The mirror symmetric centroid difference method for picosecond lifetime measurements via coincidences using very fast LaBr3(Ce) scintillator detectors

The ultra-fast timing technique was introduced in the 1980s and is capable of measuring picosecond lifetimes of nuclear excited states with about 3 ps accuracy. Very fast scintillator detectors are connected to an electronic timing circuit and detector vs. detector time spectra are analyzed by means of the centroid shift method. The very good 3% energy resolution of the nowadays available LaBr3(Ce) scintillator detectors for γ -rays has made possible an extension of the well-established fast timing technique. The energy dependent fast timing characteristics or the prompt curve, respectively, of the LaBr3(Ce) scintillator detector has been measured using a standard 152Eu γ -ray source. For any energy combination in the range of 200 keV < E γ < 1500 keV , the γ – γ fast timing characteristics is calibrated as a function of energy with an accuracy of 2–4 ps. An extension of the centroid shift method providing very attractive features for picosecond lifetime measurements is presented. The mirror symmetric centroid difference method takes advantage of the symmetry obtained when performing γ – γ lifetime measurements using a pair of almost identical very fast scintillator detectors. In particular cases, the use of the mirror symmetric centroid difference method also allows the direct determination of picosecond lifetimes, hence without the need of calibrating the prompt curve.