Measurement of the Neutron-Decay Lifetime of the 26O Ground State

The ground state of the two-neutron unbound nucleus 26 O has been speculated to have a rather long lifetime, in the picosecond regime, which would introduce potentially the first observation of a two-neutron radioactive decay. Previous measurements were able to place the lifetime at 6.5+4.6−4.8 ps (Z. Kohley et al., PRL, 110:152501, 2013.) and 7.2+3.5−4.1 ps (T. Redpath, PhD thesis, MSU, 2019.), though with large uncertainties. In order to determine the decay lifetime of the 26O ground state with high sensitivity and precision, a new method has been applied. The experiment presented in this work was performed at the Superconducting Analyzer for MUlti-particles from RAdio Isotope Beams (SAMURAI) at the Rare Isotope Beam Factory (RIBF) at RIKEN, Japan. A 27F beam was produced in the fragment separator BigRIPS and impinged on a specially designed target, consisting of tungsten and platinum sheets, where 26O was produced via a proton-knockout reaction. Following its lifetime, 26O decayed into the 24O fragment and two neutrons all measured in coincidence. The observable used in the measurement, which is sensitive to the lifetime, was the velocity difference between the fragment and the neutrons. The ratio between the number of decays occurring inside and outside of the target sheets changes according to the lifetime, strongly affecting the shape of the spectrum. Thus, the measured velocity difference delivers a characteristic spectrum from which the lifetime can be extracted. The calibration and analysis techniques used in this work are presented in detail. In particular, a dedicated GEANT4 simulation was performed, where the full reaction process is modeled from the experimental setup with the lifetime as an input parameter. The proton knockout as well as the decay are considered, where the fragment recoil momentum is treated realistically according to theoretical calculations. Finally, the lifetime is extracted from the measured spectrum by comparison to simulated spectra. With this approach, a new upper limit on the 26O lifetime could be determined with τ < 2.8 ps at 5σ confidence level. A lifetime of τ = 0 ps lies within the 1σ uncertainties (τ < 0.9 ps) and therefore the observation of a two-neutron radioactivity of 26O cannot be confirmed

A new Time-of-flight detector for the R 3 B setup

© 2022, The Author(s).We present the design, prototype developments and test results of the new time-of-flight detector (ToFD) which is part of the R3B experimental setup at GSI and FAIR, Darmstadt, Germany. The ToFD detector is able to detect heavy-ion residues of all charges at relativistic energies with a relative energy precision σΔE/ ΔE of up to 1% and a time precision of up to 14 ps (sigma). Together with an elaborate particle-tracking system, the full identification of relativistic ions from hydrogen up to uranium in mass and nuclear charge is possible.11Nsciescopu