Recoil-ion detection efficiency for complex β decays studied using the Beta-decay Paul Trap

Beta-delayed neutron emission is being studied by detecting the β particles and recoiling ions emerging from the Beta-decay Paul Trap. For β decays to the ground state or γ-emitting states of the daughter nucleus, the fraction of recoiling ions which reach the ion detector in coincidence with a β particle has been determined for 134, 135Sb, 137, 138, 140I, and 144, 145Cs. This value is needed for the determination of the β-delayed neutron emission branching ratio solely from the recoil-ion time-of-flight (TOF) spectrum. The β-particle energy and recoil-ion TOF spectra were used to constrain a simple decay model, which can be used to determine the detection efficiency. The method is compared to simulations to estimate the uncertainty introduced by incomplete knowledge of the decay pattern. By fitting the simulation results to several β-ion coincidence properties measured during the experiment, the fraction of ions which reach the microchannel plate detector can be determined to within ±4%. This result opens the possibility of using the recoil-ion TOF spectra for high precision β-delayed neutron branching-ratio measurements.