Nuclear Astrophysics addresses the role of nuclear structure and reactions on the generation and evolution of stars and stellar explosions and on the chemical evolution of our universe. A strong program in experimental nuclear astrophysics has been developed at the Nuclear Science Laboratory (NSL) addressing key questions of stellar burning and evolution as well as questions that are important for the understanding of stellar explosion. Experiments include the direct measurement of reaction and decay processes that are expected to take place in quiescent and explosive stellar burning environments, but it also entails measurements that probe these processes using indirect reaction techniques to collect additional and complementary information that is not accessible by direct experiments. The NSL is a core institution of the Joint Institute for Nuclear Astrophysics Center for Evolution of the Elements (JINA-CEE).
The nuclear astrophysics program utilizes the intense stable beams from the St. Ana 5U Pelletron accelerator for cross section measurements utilizing both forward and inverse kinematics techniques. The direct measurements in forward kinematics rely upon arrays of charged particle, neutron, or photon detectors surrounding a solid target or the use of the recirculating gas target RHINO. The inverse kinematics program uses the recirculating gas target HIPPO and the St. George recoil separator for separating and counting the reaction products. The TwinSol facility is being utilized for radioactive beam experiments at Notre Dame. AMS techniques are adopted to study the origin of long-lived galactic radioactivities from 36Cl to 60Fe.
Stellar Hydrogen Burning: Major accomplishments over the last few years have been reaction studies of relevance for stellar hydrogen burning and stellar helium burning. Key reaction of the pp-chains and the CNO reactions have been investigated towards lower energies revealing often substantial differences with respect to earlier results. New R-matrix techniques have been developed to allow a more reliable extrapolation of the experimental data towards the stellar energy range by identifying and addressing all possible reaction contributions by mapping and fitting all open reaction channels in a parallel mode. These experiments are often done in collaboration with the LUNA group to expand the measurements into the lower energy range at the LUNA underground accelerator at the INFN Gran Sasso laboratory in Italy or at the new U.S. underground laboratory CASPAR. This effort reduces significantly the uncertainties in the reaction rates associated with stellar hydrogen burning.
Stellar Helium Burning: In terms of helium burning, the focus of the program was mainly on investigating stellar neutron sources for the s-process. A number of (α,n) reactions on carbon, oxygen, neon, and magnesium isotopes has been studied in the higher energy range. The direct measurements were complemented by scattering and transfer indirect reaction studies to probe lower energy resonances near the α threshold. The present focus is to expand the (α,n) studies towards lower energies using different techniques. A direct measurement will take place at the CASPAR facility at the Sanford Underground Research Facility (SURF) in South Dakota using high intensity a beams in an underground environment to measure the weak neutron flux. Alternatively inverse kinematics experiments will be performed at the St. GEORGE recoil separator to measure directly the reaction products from the (α,n) reactions and the competing (α,γ) radiative capture processes.
Nuclear fusion in stars and neutron stars: Large uncertainties characterize stellar carbon burning. Predictions for the low energy cross sections vary by several orders of magnitude. This affects not only the late phases of stellar evolution but has also implications for type Ia supernovae ignition or superburst ignition in the crust of accreting neutron stars. In this framework, new models for nuclear reactions between very neutron rich isotopes have been developed. Such reactions are anticipated to generate fusion energy at extreme densities and heat the deeper layers of the neutron star crust, causing slow neutronization of the crust matter. A next generation of fusion experiments are being pursued at the 5U Pelletron accelerator with the newly developed SAND detector using both particle and γ-detection techniques to reduce the natural background.
Novae and X-ray bursts: A long standing effort was established in investigating critical reactions associated with the hot CNO and the hot NeNa cycles in nova explosions and the αp- and rp-process in X-ray bursts. The experimental program relies on the use of the TwinSol radioactive beam facility for the production of radioactive light ion beams. For reactions in the higher mass range, measurements will need to be studied at a future radioactive beam facility such as FRIB. The NSL therefore is directly involved in the development of critical instrumentation such as the recoil separator SECAR. Meanwhile the experimental effort concentrates on transfer and charge exchange reaction studies at the Grand Raiden Spectrometer at RCNP, Osaka, the K600 spectrometer at i_Themba, Capetown, and in the future with an Enge spectrograph being installed at the NSL to investigate key parameters in preparation for radioactive beam experiments associated with the αp- and the rp-process in X-ray bursts.
Supernova shock-front: The emerging supernova shock-front is identified as most likely environment for the astrophysical α-process as source of long-lived radioactivities observed in supernova remnants. Origin and lifetime of these long-lived radioactive isotopes are being studied using direct spectroscopy and AMS techniques at the FN tandem accelerator. The supernova shock-front environment is also considered to be a likely site for the p-process. The detailed study of the nuclear physics aspects of these processes might reveal observational signatures for the required site conditions. In that spirit, critical reactions along the p-process path have been identified and are being studied using inverse radiative capture reactions.
Neutron-star mergers: The astrophysical r-process is likely the result of neutron-star mergers and involves thousands of neutron-rich radioactive nuclei. Since it is impossible to study all of these in the laboratory, the most critical nuclei for the r-process as well as their decay and neutron capture processes have been identified in a number of theoretical studies. The nuclear masses and decay properties are being measured using isotope trapping methods. Additionally neutron-capture rates are being constrained from β-decay studies or surrogate reaction techniques at the NSCL and ATLAS.
Stellar reaction rates: In all cases, the experimental studies rely on theoretical interpretation of the reaction mechanism to interpret and reduce the uncertainties in the reaction rates within the stellar energy range. A number of new models have been developed for this purpose. For light ion reaction studies, the R-matrix code AZURE has emerged as a powerful tool for reaction cross section interpretation and extrapolation. For reaction studies on more massive nuclei within the p-process and the r-process, new Hauser Feshbach codes like CIGAR and SAPPHIRE have been developed, taking into account also non-resonant reaction contributions.