Neutron-rich Oxygen Isotopes Research Articles

Nuclear Forces and Their Impact on Neutron-Rich Nuclei and Neutron-Rich Matter

We review the impact of nuclear forces on matter at neutron-rich extremes. Recent results have shown that neutron-rich nuclei become increasingly sensitive to three-nucleon forces, which are at the forefront of theoretical developments based on effective field theories of quantum chromodynamics. These developments include the formation of shell structure, the spectroscopy of exotic nuclei, and the location of the neutron drip line. Nuclear forces also constrain the properties of neutron-rich matter, including the neutron skin, the symmetry energy, and the structure of neutron stars. First, we review our understanding of three-nucleon forces and show how chiral effective field theory makes unique predictions for many-body forces. Then, we survey results with three-nucleon forces in neutron-rich oxygen and calcium isotopes and neutron-rich matter, which have been explored with a range of many-body methods. Three-nucleon forces therefore provide an exciting link between theoretical, experimental, and observational nuclear physics frontiers.

Searching for isovector signatures in the neutron-rich oxygen and calcium isotopes

We search for potential isovector signatures in the neutron-rich oxygen and calcium isotopes within the framework of a relativistic mean-field theory with an exact treatment of pairing correlations. To probe the isovector sector we calibrate a few relativistic density functionals using the same isoscalar constraints but with one differing isovector assumption. It is found that under certain conditions, the isotopic chain in oxygen can be made to terminate at the experimentally observed 24O isotope and in the case of the calcium isotopes at 60Ca. To produce such behavior, the resulting symmetry energy must be soft, with predicted values for the symmetry energy and its slope at saturation density being J=(30.92±0.47) MeV and L=(51.0±1.5) MeV, respectively. As a consequence, the neutron-skin thickness of 208Pb is rather small: Rskin208=(0.161±0.011) fm. This same model—labeled “FSUGarnet”—predicts R1.4=(13.0±0.1) km for the radius of a “canonical” 1.4M⊙ neutron star, yet is also able to support a two-solar-mass neutron star.

Present status of nuclear cluster physics and experimental perspectives

Knowledge on nuclear cluster physics has increased considerably as nuclear clustering remains one of the most fruitful domains of nuclear physics, facing some of the greatest challenges and opportunities in the years ahead. The occurrence of "exotic" shapes in light N = Z α-like nuclei and the evolution of clustering from stability to the drip-lines are being investigated more and more accurately both theoretically and experimentally. Experimental progresses in understanding these questions were recently examined and will be further revisited in this introductory talk: clustering aspects are, in particular, discussed for light exotic nuclei with a large neutron excess such as neutron-rich Oxygen isotopes with their complete spectrocopy.

Ab InitioCoupled-Cluster Effective Interactions for the Shell Model: Application to Neutron-Rich Oxygen and Carbon Isotopes

We derive and compute effective valence-space shell-model interactions from ab initio coupled-cluster theory and apply them to open-shell and neutron-rich oxygen and carbon isotopes. Our shell-model interactions are based on nucleon-nucleon and three-nucleon forces from chiral effective-field theory. We compute the energies of ground and low-lying states, and find good agreement with experiment. In particular, our computed 2(+) states are consistent with N = 14,16 shell closures in (22,24)O, and a weaker N=14 shell closure in (20)C. We find good agreement between our coupled-cluster effective-interaction results with those obtained from standard single-reference coupled-cluster calculations for up to eight valence neutrons.

Peripheral elastic and inelastic scattering ofO17,18on light targets at 12 MeV/nucleon

A study of interaction of neutron-rich oxygen isotopes $^{17,18}\mathrm{O}$ with light targets has been undertaken in order to determine the optical potentials needed for the transfer reaction $^{13}\mathrm{C}(^{17}\mathrm{O},^{18}\mathrm{O})^{12}\mathrm{C}$. Optical potentials in both incoming and outgoing channels have been determined in a single experiment. This transfer reaction was used to infer the direct capture rate to the $^{17}\mathrm{F}(p,\ensuremath{\gamma})^{18}\mathrm{Ne}$ which is essential to estimate the production of $^{18}\mathrm{F}$ at stellar energies in ONe novae. The success of the asymptotic normalization coefficient (ANC) as indirect method for astrophysics is guaranteed if the reaction mechanism is peripheral and the distorted wave Born approximation cross-section calculations are warranted and stable against the optical model potential (OMP) used. We demonstrate the stability of the ANC method and the OMP results by using good-quality elastic and inelastic-scattering data with stable beams before extending the procedures to rare-ion beams. The peripherality of our reaction is inferred from a semiclassical decomposition of the total-scattering amplitude into barrier and internal barrier components. Comparison between elastic scattering of $^{17}\mathrm{O}$, $^{18}\mathrm{O}$, and $^{16}\mathrm{O}$ projectiles is made.

Ab Initio Coupled-Cluster Effective Interactions for the Shell Model: Application to Neutron-Rich Oxygen and Carbon Isotopes

Ab Initio Coupled-Cluster Effective Interactions for the Shell Model: Application to Neutron-Rich Oxygen and Carbon Isotopes

Beyond the neutron drip line: The unbound oxygen isotopes25O and26O

The very neutron-rich oxygen isotopes 25O and 26O are investigated experimentally and theoret- ically. In this first R3B-LAND experiment, the unbound states are populated at GSI via proton- knockout reactions from 26F and 27F at relativistic energies around 450 MeV/nucleon. From the kinematically complete measurement of the decay into 24O plus one or two neutrons, the 25O ground- state energy and lifetime are determined, and upper limits for the 26O ground state are extracted. In addition, the results provide evidence for an excited state in 26O at around 4 MeV. The ex- perimental findings are compared to theoretical shell-model calculations based on chiral two- and three-nucleon (3N) forces, including for the first time residual 3N forces, which are shown to be amplified as valence neutrons are added.

Clustering effects in reactions induced by light nuclei

Since the pioneering discovery of 12C+12C molecular resonances half a century ago, a great deal of research work has been undertaken in the α-clustering study. Our knowledge on the physics of nuclear molecules has increased considerably and nuclear clustering remained one of the most fruitful domains of nuclear physics, facing some of the greatest challenges and opportunities in the years ahead. In this work, the occurrence of "exotic" shapes in light N = Z α-like nuclei is investigated. Various approaches of the superdeformed and hyperdeformed bands associated with quasimolecular resonant structures are presented. Clustering aspects are also briefly discussed for light nuclei with neutron excess trough very recent results on neutron-rich Oxygen isotopes.

Study of two-neutron radioactivity in the decay of 26O.

A new technique was developed to measure the lifetimes of neutron unbound nuclei in the picosecond range. The decay of 26O→24O+n+n was examined as it had been predicted to have an appreciable lifetime due to the unique structure of the neutron-rich oxygen isotopes. The half-life of 26O was extracted as 4.5(-1.5)(+1.1)(stat)±3(syst) ps. This corresponds to 26O having a finite lifetime at an 82% confidence level and, thus, suggests the possibility of two-neutron radioactivity.

Chiral three-nucleon forces and bound excited states in neutron-rich oxygen isotopes

We study the spectra of neutron-rich oxygen isotopes based on chiral two- and three-nucleon interactions. First, we benchmark our many-body approach by comparing ground-state energies to coupled-cluster results for the same two-nucleon interaction, with overall good agreement. We then calculate bound excited states in 21, 22, 23O , focusing on the role of three-nucleon forces, in the standard sd shell and an extended \( sdf_{7/2}p_{3/2}\) valence space. Chiral three-nucleon forces provide important one- and two-body contributions between valence neutrons. We find that both these contributions and an extended valence space are necessary to reproduce key signatures of novel shell evolution, such as the N = 14 magic number and the low-lying states in 21O and 23O , which are too compressed with two-nucleon interactions only. For the extended space calculations, this presents first work based on nuclear forces without adjustments. Future work is needed and open questions are discussed.

Continuum Effects and Three-Nucleon Forces in Neutron-Rich Oxygen Isotopes

We employ interactions from chiral effective field theory and compute binding energies, excited states, and radii for isotopes of oxygen with the coupled-cluster method. Our calculation includes the effects of three-nucleon forces and of the particle continuum, both of which are important for the description of neutron-rich isotopes in the vicinity of the nucleus 24O. Our main results are the placement of the neutron drip line at 24O, the assignment of spins, parities and resonance widths for several low-lying states of the drip line nucleus, and an efficient approximation that incorporates the effects of three-body interactions.

Measurement of Unbound Excited States of 24O

The unbound excited states of the most neutron-rich oxygen isotope, (24)O (Z = 8, N = 16), have been investigated using the (24)O(p,p')(24)O*-->(23)O+n reaction at RIKEN, where a 63 MeV/neuclon secondary beam of (24)O was produced in a Be production target by the projectile-like fragmentation of a 95 MeV/u (40)Ar primary beam. The first 2(+) excited state was observed in the invariant mass spectrum. This experiment; will give us a deeper understanding of the new magic number at N = 16 in the neutron-rich oxygen isotopes.

Quenching of Spectroscopic Factors for Proton Removal in Oxygen Isotopes

We present microscopic coupled-cluster calculations of the spectroscopic factors for proton removal from the closed-shell oxygen isotopes (14,16,22,24,28)O with a chiral nucleon-nucleon interaction at next-to-next-to-next-to-leading order. We include coupling-to-continuum degrees of freedom by using a Hartree-Fock basis built from a Woods-Saxon single-particle basis. This basis treats bound and continuum states on an equal footing. We find a significant quenching of spectroscopic factors in the neutron-rich oxygen isotopes, pointing to enhanced many-body correlations induced by strong coupling to the scattering continuum above the neutron emission thresholds.

Ab initiocoupled-cluster approach to nuclear structure with modern nucleon-nucleon interactions

We perform coupled-cluster calculations for the doubly magic nuclei 4He, 16O, 40Ca and 48Ca, for neutron-rich isotopes of oxygen and fluorine, and employ "bare" and secondary renormalized nucleon-nucleon interactions. For the nucleon-nucleon interaction from chiral effective field theory at order next-to-next-to-next-to leading order, we find that the coupled-cluster approximation including triples corrections binds nuclei within 0.4 MeV per nucleon compared to data. We employ interactions from a resolution-scale dependent similarity renormalization group transformations and assess the validity of power counting estimates in medium-mass nuclei. We find that the missing contributions due to three-nucleon forces are consistent with these estimates. For the unitary correlator model potential, we find a slow convergence with respect to increasing the size of the model space. For the G-matrix approach, we find a weak dependence of ground-state energies on the starting energy combined with a rather slow convergence with respect to increasing model spaces. We also analyze the center-of-mass problem and present a practical and efficient solution.

CLUSTER-ORBITAL SHELL MODEL APPROACH FOR STUDY OF EXOTIC NUCLEI NEAR THE NEUTRON DRIP-LINE

We have developed an m -scheme cluster-orbital shell model approach to study neutron- and proton-rich nuclei toward the drip-lines. We discuss the property of the neutron-rich oxygen isotopes with analyzing the relation between the energy and r.m.s. radius.

Ab initiocomputation of neutron-rich oxygen isotopes

We compute the binding energy of neutron-rich oxygen isotopes and employ the coupled-cluster method and chiral nucleon-nucleon interactions at next-to-next-to-next-to-leading order with two different cutoffs. We obtain rather well-converged results in model spaces consisting of up to 21 oscillator shells. For interactions with a momentum cutoff of 500 MeV, we find that $^{28}\mathrm{O}$ is stable with respect to $^{24}\mathrm{O}$, while calculations with a momentum cutoff of 600 MeV result in a slightly unbound $^{28}\mathrm{O}$. The theoretical error estimates due to the omission of the three-nucleon forces and the truncation of excitations beyond three-particle--three-hole clusters indicate that the stability of $^{28}\mathrm{O}$ cannot be ruled out from ab initio calculations, and that three-nucleon forces and continuum effects play the dominant role in deciding this question.

Neutron transition strengths of21+states in the neutron-rich oxygen isotopes determined from inelastic proton scattering

A coupled-channel analysis of the $^{18,20,22}\mathrm{O}(p,{p}^{'})$ data has been performed to determine the neutron transition strengths of the ${2}_{1}^{+}$ states in oxygen targets, using the microscopic optical potential and inelastic form factor calculated in the folding model. A complex density- and isospin-dependent version of the CDM3Y6 interaction was constructed, based on the Brueckner-Hartree-Fock calculation of nuclear matter, for the folding model input. Given an accurate isovector density dependence of the CDM3Y6 interaction, the isoscalar (${\ensuremath{\delta}}_{0}$) and isovector (${\ensuremath{\delta}}_{1}$) deformation lengths of the ${2}_{1}^{+}$ states in $^{18,20,22}\mathrm{O}$ have been extracted from the folding model analysis of the $(p,{p}^{'})$ data. A specific $N$ dependence of ${\ensuremath{\delta}}_{0}$ and ${\ensuremath{\delta}}_{1}$ has been established which can be linked to the neutron shell closure occurring at $N$ approaching 16. The strongest isovector deformation was found for the ${2}_{1}^{+}$ state in $^{20}\mathrm{O}$, with ${\ensuremath{\delta}}_{1}$ about 2.5 times larger than ${\ensuremath{\delta}}_{0}$, which indicates a strong core polarization by the valence neutrons in $^{20}\mathrm{O}$. The ratios of the neutron/proton transition matrix elements (${M}_{n}/{M}_{p}$) determined for the ${2}_{1}^{+}$ states in $^{18,20}\mathrm{O}$ have been compared with those deduced from the mirror symmetry, using the measured $B(E2)$ values of the ${2}_{1}^{+}$ states in the proton-rich $^{18}\mathrm{Ne}$ and $^{20}\mathrm{Mg}$ nuclei, to discuss the isospin impurity in the ${2}_{1}^{+}$ excitation of the $A=18$, $T=1$ and $A=20$, $T=2$ isobars.

Neutron decay spectroscopy of neutron-rich oxygen isotopes

Neutron decay spectroscopy of neutron-rich oxygen isotopes has been performed using the two-proton knock-out reaction 9Be( 26Ne, X) 24,23,22O. A combination of the knock-out of valence and core protons can explain the three observed spectra. These knock-out processes are selective and preferentially populate hole states. The observed narrow resonance state in 23O at an excitation energy of 2.8(1) MeV was assigned to the 5 / 2 + state.

Fusion of neutron-rich oxygen isotopes in the crust of accreting neutron stars

Fusion reactions in the crust of an accreting neutron star are an important source of heat, and the depth at which these reactions occur is important for determining the temperature profile of the star. Fusion reactions depend strongly on the nuclear charge $Z$. Nuclei with $Z\ensuremath{\leqslant}6$ can fuse at low densities in a liquid ocean. However, nuclei with $Z=8$ or 10 may not burn until higher densities where the crust is solid and electron capture has made the nuclei neutron rich. We calculate the $S$ factor for fusion reactions of neutron rich nuclei including $^{24}\mathrm{O}+^{24}\mathrm{O}$ and $^{28}\mathrm{Ne}+^{28}\mathrm{Ne}$. We use a simple barrier penetration model. The $S$ factor could be further enhanced by dynamical effects involving the neutron rich skin. This possible enhancement in $S$ should be studied in the laboratory with neutron rich radioactive beams. We model the structure of the crust with molecular dynamics simulations. We find that the crust of accreting neutron stars may contain micro-crystals or regions of phase separation. Nevertheless, the screening factors that we determine for the enhancement of the rate of thermonuclear reactions are insensitive to these features. Finally, we calculate the rate of thermonuclear $^{24}\mathrm{O}+^{24}\mathrm{O}$ fusion and find that $^{24}\mathrm{O}$ should burn at densities near ${10}^{11}$ g/cm${}^{3}$. The energy released from this and similar reactions may be important for the temperature profile of the star.

Damping of collective states in an extended random-phase approximation with ground-state correlations

Applications of an extended version of the Hartree-Fock theory and the random-phase approximation derived from the time-dependent density-matrix theory (TDDM) are presented. In this TDDM-based theory, the ground state is given as a stationary solution of the TDDM equations and the excited states are calculated using the small-amplitude limit of TDDM. The first application presented is an extended Lipkin model in which an interaction term describing a particle scattering is added to the original Hamiltonian so that the damping of a collective state is taken into account. It is found that the TDDM-based theory well reproduces the ground state and excited states of the extended Lipkin model. The quadrupole excitation of the oxygen isotopes $^{16,20,22}\mathrm{O}$ is also studied as realistic applications of the TDDM-based theory. It is found that large fragmentation of the giant quadrupole resonance in $^{16}\mathrm{O}$ is reproduced, and it is pointed out that the effects of ground-state correlations are quite important for fragmentation. It is also found that the quadrupole states in neutron-rich oxygen isotopes have small spreading widths.