The generalized centroid difference method for picosecond sensitive determination of lifetimes of nuclear excited states using large fast-timing arrays

A novel method for direct electronic "fast-timing" lifetime measurements of nuclear excited states via γ-γγ-γ coincidences using an array equipped with N∈NN∈N equally shaped very fast high-resolution LaBr3(Ce) scintillator detectors is presented. Analogous to the mirror symmetric centroid difference method, the generalized centroid difference method provides two independent "start" and "stop" time spectra obtained by a superposition of the N(N-1)N(N-1)γ-γγ-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-γγ-γ cascade. Provided that the energy response and the electronic time pick-off of the detectors are almost equal, a mean prompt response difference between start and stop events is calibrated and used as a single correction for lifetime determination. These combined fast-timing arrays mean γ-γγ-γ time-walk characteristics can be determined for 40keV<Eγ<1.3MeV with an accuracy less than 5 ps using a 152Eu γ-rayγ-ray source. Due to reduction and cancellation of many possible systematic errors, the lifetime determination limit of the method over the total dynamic range is mainly determined by the statistics. The setup of an N=4 detector fast-timing array delivered an absolute time resolving power of 3 ps for 10 000 γ-γγ-γ events per total fast timing array start and stop time spectrum. The new method is tested over the total dynamic range by the measurements of known picosecond lifetimes in standard γ-rayγ-ray sources.