Nuclear astrophysics

Researchers at LLNL combine knowledge about the nuclear environment in presolar grain analysis with nuclear measurements to perform unique identification of key reaction networks.

Identifying the origin of the elements

One of the major goals in nuclear astrophysics is to understand the origin of the elements in the cosmos and our solar system. Several processes and environments are responsible for the production of the majority of these elements:

• The fusion processes during stellar evolution, such as hydrogen burning in main sequence stars, are responsible for producing the elements up to the iron–nickel region.
• Neutron-capture reactions that take place during neutron star mergers or in asymptotic giant branch stars are primarily responsible for the existence of heavier elements.
• Other means, such as photo-disintegration of other heavy nuclei, produce about 30 terrestrially stable nuclei on the proton-rich side.

Each of these nucleosynthesis processes can occur in a variety of astrophysical sites; thus, the abundance of each element is a convolution of many different nuclear astrophysical processes. The current state of knowledge prohibits the disentanglement of individual contributions due to the paucity of astronomical spectroscopic data and unconstrained nuclear data.

A three-component approach to identifying individual processes

Our approach to identifying individual contributions, and thus furthering our understanding of element origin, combines three major components.

Cosmochemistry measurements of presolar grains

Stellar condensates preserve the nucleosynthetic fingerprints of their parent stars and can provide valuable insight into these processes. We use LLNL’s microanalytical instruments, such as Nanoscale Secondary Ion Mass Spectrometry and Resonant Ionization Mass Spectrometry, to probe the isotopic signatures of multiple elements, providing complementary data to astronomical spectroscopy.

Nuclear physics experiments at radioactive beam and stable beam facilities

Measurements of astrophysically important reaction rates are challenging because of the low cross-sections. However, we are able to perform previously unfeasible experiments using new techniques such as the LLNL-developed Surrogate Reaction Method as well as new detection systems such as the MUSIC ionization chamber at Argonne National Laboratory and the SECAR recoil separator at the National Superconducting Cyclotron Laboratory.

Computational studies with extensive reaction networks

Using LLNL’s high-performance computing capabilities, we use reaction rate sensitivity studies to guide nuclear experiments through identification of the most high-impact reactions. We use the reaction networks to probe astrophysical parameters—such as temperature-density profiles—that best describe the nucleosynthesis environment inferred from the presolar grain data obtained through our cosmochemistry measurements.