States In 12C Research Articles

Decay modes of the Hoyle state in [formula omitted

Recent experimental results give an upper limit less than 0.043% (95% C.L.) to the direct decay of the Hoyle state into 3α respect to the sequential decay into Be8+α. We performed one and two-dimensional tunneling calculations to estimate such a ratio and found it to be more than one order of magnitude smaller than experiment depending on the range of the nuclear force. This is within high statistics experimental capabilities. Our results can also be tested by measuring the decay modes of high excitation energy states of 12C where the ratio of direct to sequential decay might reach 10% at E⁎(12C)=10.3 MeV. The link between a Bose Einstein Condensate (BEC) and the direct decay of the Hoyle state is also addressed. We discuss a hypothetical ‘Efimov state’ at E⁎(12C)=7.458 MeV, which would mainly sequentially decay with 3α of equal energies: a counterintuitive result of tunneling. Such a state, if it would exist, is at least 8 orders of magnitude less probable than the Hoyle's, thus below the sensitivity of recent and past experiments.

The α-decay of the Hoyle state in 12C: a new high-precision investigation

The αdecay path of the Hoyle state in 12C (Ex = 7:654MeV) represents one ofthe most challenging questions of modern nuclear physics. Its knowledge can help in theunderstanding of cluster configurations in light nuclei and the possible existence of Bose-Einstein condensates in nuclei. In stars, it is involved in the so-called 3αprocess, wherethe 12C nucleosynthesis occurs. We studied the 14N(d; α2)12C(7:654) reaction at 10:5MeVincident energy to probe its direct decay component. We found, with a precision higherof a factor 5 than any other previous experiment, an almost total absence of direct decaysby-passing the ground state of 8Be. From our analysis, a new upper limit of such decayis found at 0:043% (95% C.L.). Astrophysical 3α process reaction rate calculations haveto be accordingly revised.

The cluster states in light nuclei

The differential cross-sections of the elastic and inelastic α +11B scattering was measured at E(α) = 29 MeV. The analysis of the data by Modified Diffraction Model (MDM) showed that the root-mean- square (RMS) radius of the 11B state 3/2-, E* = 8.56 MeV is ~ 0.5 fm larger than that of the ground state. It is found that the radius of 3/2- (8.56 MeV) state in the 11B nucleus is close to the radii of the Hoyle state in 12C.

Rotational excitation of the Hoyle state in 12C

12C is synthesised in stars by fusion of three α particles. This process occurs through a resonance in the 12C nucleus, famously known as the Hoyle state. In this state, the 12C nucleus exists as a cluster of α particles. The state is the band-head for a rotational band with the 2+ rotational excitation predicted in the energy region 9 - 11 MeV. This rotational excitation can affect the triple-α process reaction rate by more than an order of magnitude at high temperatures (109 K). Depending on the energy of the resonance, the knowledge of the state can also help determine the structure of the Hoyle state. In the work presented here, the state of interest is populated by beta decay of radioactive 12N ion beam delivered by the IGISOL facility at JYFL, Jyväskylä.

Reorientation-effect measurement of the first 2+ state in 12C: Confirmation of oblate deformation

A Coulomb-excitation reorientation-effect measurement using the TIGRESS γ−ray spectrometer at the TRIUMF/ISAC II facility has permitted the determination of the 〈21+‖E2ˆ‖21+〉 diagonal matrix element in 12C from particle−γ coincidence data and state-of-the-art no-core shell model calculations of the nuclear polarizability. The nuclear polarizability for the ground and first-excited (21+) states in 12C have been calculated using chiral NN N4LO500 and NN+3NF350 interactions, which show convergence and agreement with photo-absorption cross-section data. Predictions show a change in the nuclear polarizability with a substantial increase between the ground state and first excited 21+ state at 4.439 MeV. The polarizability of the 21+ state is introduced into the current and previous Coulomb-excitation reorientation-effect analyses of 12C. Spectroscopic quadrupole moments of QS(21+)=+0.053(44) eb and QS(21+)=+0.08(3) eb are determined, respectively, yielding a weighted average of QS(21+)=+0.071(25) eb, in agreement with recent ab initio calculations. The present measurement confirms that the 21+ state of 12C is oblate and emphasizes the important role played by the nuclear polarizability in Coulomb-excitation studies of light nuclei.

Impact of off-diagonal cross-shell interaction on 14C**Supported by National Natural Science Foundation of China (11305272), Special Program for Applied Research on Super Computation of the NSFC Guangdong Joint Fund (the second phase), the Guangdong Natural Science Foundation (2014A030313217), the Pearl River S&T Nova Program of Guangzhou (201506010060), the Tip-top Scientific and Technical Innovative Youth Talents of Guangdong special support program

A shell-model investigation is performed to show the impact on the structure of 14C from the off-diagonal cross-shell interaction, 〈pp|V|sdsd〉, which represents the mixing between the 0 and 2ħω configurations in the psd model space. The observed levels of the positive states in 14C can be nicely described in 0–4ħω or a larger model space through the well defined Hamiltonians, YSOX and WBP, with a reduction of the strength of the 〈pp|V|sdsd〉 interaction in the latter. The observed B(GT) values for 14C can be generally described by YSOX, while WBP and their modifications of the 〈pp|V|sdsd〉 interaction fail for some values. Further investigation shows the effect of such interactions on the configuration mixing and occupancy. The present work shows examples of how the off-diagonal cross-shell interaction strongly drives the nuclear structure.

Multi-cluster dynamics in [formula omitted] and analogy to clustering in 12C

We investigate structure of CΛ13 and discuss the difference and similarity between the structures of C12 and CΛ13 by answering the questions if the linear-chain and gaslike cluster states, which are proposed to appear in C12, survives, or new structure states appear or not. We introduce a microscopic cluster model called, Hyper-Tohsaki–Horiuchi–Schuck–Röpke (H-THSR) wave function, which is an extended version of the THSR wave function so as to describe Λ hypernuclei. We obtained two bound states and two resonance (quasi-bound) states for Jπ=0+ in CΛ13, corresponding to the four 0+ states in C12. However, the inversion of level ordering between the spectra of C12 and CΛ13, i.e. that the 03+ and 04+ states in CΛ13 correspond to the 04+ and 03+ states in C12, respectively, is shown to occur. The additional Λ particle reduces sizes of the 02+ and 03+ states in CΛ13 very much, but the shrinkage of the 04+ state is only a half of the other states, in spite of the fact that attractive Λ-N interaction makes nucleus contracted so much when the Λ particle occupies an S-orbit. In conclusion, the Hoyle state becomes quite a compact object with BeΛ9+α configuration in CΛ13 and is no more gaslike state composed of the 3α clusters. Instead, the 04+ state in CΛ13, coming from the C12(03+) state, appears as a gaslike state composed of α+α+Λ5He configuration, i.e. the Hoyle analog state. A linear-chain state in a Λ hypernucleus is for the first time predicted to exist as the 03+ state in CΛ13 with more shrunk arrangement of the 3α clusters along z-axis than the 3α linear-chain configuration realized in the C12(04+) state. All the excited states are shown to appear around the corresponding cluster-decay threshold, reflecting the threshold rule.

Investigation of the Hoyle state in12C with a new hodoscope detector

The state in12C (7.654MeV, the Hoyle state) is important for the understanding of clustering phenomena in nuclei. The pronounced cluster nature of this state allows the triple-α process in stars with a reaction rate regulated by its structure properties. To precisely estimate the direct component in the 3α decay mechanism of the Hoyle state, we developed a new experiment using the14N(d,α)12C reaction at 10.5MeV. An anti-coincidence telescope was used to identify the α ejectiles leading the residual12C in the Hoyle state, while its decays in 3α were studied by means of a new hodoscope of silicon detectors, superOSCAR, placed in kinematical coincidence to fully reconstruct the events. Details of the experiment and preliminary results are discussed in the text.

Study of the12C excited states above the Hoyle State.

In this work we study the low-lying excited states of12C, especially above the Hoyle state (0+, 7,654 MeV) through the use of the14N(d,α)12C reaction. The EN-Tandem at ININ delivered deuteron beams between 2.5 and 7.5 MeV. Typical beam intensities were 20-50 nA. Two different compounds were used to produce thin films: Si3N4 (150 nm) and of C5H5N5 (10 μm). Angular distributions of emitted α-particles were measured at each energy. The first results of the analysis are presented including quantum number assignments (energy, spin and parity) of the excited states populated.

Current and Future Tests of the Algebraic Cluster Model of12C

A new theoretical approach to clustering in the frame of the Algebraic Cluster Model (ACM) has been developed. It predicts, in12C, rotation-vibration structure with rotational bands of an oblate equilateral triangular symmetric spinning top with a D3h symmetry characterized by the sequence of states: 0+, 2+, 3−, 4±, 5− with a degenerate 4+ and 4− (parity doublet) states. Our newly measured state in12C allows the first study of rotation-vibration structure in12C. The newly measured 5− state and 4− states fit very well the predicted ground state rotational band structure with the predicted sequence of states: 0+, 2+, 3−, 4±, 5− with almost degenerate 4+ and 4− (parity doublet) states. Such a D3h symmetry is characteristic of triatomic molecules, but it is observed in the ground state rotational band of12C for the first time in a nucleus. We discuss predictions of the ACM of other rotation-vibration bands in12C such as the (0+) Hoyle band and the (1−) bending mode with prediction of (“missing 3− and 4−”) states that may shed new light on clustering in12C and light nuclei. In particular, the observation (or non observation) of the predicted (“missing”) states in the Hoyle band will allow us to conclude the geometrical arrangement of the three alpha particles composing the Hoyle state at 7.6542 MeV in12C. We discuss proposed research programs at the Darmstadt S- DALINAC and at the newly constructed ELI-NP facility near Bucharest to test the predictions of the ACM in isotopes of carbon.

Study of resonances produced in light nuclei through two and multi particle correlations

CORRELATION experiment has been performed at INFN-LNS of Catania, using the 4π multi-detector CHIMERA, with the aim of exploring correlations between two and multi light particle produced in 12C+24Mg collisions at 35 AMeV. Particular attention has been paid to the decay mechanisms of Hoyle state, an excited resonant state of 12C produced via the triple-α process and characterized by a pronounced molecular like structure with three α particles. The study of the Hoyle state is essential for nucleosynthesis, but it also represents a clearly isolated state that can be studied as a three-α cluster system.

Evidence of enhanced radius of Hoyle rotational state in12C inelastic scattering

Nuclear radius of three α rotational state in12C with a life time of 10-21 second, which has been expected to have much more extended radius than the ground12C nucleus, is speculated from systematic analysis of the differential cross section of the α +12C inelastic scattering. Present analysis predicts about 0.6 ∼ 1.0 fm enhancement in the matter radius of the three α rotational state in comparison to the normal radius of the ground state. The spatial extension of the three α rotational state is comparable to the extended radius observed in the neutron halo phenomena.

Measurement of the 3-α decay from the Hoyle and the broad 10 MeV states in12C

The measurements of the 3-α decay from the Hoyle and the broad 10 MeV state in12C have been performed using an inverse kinematic method of the12C(12C,3α)12C reaction at E12C = 110 MeV. In the measurement of the Hoyle state, the upper limit of the direct decay was improved to be 0.2 %. This was inconsistent with the recent reported nonzero value of 0.9 %, but an order larger than the direct decay branch predicted in the 3-α model. The branching ratios of the sequtial decay for the broad 10 MeV state were also obtained. They were higher than those obtained in the RCNP experiment, since they contained the contributions of higer multipole components. Their contributions will be estimated.

Experimental progress in clarifying the excitation spectrum of12C

We briefly review recent progress in clarifying the role of α clustering in12C and discuss how experimental studies of the α + α + α breakup of excited states in12C contribute towards this goal. Finally, we show preliminary data from ongoing experimental studies of the p +11B →α + α + α reaction at the 5 MV Van dee Graaff accelerator at Aarhus University.

Alpha-Particle Condensate Structure of the Hoyle State: where do we stand?

The present understanding of the structure of the Hoyle state in12C is reviewd. It is pointed out that a crucial test of any theory is the good reproduction of the experimental results for the inelastic form factor from ground to the Hoyle state. The performances of the so-called THSR wavwe function are outlined confirming the α particle condensation hypothesis proposed 15 years back in [1].

Structure and decay pattern of linear-chain states in neutron-rich Carbon isotopes

The linear-chain states of14C are studied by using the antisymmetrized molecular dynamics. The calculated properties of the linear-chain states were compared with the observed data. The results for the π-bond linear-chain states reasonably agree with the observation. It is also shown that the linear-chain states decay to the excited states of10Be. Hence, we regard that this unique decay pattern is a strong evidence of the linear-chain formation.

Clustering effects in fusion evaporation reactions with light even-even N=Z nuclei

In the recent years, cluster structures have been evidenced in many ground and excited states of light nuclei [1, 2]. In the currently experimental campaign, the NUCL-EX collaboration has measured the12C+12C and14N+10B reactions at 95 MeV and 80 MeV respectively. The experimental data corresponding to complete fusion of target and projectile into an excited24Mg nucleus was compared to the results of a pure statistical model [3, 4]. In addition, data from12C+12C have been analyzed to investigate the decay of the Hoyle state of12C* [12] obtained as an intermediate step in the 6α decay channel of the24Mg* formed in central events.

A comparison of untagged gamma-ray and tagged-neutron yields from 241AmBe and 238PuBe sources

Untagged gamma-ray and tagged-neutron yields from 241AmBe and 238PuBe mixed-field sources have been measured. Gamma-ray spectroscopy measurements from 1 to 5MeV were performed in an open environment using a CeBr3 detector and the same experimental conditions for both sources. The shapes of the distributions are very similar and agree well with previous data. Tagged-neutron measurements from 2 to 6MeV were performed in a shielded environment using a NE-213 liquid-scintillator detector for the neutrons and a YAP(Ce) detector to tag the 4.44MeVgamma-rays associated with the de-excitation of the first-excited state of 12C. Again, the same experimental conditions were used for both sources. The shapes of these distributions are also very similar and agree well with previous data, each other, and the ISO recommendation. Our 238PuBe source provides approximately 2.6 times more 4.44MeVgamma-rays and 2.4 times more neutrons over the tagged-neutron energy range, the latter in reasonable agreement with the original full-spectrum source-calibration measurements performed at the time of their acquisition.

Quartet condensation in Fermi-systems. The Hoyle state in the 12C nucleus and life on earth

The possibility that a four component Fermi gas with attractive interaction can form a quartet condensate at low density is pointed out. It is discussed that for quartets only the Bose-Einstein Condensatio (BEC) phase exists and that the analogue to the weak coupling long coherence BCS phase of pairing is absent. Precurser phenomena in finite nuclei are presented. For instance, the present understanding of the structure of the Hoyle state in 12C being the gateway for Carbon production in the universe is reviewed. It is pointed out that a crucial test of any theory is the good reproduction of the experimental results for the inelastic form factor from ground to the Hoyle state. The performances of the so-called THSR wavwe function are outlined confirming the α particle condensation hypothesis proposed 15 years back in [1].

Colloquium: Status ofα-particle condensate structure of the Hoyle state

The present understanding of the structure of the Hoyle state in 12C is reviewed. Most of the theoretical approaches to the Hoyle state are shortly summarized. The corresponding results are analyzed with respect to whether they give evidence to the alpha particle condensation structure of the Hoyle state (and other Hoyle-like states in heavier self-conjugate nuclei) or not.