Treatment of background in γ-γ fast-timing measurements

Background characterisation in gamma-gamma fast-timing measurements is of essential importance when lifetimes of the order of tens of picoseconds are being measured. In this work, the nature, composition and behaviour of the timing background is extensively discussed and an adaptation of the background subtraction method used in E-gamma-E-gamma-E-gamma cubes to the case of E-gamma-E-gamma-Delta T cubes, is presented. This is applied to Cf-252 fission data, showing very low peak-to-background ratios (less than 0.5), collected using a hybrid array made of 51 HPGe detectors from Gammasphere and 25 LaBr3(Ce) scintillators. Two different procedures are suggested: the "Interpolation" approach and the "Three Samples" approach. Both were used to measure the lifetime of the 2(+) state in Ru-110, and gave tau = 483(38) ps and tau = 445(34) ps, respectively, both within one standard deviation of the literature value of tau(l) = 462(29) ps. The 2(+) state in Pd-114 was also measured using the three samples approach and the lifetime obtained was tau = 104(12) ps, consistent with the literature lifetime of tau(l) = 118(20) ps.