  We report a study of the relaxation time of the restoration of the resistive
superconducting state in single crystalline boron-doped diamond using
amplitude-modulated absorption of (sub-)THz radiation (AMAR). The films grown
on an insulating diamond substrate have a low carrier density of about
2.5x10^{21} cm^{-3} and a critical temperature of about 2 K. By changing the
modulation frequency we find a high-frequency rolloff which we associate with
the characterstic time of energy relaxation between the electron and the phonon
systems or the relaxation time for nonequilibrium superconductivity. Our main
result is that the electron-phonon scattering time varies clearly as T^{-2},
over the accessible temperature range of 1.7 to 2.2 K. In addition, we find,
upon approaching the critical temperature T_c, evidence for an increasing
relaxation time on both sides of T_c.
