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.