States In 12C Research Articles

New measurement of the Hoyle state radiative transition width

The radiative decay of the Hoyle state serves as the gateway to the production of heavier elements in a stellar environment. Here, we present an exclusive measurement of electric quadruple (E2) transitions of the Hoyle state to the ground state of 12C through the 12C(p, p′γγ)12C reaction. A triple coincidence measurement yields the radiative branching ratio Γrad/Γ = 4.01 (30) × 10−4. This result was corroborated by an independent experiment based on the complete kinematical measurement via12C(p, p′)12C reaction, yielding a consistent result of Γrad/Γ = 4.04 (30) × 10−4. Combining our results with the currently adopted values of Γπ(E0)/Γ and Γπ(E0), the radiative width of the Hoyle state is determined to be 3.75 (40) × 10−3 eV. It is important to note that our finding do not align with a recently reported 34% increase in the radiative decay width of the Hoyle state but is consistent with the currently accepted value.

Investigation of breakup process in 9Be(α,n)12C reaction

Neutron energies and angular distributions were measured in 9Be(α,n)12C reaction for α energies of 5.5 and 6.5 MeV. Three major neutron groups were observed in the spectrum, which correspond to the ground and first two excited states of 12C. Measured data could only be explained by the TALYS calculation if reaction at more than one location within the target is considered for a given beam energy. The preferred locations are driven by the resonance energy levels existing in 13C. Neutron yield due to the 9Be breakup process was determined which is found to be 12.6 ± 0.2% and 18.4 ± 0.5% of the total reaction cross-section for 5.5 and 6.5 MeV respectively.

Effects of Hoyle state de-excitation on νp–process nucleosynthesis and Galactic chemical evolution

The particle-induced hadronic de-excitation of the Hoyle state in 12C induced by inelastic scattering in a hot and dense plasma can enhance the triple-alpha reaction rate. This prevents the production of heavy nuclei within the neutrino-driven winds of core-collapse supernovae and raises a question as to the contribution of proton-rich neutrino-driven winds as the origin of p–nuclei in the solar-system abundances. Here we study νp-process nucleosynthesis in proton-rich neutrino-driven winds relevant to the production of Mo92,94 and Ru96,98 by considering such particle-induced de-excitation. We find that the enhanced triple-alpha reaction rate induced by proton inelastic scattering can increase the production of Mo92,94 and Ru96,98 in an energetic hypernova wind model with a massive proto-neutron star although the abundances of these p–nuclei decrease in a wind model of ordinary core-collapse supernovae. The enhancement of the triple-alpha reaction rate induced by the neutron scattering hardly affects the νp-process at low temperature. We show the new result that, contrary to the results for ordinary core-collapse supernovae, the hypernova νp-process can contribute to the Galactic chemical evolution of p–nuclei regardless of the particle-induced Hoyle state de-excitation and even enhance the solar isotopic fractions of Mo92,94 and Ru96,98.

Algebraic model studies of the structure and reactions of 12C and 16O

The role of algebraic models and geometric cluster models of alpha clusters in the structure of ground and first excited 0+ and second excited 2+ states in 12C are evaluated with the aim of establishing if the rotational bands are compatible with rigid structures or if they are instead overlaps of different configurations. A rigid equilateral triangle shape is assumed in Ref.[2, 3]: the energy spectrum, electromagnetic observables are calculated and the transition densities are compared with elastic and inelastic cross-sections for various processes. In a different approach [4, 5], we solve the three-body quantum problem for 3 alphas using the orthogonality condition and the stochastic variational method with correlated Gaussian basis functions. The two-body density distributions indicate that the main configurations of both the second 0+ and 2+ states are acute iscosceles triangle shapes with a 8Be(0+)+α configuration. We also find clues that the second 2+ state is probably not a member of the rigid rotational band built on the Hoyle state.

Radiative Decay Branching Ratio of the Hoyle State in 12C via Charged Particle Coincidence Techniques

The properties of the Hoyle state in 12C (7.654 MeV, 0+) affect the rate at which carbon, one of the most abundant elements in the Universe, is forged in stars. Recent experiments reported values of its radiative decay branching ratio that are in tension, posing major implications especially in the astrophysical domain. This work reports on an almost background-free measurement of the radiative decay branching ratio of the Hoyle state that exploits charged particle coincidence techniques. The experiment adopts several methodologies to minimize the background and identify the rare signal associated with the radiative decay. Large care is devoted to having under full control two of the major sources of systematic errors in particle-coincidence experiments: the coincidence efficiency and the spurious coincidence rate. We find a radiative decay branching ratio of Γrad/Γtot = 4.2(6) · 10−4. The new finding helps to resolve the tension between recent data published in the literature.

Signatures of clustering accessible with a Time Projection Chamber: TexAT

Many experimental observables in clustering require high-sensitivity, almost background-free measurements. The use of Time Projection Chambers (TPCs) over the past 15 years have demonstrated their capability as a step change in the accessibility of many of these observables, as well as the possibility to study clustering via new techniques. A summary of the difficulties and pitfalls of determining the type of clustering from observables is briefly discussed, with a focus on α-condensation and the possibility of an additional (Efimov) 0+ state in 12C, below the Hoyle state. The expansion of the technique used to tackle this challenge to study 3α + p clustering in 13N is also discussed, highlighting the exciting opportunities TPCs provide to study clustering inaccessible by conventional means. Finally, an overview of the recent and upcoming advances in TPC technology is given as well as listing additional future challenges that are needed to be overcome, in the context of discussing the next-generation upgrade to the existing TexAT TPC, known as TeBAT (TExas Birmingham Active Target).

Exploring cluster structures in the high excitation energy region of Be, B, and C isotopes via 10B + 10B nuclear reactions

The unique opportunity presented by 10B + 10B reactions to study high-energy, high-spin states in the A=10 mass region is explored. Results from the measurement at 72 MeV are presented, the most important being new and rarely seen states in the 12C [1] and 13C [2], which motivate targeted future experiments. In particular, a new state of 12C at Ex = 24.4 MeV is strongly populated in the triple α-particle coincidences, while the rarely seen state at Ex = 30.3 MeV is found to be strong in the d+10B decay channel, reinforcing the previous suggestions that it has the exotic 2α+2d molecular structure [3]. Regarding the 13C nucleus, a potentially novel state at Ex = 19.0 MeV is prominently observed in α+ 9Be coincidences and demonstrates a well-defined cluster structure. Lastly, high-spin states in mirror nuclei pairs 9Be-9B, 10Be-10C and 11B-11C populated in the presented measurement are explored.

Search for the 6α condensed state in 24Mg using the 12C+12C scattering

We searched for the 6α-condensed state in 24Mg by measuring the C12+12C scattering with the SAKRA Si detector array at Ecm=17.5–25.0 MeV. By using the invariant-mass method for the detected 3α particles, the inclusive cross sections for the C12+12C→12C(02+)+X and C12(31−)+X reactions were determined. In addition, the missing-mass spectroscopy was successfully utilized to determine the excitation energy of the residual C12 nucleus and the exclusive cross sections for the C12+12C→12C(02+)+12C(01+), C12(02+)+12C(21+), and C12(02+)+12C(02+) reactions. In both the inclusive C12(02+)+X channel and the exclusive C12(02+)+12C(01+) channel, the cross section peaked at Ecm=19.4 MeV, which correspond to the excitation energy of Ex=33.3 MeV in 24Mg. This 19.4-MeV state is a candidate for the 6α-condensed state because of the agreement of the excitation energy with the theoretical value and its decay property. In the exclusive C12(02+)+12C(02+) channel, a broad state was observed at Ecm=22.5 MeV, which correspond to the excitation energy of Ex=36.4 MeV in 24Mg. From the angular distribution of the differential cross section, the spin and parity of this 22.5-MeV state was assigned to be 4+. In addition, a 2+ state was suggested at the low-energy side of the 22.5-MeV state. Because their excitation energies are higher than the theoretical value of the 6α-condensed state, these states might be excited states of the 6α-condensed state such as the 22+ and 41+ states in C12.

The spectroscopic quadrupole moment of the [formula omitted] state of 12C: A benchmark of theoretical models

The spectroscopic quadrupole moment of the first 2+ state of 12C has been measured employing the Coulomb-excitation re-orientation technique. Our result of Qs(21+)=+9.3−3.8+3.5efm2 suggests a larger oblate deformation than previously reported. Combining this with the consistently re-analyzed adopted value, we present the most precise value to date of Qs(21+)=+9.5(18)efm2, which is consistent with a geometrical rotor description. This simple outcome is compared to state-of-the-art shell-model, mean-field, ab initio calculations, cluster-based and geometrical-like theories, which show varying degrees of emergent quadrupole collectivity.

Lineshape response of plastic scintillator to pair production of 4.44 MeV [formula omitted]’s

We measure the distribution of energy deposited in a 40 × 88 mm plastic scintillator by e+e− pair production of 4.44 MeV γ-rays. We observe the double-escape peak of 3.42 MeV from pair production by tagging 511 keV annihilation radiation in two high-Z scintillators. The source is a standard commercial neutron source using α-emitting 241Am encapsulated with 9Be, which has a reaction branch feeding the first Iπ=2+ state of 12C making the 4.44 MeV γ-rays. We demonstrate the extraction of the double-escape peak from the large neutron-produced backgound, and explore some of the features and difficulties of this technique with our apparatus.

Search for cluster effects in p+12C elastic scattering by a diffractional method

We investigate medium-energy angular distribution data of proton elastic scattering on 12C, with the aim to probe the existence of cluster distributions in the ground state of 12C. In our approach, we exploit Coupled-channel calculations to describe the scattering from a spheroidal-like structure, and we include a further contribution, based on a diffraction scattering formula, to explicitly take into account a possible triangular α-cluster structure of the target. From the present analysis, we find a quite small cluster component in 12C ground state (with an upper limit on the occurrence probability of 1%, 99.75% confidence level) and an inter-cluster distance of ≃3.9 fm. These values are compared with several theoretical predictions reported in the literature.

Potential experimental evidence of an Efimov state in 12C and its influence on astrophysical carbon creation

We describe the search of the Efimov state in 12C performed by analysing a large set of 3-α particles coincidence data measured at the LNS laboratories with the CHIMERA detector. The data are compatible with the presence of an Efimov state at the excitation energy of 7.485 MeV, with an excitation probability smaller than 0.3% of the population probability of the Hoyle state. The Efimov state gamma decay probability should be around 30%, much larger than the one of the Hoyle state. The population of this level is compatible with the Suda limit for the 3-α reaction rate in stars because its population should be possible only through direct 3-α reaction. More investigations improving the detector angular resolution are needed to confirm these preliminary observations.

Study of elastic and inelastic scattering of 7Be + 12C at 35 MeV

The elastic and inelastic scattering of 7Be from 12C have been measured at an incident energy of 35 MeV. The inelastic scattering leading to the 4.439 MeV excited state of 12C has been measured for the first time. The experimental data cover an angular range of θcm = 15∘-120∘. Optical model analyses were carried out with Woods-Saxon and double-folding potential using the density dependent M3Y (DDM3Y) effective interaction. The microscopic analysis of the elastic data indicates breakup channel coupling effect. A coupled-channel analysis of the inelastic scattering, based on collective form factors, shows that mutual excitation of both 7Be and 12C is significantly smaller than the single excitation of 12C. The larger deformation length obtained from the DWBA analysis could be explained by including the excitation of 7Be in a coupled-channel analysis. The breakup cross section of 7Be is estimated to be less than 10% of the reaction cross section. The intrinsic deformation length obtained for the 12C⁎ (4.439 MeV) state is δ2 = 1.37 fm. The total reaction cross section deduced from the analysis agrees very well with Wong's calculations for similar weakly bound light nuclei on 12C target.

Neutron-upscattering enhancement of the triple-alpha process

The neutron inelastic scattering of carbon-12, populating the Hoyle state, is a reaction of interest for the triple-alpha process. The inverse process (neutron upscattering) can enhance the Hoyle state’s decay rate to the bound states of 12C, effectively increasing the overall triple-alpha reaction rate. The cross section of this reaction is impossible to measure experimentally but has been determined here at astrophysically-relevant energies using detailed balance. Using a highly-collimated monoenergetic beam, here we measure neutrons incident on the Texas Active Target Time Projection Chamber (TexAT TPC) filled with CO2 gas, we measure the 3α-particles (arising from the decay of the Hoyle state following inelastic scattering) and a cross section is extracted. Here we show the neutron-upscattering enhancement is observed to be much smaller than previously expected. The importance of the neutron-upscattering enhancement may therefore not be significant aside from in very particular astrophysical sites (e.g. neutron star mergers).

Study of the 13C(3He, α)12C reaction at energies of 50 and 60 MeV

Cross sections for the 13C(3He, α)12C reaction were measured for 12C states up to excitation energies of about 20 MeV for 3He energies of 50 and 60 MeV. An analysis of the measured angular distributions was carried out using the Coupled Channels Born Approximation method under the assumption of a direct one-step mechanism of picking up a neutron from the 13C nucleus. In these calculations, the real parts of the implemented potentials were constructed within the framework of the double folding model. This analysis made it possible to extract the values of the spectroscopic factors for the 13C g.s. → 12C g.s. (12C*) + n configurations, which were found to be in reasonable agreement with theoretical predictions.

Inelastic Electron Scattering Form Factor of Isoscalar (T = 0) and Isovector (T = 1) Particle-hole States in 12C and 16O

Inelastic longitudinal and transverse electron scattering form factors of low-lying T = 0, T = 1 particle-hole states of 12C and 16O are studied in the framework of the Tamm–Dancoff approximation (TDA). The Hamiltonian with the Michigan-three-Yukawa (M3Y) potential is diagonalized. To obtain a good agreement with the experimental data, the ground state is corrected by including the admixture from higher orbits with regard for the core polarization effects.

Multiprobe study of excited states in C12 : Disentangling the sources of monopole strength between the energy of the Hoyle state and Ex=13 MeV

Background: The Hoyle state is the archetypal α-cluster state which mediates the 3α reaction to produce C12 and is of great interest for both nuclear structure and astrophysics. Recent theoretical calculations predict a breathing-mode excitation of the Hoyle state at Ex≈9 MeV. Its observation is hindered by the presence of multiple broad states and potential interference effects. An analysis with Gaussian line shapes of measurements at the Research Center for Nuclear Physics (Osaka University) with the Grand Raiden spectrometer suggested that additional strength was needed at Ex≈9 MeV to reproduce the data; this analysis did not account for the well-known threshold effects observed in C12. Nevertheless, various theoretical studies have since concluded that this additional strength corresponds to the predicted breathing-mode excitation of the Hoyle state. To meaningfully identify a new source of monopole strength in this astrophysically significant region, a more appropriate phenomenological analysis which accounts for penetrability and interference effects must be used to determine whether the data can be explained with previously established states. Purpose: We aim to investigate the monopole strength in the astrophysically important excitation-energy region of C12 between Ex=7 and 13 MeV to determine whether the previously established sources of monopole strength are able to reproduce the data. Method: The C12(α,α′)C12 and C14(p,t)C12 reactions, which are expected to exhibit contrasting selectivity towards different monopole excitations, were employed at various detection angles and beam energies to populate states in C12. The inclusive excitation-energy spectra were simultaneously analyzed with multilevel, multichannel line shapes. Various scenarios with different sources of monopole strength and interference effects were considered to determine whether the ghost of the Hoyle state and the previously established broad 03+ state at Ex≈10 MeV are able to reproduce the observed monopole strength. Results: Clear evidence was found for excess monopole strength at Ex≈9 MeV, particularly in the C12(α,α′)C12 reaction at 0∘. This additional strength cannot be reproduced by the previously established monopole states between Ex=7 and 13 MeV. Coincident charged-particle decay data suggest that the strength at Ex≈9 MeV is dominantly monopole, with no evidence of a J>0 contribution. Conclusions: The data support a new source of monopole strength at Ex≈9 MeV, which cannot be described with a phenomenological parametrization of all previously established states. An additional 0+ state at Ex≈9 MeV yielded a significantly improved fit of the data and is a clear candidate for the predicted breathing-mode excitation of the Hoyle state. Alternatively, the results may suggest that a more sophisticated, physically motivated parametrization of the astrophysically important monopole strengths in C12 is required.17 MoreReceived 19 November 2020Revised 21 May 2021Accepted 16 June 2021DOI:https://doi.org/10.1103/PhysRevC.105.024308©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAlpha decayCollective levelsDirect reactionsNuclear astrophysicsNuclear reactionsNuclear structure & decaysProperties6 ≤ A ≤ 19TechniquesCluster modelsNuclear PhysicsGravitation, Cosmology & Astrophysics

Investigating the predicted breathing-mode excitation of the Hoyle state

Knowledge of the low-lying monopole strength in C12—the Hoyle state in particular—is crucial for our understanding of both the astrophysically important 3α reaction and of α-particle clustering. Multiple theoretical models have predicted a breathing mode of the Hoyle State at Ex≈9 MeV, corresponding to a radial in-phase oscillation of the underlying α clusters. The C12(α,α′)C12 and C14(p,t)C12 reactions were employed to populate states in 12C in order to search for this predicted breathing mode. A self-consistent, simultaneous analysis of the inclusive spectra with R-matrix lineshapes, together with angular distributions of charged-particle decay, yielded clear evidence for excess monopole strength at Ex≈9 MeV which is highly collective. Reproduction of the experimentally observed inclusive yields using a fit, with consistent population ratios for the various broad states, required an additional source of monopole strength. The interpretation of this additional monopole resonance as the breathing-mode excitation of the Hoyle state would provide evidence supporting a D3h symmetry for the Hoyle state itself. The excess monopole strength may complicate analysis of the properties of the Hoyle state, modifying the temperature dependence of the 3α rate at T9≳2 and ultimately, the predicted nucleosynthesis in explosive stars.

Search for rare 3-α decays in the region of the Hoyle state of 12C

We performed a refined analysis of the data collected with the CHIMERA detector with the aim of searching rare 3-α decay modes near the Hoyle state of 12C. In particular we investigated the direct and sequential decay modes of the Hoyle state including also the recently suggested Efimov state. The comparison of selected events with simulations constrains the precision achievable with measurements in different experimental conditions. We observe good sensitivities to different decay scenarios also in the CHIMERA spherical region notwithstanding its limited angular resolution. The search for the decay of the Efimov state in 12C was performed by using new selection strategies as suggested by simulations. The obtained results are not conclusive, however, we are able to exclude the presence of a sequential decay of this state, while our data could be compatible with its direct decay.

Measurement of 12C(n,n’) reaction cross section to determine triple-alpha reaction rate in high-density environments

The reaction rate of the triple-alpha reaction can be enhanced in hot and dense environments due to the deexcitation of the Hoyle state in 12C by neutrons. The cross section of the deexcitation should be determined for the enhanced reaction rate. We plan to obtain the cross section by measuring the inverse reaction using a neutron beam around 10 MeV and an active target system. In the present paper, we report a proof-of-principle experiment using a neutron beam at 14 MeV. The obtained cross section is consistent with a previous result, demonstrating the validity of our method.