States In 16O Research Articles

Erratum: Constraining theSfactor ofN15(p,γ)O16at astrophysical energies [Phys. Rev. C82, 055804 (2010)

The 15N(p,g)16O reaction represents a break out reaction linking the first and second cycle of the CNO cycles redistributing the carbon and nitrogen abundances into the oxygen range. The reaction is dominated by two broad resonances at Ep = 338 keV and 1028 keV and a Direct Capture contribution to the ground state of 16O. Interference effects between these contributions in both the low energy region (Ep < 338 keV) and in between the two resonances (338 <Ep < 1028 keV) can dramatically effect the extrapolation to energies of astrophysical interest. To facilitate a reliable extrapolation the 15N(p,g)16O reaction has been remeasured covering the energy range from Ep=1800 keV down to 130 keV. The results have been analyzed in the framework of a multi-level R-matrix theory and a S(0) value of 39.6 keV b has been found.

The [formula omitted] band of 16O 17O, 17O 18O and 17O 2 by high sensitivity CRDS near 1.27 μm

The very weak a 1 Δ g − X 3 Σ g − system of the three 17O isotopologues of oxygen – 16O 17O, 17O 18O and 17O 2 – was studied by high sensitivity CW-Cavity Ring Down Spectroscopy. The spectra of a 17O highly enriched sample were recorded at room temperature between 7640 and 7917 cm −1 and at liquid nitrogen temperature in the 7876–7893 cm −1 region. The magnetic dipole (0–0) band was observed for all three 17O isotopologues. At liquid nitrogen temperature, some of the transitions were observed with partially resolved hyperfine splitting due to the 17O nuclear spin. The electric quadrupole (0–0) band and the (1–1) hot band were also observed for the 16O 17O and 17O 2 species. The rotational and hyperfine spectroscopic parameters of the X 3 Σ g − and a 1Δ g states of the three studied isotopologues were derived from a global fit of the measured line positions and microwave and Raman measurements available in the literature. The spectroscopic constants of the a 1Δ g ( v=0, 1) states of 17O 2 are reported for the first time.

Rigorous spectral representation of relativistic random phase approximation for finite nuclei

By using the rigorous spectral representation of relativistic random phase approximation, the low-lying excitation of finite nuclei and its longitudinal response function for quasielastic electron scattering are calculated in the σ-ω model of quantum hadrodynamics. It is shown that the reproduction of the correct order of the 1− and 3− excitation states of 16O is due to the contribution of the exchange vertex. There is no significant influence of the retardation effect on the low-lying excitation states. In contrast, the retardation effect plays an important role in the electron scattering process of nuclei. The theoretical longitudinal responses of 12C and 40Ca, including the contributions of the exchange vertex and the retardation effect, are suppressed and reproduce the experimental data better than the results excluding them.

Ab InitioComputation of theF17Proton Halo State and Resonances inA=17Nuclei

We perform coupled-cluster calculations of the energies and lifetimes of single-particle states around the doubly magic nucleus (16)O based on chiral nucleon-nucleon interactions at next-to-next-to-next-to-leading order. To incorporate effects from the scattering continuum, we employ a Gamow-Hartree-Fock basis. Our calculations for the J(pi)=1/2(+) proton halo state in (17)F and the 1/2(+) state in (17)O agree well with experiment, while the calculated spin-orbit splitting between 5/2(+) and 3/2(+) states is too small due to the lack of three-nucleon forces. Continuum effects yield a significant amount of additional binding energy for the 1/2(+) and 3/2(+) states in (17)O and (17)F.

Alpha decay widths of excited states of 16O

The 12C(13C, 9Be)16O reaction has been used to populate excited states in 16O. The 9Be nuclei were detected in a magnetic spectrometer and the 12C decay product of the recoiling excited 16O nucleus was detected in an array of silicon strip detectors. The large angular coverage of the strip detector array allowed the α-decay widths of the 14.66 MeV, 5−, and 16.275 MeV, 6+, states to be determined with good accuracy. The present results, together with earlier measurements, allow precise values for the branching ratios to be calculated: Ex(16O) = 14.66 MeV, Jπ = 5−, Γα0/Γ = 0.951 ± 0.049 and Γα1/Γ < 0.05; Ex(16O) = 16.275 MeV, Jπ = 6+, Γα0/Γ = 0.982 ± 0.048 and Γα1/Γ < 0.02.

α-particle condensate states in 16O

The existence of a rotational band with the α+C12(02+) cluster structure, in which three α particles in 12C(02+) are locally condensed, is demonstrated near the four-α threshold of 16O in agreement with experiment. This is achieved by studying structure and scattering for the α+C12(02+) system in a unified way. A drastic reduction (quenching) of the moment of the inertia of the 0+ state at 15.1 MeV just above the four-α threshold in 16O suggests that it could be a candidate for the superfluid state in α-particle condensation.

Asymptotic normalization coefficient and important astrophysical process 15N(p,γ)160

In this work we report the application of the ANC method for the determination of the non-resonant radiative capture amplitude for the important astrophysical CNO cycle reaction 15N(p, γ) 16O, which provides a leak from the CN cycle into the CNO bi-cycle and CNO tri-cycle. It is contributed by the resonance capture to the ground state through two strong 1− resonances and non-resonant capture to the ground state, which interferes with the resonant capture terms. To determine more accurately the contribution from the non-resonant capture we determined the proton ANCs for the ground and seven excited states of 16O by measuring the angular distributions of the peripheral 15N(3He, d)16O proton transfer reaction. Using these ANCs and proton and α resonance widths determined from an R-matrix fit to the data from the 15N(p, α)12C reaction, we calculated the astrophysical S factor for the 15N(p, γ)16O reaction. The results indicate that the direct capture contribution was previously overestimated. We find the astrophysical factor to be S(0) = 36.0 ± 6.0 keVb, which is about a factor of two lower than the presently accepted value. We conclude that for every 2200 ± 300 cycles of the main CN cycle one CN catalyst is lost due to this reaction.

Concepts of nuclearα-particle condensation

Certain aspects of the recently proposed antisymmetrised alpha particle product state wave function, or THSR alpha cluster wave function, for the description of the ground state in 8Be, the Hoyle state in 12C, and analogous states in heavier nuclei, are elaborated in detail. For instance, the influence of antisymmetrisation in the Hoyle state on the bosonic character of the alpha particles is studied carefully. It is shown to be weak, so that bosonic aspects are predominant. The de Broglie wave length of alpha particles in the Hoyle state is shown to be much larger than the inter-alpha distance. It is pointed out that the bosonic features of low density alpha gas states have measurable consequences, one of which, that is enhanced multi-alpha decay properties, likely already have been detected. Consistent with experiment, the width of the proposed analogue to the Hoyle state in 16O at the excitation energy of E_x=15.1 MeV is estimated to be very small (34 keV), lending credit to the existence of heavier Hoyle-like states. The intrinsic single boson density matrix of a self-bound Bose system can, under physically desirable boundary conditions, be defined unambiguously. One eigenvalue then separates out, being close to the number of alpha's in the system. Differences between Brink and THSR alpha cluster wave functions are worked out. No cluster model of the Brink type can describe the Hoyle state with a single configuration. On the contrary, many superpositions of the Brink type are necessary, implying delocalisation towards an alpha product state. It is shown that single alpha particle orbits in condensates of different nuclei are almost the same. It is thus argued that alpha particle antisymmetrised product states of the THSR type are a very promising novel and useful concept in nuclear physics.

Molecular and cluster structures in 18O

We have studied the multi-nucleon transfer reaction 12C ( 7Li ,p) at E lab = 44 MeV, populating states in the oxygen isotope 18O . The experiments were performed at the Tandem accelerator of the Maier-Leibniz Laboratory in Munich using the high-resolution Q3D magnetic spectrograph. States were populated up to an excitation energy of 21.2MeV with an overall energy resolution of 45keV, and 30 new states of 18O have been identified. The structure of the rotational bands observed is discussed in terms of cluster bands with the underlying cluster structures: 14C$ \(\displaystyle \otimes\) \(\displaystyle \alpha\)$and 12C ⊗ 2n ⊗ \( \alpha\) . Because of the broken intrinsic reflection symmetry in these structures the corresponding rotational bands appear as parity doublets.

The study of the reduced α-width and ANC of 16O states through its sequential breakup

In this work we theoretically study using continuum-discretized-coupled channel (CDCC) theory, the resonant breakup of 16O by comparison with a recent measurement. The ground state Asymptotic Normalization Coefficient (ANC) and the alpha spectroscopic factor of 16O are evaluated in this work. The peripheral aspect of resonance breakup through a 2+ unbound state has been also studied.

D 216O: ICLAS between 13 600 and 14 020 cm −1 and normal mode labeling of the vibrational states

The high resolution absorption spectrum of dideuterated water, D 2 16O, has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) in the 13 600–14 020 cm −1 spectral region which is the highest energy region reported so far for this water isotopologue. Because the HD 16O absorption is stronger by three orders of magnitude in the region under study, it was necessary to use high deuterium enrichment in order to minimize the HD 16O absorption lines overlapping the D 2 16O spectrum. With the high sensitivity achieved (noise equivalent absorption α min ∼10 −9 cm −1), transitions with line strengths on the order of 5 × 10 −28 cm molecule −1 could be detected. The spectrum analysis, based on recent variational calculations has provided a set of 177 new rovibrational energy levels belonging to six vibrational states. The most complete set of 53 vibrational energy levels of D 2 16O, including the three newly determined band origins, was constructed from an exhaustive review of the literature data. The fitting of the parameters of the vibrational effective Hamiltonian has allowed to reproduce the whole set of vibrational energies with an rms deviation of 0.055 cm −1. This simple model gave consistent vibrational labels of the D 2 16O states up to 18 000 cm −1. Above 15 000 cm −1, Fermi and Darling-Dennison resonance interaction were found to induce strong vibrational mixings of the wave functions in the normal mode basis, leading to ambiguous vibrational labeling.

Precision study of ground state capture in the14N(p,γ)15Oreaction

The rate of the hydrogen-burning carbon-nitrogen-oxygen (CNO) cycle is controlled by the slowest process, 14N(p,γ )15O, which proceeds by capture to the ground and several excited states in 15O. Previous extrapolations for the ground state contribution di

Resonance states in 16O+16O, 12C+16O, α+16O and α+12C with modified Morse potentials

The resonance states in 16O+16O, 12C+16O, α+16O and α+12C are described using modified Morse potential proposed earlier whose success has already been demonstrated in the case of 12C+12C system. The general validity of such a potential with long range, shallow depth and repulsive soft core determined from the resonance data itself is being examined through the present study of the resonances in the above four systems. In each system, the experimental data of a large number of states have been successfully described with a modified Morse potential. The success points out a common mechanism of the origin of these states, and reaffirms authentically the diatomic-like rotational and vibrational picture of the nuclear molecular resonances proposed previously. The close resemblance between the physics of diatomic molecules and nuclear molecular resonances extending to the level of potential which is Morse type in both the cases — although belong to two different areas of physics — is further strengthened through the present study.

α-cluster states and 4α-particle bose condensate in 16O

In order to explore the 4α-particle condensate state in 16O, we solve a full four-body equation of motion based on the 4α OCM (Orthogonality Condition Model) in a large 4α model space spanned by Gaussian basis functions. A full spectrum up to the 06+ state is reproduced consistently with the lowest six 0+ states of experimental spectrum. The 06+ state is obtained at 2 MeV above the 4α breakup threshold and has a dilute density structure, where the rms radius is more than 5 fm. The state has an appreciably large α condensate fraction 61%, and a highly amount of α+12C(02+) components, both of which are strong pieces of evidence of the state being the 4α condensate state.

Measurement and DWBA analysis of the 12C( 6Li, d) 16O α-transfer reaction cross sections at 48.2 MeV. R-matrix analysis of 12C( [formula omitted]) 16O direct capture reaction data

The angular distributions for low lying states of 16O produced in the 12C( 6Li, d) 16O α-transfer reaction at E Li 6 ( lab ) = 48.2 MeV have been measured using a high energy resolution position sensitive detection system. The measured cross section data have been analyzed by the FRDWBA theory with a particular emphasis put on the states of astrophysical interest mainly the 1 − (7.12 MeV) state. Extracted values of nuclear level parameters (excitation energy, E x , line width, Γ c . m . , α-spectroscopic factor, S α , and reduced α-width, γ α 2 or θ α 2 ) are reported and discussed in comparison to previous ones. The used binding potential corresponds to a long nuclear interaction range (up to a channel radius a = 7.7 fm ), suggesting that the channel radius a = 6.5 fm is likely suitable for the calculations of reduced α-widths near the nuclear surface. However, given that previous DWBA results are available for comparison only near the channel radius a = 5.5 fm , calculations have been carried out here for both three preceding a values. Formal values, 0.0036 ⩽ θ α 2 ( 7.12 ) ⩽ 0.152 and 0.019 ⩽ θ α 2 ( 6.92 ) ⩽ 0.096 of the reduced α-widths ( a = 6.5 fm ) of the 1 − (7.12 MeV) and 2 + (6.92 MeV) states have thus been derived. Furthermore, the R-matrix analysis of various data sets of importance to the 12C( α , γ ) 16O direct capture reaction has been carried out, leading to the range of values for the 1 − (7.12 MeV) state, 0.0096 ⩽ θ α 2 ( 7.12 ) ⩽ 0.0166 , which is consistent with the preceding DWBA interval, and to a result for the E1 component of the reaction astrophysical factor, S E 1 ( 0.3 ) = 80.6 −16 +17 keV b , very consistent with most constrained values previously reported in the literature.

An exact microscopic multiphonon approach to collective modes in nuclei

We propose a method for solving the nuclear eigenvalue problem using a Hamiltonian of general form in a space spanned by multiphonon states built out of particle-hole TammDancoff phonons. The method consists in deriving for a given n-phonon subspace a set of equations of motion of simple structure, where all quantities are expressed in terms of energies and one-body density matrices determined in the (n − 1)-phonon subspace. Their solution, therefore, yields states built out of particle-hole operators applied on top of the (n−1)phonons basis states. Because of this peculiar structure, the states so generated form an overcomplete set. The redundancy, however, is removed completely and efficiently by a procedure based on the Choleski decomposition. The phonon composition of the basis states allows to remove naturally and maximally the spurious admixtures induced by the center of mass motion. The iteration of the equations from n = 0 up to an arbitrary number N of phonons generates an orthogonal basis spanning a space which is the direct sum of 0+1+..+N phonon subspaces. This basis decomposes the full Hamiltonian into N diagonal blocks plus off-diagonal terms coupling the n-phonons with the (n ± 1)and (n ± 2)-phonon subspaces. These off-diagonal terms are easily computed by recursive relations. The Hamiltonian can, therefore, be easily diagonalized by resorting to importance sampling techniques [1,2], which allow to truncate the multiphonon space while monitoring the accuracy of the solutions. The method can be easily upgraded so as to generate a particle-hole or quasiparticle RPA multiphonon basis. It is suitable for describing with high accuracy low-lying multiphonon spectra, whose experimental evidence is rapidly growing, as well as high-energy resonances like the recently discovered double dipole giant resonance. It can account with great accuracy for the anharmonicities associated to all collective modes. Last, but not least, it yields a highly correlated ground state which contains explicitly up to N particle N hole configurations and, therefore, applies to the cases where the quasiboson approximation breaks down. For illustrative purposes, we apply the method to 16O, whose low-lying states are dominated by complex configurations containing up to 4 particle 4 hole states. Such a calculation probes the extreme accuracy of the method, on the one hand, and, on the other hand, provides a complete and exhaustive information on the microscopic structure of low and high energy states in 16O.

Measurement of the Cascade Transition via the First Excited State ofO16in theC12(α,γ)O16Reaction, and ItsSFactor in Stellar Helium Burning

Radiative alpha-particle capture into the first excited, J(pi)=0+ state of 16O at 6.049 MeV excitation energy has rarely been discussed as contributing to the 12C(alpha,gamma)16O reaction cross section due to experimental difficulties in observing this transition. We report here measurements of this radiative capture in 12C(alpha,gamma)16O for center-of-mass energies of E=2.22 MeV to 5.42 MeV at the DRAGON recoil separator. To determine cross sections, the acceptance of the recoil separator has been simulated in GEANT as well as measured directly. The transition strength between resonances has been identified in R-matrix fits as resulting both from E2 contributions as well as E1 radiative capture. Details of the extrapolation of the total cross section to low energies are then discussed [S6.0(300)=25(-15)(+16) keV b] showing that this transition is likely the most important cascade contribution for 12C(alpha,gamma)16O.

Inelastic form factors to alpha-particle condensate states in 12C and 16O: What can we learn?

In order to discuss the spatial extention of the 02 +-state of 12C (Hoyle state), we analyze the inelastic form factor of electron scattering to the Hoyle state, which our 3α-condensate wave function reproduces very well like previous 3α RGM/GCM models. The analysis is made by varying the size of the Hoyle state artificially. As a result, we find that only the maximum value of the form factor sensitively depends on its size, while the positions of maximum and minimum are almost unchanged. This size dependence is found to come from a characteristic feature of the transition density from the ground state to the Hoyle state. We further show the theoretical predictions of the inelastic form factor to the 22 +-state of 12C, which was recently observed above the Hoyle state, and of the inelastic form factor to the calculated 03 +-state of 16O, which was conjectured to correspond to the 4α condensed state in previous theoretical work by the present authors.

CNO hydrogen burning studied deep underground

In stars, four hydrogen nuclei are converted into a helium nucleus in two competing nuclear fusion processes, namely the proton-proton chain (p-p chain) and the carbon-nitrogen-oxygen (CNO) cycle. For temperatures above 20 million kelvin, the CNO cycle dominates energy production, and its rate is determined by the slowest process, the 14N(p,γ)15O radiative capture reaction. This reaction proceeds through direct and resonant capture into the ground state and several excited states in 15O. High energy data for capture into each of these states can be extrapolated to stellar energies using an R-matrix fit. The results from several recent extrapolation studies are discussed. A new experiment at the LUNA (Laboratory for Underground Nuclear Astrophysics) 400kV accelerator in Italy's Gran Sasso laboratory measures the total cross section of the 14N(p,γ)15O reaction with a windowless gas target and a 4π BGO summing detector, down to center of mass energies as low as 70keV. After reviewing the characteristics of the LUNA facility, the main features of this experiment are discussed, as well as astrophysical scenarios where cross section data in the energy range covered have a direct impact, without any extrapolation.

Relation between the 16O(α,γ)20Ne reaction and its reverse 20Ne(γ,α)16O reaction in stars and in the laboratory

The astrophysical reaction rates of the 16O(a,g)20Ne capture reaction and its inverse 20Ne(g,a)16O photodisintegration reaction are given by the sum of several narrow resonances and a small direct capture contribution at low temperatures. Although the thermal population of low lying excited states in 16O and 20Ne is extremely small, the first excited state in 20Ne plays a non-negligible role for the photodisintegration rate. Consequences for experiments with so-called quasi-thermal photon energy distributions are discussed.