N=16Spherical Shell Closure inO24

Abstract

The unbound excited states of the neutron drip-line isotope 24 O have been investigated via the 24 O(p,p ) 23 O+n reaction in inverse kinematics at a beam energy of 62 MeV/nucleon. The decay energy spectrum of 24 O * was reconstructed from the momenta of 23 O and the neutron. The spinparity of the first excited state, observed at Ex = 4.65 ± 0.14 MeV, was determined to be J π = 2 + from the angular distribution of the cross section. Higher-lying states were also observed. The quadrupole transition parameter β2 of the 2 Otsuka et al. have investigated theoretically the structural evolution of the oxygen isotopes with increasing neutron number (N ) and attributed the development of the shell closure at N = 16 to the strong neutron-proton tensor interaction [10,11].In this Letter we report on the first spectroscopic study of 24 O by proton inelastic scattering. In addition to the excitation energies of the states populated, the wellknown character of proton inelastic scattering also permits the spins-parities, as well as the quadrupole transition parameter (β 2 ) of the first 2 + state to be deduced. As described below, we have been able to provide a firm 2 + assignment for the state at E x = 4.65 ± 0.14 MeV and determine the β 2 , the small value of which is indicative of the spherical closed-shell character of 24 O. A comparison of the E x (2 + 1 ) and β 2 for the chain of oxygen isotopes shows strong evidence for a large shell gap at N = 16.The experiment was performed at the RIPS facility [17] at RIKEN. A schematic view of the downstream section of RIPS and the experimental setup is shown in Fig. 1

Two-neutron sequential decay ofO24

“…First predicted by Goldansky in 1960 [4], two-proton radioactivity has been observed in 45 Fe [5], and genuine threebody decays along with their angular correlations have been observed in many two-proton unbound systems, e.g. 19 Mg, 16 Ne [6] and 6 Be [7]. Analogously, two-neutron unbound systems and their three-body correlations have been measured in several neutron-rich nuclei including 5 O [14], with the last one showing potential for two-neutron radioactivity [15,16].…”

“…So far, there has been no correlation measurement of a state decaying by sequential emission of two neutrons, although evidence for sequential decay has been reported for the high-energy continuum of 14 Be [12]. The structure of 24 O, the heaviest bound isotope for which the neutron drip-line is established [17], has been well studied and there is substantial evidence for the appearance of a new magic number N = 16 [18][19][20][21][22]. Two unbound resonances have been observed in 24 O above the one-neutron separation energy [19,23,24], and there is also evidence for resonances above the two-neutron separation energy [19,23] around 7.5 MeV.…”

Mapping the deformation in the “island of inversion”: Inelastic scattering ofNe30andMg36at intermediate energies

“…These studies have revealed that far away from the line of β stability traditional closed-shell numbers may vanish, while others can emerge [1]. For example, neutron shell quenching occurs around N = 20 [2][3][4][5] and N = 28 [6,7], while neutron numbers N = 16 and N = 32 and 34 have been found to feature doubly closed-shell signatures for 24 O and 52,54 Ca [8][9][10][11]. It has been shown that the creation and disappearance of magic numbers are mutually connected [12], raising the question of their respective region of validity.…”

Beyond the neutron drip line: The unbound oxygen isotopes25O and26O

“…Another striking feature in the oxygen isotopic chain is the doubly magic nature of 22 O and 24 O [6][7][8][9][10][11] in strong contrast to the lighter elements, where the drip line is marked by nuclei exhibiting a loosely bound halo structure. The neutron-rich oxygen isotopes also provide interesting insights, when viewed coming from their stable isotones.…”