Astrophysical Nuclear Reactions Research Articles

Re-examining the Lithium abundance problem in Big-Bang nucleosynthesis

One of the three testaments in favor of the big bang theory is the prediction of the primordial elemental abundances in the big-bang nucleosynthesis (BBN). The Standard BBN is a parameter-free theory due to the precise knowledge of the baryon-to-photon ratio of the Universe obtained from studies of the anisotropies of cosmic microwave background radiation. Although the computed abundances of light elements during primordial nucleosynthesis and those determined from observations are in good agreement throughout a range of nine orders of magnitude, there is still a disparity of 7Li abundance overestimated by a factor of ∼2.5 when calculated theoretically. The number of light neutrino flavors, the neutron lifetime and the baryon-to-photon ratio in addition to the astrophysical nuclear reaction rates determine the primordial abundances. We previously looked into the impact of updating baryon-to-photon ratio and neutron lifetime and changing quite a few reaction rates on the yields of light element abundances in BBN. In this work, calculations are performed using new reaction rates for 3H(p, γ)4He, 6Li(p, γ)7Be, 7Be(p, γ)8B, 13N(p, γ)14O, 7Li(n, γ)8Li and 11B(n, γ)12B along with the latest measured value of neutron lifetime. We observe from theoretical calculations that these changes result in improvement by causing further reduction in the abundance of 7Li than calculated earlier.

Pynucastro: A Python Library for Nuclear Astrophysics

We describe pynucastro 2.0, an open-source library for interactively creating and exploring astrophysical nuclear reaction networks. We demonstrate new methods for approximating rates and use detailed balance to create reverse rates, show how to build networks and determine whether they are appropriate for a particular science application, and discuss the changes made to the library over the past few years. Finally, we demonstrate the validity of the networks produced and share how we use pynucastro networks in simulation codes.

Transfer Reactions in Nuclear Astrophysics

Transfer reactions are important tool in nuclear astrophysics. These reactions allow us to identify states in nuclei and to find the corresponding energies, to determine if these states can contribute to astrophysical nuclear reactions and ultimately to determine the strength of that contribution. In this paper,the basic details of how transfer reactions may be used in nuclear astrophysics are set out along with some common pitfalls to avoid.

An Improved Method for Coupling Hydrodynamics with Astrophysical Reaction Networks

Reacting astrophysical flows can be challenging to model, because of the difficulty in accurately coupling hydrodynamics and reactions. This can be particularly acute during explosive burning or at high temperatures where nuclear statistical equilibrium is established. We develop a new approach, based on the ideas of spectral deferred corrections (SDC) coupling of explicit hydrodynamics and stiff reaction sources as an alternative to operator splitting, that is simpler than the more comprehensive SDC approach we demonstrated previously. We apply the new method to a double-detonation problem with a moderately sized astrophysical nuclear reaction network and explore the time step size and reaction network tolerances, to show that the simplified-SDC approach provides improved coupling with decreased computational expense compared to traditional Strang operator splitting. This is all done in the framework of the Castro hydrodynamics code, and all algorithm implementations are freely available.

Description of isospin mixing by a generator coordinate method

Background: Isospin mixing is an interesting feature of atomic nuclei that plays a crucial role in the astrophysical nuclear reactions. However, variational nuclear structure models cannot describe it in a straightforward manner.Purpose: We propose a tractable method to describe isospin mixing within the framework of a generator coordinate method and demonstrate its usability.Method: We generate basis wave functions by applying the Fermi transition operator to the wave functions of isobars. The superposition of these basis wave functions and variationally obtained wave functions quantitatively describes isospin mixing.Results: We apply our method to $^{14}\mathrm{N}$ and show that it reasonably describes both $T=0$ and 1 states and their mixing. The energy spectrum and $E1$ transition strengths are compared with the experimental data.Conclusion: The proposed method is effective in describing isospin mixing and is particularly useful for discussing of $\ensuremath{\alpha}$-capture reactions of $N=Z$ nuclei.

Transfer Reactions As a Tool in Nuclear Astrophysics

Nuclear reaction rates are one of the most important ingredients in describing how stars evolve. The study of the nuclear reactions involved in different astrophysical sites is thus mandatory to address most questions in nuclear astrophysics. Direct measurements of the cross-sections at stellar energies are very challenging–if at all possible. This is essentially due to the very low cross-sections of the reactions of interest (especially when it involves charged particles), and/or to the radioactive nature of many key nuclei. In order to overcome these difficulties, various indirect methods such as the transfer reaction method at energies above or near the Coulomb barrier are used to measure the spectroscopic properties of the involved compound nucleus that are needed to calculate cross-sections or reaction rates of astrophysical interest. In this review, the basic features of the transfer reaction method and the theoretical concept behind are first discussed, then the method is illustrated with recent performed experimental studies of key reactions in nuclear astrophysics.

Bound-to-continuum potential model for the ( p,γ ) reactions of the CNO nucleosynthesis cycle

The study of CNO cycle involves the examination of the proton radiative capture, or the $(p,\gamma)$ reactions below 2 MeV. The astrophysical $\mathcal{S}$ factor characterizing the $(p,\gamma)$ reaction is usually reduced to the electric dipole transition $E1$ from the scattering state to the bound state. In this work, the partial scattering and the single-particle bound wave functions in the reduced matrix element of the transition are obtained from the single self-consistent mean-field potential deduced from the Skyrme Hartree-Fock calculation. The astrophysical $\mathcal{S}$ factors of the $(p,\gamma)$ reactions in the CNO cycle were successfully reproduced. The self-consistent Hartree-Fock calculation from the discrete to the continuum is a promising approach for the microscopic analysis of the nucleon-induced reactions in nuclear astrophysics.

Nuclear Reactions in Astrophysics: A Review of Useful Probes for Extracting Reaction Rates

Astrophysical simulations require knowledge of a wide array of reaction rates. For a number of reasons, many of these reaction rates cannot be measured directly and instead are probed with indirect nuclear reactions. We review the current state of the art regarding the techniques used to extract reaction information that is relevant to describe stars, including their explosions and collisions. We focus on the theoretical developments over the last decade that have had an impact on the connection between the laboratory indirect measurement and the astrophysical desired reaction. This review includes three major probes that have been, and will continue to be, widely used in our community: transfer reactions, breakup reactions, and charge-exchange reactions.

Trojan horse method as an indirect approach to study resonant reactions in nuclear astrophysics

We review the Trojan horse method and its applications to resonant reactions in nuclear astrophysics.

Analytic Continuation of Scattering Data, Asymptotic Normalization Coefficients, and Astrophysics

The formalism of asymptotic normalization coefficients and methods for determining them from experimental nuclear data are discussed. A new method is proposed that uses the continuation of the modulus squared of a partial-wave amplitude. It is shown that the recently proposed $$\Delta$$ method for the continuation of elastic scattering data is not strictly correct. It is argued that, unlike asymptotic normalization coefficients, the values of spectroscopic factors cannot be unambiguously determined from experimental data on nuclear reactions. The Trojan horse method is discussed, which allows one to determine cross sections of binary astrophysical nuclear reactions from the data of reactions with three particles in the final state.

Constraining the External Capture to the ^{16}O Ground State and the E2 S Factor of the ^{12}C(α,γ)^{16}O Reaction.

The ^{12}C(α,γ)^{16}O reaction is one of the most crucial reactions in nuclear astrophysics. The E2 external capture to the ^{16}O ground state (GS) has not been emphasized in previous analyses but may make a significant contribution to the ^{12}C(α,γ)^{16}O cross section depending on the value of the GS asymptotic normalization coefficient (ANC). In the present work, we determine this ANC to be 337±45 fm^{-1/2} through the ^{12}C(^{11}B,^{7}Li)^{16}O reaction using a high-precision magnetic spectrograph. This sheds light on the existing large discrepancy of more than 2 orders of magnitude between the previously reported ANC values. Based on the new ANC, we experimentally constrain the GS external capture and show that through interference with the high energy tail of the 2^{+} subthreshold state, a substantial enhancement in the GS S_{E2}(300) factor can be obtained (70±7 keV b) compared to that of a recent review (45keV b), resulting in an increase of the total S factor from 140 to 162keV b, which is now in good agreement with the value obtained by reproducing supernova nucleosynthesis calculations with the solar-system abundances. This work emphasizes that the external capture contribution for the ground state transition cannot be neglected in future analyses of the ^{12}C(α,γ)^{16}O reaction.

Atomic effects in astrophysical nuclear fusion reactions

The electron-screening acceleration of laboratory fusion reactions at astrophysical' energies is an unsolved problem of great importance to astrophysics. That effect is modeled here by considering the fusion of hydrogen-like atoms whose electron probability density is used in Poisson 's equation in order to derive the corresponding screened Coulomb potential energy. That way atomic excitations and deformations of the fusing atoms can be taken into account. Those potentials are then treated semiclassically in order to obtain the screening (accelerating) factor of the reaction. By means of the proposed model the effect of a superstrong magnetic field on laboratory Hydrogen fusion reactions is investigated here for the first time showing that, despite the considerable increase in the cross section of the dd reaction, the pp reaction is still too slow to justify experimentation. The proposed model is finally applied on the H2 (d, p) H3 fusion reaction describing satisfactorily the experimental data although some ambiguity remains regarding the molecular nature of the deuteron target. Notably, the present method gives a sufficiently high screening energy for Hydrogen fusion reactions so that the take-away energy of the spectator nucleus can also be taken into account.

Potential model for nuclear astrophysical fusion reactions with a square-well potential

The potential model for nuclear astrophysical reactions requires a considerably shallow nuclear potential when a square-well potential is employed to fit experimental data. We discuss the origin of this apparently different behavior from that obtained with a smooth Woods-Saxon potential, for which a deep potential is often employed. We argue that due to the sharp change of the potential at the boundary the radius parameter tends to be large in the square-well model, which results in a large absorption radius. The wave function then needs to be suppressed in the absorption region, which can eventually be achieved by using a shallow potential. We thus clarify the reason why the square-well potential has been able to reproduce a large amount of fusion data.

Big-Bang Nucleosynthesis and Primordial Lithium Abundance Problem

Prediction of the primordial abundances of elements in the big-bang nucleosynthesis (BBN) is one of the three strong evidences for the big bang model. Precise knowledge of the baryon-to-photon ratio of the Universe from observations of the anisotropies of cosmic microwave background radiation has made the Standard BBN a parameter-free theory. Although, there is a good agreement over a range of nine orders of magnitude between abundances of light elements deduced from observations and calculated in primordial nucleosynthesis, there remains a yet-unexplained discrepancy of $^7$Li abundance higher by a factor of $\sim 3$ when calculated theoretically. The primordial abundances depend on the astrophysical nuclear reaction rates and on three additional parameters, the number of light neutrino flavours, the neutron lifetime and the baryon-to-photon ratio in the universe. The effect of the modification of thirty-five reaction rates on light element abundance yields in BBN was investigated earlier by us. In the present work we have incorporated the most recent values of neutron lifetime and the baryon-to-photon ratio and further modified $^3$He($^4$He,$\gamma$)$^7$Be reaction rate which is used directly for estimating the formation of $^7$Li as a result of $\beta^+$ decay as well as the reaction rates for t($^4$He,$\gamma$)$^7$Li and d($^4$He,$\gamma$)$^6$Li. We find that these modifications reduce the theoretically calculated abundance of $^7$Li by $\sim 12\%$.

Characterization of the CATRiNA neutron detector system

Abstract The Compound Array for Transfer Reactions in Nuclear Astrophysics (CATRiNA) is an array of 16 liquid deuterated-benzene (C 6D6 ) neutron detectors . The fast-response and pulse-shape-discrimination (PSD) capabilities of liquid scintillators allow for the extraction of neutron energies via time-of-flight and for the identification and separation of neutron/gamma interactions. In addition to these properties, deuterated liquid scintillators have a structured pulse-height spectrum with promising capabilities for neutron spectroscopy . In this paper, we report on the development and characterization of the CATRiNA neutron detector system at Florida State University (FSU). Monte-Carlo simulations and measurements with neutron and γ -ray sources, as well as mono-energetic neutrons (E n = 2.3–7.3 MeV) from in-beam measurements using the 7 Li(p,n) reaction, were performed to characterize the detectors. CATRiNA is envisioned to be used to measure several nuclear reactions like (3 He,n), (d,n), (p,n), relevant for nuclear structure and nuclear astrophysics studies.

Double-folding potentials from chiral effective field theory

The determination of nucleus–nucleus potentials is important not only to describe the properties of the colliding system, but also to extract nuclear-structure information and for modelling nuclear reactions for astrophysics. We present the first determination of double-folding potentials based on chiral effective field theory at leading, next-to-leading, and next-to-next-to-leading order. To this end, we construct new soft local chiral effective field theory interactions. We benchmark this approach in the 16O–16O system, and present results for cross sections computed for elastic scattering up to 700 MeV in energy, as well as for the astrophysical S-factor of the fusion reaction.

Laboratory study of astrophysical collisionless shock at SG-II laser facility

Astrophysical collisionless shocks are amazing phenomena in space and astrophysical plasmas, where supersonic flows generate electromagnetic fields through instabilities and particles can be accelerated to high energy cosmic rays. Until now, understanding these micro-processes is still a challenge despite rich astrophysical observation data have been obtained. Laboratory astrophysics, a new route to study the astrophysics, allows us to investigate them at similar extreme physical conditions in laboratory. Here we will review the recent progress of the collisionless shock experiments performed at SG-II laser facility in China. The evolution of the electrostatic shocks and Weibel-type/filamentation instabilities are observed. Inspired by the configurations of the counter-streaming plasma flows, we also carry out a novel plasma collider to generate energetic neutrons relevant to the astrophysical nuclear reactions.

Study of background processes with the formation of neutrons in nuclear reactions in the energy range of 26–32 kev

The nature of background processes accompanying astrophysical nuclear reactions induced by hydrogen, helium, and neon ions in deuterated targets with small cross sections has been studied in calculations and experiments. The experiments have been performed at a Hall pulsed plasma accelerator in the ion energy range of 26–32 keV. The yield of background neutrons and γ-quanta with energies below 4 MeV in the proton-induced D(p, γ)3He reaction is primarily due to the presence of a natural impurity of gaseous deuterium in gaseous hydrogen and the chain of D(D, 3He)n → (n, γ) or (n, n'γ) reactions. A small contribution comes from the chain of D(1H, 1H)D → D(D, 3He)n → (n, γ) or (n, n'γ) reactions. It has been shown that background neutrons and γ-quanta from the D(4He, γ)6Li reaction are entirely due to the chain of D(4He, 4He)D → D(D, 3He)n → (n, γ) or (n, n'γ) reactions. It has been shown that the yield of neutrons and γ-ray photons detected at the interaction of neon ions with deuterated targets is also entirely due to the chain of elastic- scattering reactions of neon ions on deuterons in the target and to subsequent inelastic processes of interaction of deuterons accelerated at elastic scattering with other deuterons of the target. The main contribution to the yields of background neutrons and γ-quanta comes from doubly charged neon ions. The main conclusion is that the explanation of the yield of neutrons and γ-quanta at the interaction of hydrogen, helium, and neon ions with deuterated targets does not require “exotic” theoretical models.

Target characterizations for a 14N(p,γ)15O cross section measurement

The 14 N(p, γ ) 15 O reaction controls the rate of CNO cycle hydrogen burning in various astrophysical sites and it is therefore one of the most important reactions in nuclear astrophysics. An experimental program is in progress to measure the 14 N(p, γ ) 15 O cross section in a wide energy range using a novel approach. A crucial quantity for the cross section determination is the number of N atoms in the target. In this paper the results of different experiments used for N target characterization are presented.

Nuclear clustering and the electron screening puzzle

Electron screening changes appreciably the magnitude of astrophysical nuclear reactions within stars. This effect is also observed in laboratory experiments on Earth, where atomic electrons are present in the nuclear targets. Theoretical models were developed over the past 30 years and experimental measurements have been carried out to study electron screening in thermonuclear reactions. None of the theoretical models were able to explain the high values of the experimentally determined screening potentials. We explore the possibility that the "electron screening puzzle" is due to nuclear clusterization and polarization effects in the fusion reactions. We will discuss the supporting arguments for this scenario.