Direct population and lifetime measurements of the 2₁+ and 4₁+ states in 40Ca using an alpha-transfer reaction

At TRIUMF, Canada's particle accelerator centre, the TIGRESS Integrated Plunger (TIP) and its configurable detector systems have been used for charged-particle tagging and lightion identification in Doppler-shift lifetime measurements using gamma-ray spectroscopy with the TIGRESS array of HPGe detectors. An experiment using these devices to measure the lifetime of the 2₁+ and 4₁+ states of 40Ca has been performed by projecting an 36Ar beam onto a ⁿᵃᵗ.C target. Analysis of the experimental gamma-ray spectra confirmed the direct population of these states. Kinematics of the reaction mechanism were identified using Monte-Carlo simulations, which also enabled the use of charged-particle correlations to select reactions that populated a specific excited state in the 40Ca immediately after its production. Selection of the 2₁+ state with this additional sensitivity further eliminated feeding cascades, and therefore restricted the decay kinetics predominantly to first order. Direct population of the 2₁+ state was achieved with a direct-population-to-feeding ratio of 21(±2) : 1. The 2₁+ lifetime that was measured in this experiment was 42 ± 5 fs, and the 4₁+ lifetime was 270 ± 10 fs. Both uncertainties quoted here are statistical only.