$N=16$ Spherical Shell Closure in ${}^{24}\mathbf{O}$

The unbound excited states of the neutron drip-line isotope ${}^{24}\mathrm{O}$ have been investigated via the ${}^{24}\mathrm{O}(p,p^{\prime }){}^{23}\mathrm{O}+n$ reaction in inverse kinematics at a beam energy of $62\mathrm{MeV}/\mathrm{nucleon}$. The decay energy spectrum of ${}^{24}\mathrm{O}^{*}$ was reconstructed from the momenta of ${}^{23}\mathrm{O}$ and the neutron. The spin parity of the first excited state, observed at $E_x=4.65\pm 0.14\mathrm{MeV}$, was determined to be $J^{\pi }=2^{+}$ from the angular distribution of the cross section. Higher-lying states were also observed. The quadrupole transition parameter ${\beta }_2$ of the $2_1^{+}$ state was deduced, for the first time, to be $0.15\pm 0.04$. The relatively high excitation energy and small ${\beta }_2$ value are indicative of the $N=16$ shell closure in ${}^{24}\mathrm{O}$.

Figure 1

Schematic view of the experimental setup.

Figure 2

(a) Coincidence yield of ${}^{23}\mathrm{O}$ and a neutron in decay energy and (b) the cross section, $d\sigma /d{E}_{\mathrm{decay}}$. The error bars are statistical only. The (shaded) red solid line and blue dashed line histograms are the results of fits for resonances at $E_{\mathrm{decay}}=0.56$ and 1.06 MeV, respectively. The dotted Gaussian function at $E_{\mathrm{decay}}\sim 3.2\mathrm{MeV}$ represents higher-lying state(s). The dot-dashed line corresponds to the nonresonant continuum [19]. The black solid line histogram is the sum of all the contributions.

Figure 3

Angular distributions for the resonances at $E_{\mathrm{decay}}=0.56$ (filled circles) and 1.06 MeV (open circles in the inset). The error bars are statistical only. The curves represent the results of microscopic (black curves) and phenomenological (red dashed and red dot-dashed curves) DWBA calculations (see text).

Figure 4

Neutron number dependencies of (a) $E_x(2_1^{+})$ and (b) ${\beta }_2(2_1^{+})$. The present results are shown by the filled circles. The $E_x(2_1^{+})$ data for $N=10–14$ were taken from Refs. 6, 39. The ${\beta }_2$ data for $N=10–14$ were taken from Refs. 7, 40. The dotted lines represent the USDB shell-model predictions [24].