Congratulations to Dr. Thomas Redpath on his recent publication describing the MoNA segemented target system and his measurement of the two neutron decay of the 26O ground state!
The study of neutron-unbound systems using invariant mass spectroscopy is often performed using low-intensity radioactive ion beams. Low reaction yields can be countered by using thick targets but at the expense of larger uncertainties in the reconstructed invariant mass. The MoNA Collaboration developed a new segmented target designed to address this trade-off. It is composed of three ∼4 mm thick passive beryllium targets interleaved between four 140 μm thick position sensitive silicon detectors. Energy loss measurements from the silicon detectors allow the reaction/decay vertex to be localized to one of the beryllium segments which enables the fragment momentum to be measured with a higher accuracy than for a single thick beryllium target. In the first experiment to use this new system the three-body decay energy of two-neutron unbound 26O was measured. The improvement over a single thick target in decay energy resolution is demon- strated for the 18 keV 26O ground state resonance.
Figure 1: The decay energy spectrum for 24O + 2n was reconstructed to demonstrate the ability of the segmented target to preserve the resolution of the decay energy measurement. In both panels the measured three body decay energy spectra are plotted as black points with red error bars. The smooth curves represent spectra reconstructed from simulations of the 26O ground state decay (red curves) and the decay of an 26O excited state (see inset in top panel); the black curve is the sum of the two simulations. In the top panel, the energy loss measurements from the silicon detectors are used to determine in which beryllium target the reaction occured event-by-event. This information informs the reconstruction of the fragment momentum. In the bottom panel, all events are reconstructed assuming the reaction occured in the middle beryllium target.