TPEN: a triple-foil differential plunger for lifetime measurements of excited states in exotic nuclei

A Triple-foil differential Plunger for Exotic Nuclei (TPEN) has been developed to measure the lifetimes of excited states in nuclei with small production cross-sections. TPEN utilises one target foil and two degrader foils to make differential lifetime measurements: directly determining the decay function and its derivative at a single plunger distance setting. The direct measurement of the decay function and its derivative removes the requirement to measure γ-ray intensities at several target-to-degrader distances, thereby reducing the beam-time required relative to a conventional plunger with a single-degrader foil. This paper describes the commissioning of TPEN in the lifetime measurement of the first excited 4⁺ state in ¹⁵⁶Dy using this differential lifetime technique. The 46(2) ps measured lifetime was found to be in good agreement with the 45.6(5) ps deduced from a previous high-statistics experiment using a conventional two-foil plunger. A comparison between the differential mode with TPEN, and that using TPEN as a conventional two-foil plunger reveals that it achieves the same statistical accuracy with a 4-fold reduction in beam-time in the differential mode, for the reaction and experimental setup used in this work. TPEN opens up new opportunities to study exotic nuclei with lower production cross-sections that were previously not accessible.