  Fluorescent nanodiamonds containing negatively-charged nitrogen-vacancy
(NV$^-$) centers are promising for a wide range of applications, such as for
sensing, as fluorescence biomarkers, or to hyperpolarize nuclear spins. NV$^-$
centers are formed from substitutional nitrogen (P1 centers) defects and
vacancies in the diamond lattice. Maximizing the concentration of NVs is most
beneficial, which justifies the search for methods with a high yield of
conversion from P1 to NV$^-$. We report here the characterization of surface
cleaned fluorescent micro- and nanodiamonds, obtained by irradiation of
commercial diamond powder with high-energy (10 MeV) electrons and simultaneous
annealing at 800{\deg}C. Using this technique and increasing the irradiation
dose, we demonstrate the creation of NV$^-$ with up to 25 % conversion yield.
Finally, we monitor the creation of irradiation-induced spin-1 defects in
microdiamond particles, which we associate with W16 and W33 centers, and
investigate the effects of irradiation dose and particle size on the coherence
time of NV$^-$.
