AbstractA novel Monte Carlo technique has been developed to determine lifetimes of excited states in the tens-to-hundreds femtoseconds range in products of low-energy heavy-ion binary reactions, with complex velocity distributions. The method is based on a detailed study of Doppler-broadened $γ $ γ -ray lineshapes. Its relevance is demonstrated in connection with the $^{18}\text {O}(7.0\, \text {MeV/u})+\,^{181}\text {Ta}$ 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 $^{17}\text {O}$ 17 O and $^{19}\text {O}$ 19 O , with known lifetimes, are used to validate the method over the $∼ 20{-}400\,\text {fs}$ ∼ 20 - 400 fs lifetime-sensitivity range. Emphasis is given to the unprecedented position resolution provided by $γ $ γ -tracking arrays, which turns out to be essential for reaching the required accuracy in Doppler-shift correction. The technique is anticipated to be an important tool for lifetime investigations in exotic neutron-rich nuclei, produced with intense ISOL-type beams.