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 gamma-gamma coincidences using an array equipped with N is an element of 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) gamma-gamma 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 gamma-gamma 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 gamma-gamma time-walk characteristics can be determined for 40 keV < E-gamma < 1.3 MeV with an accuracy less than 5 ps using a Eu-152 gamma-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 gamma-gamma 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 gamma-ray sources. (C) 2013 Elsevier B.V. All rights reserved