  A novel Monte Carlo technique has been developed to determine lifetimes of
excited states in the tens-to-hundreds femtoseconds range. The method is
applied to low-energy heavy-ion binary reactions populating nuclei with complex
velocity distributions. Its relevance is demonstrated in connection with the
$^{18}$O(7.0 MeV/u) + $^{181}$Ta experiment, performed at GANIL with the
AGATA+VAMOS+PARIS setup, to study neutron-rich O, C, N, ... nuclei. Excited
states in $^{17}$O and $^{19}$O, with known lifetimes, are used to validate the
method over the $\sim$20-400 fs lifetime-sensitivity range. Emphasis is given
to the unprecedented position resolution provided by $\gamma$-tracking arrays,
which turns out to be essential for reaching the required accuracy in
Doppler-shift correction, at the basis of the detailed analysis of $\gamma$-ray
lineshape and resulting state lifetime determination. The technique is
anticipated to be an important tool for lifetime investigations in exotic
neutron-rich nuclei, produced with intense ISOL-type beams.
