One-Neutron Removal Measurement Reveals ${}^{24}\mathbf{O}$ as a New Doubly Magic Nucleus

The first measurement of the momentum distribution for one-neutron removal from ${}^{24}\mathrm{O}$ at $920A\mathrm{MeV}$ performed at GSI, Darmstadt is reported. The observed distribution has a width (FWHM) of $99\pm 4\mathrm{MeV}/c$ in the projectile rest frame and a one-neutron removal cross section of $63\pm 7\mathrm{mb}$. The results are well explained with a nearly pure $2s_{1/2}$ neutron spectroscopic factor of $1.74\pm 0.19$ within the eikonal model. This large $s$-wave probability shows a spherical shell closure thereby confirming earlier suggestions that ${}^{24}\mathrm{O}$ is a new doubly magic nucleus.

Figure 1

Schematic setup of the experiment on precise momentum measurements of ${}^{23}\mathrm{O}$ fragments after neutron removal in a secondary target placed at the central focal plane of the FRS, GSI.

Figure 2

The longitudinal momentum distribution data for ${}^{24}\mathrm{O}\to {}^{23}\mathrm{O}$ (filled circles) and the momentum resolution (open circles). The dash-dotted line shows the calculated distribution for neutron knocked out from $1d_{5/2}$ orbital. The solid, dashed, and dotted lines show calculated distributions for neutrons removed from $2s_{1/2}$ orbital with $S=1.74$, 1.93, 1.55, respectively.