The Generalized Centroid Difference method for lifetime mesaurements via $\gamma$-$\gamma$ coincidences using large fast-timing arrays
Résumé
A novel method for direct electronic "fast-timing" lifetime measurements of nuclear excited states
via γ-γ coincidences using an array equipped with N very fast high-resolution LaBr3(Ce) scintillator detectors
is presented. The generalized centroid difference method provides two independent “start” and “stop” time
spectra obtained without any correction by a superposition of the N(N − 1)/2 calibrated γ-γ time difference
spectra of the N detector fast-timing system. The two fast-timing array time spectra correspond to a forward
and reverse gating of a specific γ-γ cascade and the centroid difference as the time shift between the centroids
of the two time spectra provides a picosecond-sensitive mirror-symmetric observable of the set-up. The energydependent
mean prompt response difference between the start and stop events is calibrated and used as a single
correction for lifetime determination. These combined fast-timing array mean γ-γ zero-time responses can be
determined for 40 keV < Eγ < 1.4 MeV with a precision better than 10 ps using a 152Eu γ-ray source. The
new method is described with examples of (n,γ) and (n,f,γ) experiments performed at the intense cold-neutron
beam facility PF1B of the Institut Laue-Langevin in Grenoble, France, using 16 LaBr3(Ce) detectors within
the EXILL&FATIMA campaign in 2013. The results are discussed with respect to possible systematic errors
induced by background contributions.