Experiment

The nucleus $^{156}$Gd was produced by using the fusion-evaporation reaction $^{154}$Sm($\alpha$,2n) and then studied by using the JUROGAM $\gamma$-ray detector, Ref. [6], at Jyväskylä. The optimal bombarding energy (27 MeV) was deduced from the excitation function measured for this reaction at the Orsay Tandem during a pilot experiment. This bombarding energy enabled us to optimize the population at low and medium spins in $^{156}$Gd and to minimize the contaminations from other channels (e.g. mainly $^{155}$Gd) below 8%. In this experiment, 43 Anti-Compton suppressed HP-Ge detectors were used, giving a total photopeak efficiency of 4.2%. We used self-supporting, 99.2% enriched, $^{154}$Sm targets with a thickness of 2 mg/cm$^{2}$. The acquisition was performed by both analogue and digital system in triggerless mode. The TNT2 digital acquisition cards from the IPHC, Strasbourg, were used to record data from prompt gamma-ray emissions from the Germanium detectors. The digital acquisition allows a higher count-rate (up to 100 kHz) due to shorter deadtime [7]. The digitization of the ADC pulse via the Jordanov algorithm [8] provides a stable energy measurement and fast baseline restoration. These features provide access to a wider range of beam intensities and therefore to phenomena with lower cross sections. At a similar count-rate, the digital acquisition records 36% more statistics than the analogue system and shows a better linearity in energy, specifically under 300 keV. In our study, a total of $228 \times 10^{6}$ $\gamma\gamma\gamma$ coincidence-events have been collected (i.e. pure unfolded coincidences after Compton-suppresion).