  We studied the parameters to optimize the production of negatively-charged
nitrogen-vacancy color centers (NV-) in type~1b single crystal diamond using
proton irradiation followed by thermal annealing under vacuum. Several samples
were treated under different irradiation and annealing conditions and
characterized by slow positron beam Doppler-broadening and photoluminescence
(PL) spectroscopies. At high proton fluences another complex vacancy defect
appears limiting the formation of NV-. Concentrations as high as 2.3 x 10^18
cm^-3 of NV- have been estimated from PL measurements. Furthermore, we inferred
the trapping coefficient of positrons by NV-. This study brings insight into
the production of a high concentration of NV- in diamond, which is of utmost
importance in ultra-sensitive magnetometry and quantum hybrid systems
applications.
