Development of a New High-Resolution Neutron Detector and Beta-delayed Neutron Spectroscopy of 24O.

An efficient neutron detection system with good energy resolution is needed to correctly characterize the decays of neutron-rich nuclei where beta-delayed neutron emission is a dominant decay mode. Precision neutron spectroscopy probes nuclear structure effects in neutron-rich nuclei and is essential to exploit the opportunities in new-generation radioactive beam facilities. A new high-resolution neutron detector, Neutron dEtector with Xn Tracking (NEXT), has been constructed, characterized, and tested in decay and reaction experiments. Its essential capability is the neutron interaction position localization, which enables improvement in energy resolution without compromising detection efficiency in the time-of-flight measurements. Neutron-gamma discrimination capability of NEXT allows for performing experiments even in high background conditions. First measurements were performed with beta-delayed neutron emitters using NEXT at Argonne National Laboratory (ANL), focussing on fission fragments. At National Superconducting Cyclotron Laboratory (NSCL), NEXT was used alongside the Versatile Array of Neutron Detectors at Low Energy (VANDLE) to study light drip-line nuclei at and below the island of inversion with first-ever neutron spectroscopy performed for several isotopes.

The neutron energy spectrum measurement of the beta-delayed neutron precursor 24O was performed for the first time at NSCL using two different neutron detector arrays: VANDLE and NEXT, accompanied by gamma spectroscopy. The beta-gamma and beta- delayed neutron measurements following the decay of 24O provided the beta decay strength distribution extending to neutron unbound states in 24F, serving as an excellent case to test the nuclear model calculations near the neutron drip line. The experimental results are compared with the shell model calculations using the standard USDA and USDB interactions and IMSRG.