Recoil distance method lifetime measurements at TRIUMF-ISAC using the TIGRESS Integrated Plunger

Abstract

The TIGRESS Integrated Plunger device (TIP) has been developed for recoil distance method (RDM) lifetime measurements using the TIGRESS array of HPGe γ-ray detectors at TRIUMF’s ISAC-II facility. A commissioning experiment was conducted utilizing a 250 MeV 84Kr beam at ≈2×108 particles per second. The 84Kr beam was Coulomb excited to the 21+ state on a movable 27Al target. A thin Cu foil fixed downstream from the target was used as a degrader. Excited nuclei emerged from the target and decayed by γ-ray emission at a distance determined by their velocity and the lifetime of the 21+ state. The ratio of decays which occur between the target and degrader to those occurring after traversing the degrader changes as a function of the target–degrader separation distance. Gamma-ray spectra at 13 target–degrader separation distances were measured and compared to simulated lineshapes to extract the lifetime. The result of τ=5.541±0.013(stat.)±0.063(sys.) ps is shorter than the literature value of 5.84±0.18 ps with a reduction in uncertainty by a factor of approximately two. The TIP plunger device, experimental technique, analysis tools, and result are discussed.