  We report depth-resolved photoluminescence measurements of nitrogen-vacancy
(NV$^-$) centers formed along the tracks of swift heavy ions (SHIs) in type Ib
synthetic single crystal diamonds that had been doped with 100 ppm nitrogen
during crystal growth. Analysis of the spectra shows that NV$^-$ centers are
formed preferentially within regions where electronic stopping processes
dominate and not at the end of the ion range where elastic collisions lead to
formation of vacancies and defects. Thermal annealing further increases NV
yields after irradiation with SHIs preferentially in regions with high vacancy
densities. NV centers formed along the tracks of single swift heavy ions can be
isolated with lift-out techniques for explorations of color center qubits in
quasi-1D registers with an average qubit spacing of a few nanometers and of
order 100 color centers per micrometer along 10 to 30 micrometer long
percolation chains.
