Nuclei: From Structure to Exploding Stars

Nuclear processes generate the energy that makes stars shine and are responsible for the synthesis of the elements. When stars eject part of their matter through various means, they enrich the interstellar medium with the nuclear ashes and thereby provide the building blocks for the birth of new stars, of planets, and of life itself. Element synthesis and nuclear energy generation in stars are the two key questions in the field of nuclear astrophysics. Both require accurate knowledge of charged-particle- and neutron-induced nuclear reactions that take place in the hot stellar plasma. It is indeed remarkable how the quantum mechanical properties of atomic nuclei determine the properties of stars.

A recent review of the field has summarized fifteen key questions in nuclear astrophysics: (i) Why do predictions of helioseismology disagree with those of the standard solar model? (ii) What is the solution to the lithium problem in Big Bang nucleosynthesis? (iii) What do the observed light-nuclide and s-process abundances tell us about convection and dredge-up in massive stars and AGB stars? (iv) What are the production sites of the gamma-ray emitting radioisotopes Al-26, Ti-44 and Fe-60? (v) What is the origin of about thirty rare and neutron deficient nuclides beyond the iron peak (p-nuclides)? (vi) What causes core-collapse supernovae to explode? (vii) What is the extend of neutrino-induced nucleosynthesis (nu-process)? (viii) What is the extent of the nucleosynthesis in proton-rich outflows in the early ejecta of core-collapse supernovae (nu- p-process)? (ix) What are the sites of the r-process? (x) What causes the discrepancy between models and observations regarding the mass ejected during classical nova outbursts? (xi) Which are the physical mechanisms driving convective mixing in novae? (xii) What are the progenitors of type Ia supernovae? (xiii) What is the nucleosynthesis endpoint in type I X-ray bursts? Is there any matter ejected from those systems? (xiv) What is the impact of stellar mergers on galactic chemical abundances? (xv) What are the production and acceleration sites of galactic cosmic rays? Research at TUNL, a recognized leader in the field of nuclear astrophysics, addresses about half of these questions.

The Laboratory for Experimental Nuclear Astrophysics (LENA) is among only a few accelerator facilities in the world dedicated entirely to nuclear astrophysics experiments. It has two low-energy electrostatic accelerators that are capable of delivering high-current charged-particle beams to a common target. One is an ECR source on a 200-kV platform and the other one is a 1-MV JN Van de Graaff accelerator. Both accelerators are fully computer-controlled and transport ion beams to a common target. LENA is unique in the sense that it will cover the whole energy range below 1~MeV with ion beams of high intensity and stability.

TUNL Program in the National Context

The current Long Range Plan "The Frontiers of Nuclear Science" identifies three fairly broad questions for nuclear astrophysics: (i) What is the nature of neutron stars and dense nuclear matter? (ii) What is the origin of the elements in the cosmos? and (iii) What are the nuclear reactions that drive stars and stellar explosions? The TUNL program is focused on questions (ii) and (iii). In addition, the experiments we propose for classical novae and our work on reaction rates directly support ongoing studies at present-generation RIB facilities and planned research at FRIB. Finally, our work on detector development and experience with high-current beams at LENA are being directly applied to the initiative to build an underground accelerator laboratory in the U.S. Thus, the TUNL program continues to be well-situated in the national context.

Many of our innovative experiments are performed at our local world-class facilities, the Laboratory for Experimental Nuclear Astrophysics (LENA), and the High Intensity Gamma-Ray Source (HIGS). In addition, our theoretical work in this area has not only provided a numerical foundation for modern stellar computer models, but has also guided and motivated many other researchers at radioactive ion-beam and stable-beam facilities around the world. Our expertise is also being applied to the initiative to build a U.S. underground accelerator laboratory. Indeed, since cosmic-ray interactions are just one source of background, our existing and planned detection technologies have direct applications to underground measurements. In the following, we describe our future plans to address questions concerning stellar evolution and stellar explosions from the nuclear perspective.

On the theoretical side, we have completed a major project and published a new evaluation of experimental charged-particle thermonuclear reaction rates. These results are obtained using a new method, employing Monte Carlo techniques, to derive statistically meaningful reaction rates and associated probability density functions. Based on these results we are now in the process of constructing a state-of-the-art nuclear-reaction-rate library for use in modern stellar models of massive stars, AGB stars, classical novae, type I X-ray bursts, and of other sites. We are also in the process of making this new reaction rate library widely available to the community of stellar modelers.