  NV centers in diamond recommend themselves as good sensors of environmental
fields as well as detectors of diamond impurities. In particular, C-centers,
often also called ${{p}_{1}}$-centers, can be detected via double
electron-electron resonance. This resonance can be used to measure the C-center
concentration. Here, we measured the concentration of C-centers in several
diamond plates and investigated the influence of the free precession time of
the NV center on the observed contrast in the measured double electron-electron
resonance spectrum. The dependence of the resonance amplitudes and widths on
the concentration of C-centers as well as the length of the combined C-center
driving and NV-center $\pi$-pulse is also discussed. The optimal contrast-free
precession time was determined for each C-center concentration, showing a
strong correlation with both the concentration of C-centers and the NV-center
${{T}_{2}}$ time.
