Breakdown of theZ=8Shell Closure in UnboundO12and its Mirror Symmetry

Abstract

An excited state in the proton-rich unbound nucleus 12 O was identified at 1.8(4) MeV via missing-mass spectroscopy with the 14 Oðp; tÞ reaction at 51 AMeV. The spin-parity of the state was determined to be 0 þ or 2 þ by comparing the measured differential cross sections with distorted-wave calculations. The lowered location of the excited state in 12 O indicates the breakdown of the major shell closure at Z ¼ 8 near the proton drip line. This demonstrates the persistence of mirror symmetry in the disappearance of the magic number 8 between 12 O and its mirror partner 12 Be. DOI: 10.1103/PhysRevLett.103.152503 PACS numbers: 25.40.Hs, 21.10.Hw, 24.10.Eq, 27.20.+n Symmetry and its breaking have played an important role in physics. The CP violation in particle physics led to the discovery of the third generation of quarks [1]; superconductivity of solid states is a manifestation of the gauge symmetry breaking in electron motion [2].Mirror symmetry in atomic nuclei is a unique feature of the two-fermionic quantum system comprised of protons and neutrons. Because of the charge invariance of the nuclear force, ''mirror'' nuclei, a pair of nuclei where numbers of protons and neutrons are interchanged, show a marked similarity in their level schemes. However, the presence of the strong Coulomb field in the proton-rich mirror partner can degrade the symmetry, enhancing complex but rich aspects of the finite system.The increasing availability of radioactive ion (RI) beams opens new possibilities to test mirror symmetry among nuclei with large isospin even beyond the drip lines. The isospin degree of freedom of the nuclear shell structure makes a sharp contrast to other quantum systems such as the quantum dots [3] and the metal clusters [4], where the electromagnetic or spatial degree of freedom is employed in manipulating the shell structure. Recent experimental studies on exotic nuclei have shown that the conventional magic numbers disappear in the neutron-rich regions at N ¼ 8, 20 and 28 [5][6][7][8][9][10][11], and possibly in superheavy elements [12]. Theoretical works point to various underlying mechanisms in terms of the isospin-dependent part of the nuclear effective interaction [13,14], reduction of the spin-orbit potential [15], coupling to the continuum [16], and deformation [17] or clustering [18,19]. However, the validity of the mirror symmetry of these effects at extreme conditions of isospin and binding energies remains an open question, limiting predictions for very proton-rich nuclei. The present Letter presents a study of shell quenching at Z ¼ 8 in the proton-unbound nucleus 12 O. Mirror symmetry in the shell quenching phenomena between 12 8 O 4 and its mirror partner 12 4 Be 8 is investigated experimentally from the low-lying excitation properties.We studied the structure of 12 O via missing-mass spectroscopy with the 14 Oðp; tÞ reaction at 51 AMeV. The systematics of the low-lying excited states in even-even nuclei provides a sensitive probe for the evolution of the shell structure. In 12 Be, the a...

Two-proton decay of12O and its isobaric analog state in12N

“…A broad excited state is also observed at E T =3.606 MeV (E * =1.968 MeV). This might be same state observed by Suzuki et al [13], although Sherr and Fortune suggest that both a 2 + and a 0 + state are expected near this energy [26]. Clearly the statisti- cal uncertainties are not sufficient to determine whether this structure is a single or doublet.…”

“…2(a). For the 2p+ 10 C events, there was contamination from two previously unknown levels in 13 O (created via inelastic excitation of the projectile) which undergo sequential two-proton decay with E T =0.9 and 1.6 MeV. The latter is more intense and also has the same E T value as 12 O g.s.…”

“…The isobaric analog state of 12 O g.s. in 12 N was observed for the first time, produced via proton knockout reactions from the 13 O beam. While single-nucleon decay of this state cannot conserve both energy and isospin, twoproton decay to the isobaric analog in 10 B does.…”

“…itself is also not well understood. The experimental values of 400(250) keV [12], * Permanent address: INFN, Laboratori Nazionali del Sud, Catania, Italy 578(205) keV [1], and 600(500) keV [13] are all large and required significant corrections due to the detector response. However, Sherr and Fortune [14] quote an unpublished value of less than 100 keV from double-chargeexchange studies using π + ions [15].…”

“…Following neutron knockout from a 13 O beam, 12 O fragments were created and the three decay products following two-proton decay were detected. A new ground-state mass was determined by the invariant mass method implying a decay kinetic energy of 1.638 (24) MeV, and the width was found to be less than 72 keV.…”