States In 16O Research Articles

Probing new physics with underground accelerators and radioactive sources

New light, weakly coupled particles can be efficiently produced at existing and future high-intensity accelerators and radioactive sources in deep underground laboratories. Once produced, these particles can scatter or decay in large neutrino detectors (e.g. Super-K and Borexino) housed in the same facilities. We discuss the production of weakly coupled scalars ϕ via nuclear de-excitation of an excited element into the ground state in two viable concrete reactions: the decay of the 0+ excited state of 16O populated via a (p,α) reaction on fluorine and from radioactive 144Ce decay where the scalar is produced in the de-excitation of 144Nd⁎, which occurs along the decay chain. Subsequent scattering on electrons, e(ϕ,γ)e, yields a mono-energetic signal that is observable in neutrino detectors. We show that this proposed experimental setup can cover new territory for masses 250 keV≤mϕ≤2me and couplings to protons and electrons, 10−11≤gegp≤10−7. This parameter space is motivated by explanations of the “proton charge radius puzzle”, thus this strategy adds a viable new physics component to the neutrino and nuclear astrophysics programs at underground facilities.

Theory for Quartet Condensation in Fermi Systems with Applications to Nuclei and Nuclear Matter

The theory of quartet condensation is further developed. The onset of quartet- ting in homogeneous fermionic matter is studied with the help of an in-medium modified four fermion equation. It is found that at very low density quartetting wins over pairing. At zero temperature, in analogy to pairing, a set of equations for the quartet order parameter is given. Contrary to pairing, quartetting only exists for strong coupling and breaks down for weak coupling. Reasons for this finding are detailed. In an application to nuclear matter, the critical temperature for α particle condensation can reach values up to around 8 MeV. The disappearance of α-particles with increasing density, i.e. the Mott transition, is investigated. In finite nuclei the Hoyle state, that is the 02+ of 12C is identified as an 'α-particle condensate' state. It is conjectured that such states also exist in heavier na-nuclei, like 16O, 20Ne, etc. The sixth 0+ state in 16O is proposed as an analogue to the Hoyle state. The Gross-Pitaevski equation is employed to make an estimate of the maximum number of a particles a condensate state can contain. Possible quartet condensation in other systems is discussed briefly.

Alpha Cluster Structures and Isoscalar Monopole Excitations in Light Nuclei

Isoscalar monopole excitations to cluster states in light nuclei are in general strong as to be comparable with the single particle strength. Their experimental strengths share about 20 % of the sum rule value in the case of 4He, 11B, 12C, and 16O etc. We discuss the isoscalar monopole strength function in 16O, which is typical in light nuclei, is discussed up to Ex ~ 40 MeV. It is found that 1) two different types of monopole excitations exist in 16O; one is the monopole excitation to cluster states which is dominant in the lower energy part, and the other is the monopole excitation of the mean-field type such as one-particle one-hole (1p1h) which is attributed mainly to the higher energy part, and 2) this character of the monopole excitations originates from the fact that the ground state of 16O with the dominant doubly closed shell structure has a duality of the mean-field-type as well as alpha-clustering character.

Probing effect of tensor interactions in 16O via (p, d) reaction

We have measured the 16O(p, d) reaction using 198-, 295- and 392-MeV proton beams to search for a direct evidence on an effect of the tensor interactions in light nucleus. Differential cross sections of the one-neutron transfer reaction populating the ground states and several low-lying excited states in 15O were measured. Comparing the ratios of the cross sections for each excited state to the one for the ground state over a wide range of momentum transfer, we found a marked enhancement of the ratio for the positive-parity state(s). The observation is consistent with large components of high-momentum neutrons in the initial ground-state configurations due to the tensor interactions.

Isoscalar Monopole Excitations and Cluster Structures in Light Nuclei

Isoscalar monopole excitation to cluster states in light nuclei is in general strong as to be comparable with the single particle strength and shares about 20 % of the sum rule value. The isoscalar monopole strength function in 16O is discussed up to \({E_x \lesssim 40 \, {\rm MeV}.}\) We found that (1) two different types of monopole excitations exist in 16O; one is the monopole excitation to cluster states which is dominant in the lower energy part, and the other is the monopole excitation of the mean-field type such as one-particle one-hole (1p1h) which is attributed mainly to the higher energy part, and (2) this character of the monopole excitations originates from the fact that the ground state of 16O with the dominant doubly closed shell structure has a duality of the mean-field-type as well as alpha-clustering character.

Gas-like alpha-cluster states and condensates in nuclei

Gas-like α-cluster states are investigated in l6O with the use of 4α OCM (Orthogonality Condition Model). The 0+6 state in 16O is shown to have a character of a 4α condensate, an analogue to the Hoyle state in 12C. The decay properties of this state and the α+Hoyle-like rotatinal states are also investigated.

Cluster structure and isoscalar monopole excitation in light nuclei

Isoscalar monopole excitation to cluster states in light nuclei is in general strong as to be comparable with the single particle strength and shares about 20 % of the sum rule value. In the present paper, the isoscalar monopole strength function in 16O is discussed up to Ex ≲ 40 MeV as a typical example. We found that 1) two different types of monopole excitations exist in 16O; one is the monopole excitation of cluster states which is dominant in the lower energy part, and the other is the monopole excitation of the mean-field type such as one-particle one-hole (1p1h) which is attributed mainly to the higher energy part, 2) this character of the monopole excitations originates from the fact that the ground state of 16O with the dominant doubly closed shell structure has a duality of the mean-field-type as well as alpha-clustering character, and 3) the monopole strength is much enhanced by the α-type ground state correlation.

Causality constraints for charged particles

In quantum systems with short-range interactions, causality imposes nontrivial constraints on low-energy scattering parameters. We investigate these causality constraints for systems where a long-range Coulomb potential is present in addition to a short-range interaction. The main result is an upper bound for the Coulomb-modified effective range parameter. We discuss the implications of this bound to the effective field theory for nuclear halo systems. In particular, we consider several examples of proton–nucleus and nucleus–nucleus scattering. For the bound-state regime, we find relations for the asymptotic normalization coefficients (ANCs) of nuclear halo states. As an application of these relations, we extract the ANCs of the excited 2+ and 1− states in 16O from α − 12C scattering data.

Alpha-Particle Condensation in Nuclear Systems

The onset of quartetting, i.e. α-particle condensation, in symmetric and asymmetric nuclear matter is studied with the help of an in-medium modified four nucleon equation. It is found that at very low density quartetting wins over pairing, because of the strong binding of the α-particles. The critical temperature can reach values up to around 8 MeV. The disappearance of α-particles with increasing density, i.e. the Mott transition, is investigated. In finite nuclei the Hoyle state, that is the 02+ of 12C is identified as an 'α-particle condensate' state. It is conjectured that such states also exist in heavier nα-nuclei, like 16O, 20Ne, etc. The sixth 0+ state in 16O is proposed as an analogue to the Hoyle state. The Gross-Pitaevski equation is employed to make an estimate of the maximum number of α particles a condensate state can contain. Possible quartet condensation in other systems is discussed briefly.

High-spin resonant states in 16O

α+12C elastic scattering in the energy range Ec.m. = 21.15–26.625 MeV is analyzed by the optical model with parity-dependent potential. The characteristic features enhanced by Coulomb scattering appear to be reproduced. The volume integral of the potential, which indicates the net strength of interactions between nuclei, seems to be consistent with those from other studies and it gives a smooth energy variation from high to low energies. The 9− state is found to occur at Ec.m. ≈ 23.0 MeV, corresponding to the excitation energy Ex ≈ 30.2 MeV. It is possible that the 8+ resonance was observed at Ex ≈ 29 MeV with a large width. The α+12C rotational band can survive even at high excitation energies. The rotational band starting from the 0+2 state (Ex = 6.05 MeV) is confirmed to have N = 8; the odd-parity band, N = 9.

Clustering in nuclei

I stress a role of clustering in describing the structure of such state that is difficult to understand from the mean field. The examples include the neutron-halo of 11Li, 3H+p and 3He+n clustering in 4He and a unified description of the ground and first excited 0+ states of 16O.

Microscopic description of the elusive Hoyle state

Within a symmetry-guided shell-model framework and using a fraction of the model space extended beyond current no-core shell-model limits along with a schematic effective many-nucleon interaction, we gain new insights into the many-body dynamics that give rise to the ground state and low-lying 0+ states of 12C and 16O. In particular, we gain further understanding of the alpha-clustering nature of the challenging Hoyle state and its first 2+ excitation in 12C, as well as the corresponding states in 16O. This provides guidance for ab initio shell models by informing key features of the underlying nuclear structure and interaction.

Absolute decay width measurements in 16O

The reaction 126C(63Li, d)168O* at a 6Li bombarding energy of 42 MeV has been used to populate excited states in 16O. The deuteron ejectiles were measured using the high-resolution Munich Q3D spectrograph. A large-acceptance silicon-strip detector array was used to register the recoil and break-up products. This complete kinematic set-up has enabled absolute α-decay widths to be measured with high-resolution in the 13.9 to 15.9 MeV excitation energy regime in 16O; many for the first time. This energy region spans the 14.4 MeV four-α breakup threshold. Monte-Carlo simulations of the detector geometry and break-up processes yield detection efficiencies for the two dominant decay modes of 40% and 37% for the α+12C(g.s.) and a+12C(2+1) break-up channels respectively.

Phase transitions for excited states in 16O+α → 20Ne within the SACM

We present the cranking of the Semimicroscopic Algebraic Cluster Model (SACM) for two spherical clusters. A geometrical mapping is applied and a discussion on phase transition as a function of the cranking parameter is given. This parameter can be related to the average angular momentum of the nucleus. The particular cluster system considered is 16O+α → 20Ne. We also investigate the phase transition property when the Pauli exclusion principle is not observed. We show that phase transitions may occur within the same dynamical symmetry limit.

Concerning positive-parity collective states in 18O

I discuss collective positive-parity states in 18O in light of two recent experiments. Single-particle widths have been calculated and used to extract spectroscopic factors. Special attention is given to 4p -2h and 6p -4h cluster bands.

Indirect Measurement of Astrophysical 13C(α, n)16O S-Factors

The 13C(α, n)16O reaction is believed to be the main neutron source reaction for the s-process in asymptotic giant branch (AGB) stars. The astrophysical S-factors of this reaction have been determined based on an evaluation of the α spectroscopic factor of the 1/2+ subthreshold state in 17O (Ex = 6.356 MeV) by using the 13C(11B, 7Li)17O α transfer reaction. Our result confirms that the 1/2+ subthreshold resonance is dominant for the 13C(α, n)16O reaction at low energies of astrophysical interest.

Resonance States and Alpha Condensation in 12C and 16O

Low density states near the 3α and 4α breakup thresholds in 12C and 16O, respectively, are discussed in terms of the α-particle condensation. Calculations are performed in Orthogonality Condition Model and Tohsaki–Horiuchi–Schuck–Ropke approaches. The \({0_2^+}\) state in 12C and the \({0_6^+}\) state in 16O are shown to have dilute density structures and give strong enhancement of the occupation of the S-state c.o.m. orbital of the α-particles. The possibility of the existence of α-particle condensed states in heavier nα nuclei is also discussed.

Deducing physical properties of weakly bound states from low-energy scattering data. Application to 16O and 12C+α

Possibilities of deducing the asymptotic normalisation constant Cb of the 21+ subthreshold bound state of 16O from the low-energy 12C+α d-wave scattering phase shifts are studied. Within the potential model, it is shown that such a connection exists in principle, either by studying the range of potentials fitting the phase shifts, or by using the effective range expansion. However, present-day data do not seem sufficient to strongly constrain Cb, as values ranging from 20 to 180 × 103 fm−1/2 are obtained, depending on the order of the chosen effective-range expansion.

Alpha clustering and condensation in nuclei

Low density states near the 3α and 4α breakup thresholds in 12C and 16O, respectively, are discussed in terms of the α-particle condensation. Calculations are performed in OCM (Orthogonality Condition Model) and THSR (Tohsaki-Horiuchi-Schuck-Ropke) approaches. The 0+2 state in 12C and the 0+6 state in 16O are shown to have dilute density structures and give strong enhancement of the occupation of the S-state c.o.m. orbital of the α-particles. The possibility of the existence of α-particle condensed states in heavier nα nuclei is also discussed.

The ANC of 16O subthreshold states from 12C(6Li, d) reaction at energies near the barrier

The ANC of the 2+ (6.92 MeV) and 1− (7.12 MeV) subthreshold states of 16O have been extracted from the normalization of 12C(6Li, d) angular distribution to a Finite Range Distorted Wave Born Approximation (FRDWBA) calculation. The theoretical analysis indicates a peripheral reaction and the extracted ANCs are not sensitive to the number of nodes in the bound state potential. The uncertainty from the entrance channel potential is minimized to 8% for the 6.92 and 11% for the 7.12 MeV state if the normalization is performed at the grazing angle. The uncertainty from the exit channel potential at the grazing angle is found to be 10% and 12% respectively for the 7.12 and 6.92 MeV states.