Stellar Burning Research Articles

Nuclear Astrophysics deep underground and the LUNA experiment

The cross sections of nuclear reactions relevant for astrophysics are crucial ingredients to understand the energy generation inside stars and the synthesis of the elements. In stars, nuclear reactions take place at energies well below the Coulomb barrier. As a result, their cross sections are often too small to be measured in laboratories on the Earth’s surface, where the signal would be overwhelmed by the cosmic-ray induced background. An effective way to suppress the cosmic-ray induced background is to perform experiments in underground laboratories. LUNA is a unique facility located at Gran Sasso National Laboratories (Italy) and devoted to Nuclear Astrophysics. The extremely low background achieved at LUNA allows to measure nuclear cross sections directly at the energies of astrophysical interest. Over the years, many crucial reactions involved in stellar hydrogen burning as well as Big Bang Nucleosynthesis have been measured at LUNA. This paper provides a short overview on underground Nuclear Astrophysics and discusses the latest results and future perspectives of the LUNA experiment.

THM applied to the investigation of explosive astrophysical scenarios

The Trojan Horse Method (THM) makes use of quasi-free reactions to deduce the cross section of nuclear reactions relevant for astrophysics at the energies of interest. Thanks to the suppression of the Coulomb barrier, the THM cross section does not exponentially vanishes at astrophysical energies. Here we will briefly summarise the fundamentals of the method, then we will discuss two applications of the method to reactions that have a pivotal role in the latest stages of stellar evolution, leading to explosive scenarios. In particular, we will focus on the indirect investigation of the 18F(p,α)15O reaction, which is the most important 18F destruction channel in novae, and the 12C + 12C reaction, which plays a critical role in astrophysics to understand stellar burning scenarios in carbon-rich environments, including supernovae.

Theoretical exploration of S-factors for nuclear reactions of astrophysical importance

We present here a robust analytical model based on nuclear reaction theory for non-resonant fusion cross sections near Coulomb barrier. The astrophysical S-factors involving stable and neutron rich isotopes of C, O, Ne, Mg and Si for fusion reactions have been calculated in the centre of mass energy range of 2-30 MeV. The model is based on the tunneling through barrier arising out of nuclear, Coulomb and centrifugal potentials. Our formalism predicts precisely the suppression of S-factor at sub-barrier energies which are of astrophysical interest by using only five parameters for asymmetric systems and just four parameters for symmetric systems and thus providing an accurate and very compact description of S-factor. The cross sections can be convoluted with Maxwell-Boltzmann distribution of energies to obtain thermo- or pycno-nuclear reaction rates relevant to nucleosynthesis at high density environments and stellar burning at high temperatures as well as for 34Ne + 34Ne fusion occurring in the inner crust of accreting neutron stars.

Impact ofO16(γ,α)C12measurements on theC12(α,γ)O16astrophysical reaction rate

The $^{12}$C($\alpha,\gamma$)$^{16}$O reaction, an important component of stellar helium burning, plays a key role in nuclear astrophysics. It has direct impact on the evolution and final state of massive stars, while also influencing the elemental abundances resulting from nucleosynthesis in such stars. Providing a reliable estimate for the energy dependence of this reaction at stellar helium burning temperatures has been a major goal for the field. In this work, we study the role of potential new measurements of the inverse reaction, $^{16}$O($\gamma,\alpha$)$^{12}$C, in reducing the overall uncertainty. A multilevel R-matrix analysis is used to make extrapolations of the astrophysical S factor for this reaction to the stellar energy of 300 keV. The statistical precision of the S-factor extrapolation is determined by performing multiple fits to existing E1 and E2 ground state capture data, including the impact of possible future measurements of the $^{16}$O($\gamma,\alpha$)$^{12}$C reaction. In particular, we consider a proposed JLab experiment that will make use of a high-intensity low-energy bremsstrahlung beam that impinges on an oxygen-rich single-fluid bubble chamber in order to measure the total cross section for the inverse reaction. The importance of low energy data as well as high precision data is investigated.

Nuclear astrophysics and resonant reactions: Exploring the threshold region with the Trojan Horse Method

Resonant reactions play an important role in astrophysics as they might significantly enhance the cross section with respect to the direct reaction contribution and alter the nucleosynthetic flow. Moreover, resonances bear information about states in the intermediate compound nucleus formed in the reaction. However, nuclear reactions in stars take place at energies well below [Formula: see text] MeV and the Coulomb barrier, exponentially suppressing the cross section, and the electron screening effect, due to the shielding of nuclear charges by atomic electrons, make it very difficult to provide accurate input data for astrophysics. Therefore, indirect methods have been introduced; in particular, we will focus on the Trojan Horse Method. We will briefly discuss the theory behind the method, to make clear its domain of applicability, the advantages and the drawbacks, and two recent cases will be shortly reviewed: the [Formula: see text] reaction, which is an important fluorine destruction channel in the proton-rich outer layers of asymptotic giant branch (AGB) stars, and the [Formula: see text] reactions, which play a critical role in astrophysics to understand stellar burning scenarios in carbon-rich environments.

Research at the Institute of Astronomy and Astrophysics of the Université Libre de Bruxelles

Over the years, a coherent research strategy has developed in the field of stellar physics at the Institute of Astronomy and Astrophysics (IAA). It involves observational studies (chemical composition of giant stars, binary properties, tomography of stellar atmospheres) that make use of the large ESO telescopes as well as of other major instruments. The presence of a high-resolution spectrograph on the 3.6-m Devasthal Optical Telescope (DOT) would therefore be highly beneficial to IAA research. These observations are complemented and supported by theoretical studies of mass transfer in binary systems, of standard and non-standard stellar evolution (including the modelling of stellar hydrodynamical nuclear burning for application to certain thermonuclear supernovae) and of nuclear astrophysics (a field in which IAA has been recognized for a long time as an international centre of excellence), including the theory of nucleosynthesis. IAA also addresses the end-points of stellar evolution as it is carrying out research on the compact remnants of stellar evolution of massive stars: neutron stars.

Felsenkeller 5 MV underground accelerator: Towards the Holy Grail of Nuclear Astrophysics 12C(α, γ)16O

Low-background experiments with stable ion beams are an important tool for putting the model of stellar hydrogen, helium, and carbon burning on a solid experimental foundation. The pioneering work in this regard has been done by the LUNA collaboration at Gran Sasso, using a 0.4 MV accelerator. The present contribution reviews the status of the project for a higher-energy underground accelerator in Felsenkeller, Germany. Results from γ-ray, neutron, and muon background measurements in the Felsenkeller underground site in Dresden, Germany, show that the background conditions are satisfactory. Two tunnels of the Felsenkeller site have recently been refurbished for the installation of a 5MV high-current Pelletron accelerator. Civil construction work has completed in March 2018. The accelerator will provide intense, 50 μA, beams of 1H+, 4He+, and 12C+ ions, enabling research on astrophysically relevant nuclear reactions with unprecedented sensitivity.

Nuclear Astrophysics Deep Underground

Cross sections of nuclear reactions relevant for astrophysics are crucial ingredients to understand the energy generation inside stars and the synthesis of the elements. At astrophysical energies, nuclear cross sections are often too small to be measured in laboratories on the Earth surface, where the signal would be overwhelmed by the cosmic-ray induced background. LUNA is a unique Nuclear Astrophysics experiment located at Gran Sasso National Laboratories. The extremely low background achieved at LUNA allows to measure nuclear cross sections directly at the energies of astrophysical interest. Over the years, many crucial reactions involved in stellar hydrogen burning as well as Big Bang nucleosynthesis have been measured at LUNA. The present contribution provides an overview on underground Nuclear Astrophysics as well as the latest results and future perspectives of the LUNA experiment.

Experimental measurement of the 12C+16O fusion cross sections at astrophysical energies

The total cross sections of the 12C+16O fusion have been experimentally determined at low energies to investigate the role of this reaction during late stellar evolution burning phases. A high-intensity oxygen beam was produced by the 5MV pelletron accelerator at the University of Notre Dame impinging on a thick ultra-pure graphite target. Protons and γ-rays were measured simultaneously in the center-of-mass energy range from 3.64 to 5.01 MeV, using strip silicon and HPGe detectors. Statistical model calculations were employed to interpret the experimental results. A new broad resonance-like structure is observed for the 12C+16O reaction, and a decreasing trend of its S-factor towards low energies is found.

Experimental measurement of C12+O16 fusion at stellar energies

The total cross section of the $^{12}\mathrm{C}+^{16}\mathrm{O}$ fusion reaction has been measured at low energies to investigate the role of this reaction during late stellar evolution burning phases. A high-intensity oxygen beam, produced by the 5 MV pelletron accelerator at the University of Notre Dame, impinged on a thick, ultrapure graphite target. Protons and $\ensuremath{\gamma}$ rays were simultaneously measured in the center-of-mass energy range from 3.64 to 5.01 MeV for singles and from 3.73 to 4.84 MeV for coincidence events, using silicon and Ge detectors. Statistical model calculations were employed to interpret the experimental results. The emergence of a new resonance-like broad structure and a decreasing trend in the $S$-factor data towards lower energies (opposite to previous data) are found for the $^{12}\mathrm{C}+^{16}\mathrm{O}$ fusion reaction. Based on these results the uncertainty range of the reaction rate within the temperature range of late stellar burning environments is discussed.

Nanostructured targets irradiation by ns-laser for nuclear astrophysics applications: first results

The studies discussed in this work are related to a scientific program that aims to reproduce astrophysical-plasmas in laboratory in order to better understand the nuclear processes involved in the stellar burning. An experimental campaign aiming to investigate the effects of innovative nanostructured targets based on Ni, Fe and Co nanowires on laser energy absorption in the ns time domain has been carried out at the LENS (Laser Energy for Nuclear Science) laboratory of INFN-LNS, Catania. Nanowires structures are tuned to increase the light absorption in the visible and infrared range due possibly to plasmonic excitation driven by the incoming photons. Different diagnostics techniques permit to monitor the plasma and to determine its reproducibility. Targets were then irradiated by Nd:YAG 2J, 6 ns infrared laser (λ = 1064 nm) at different pumping energies. Some preliminary results will be illustrated.

23Na(p,γ)24Mg Cross Section Measurements

As a link between the NeNa and MgAl cycles in stellar burning, the reaction 23 Na(p,γ )24 Mg is of interest for various astrophysical scenarios, such as AGB stars. A combined effort at the Laboratory for Underground Nuclear Astrophysics (LUNA) and the Nuclear Science Laboratory (NSL) at the University of Notre Dame aims at a cross section determination for the this reaction, to constrain the astrophysical reaction rate by improving the knowledge of the resonance strengths and the non-resonant component. Experiments at LUNA benefit from the underground location at the Gran Sasso National Laboratory which allows for the measurement of resonances at low energies with high sensitivity in a low background environment. Measurements at the University of Notre Dame pursue a determination of the non-resonant cross section at higher energies. We present the two experiments and the status of the data analysis.

Stellar helium burning in other universes: A solution to the triple alpha fine-tuning problem

Motivated by the possible existence of other universes, with different values for the fundamental constants, this paper considers stellar models in universes where 8Be is stable. Many previous authors have noted that stars in our universe would have difficulty producing carbon and other heavy elements in the absence of the well-known 12C resonance at 7.6 MeV. This resonance is necessary because 8Be is unstable in our universe, so that carbon must be produced via the triple alpha reaction to achieve the requisite abundance. Although a moderate change in the energy of the resonance (200–300 keV) will indeed affect carbon production, an even smaller change in the binding energy of beryllium (∼100 keV) would allow 8Be to be stable. A stable isotope with A=8 would obviate the need for the triple alpha process in general, and the 12C resonance in particular, for carbon production. This paper explores the possibility that 8Be can be stable in other universes. Simple nuclear considerations indicate that bound states can be realized, with binding energy ∼0.1−1 MeV, if the fundamental constants vary by a ∼few−10%. In such cases, 8Be can be synthesized through helium burning, and 12C can be produced later through nuclear burning of beryllium. This paper focuses on stellar models that burn helium into beryllium; once the universe in question has a supply of stable beryllium, carbon production can take place during subsequent evolution in the same star or in later stellar generations. Using both a semi-analytic stellar structure model as well as a state-of-the-art stellar evolution code, we find that viable stellar configurations that produce beryllium exist over a wide range of parameter space. Finally, we demonstrate that carbon can be produced during later evolutionary stages.

Indirect methods in nuclear astrophysics

We discuss recent developments in indirect methods used in nuclear astrophysics to determine the capture cross sections and subsequent rates of various stellar burning processes, when it is difficult to perform the corresponding direct measurements. We discuss in brief, the basic concepts of Asymptotic Normalization Coefficients, the Trojan Horse Method, the Coulomb Dissociation Method, (d,p), and charge-exchange reactions.

Experimental study of the 13C+12C fusion reaction at deep sub-barrier energies

Heavy-ion fusion reactions between light nuclei such as carbon and oxygen isotopes have been studied because of their significance for a wide variety of stellar burning scenarios. One important stellar reaction is 12C+12C, but it is difficult to measure it in the Gamow window because of very low cross sections and several resonances occurring. Hints can be obtained from the study of 13C+12C reaction. We have measured this process by an activation method for energies down to Ecm=2.5 MeV using 13C beams from the Bucharest 3 MV tandetron and gamma-ray deactivation measurements in our low and ultralow background laboratories, the latter located in a salt mine about 100 km north of Bucharest. Results obtained so far are shown and discussed in connection with the possibility to go even further down in energy and with the interpretation of the reaction mechanism at such deep sub-barrier energies.

Cross Section Measurements for the 23Na(p,γ)24Mg Reaction at LUNA

LUNA, the Laboratory for Underground Nuclear Astrophysics, is an accelerator facility for measurements of nuclear cross sections of astrophysical interest. The greatly reduced cosmic ray background at LUNA's underground location in the Gran Sasso National Laboratory (LNGS) allows direct measurements of weak reactions at low energies.One of the reactions currently under study at LUNA is 23Na(p,γ)24Mg, which links the NeNa and MgAl cycles in stellar burning. The LUNA facility is presented, with a focus on the current experimental efforts to study the reaction 23Na(p,γ)24Mg.

NEW ASTROPHYSICAL REACTION RATE FOR THE 12C(α, γ)16O REACTION

ABSTRACT A new astrophysical reaction rate for 12C(α, γ)16O has been evaluated on the basis of a global R-matrix fitting to the available experimental data. The reaction rates of 12C(α, γ)16O for stellar temperatures between 0.04 ≤ T 9 ≤ 10 are provided in a tabular form and by an analytical fitting expression. At T 9 = 0.2, the reaction rate is (7.83 ± 0.35) × 1015 cm3 mol−1 s−1, where stellar helium burning occurs.

Underground nuclear astrophysics studies with CASPAR

The drive of low-energy nuclear astrophysics laboratories is to study the reactions of importance to stellar burning processes and elemental production through stellar nucleosynthesis, over the energy range of astrophysical interest. As laboratory measurements approach the stellar burning window, the rapid drop off of cross-sections is a significant barrier and drives the need to lower background interference. The natural background suppression of underground accelerator facilities enables the extension of current experimental data to lower energies. An example of such reactions of interest are those thought to be sources of neutrons for the s-process, the major production mechanism for elements above the iron peak. The reactions 13 C(α,n)16 O and 22 Ne(α,n)25 Mg are the proposed initial focus of the new nuclear astrophysics accelerator laboratory (CASPAR) currently under construction at the Sanford Underground Research Facility, Lead, South Dakota

The12C(12C,α)20Ne and12C(12C,p)23Na reactions at the Gamow peak via the Trojan Horse Method

A measurement of the 12 C( 14 N, α 20 Ne) 2 H and 12 C( 14 N,p 23 Na) 2 Hreactions has been performed at a 14 N beam energy of 30.0 MeV. The experiment aims to explore the extent to which contributing 24 Mg excited states can be populated in the quasi-free reaction off the deuteron in 14 N. In particular, the 24 Mg excitation region explored in the measurement plays a key role in stellar carbon burning whose cross section is commonly determined by extrapolating high-energy fusion data. From preliminary results, α and proton channels are clearly identified. In particular, ground and first excited states of 20 Ne and 23 Na play a major role.

China carves out larger role in underground science

As it is doing in so many areas of science, China is racing onto the world stage of underground astroparticle physics.