The sudden change of the neutron dripline from 24O (N=16) to 31F (N=22), called oxygen anomaly, is one of the exotic phenomena. Recent theoretical studies suggest importance of three nucleon forces on the binding energies of the oxygen isotopes, especially for N>16, while available experimental data are limited because the measurement requires production of extremely neutron rich nuclei.
The region of the oxygen anomaly is also interesting in terms of the shell evolution. It is well known that the shell closure of the N=20 nuclei disappears in the island of inversion. Recent in-beam gamma-ray spectroscopy suggests that the N=20 shell gap is quenched at 29F. The experimental study of 28O is strongly desired to clarify the shell evolution along N=20 isotonic chain down to Z=8.
The SAMURAI21 collaboration studied 27O and 28O with SAMURAI spectrometer at RIKEN-RIBF. These unbound nuclei are produced by two- and one-proton removal reaction on a liquid hydrogen target from 29Ne and 29F, respectively. Decay products, 24O and neutrons, are detected in coincidence to reconstruct the invariant mass of the 27O and 28O. The experimental results will be discussed in the presentation.