Measurement of the one-neutron removal momentum distribution characterizes 24O as a new doubly magic nucleus

First results are reported on the momentum distribution after one-neutron removal from 24O at 920 MeV/u. The investigated isotopes were produced by projectile fragmentation of a 1 GeV/u 48Ca beam in a beryllium production target placed at the entrance of the FRagment Separator (FRS). They were separated and identified in-flight at the mid-focal plane of the separator, where a carbon reaction target was placed. Due to the high resolution of the FRS, operated in dispersion matched-mode, precise momentum measurements of secondary fragments could be performed. The secondary fragments produced in the C target were detected at the final achromatic focal plane of the FRS thus providing the measurements independently of the large momentum spread of the primary fragments. The 23O momentum distribution in the projectile rest frame and the one-neutron removal cross section can be well explained by using an eikonal model with a nearly pure 2s1/2 occupation probability. This large s-wave probability indicates a spherical shell closure at N=16, thereby experimentally confirming earlier suggestions that 24O is a new doubly magic nucleus.