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At the Institut Laue-Langevin in Grenoble, germanium-gated γ-γ fast-timing lifetime measurements of nuclear excited states in neutron-rich nuclei have been performed within a prompt γ-ray spectroscopy experimental campaign. We report on results obtained from the cold-neutron induced fission of 235 U. The excited secondary fission products were stopped almost instantaneously within the thick target and the γ rays emitted were collected triggerlessly using the EXILL&FATIMA mixed array of HPGe and LaBr 3 (Ce) detectors. Precise lifetimes could be determined by analysing the γ-γ time difference spectra using the generalized centroid difference method. This picosecondsensitive method provides many advantages and is briefly explained. Still, the major source of systematic errors is related to the contribution of time-correlated Compton background. The EXILL&FATIMA results are discussed with respect to the typical energy-dependent timing behaviour of the background. According to the time response of the background, appropriate methods and a time correction for the sub-nanosecond regime are proposed.

This is an Open Access article distributed under the terms of the Creative Common Attribution Licence 4.0 which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.; International audience; A novel method for direct electronic "fast-timing" lifetime measurements of nuclear excited statesvia γ-γ coincidences using an array equipped with N very fast high-resolution LaBr3(Ce) scintillator detectorsis presented. The generalized centroid difference method provides two independent “start” and “stop” timespectra obtained without any correction by a superposition of the N(N − 1)/2 calibrated γ-γ time differencespectra of the N detector fast-timing system. The two fast-timing array time spectra correspond to a forwardand reverse gating of a specific γ-γ cascade and the centroid difference as the time shift between the centroidsof the two time spectra provides a picosecond-sensitive mirror-symmetric observable of the set-up. The energydependentmean prompt response difference between the start and stop events is calibrated and used as a singlecorrection for lifetime determination. These combined fast-timing array mean γ-γ zero-time responses can bedetermined for 40 keV < Eγ < 1.4 MeV with a precision better than 10 ps using a 152Eu γ-ray source. Thenew method is described with examples of (n,γ) and (n,f,γ) experiments performed at the intense cold-neutronbeam facility PF1B of the Institut Laue-Langevin in Grenoble, France, using 16 LaBr3(Ce) detectors withinthe EXILL&FATIMA campaign in 2013. The results are discussed with respect to possible systematic errorsinduced by background contributions.