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We report on systematic γ - γ fast-timing measurements by using four cylindrical 1.5 ′ ′ × 1. 5 ′ ′ LaBr 3 (Ce) scintillator detectors which were installed in compact geometry around the focal plane of the Lohengrin fission-fragment separator at the Institut Laue–Langevin in Grenoble, France. Unconventional γ -ray sources as 185Os and 187W were produced by thermal-neutron activation to provide nearly prompt low-energy γ and K-X rays with average γ - and X-ray multiplicity equal to two. Due to practically no contribution of Compton background, highly precise results of time-walk measurements down to 40 keV are presented. Timing deviations related to different phenomena have been investigated, such as the geometry of an extended γ -ray source and the detector arrangement, long-term timing shifts and the timing contributions of the Compton background and the inter-detector Compton-scattering. The geometrical timing deviations are shown to be minimized using a multi-element detector array with a centrally symmetric arrangement relative to the center of the focal plane. Time-correction formula are proposed as analytical corrections for long-term time shifts, the time contributions of the Compton background and the position-dependent change of the time walk for cases where the calibration source cannot be placed at the center of the focal plane. [ABSTRACT FROM AUTHOR]