Study of neutron unbound states in ¹⁸O by β-delayed neutron decay of ¹⁸N

The neutron-unbound states in ¹⁸O have been investigated in various types of experiments, however, the data are still in disagreement with each other. The latest β-delayed neutron data in 2005 showed a large missing neutron intensity. We have performed an experiment to measure the β-delayed neutron decay of ¹⁸N for the unbound states in ¹⁸O with a newly developed TOF detector system consisting of large and small plastic scintillators for a wide neutron energy range (100 keV- 4 MeV), which was designed for spectroscopy of very neutron-rich nuclei far from the stability.

The radioactive nucleus ¹⁸N was produced at RCNP, Osaka University by the direct reaction of ⁹Be(¹⁸O,¹⁸N)⁹B at 9.4 MeV/u. The secondary ¹⁸N beam was isotope-separated by the fragment separater and transported to a gold stopper foil. The ¹⁸N beam intensity of ~1300 pps with purity of ~98% was obtained with the primary beam intensity of 3.2 eμA. The β ray and the γ ray were detected by thin plastic scintillators and Ge detectors, respectively.

With such system we could find a new low-energy neutron peak (E_n~355 keV) with large intensity, which may solve the missing neutron intensity problem. Search for other small peaks is in progress. A revised decay scheme of ¹⁸N is presented based on β-n-γ coincidence relation.