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In this review, we emphasize the interplay between astrophysical observations, modeling, and nuclear physics laboratory experiments. Several important nuclear cross sections for astrophysics have long been identified e.g. 12C(alpha,gamma)16O for stellar evolution, or 13C(alpha,n)16O and 22Ne(alpha,n)25Mg as neutron sources for the s-process. More recently, observations of lithium abundances in the oldest stars, or of nuclear gamma-ray lines from space, have required new laboratory experiments. New evaluation of thermonuclear reaction rates now includes the associated rate uncertainties that are used in astrophysical models to i) estimate final uncertainties on nucleosynthesis yields and ii) identify those reactions that require further experimental investigation. Sometimes direct cross section measurements are possible, but more generally the use of indirect methods is compulsory in view of the very low cross sections. Non-thermal processes are often overlooked but are also important for nuclear astrophysics, e.g. in gamma-ray emission from solar flares or in the interaction of cosmic rays with matter, and also motivate laboratory experiments. Finally, we show that beyond the historical motivations of nuclear astrophysics, understanding i) the energy sources that drive stellar evolution and ii) the origin of the elements can also be used to give new insights into physics beyond the standard model.
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The origin of the elements is a fascinating question that scientists have been trying to answer for the last seven decades. The formation of light elements in the primordial universe and heavier elements in astrophysical sources occurs through nuclear reactions. We can say that nuclear processes are responsible for the production of energy and synthesis of elements in the various astrophysical sites. Thus, nuclear reactions have a determining role in the existence and evolution of several astrophysical environments, from the Sun to the spectacular explosions of supernovae. Nuclear astrophysics attempts to address the most basic and important questions of our existence and future. There are still many issues that are unresolved such as, how stars and our Galaxy have formed and how they evolve, how and where are the heaviest elements made, what is the abundance of nuclei in the universe and what is the nucleosynthesis output of the various production processes and why the amount of lithium-7 observed is less than predicted. In this paper, we review our current understanding of the different astrophysical nuclear processes leading to the formation of chemical elements and pay particular attention to the formation of heavy elements occurring during high-energy astrophysical events. Thanks to the recent multi-messenger observation of a binary neutron star merger, which also confirmed production of heavy elements, explosive scenarios such as short gamma-ray bursts and the following kilonovae are now strongly supported as nucleosynthesis sites.
Knowledge on nuclear cluster physics has increased considerably since the pioneering discovery of 12C+12C resonances half a century ago and nuclear clustering remains one of the most fruitful domains of nuclear physics, facing some of the greatest challenges and opportunities in the years ahead. The occurrence of exotic shapes and/or Bose-Einstein alpha condensates in light N-Z alpha-conjugate nuclei is investigated. Evolution of clustering from stability to the drip-lines examined with clustering aspects persisting in light neutron-rich nuclei is consistent with the extension of the Ikeda-diagram to non alpha-conjugate nuclei.
Nuclear astrophysics, the union of nuclear physics and astronomy, went through an impressive expansion during the last twenty years. This could be achieved thanks to milestone improvements in astronomical observations, cross section measurements, powerful computer simulations and much refined stellar models. Italian groups are giving quite important contributions to every domain of nuclear astrophysics, sometimes being the leaders of worldwide unique experiments. In this paper we will discuss the astrophysical scenarios where nuclear astrophysics plays a key role and we will provide detailed descriptions of the present and future of the experiments on nuclear astrophysics which belong to the scientific programme of INFN (the National Institute for Nuclear Physics in Italy).
A bubble chamber has been developed to be used as an active target system for low energy nuclear astrophysics experiments. Adopting ideas from dark matter detection with superheated liquids, a detector system compatible with gamma-ray beams has been developed. This detector alleviates some of the limitations encountered in standard measurements of the minute cross sections of interest to stellar environments. While the astrophysically relevant nuclear reaction processes at hydrostatic burning temperatures are dominated by radiative captures, in this experimental scheme we measure the time-reversed processes. Such photodisintegrations allow us to compute the radiative capture cross sections when transitions to excited states of the reaction products are negligible. Due to the transformation of phase space, the photodisintegration cross sections are up to two orders of magnitude higher. The main advantage of the new target-detector system is a density several orders of magnitude higher than conventional gas targets. Also, the detector is virtually insensitive to the gamma-ray beam itself, thus allowing us to detect only the products of the nuclear reaction of interest. The development and the operation as well as the advantages and disadvantages of the bubble chamber are discussed.
This paper follows the inaugural talk one of the authors (LT) gave at the opening of the ECT* workshop with the same title, which he co-organized in Trento, Italy, November 5-9, 2018. As such it follows the ideas expressed there, which were to out-line the discussions that the organizers intended for that meeting. Therefore, the paper will review the indirect methods in nuclear astrophysics, their use and their specific problems, old and new, the need to further developments rather than giving complete treatments of each method or reviewing exhaustively the existing literature. The workshop was from its inception aiming also at reviewing the status of the field of nuclear astrophysics and its connections with adjacent branches of physics. Some lines on these are included here.
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