USNDP Astrophysics Task Force Progress Report

Michael Smith (ORNL)
April 16, 1999

Research programs in nuclear astrophysics address some of the most fundamental questions in nature: What are the origins of the elements that make up our bodies and our world? How did the solar system, the sun, the stars, and the galaxy form, and how do they evolve? Measurements in the nuclear laboratory form the empirical foundation for the sophisticated theoretical models of these astrophysical systems. In many cases, however, new nuclear physics measurements are not rapidly disseminated to the research community nor rapidly incorporated into astrophysical models.

Additionally, progress in laboratory nuclear physics coupled with the evolution of fast, computers, has enabled extremely complex astrophysical calculations of increasing realism - for example, calculations coupling the time-dependent synthesis of hundreds of isotopes via thousands of coupling reactions with multi-dimensional simulations of stellar and explosive environments. These new codes require significantly more, and more accurate, nuclear data than before to keep pace with the new observational capabilities of large telescopes. Progress in many fundamental problems in astrophysics requires the best predictive power of astrophysical models. However, this predictive power has, in many instances, a strong dependence on the input nuclear data. For these reasons, progress in astrophysics can be significantly aided by more providing usable, accurate, and significant amounts of nuclear data in a timely fashion, and in formats that the astrophysics research community can easily implement into their models.

An Astrophysics Task Force was formed in Spring 1998 to coordinate the ongoing work in this area, and to propose (to the Department of Energy) a new, expanded, coordinated US effort in nuclear astrophysics data. This report gives a brief overview of the nuclear astrophysics data projects being carried out by Task Force members. Many of these projects are described in greater detail in the reports submitted to this USNDP meeting by the Nuclear Reactions Working Group, the Nuclear Structure Working Group, the Dissemination Working Group, or by the individual USNDP Data Centers.

1. Argonne National Laboratory (ANL) - Donald Smith, in collaboration with Laura Van Wormer, Hiram College

   Information on level properties, resonance parameters, and cross sections have been compiled for the ^33,34,36S(p,) and ^32,33,34,36S(p,) reactions in preparation for a detailed evaluation of these processes for applications in nuclear astrophysics. Most of this work was performed by a student from Hiram College, Ohio, through the Argonne National Laboratory Student Research Participation Program.

   In the next stage of this project, the compiled data for ^33,34,36S(p,) and ^32,33,34,36S(p,) will be organized. Summaries of the references and tabular information will be issued in Argonne reports and this information will also be made available on the World Wide Web. Procedures will be developed for calculating the direct proton capture strength associated with (p,) and (p,) reactions on A = 30-50 nuclei. A search will be made for possible missing low-energy proton resonances through consideration of more complex transfer reactions such as (³He,d). Knowledge of the existence of these resonances is needed in order to produce reliable evaluations of the hydrogen burning reactions which are required for nuclear astrophysics applications - such as explaining the energy generation and element synthesis occurring in stellar explosions.

2. Lawrence Berkeley National Laboratory (LBNL) - Eric Norman

   At LBNL, decay data that is of interest to nuclear astrophysics has recently been evaluated. An international Decay Data Project (DDP), headed in the US by Dick Helmer, does decay data evaluations for nuclides of interest in detector calibration, nuclear medicine, and other applications. We have added some nuclides to their list that are of interest in astrophysics. Eddie Browne from LBNL, a member of this project, and Alice Wu, an LBNL visitor, have done evaluations of ²⁶Al, ⁴⁴Ti, ⁵⁷Ni, ⁶⁰Fe. They are now working on several additional decay data sets of astrophysical interest. Once these data sets are evaluated, they are included in the DDP lists and are also entered into ENSDF.

3. Joint Report: University of California at Santa Cruz (UCSC) - Stan Woosley; and Lawrence Livermore National Laboratory (LLNL) - Roger White and Rob Hoffman

   A lengthy and thorough review of the Hauser Feshbach cross sections employed in modern studies of nucleosynthesis has been completed by Hoffman, Woosley, Weaver, Rauscher, and Thielemann. This study examines the sensitivity of the nucleosynthesis of intermediate mass elements 28<A<80 in supernovae derived from massive stars to the nuclear reaction rates employed in the model. The final yields for the two rate sets are compared and found to differ in most cases by less than a factor of two over the entire range of nuclei studied. Reasons for the major discrepancies are discussed in detail along with the physics underlying the two reaction rate sets employed. The nucleosynthesis results are relatively robust and less sensitive than might be expected to uncertainties in nuclear reaction rates, though they are sensitive to the stellar model employed.

   Hoffman and Woosley have begun the implementation of the new weak interaction rates of Langanke and collaborators into the stellar model calculations to see what their effect will be. Rates are currently being fit and tabular values are being read for these reactions. Assistance has been given to Langanke and collaborators to find a good representation for their tables. It will soon be determined if these new rates result in gross modifications of pre-Supernova models.

   A new set of reaction rates from the NACRE collaboration is being converted from TeX into fortran and implemented into nucleosynthesis codes. Finally, the nucleosynthesis that occurs in a variety of new stellar and supernova models that include improved opacities, rotation, mass loss, etc., is being investigated.

4. Los Alamos National Laboratory (LANL) - Gerry Hale

   At Los Alamos, we have continued evaluations of light-element cross sections important in astrophysics. From R-matrix analyses of the ⁵He, ¹⁶O, and ¹⁷O systems, new evaluated cross sections have been obtained for the reactions ³He(d,p)4He, ¹²C(,)¹⁶O, and ¹³C(,n)¹⁶O. We also recently restarted an analysis of low-energy N+N scattering with deuteron photodisintegration data added, in order to obtain an improved evaluation for n + p capture in the region of importance for big bang nucleosynthesis.

5. National Nuclear Data Center (NNDC) - Vicki McLane

   Charged-particle nuclear data back to 1980 continues to be compiled and entered into the NNDC databases. Presently there are more than 3,400 CPND references entered onto the CSISRS library; about 600K data points.

   The experimental data used in the NACRE collection of reaction rates is in the process of being entered into the NNDC system. The US data should be complete by the end of May.

   Finally, the NNDC is entering into a cooperative agreement with VNIIEF, Sarov, whereby VNIIEF will scan in data from figures in the articles when we are unable to obtain the numerical data.

6. Oak Ridge National Laboratory (ORNL) - Michael Smith

   The ORNL effort involves making new evaluations of important reactions and disseminating them to the research community in user-friendly formats that are easily incorporated into astrophysics models. Our evaluation work is focused primarily on capture reactions on radioactive isotopes on the proton-rich side of stability - reactions that are important for understanding the element synthesis and energy generation in stellar explosions such as novae, supernovae, and X-ray bursters. For example, work has recently begun on evaluating the ¹⁸F(p,)¹⁵O and ¹⁸F(p,)¹⁹Ne reactions. These reactions are part of a group identified by the Astrophysics Data Steering Committee in 1996 as having a high priority for evaluation. This evaluation work is closely coupled to the measurement program with proton-rich radioactive isotopes at ORNL's Holifield Radioactive Ion Beam Facility. We are also working on the ¹⁷O(p,)¹⁴N and ¹⁷O(p,)¹⁸F reactions that are important for understanding the abundance of oxygen isotopes in the envelopes of Red Giant stars. These abundances can be used as a tracer of the convection process occurring in Red Giants. Finally, we are examining and comparing recent assessments of solar fusion cross sections and determining their astrophysical implications for the Solar Neutrino Problem.

   Our WWW dissemination work has centered around improving the usefulness of our posting of one of the most important reaction rate collections, by Caughlan and Fowler, and on creating a Nuclear Astrophysics Bibliography. The rate collection was improved by completing a graphical user interface based on the chart of the nuclides which allows users to search for rates of interest. This collection is a central part of the nuclear astrophysics data that we have posted at ORNL at the address http://www.phy.ornl.gov/astrophysics/data/data.html. This new improvement is in addition to improvements made in early 1998 of adding GIF and Postscript plots of each of the rates, along with a technique to automatically regenerate plots if future modifications are needed. The Nuclear Astrophysics Bibliography is a useful resource for producing evaluations of nuclear reaction and structure information important for astrophysics. It includes references to astrophysical journals and reports which are outside the normal scope of Nuclear Science References. The first phase of this project, which includes over 1200 references, has recently been completed.