Selected topics in nuclear astrophysics

In this lectures after a brief introduction to stellar reaction rates and its implementation in nuclear networks I discuss the nuclear aspects of the collapse of the inner core of massive stars once it has run out of its nuclear energy source and of the star's explosion as a type II supernova and the explosive nucleosynthesis occurring during this explosion which leads to the production of heavy elements by the rapid neutron capture process and potentially also by the recently discovered νp process.     

核天体物理实验研究

研究微观世界的核物理与研究宏观世界的天体物理自然融合形成了前沿交叉学科——核天体物理。核天体物理的主要研究目标是应用核物理的知识和规律来阐释宇宙演化进程中化学元素合成及演化过程、时间标度、天体环境和天体场所,来说明恒星中核燃烧产生的能量及其对恒星结构和演化的影响,来认识致密天体的结构和性质。文章阐述了核天体物理研究的意义和现状,介绍了有关基本概念和天体网络模型所需的核物理输入量以及广泛应用的一些实验方法,同时扼要地分析了利用国内大科学装置开展核天体物理研究的可行性。最后对未来我国深地科学与工程实验室的建设前景做了展望。     

核天体物理实验研究

研究微观世界的核物理与研究宏观世界的天体物理自然融合形成了前沿交叉学科——核天体物理。核天体物理的主要研究目标是应用核物理的知识和规律来阐释宇宙演化进程中化学元素合成及演化过程、时间标度、天体环境和天体场所,来说明恒星中核燃烧产生的能量及其对恒星结构和演化的影响,来认识致密天体的结构和性质。文章阐述了核天体物理研究的意义和现状,介绍了有关基本概念和天体网络模型所需的核物理输入量以及广泛应用的一些实验方法,同时扼要地分析了利用国内大科学装置开展核天体物理研究的可行性。最后对未来我国深地科学与工程实验室的建设前景做了展望。     

Microscopic cluster models in nuclear astrophysics

We present a brief overview of cluster models in nuclear astrophysics, with emphasis on microscopic approaches. General problems of the field are outlined, and we develop the specificities of the resonating group method. The ⁷Be(p,γ)⁸B reaction is treated as a typical example.     

The Trojan Horse Method in nuclear astrophysics

The study of energy production and nucleosynthesis in stars requires an increasingly precise knowledge of the nuclear reaction rates at the energies of interest. To overcome the experimental difficulties arising from the small cross sections at those energies and from the presence of the electron screening, the Trojan Horse Method has been introduced. The method provides a valid alternative path to measure unscreened low-energy cross sections of reactions between charged particles, and to retrieve information on the electron screening potential when ultra-low energy direct measurements are available.     

Fast neutron capture reactions in nuclear astrophysics

The cross sections of the neutron capture reaction on light nuclei, protons,¹²C and¹⁶O, were measured at astrophysically relevant energies between 10 and 300 keV. They are not only important for estimating yields of elements in the primordial nucleosynthesis, stellar nucleosynthesis of s-, r-, and p-processes, but also to study the role of meson exchange currents, the nuclear structure and the reaction mechanism. In the measurement, we used a prompt γ-ray detection method, combined with a pulsed neutron beam, and a recently developed Monte-Carlo code, TIME-MULTI, to correct for neutron multiple-scattering effects in a sample.     

Measurements of stellar and explosive nuclear astrophysics reactions

This paper described the nuclear astrophysical studies using the unstable ion beam facility GIRAFFE in CIAE, by indirect measurements. We measured the angular distributions for some single proton or neutron transfer reactions, such as ⁷Be(d,n)⁸B, ¹¹C(d,n)¹²N, ⁸Li(d,p)⁹Li and ¹³N(d,n)¹⁴O in inverse kinematics, and derived the astrophysical S-factors or reaction rates of ⁷Be(p,γ)⁸B, ¹¹C(p,γ)¹²N, ⁸Li(n,γ)⁹Li, ¹³N(p,γ)¹⁴O by asymptotic normalization coefficient, spectroscopic factor, and R-matrix approach at astrophysically relevant energies. Some most recent progress of nuclear astrophyiscal work in CIAE are also summarized.     

On the nuclear modification factor at RHIC and LHC

We show that pQCD factorization incorporated with pre-hadronization energy-loss effect naturally leads to flatness of the nuclear modification factor RAA$R_{\mathit{AA}}$ for produced hadrons at high transverse momentum pT$p_T$. We consider two possible scenarios for the pre-hadronization: In scenario 1, the produced gluon propagates through dense QCD medium and loses energy. In scenario 2, all gluons first decay to quark–antiquark pairs and then each pair loses energy as propagating through the medium. We show that the estimates of the energy-loss in these two different models lead to very close values and is able to explain the suppression of high-pT$p_T$ hadrons in nucleus–nucleus collisions at RHIC. We show that the onset of the flatness of RAA$R_{\mathit{AA}}$ for the produced hadron in central collisions at midrapidity is about pT≈15$p_T\approx 15$ and 25 GeV at RHIC and the LHC energies, respectively. We show that the smallness (RAA<0.5$R_{\mathit{AA}}<0.5$ ) and the high-pT$p_T$ flatness of RAA$R_{\mathit{AA}}$ obtained from the kT$k_T$ factorization supplemented with the Balitsky–Kovchegov (BK) equation is rather generic and it does not strongly depend on the details of the BK solutions. We show that energy-loss effect reduces the nuclear modification factor obtained from the kT$k_T$ factorization about 30–50% at moderate pT$p_T$.     

Complexity methods applied to turbulence in plasma astrophysics

In this review many of the well known tools for the analysis of Complex systems are used in order to study the global coupling of the turbulent convection zone with the solar atmosphere where the magnetic energy is dissipated explosively. Several well documented observations are not easy to interpret with the use of Magnetohydrodynamic (MHD) and/or Kinetic numerical codes. Such observations are: (1) The size distribution of the Active Regions (AR) on the solar surface, (2) The fractal and multi fractal characteristics of the observed magnetograms, (3) The Self-Organised characteristics of the explosive magnetic energy release and (4) the very efficient acceleration of particles during the flaring periods in the solar corona. We review briefly the work published the last twenty five years on the above issues and propose solutions by using methods borrowed from the analysis of complex systems. The scenario which emerged is as follows: (a) The fully developed turbulence in the convection zone generates and transports magnetic flux tubes to the solar surface. Using probabilistic percolation models we were able to reproduce the size distribution and the fractal properties of the emerged and randomly moving magnetic flux tubes. (b) Using a Non Linear Force Free (NLFF) magnetic extrapolation numerical code we can explore how the emerged magnetic flux tubes interact nonlinearly and form thin and Unstable Current Sheets (UCS) inside the coronal part of the AR. (c) The fragmentation of the UCS and the redistribution of the magnetic field locally, when the local current exceeds a Critical threshold, is a key process which drives avalanches and forms coherent structures. This local reorganization of the magnetic field enhances the energy dissipation and influences the global evolution of the complex magnetic topology. Using a Cellular Automaton and following the simple rules of Self Organized Criticality (SOC), we were able to reproduce the statistical characteristics of the observed time series of the explosive events, (d) finally, when the AR reaches the turbulently reconnecting state (in the language of the SOC theory this is called SOC state) it is densely populated by UCS which can act as local scatterers (replacing the magnetic clouds in the Fermi scenario) and enhance dramatically the heating and acceleration of charged particles.     

Topology of nuclear reaction networks of interest for astrophysics

正Our understanding of the observed elemental abundance in the universe, stemming from nuclear reactions during the big bang or from nucleosynthesis within stellar environments,requires theoretical analyses based on multidimensional nucleosynthesis calculations involving hundreds of nuclei connected via thousands of nuclear processes. Up to recently,full nucleosynthesis network calculations remained computationally expensive and prohibitive. A recent publication by a Chinese group led by YuGang Ma [1] has proved that     

The evaluation of half-lives and other decay data used in nuclear astrophysics and cosmochronology

The current status of some decay data used in nuclear astrophysics and cosmochronology is presented. The half-life of ⁷⁹Se has been evaluated as 3.6(3) × 10⁵ yr. The total energy of non-neutrino radiation released in act of ³⁷Ar decay has been obtained being 2.709 (16) keV per disintegration. The recommended half-life values of the long-lived radionuclides (T _1/2 ≳ 10⁶ yr) of ²⁶Al, ⁴⁰K, ⁵³Mn, ⁶⁰Fe, ⁸⁷Rb, ⁹³Zr, ⁹⁸Tc, ¹⁰⁷Pd, ¹²⁹I, ¹³⁵Cs, ¹⁴⁶Sm, ¹⁷⁶Lu, ¹⁸²Hf, ¹⁸⁷Re, ²⁰⁵Pb, ²³²Th, ²³⁵U, ²³⁸U, ²⁴⁴Pu, and ²⁴⁷Cm are given based on the evaluations published until 2010.     

Beta-delayed neutron spectra for application in reactor technology,nuclear physics and astrophysics

Recent developments in ß-delayed neutron (DN) spectroscopy are reviewed, and the importance of DN energy spectra for various problems in reactor physics, nuclear physics and astrophysics is discussed.     

Shallow-underground accelerator sites for nuclear astrophysics: Is the background low enough?

In order to reliably estimate the rate of a charged particle induced nuclear reaction in a non-explosive astrophysical scenario, its cross-section must be measured far below the Coulomb barrier. However, at the corresponding energies the cross-section values are very low, so that the experimental counting rate is dominated by cosmic-ray induced background, even if a suitable anticoincidence shield is applied. This problem can be overcome by performing an accelerator-based experiment in a deep underground site, as has been done with great success at the LUNA 0.4MV accelerator in Gran Sasso, Italy. Several underground accelerators with higher beam energy are in the planning phase worldwide. All of them are shielded by over 1000m of rock, a depth at which cosmic-ray effects are negligible for the purposes of nuclear astrophysics experiments. It is shown here that a combined approach, using a shallow-underground laboratory below 47m of rock and an active shield to veto surviving muons in simple detectors, results in a background level that is not far from that of deep underground sites. Data have been obtained using two ??traveling?? ??-detectors. They have been transported both shallow underground, to the Dresden Felsenkeller in Germany, and deep underground, to the Gran Sasso laboratory in Italy. As shallow-underground facilities are more easily accessible than deep-underground ones, the present finding holds the promise of greatly accelerated progress in the field of cross-section measurements for nuclear astrophysics.     

Promising lines of research in the realms of laboratory nuclear astrophysics by means of powerful lasers

Basic nuclear-astrophysics problems that can be studied under laboratory conditions at a laserradiation intensity of 10¹⁸ W/cm² or more are specified. These are the lithium problem, the problem of determining neutron sources for s-processes of heavy-element formation, the formation of bypassed stable p-nuclei, and nuclear reactions involving isotopes used by astronomers for diagnostics purposes. The results of experiments at the Neodym laser facility are presented, and proposals for further studies in these realms are formulated.