Recoil distance method lifetime measurements of the 2⁺₁ excited states in ⁸⁴Kr and ⁹⁴Sr

Intense re-accelerated beams delivered by the Isotope Separator and Accelerator (ISAC-II) facility at TRIUMF, Canada’s national laboratory for particle and nuclear physics, permit access to nuclear structure information for a wide range of radionuclides via in-beam γ-ray spectroscopy with the TRIUMF-ISAC Gamma-Ray Escape Suppressed Spectrometer (TIGRESS), a high-efficiency and Compton-suppressed segmented high-purity germanium (HPGe) detector array. Electromagnetic transition rates measured via Doppler-shift lifetime techniques such as the recoil distance method (RDM) are recognized as a sensitive probe of collective behavior and shape deformation and can be used to discriminate between model calculations. To take advantage of this opportunity, the TIGRESS Integrated Plunger (TIP) has been constructed at Simon Fraser University (SFU). The TIP infrastructure supports Doppler-shift lifetime measurements via the RDM using a 24-element TIP CsI(Tl) wall for charged-particle identification. A commissioning experiment aimed towards a high-precision measurement of the lifetime of the 2⁺₁→ 0⁺₁ transition in ⁸⁴Kr was performed using Coulomb excitation (Coulex) coupled with an RDM lifetimem easurement of the stable ⁸⁴Kr beam. A rare isotope beam (RIB) experiment was also performed to measure the lifetime of the 2⁺₁→ 0⁺₁ transition as well as the reduced transition probability B(E2; 2⁺₁ → 0⁺₁) in ⁹⁴Sr near the shape transition region around A = 100 and N = 60. A Monte Carlo code for simulating γ-ray spectra has been coupled with a likelihood ratio χ2Λ data analysis method in order to determine the lifetime in the two presented experiments. The device, experimental approach, analysis, and results are presented and discussed.