γ-γ fast timing with high-performance LaBr3(Ce) scintillators

Abstract

We present a review of the electronic $\gamma$-$\gamma$ “fast-timing” technique in combination with LaBr${}_3$(Ce) scintillator detectors. The $\gamma$-$\gamma$ fast-timing technique has increased in popularity since the commercial introduction of the LaBr${}_3$(Ce) scintillators in 2005. The use of LaBr${}_3$(Ce) for measurements of lifetimes of nuclear excited states has rapidly spread out over the world and also the setups have grown from a few detectors to large-scale fast-timing arrays. The LaBr${}_3$(Ce) is one of the fastest scintillators available with good relative energy resolution of about 3%. Due to high energy selectivity, lifetimes of nuclear excited states down to 1 ps in the best case can be determined directly via electronic $\gamma$-$\gamma$ time-difference measurements. The use of the high-performance LaBr${}_3$(Ce) detectors made it possible to systematically investigate the $\gamma$-$\gamma$ fast-timing technique over the total dynamic range corresponding to $\gamma$-ray energies of 40 keV up to 6.8 MeV with precision of 2(1) ps. A non-linear energy-dependent time difference between the signals of full-energy peak and Compton events is given. Related to this finding, a new procedure to calibrate the time response of full-energy peak events has been introduced as well as time-correction formulae to account for the Compton contributions in the total experimental $\gamma$-$\gamma$ time-difference distribution. We present a review of the $\gamma$-$\gamma$ fast-timing technique including the performance of the LaBr${}_3$(Ce) detectors, the electronic timing principles, the methods to analyze the experimental $\gamma$-$\gamma$ time-difference distributions and the possible energy-dependent time deviations that can be observed using the $\gamma$-$\gamma$ fast-timing technique with many LaBr${}_3$(Ce) detectors. The use of a centrally symmetric detector arrangement with respect to the center of an extended $\gamma$-ray emission area reduces any possible energy-dependent time shifts rapidly to negligible values with the number of detectors. Moreover, a transition from the conventional analog to the digital timing technique is observed, worldwide. We present the promising results of nowadays available digitizers, where the programmable timing algorithm is used onboard to extract timing information with comparable or even better accuracy than the use of analog-electronic timing modules.