S11, which have initial nitrogen impurity concentrations of about 200 ppm, NV concentrations are roughly proportional to the irradiation dose. In samples S6–S8, whose initial nitrogen impurity concentrations are about 1 ppm. The NV concentration increases only about 30% as the irradiation dose increases 9 times. This suggests that the NV concentration is saturated at these doses for single electron irradiation.