Nuclear data on unstable nuclei for astrophysics

Michael S. Smitha, Richard A. Meyerb, Daniel W. Bardayana, Jeffery C. Blackmona,Kyungyuk Chaec,a, Michael W. Guidryc,a, W. Raphael Hixc,a, R. L. Kozubd, Eric J.Lingerfeltc,a, Zhanwen Mac,a, Jason P. Scottc,a

aPhysics Division, Oak Ridge National Laboratory, Oak Ridge, TNbRAME’ Inc., Teaticket, MAcDept. of Physics and Astronomy, Univ. of Tennessee, Knoxville, TNdPhysics Dept., Tennessee Technological Univ., Cookeville, TN

Recent measurements with radioactive beams at ORNL’s Holifield Radioactive IonBeam Facility (HRIBF) have prompted the evaluation of a number of reactions involvingunstable nuclei needed for stellar explosion studies. We discuss these evaluations, as wellas the development of a new computational infrastructure to enable the rapid incorpo-ration of the latest nuclear physics results in astrophysics models. This infrastructureincludes programs that simplify the generation of reaction rates, manage rate databases,and visualize reaction rates, all hosted at a new website www.nucastrodata.org.

1. NUCLEAR DATA EVALUATIONS FOR STELLAR EXPLOSIONSTUDIES

Thousands of different isotopes of neutron-rich nuclei are believed to be synthesized insupernova explosions. To simulate these cataclysmic events, a knowledge of the structureof neutron-rich nuclei and the reactions involving them is essential. Similarly, informationon proton-rich unstable nuclei is needed to understand nova explosions occurring on thesurface of white dwarf stars and X-ray bursts on the surface of neutron stars. Recentmeasurements with radioactive beams at ORNL’s Holifield Radioactive Ion Beam Facility(HRIBF)[1] have prompted the evaluation of a number of reactions involving unstablenuclei, and the associated level structures, that are necessary to probe the details of thesespectacular astrophysical explosions. A number of reactions are also being investigatedto prepare for possible future measurements at HRIBF. The results of our evaluations –new cross sections and level schemes – will be converted into new thermonuclear reactionrates using software tools discussed below.

Recent reactions and nuclei being assessed at ORNL include:

• 18F(p,α)15O and 18F(p,γ)19Ne and the level structure of 19Ne above the 18F + pthreshold; this work is illustrated in Fig. 1 and discussed below

• 14O(α,p)17F and the level structure of 18Ne above the 14O + α and 17F + p thresh-olds; R-matrix fits to the measurements of four different reaction yields are currentlybeing made[2]

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0.03 0.1 0.5 1

10-3

10-2

10-1

100

101

18F(p,α)15O18F(p,γ)19Ne

Rat

e / P

rese

nt

Temperature (T9)

0.1 0.5 1 3

Figure 1. The ratio of the 18F+p reaction rates to the present nominal rate. The solidlines correspond to the low and high rates in the present study, and the shaded regionsare the rates in Ref. [4].

• 33,34Cl(p,γ)34,35Ar and the level structure of 34,35Ar above the 33,34Cl + p thresholds,for a future HRIBF measurement

• 30P(p,γ)31S and the level structure of 31S above the 30S + p threshold; this workincludes results from a recent HRIBF measurement of 32S(p,d)31S

The highlights of our first evaluation of the 18F(p,α)15O and 18F(p,γ)19Ne reactionsand the level structure of 19Ne above the 18F + p threshold are given in Shu et al. [3],and a longer paper updated with recent results from a 18F(d,p)19F measurement is inprogress. Our new thermonuclear reaction rates based on this evaluation are shown inFig. 1, where the “low” and “high” rates were calculated by varying the contributionof each of 21 resonances within their uncertainty and adding the resultant deviation inquadrature. Our updated rate is consistent with the rates in Ref. [4] within uncertainties,except at temperatures of 2–3 GK and near 0.3 GK. The difference near 0.3 GK resultsfrom a smaller value of the strength of the 330-keV 19Ne resonance in the present work.At 2–3 GK, our rate is larger by ∼ 20–60% primarily because of the contributions ofhigher energy resonances included in the present work. The uncertainty in our (p,α) rateis smaller than in Ref. [4] primarily because of the precision of the newer measurements

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of the 330-keV and 665-keV resonance parameters, as well as the new upper limits on Γp

for the 8, 38, and 287-keV levels in 19Ne.

2. STRATEGIES FOR FUTURE NUCLEAR ASTROPHYSICS DATAACTIVITIES

Measurements and theoretical descriptions of nuclei and their interactions provide afoundation for sophisticated models of stellar explosions, as well as for other astrophysi-cal systems ranging from the Big Bang to the inner workings of our own Sun. In manyinstances, the ability of astrophysical models to accurately describe the latest, spectacularobservations of the cosmos strongly depends on the input nuclear data, and more extensiveand precise nuclear data is required for advances in astrophysics. However, to be utilizedfor astrophysical studies, state-of-the-art nuclear measurements and theoretical calcula-tions have to be appropriately processed for input into astrophysics simulation codes. Thisrequires dedicated efforts in data compilation, evaluation, processing, dissemination, andcoordination. Unfortunately, the current worldwide effort in nuclear astrophysics datadoes not meet the data needs of the astrophysics community. As a result, the latestnuclear measurements or model calculations are frequently not utilized in studies of thevery astrophysical puzzles that motivated their generation. The situation is getting worseas more nuclear measurements are being made but not incorporated into reaction ratelibraries and other astrophysical datasets that are in the public domain.

Fortunately, there are a number of strategies that will enable a more effective utilizationof nuclear physics information in astrophysics simulations. These include the developmentof software to facilitate the connection between the nuclear laboratory and stellar models,as well as initiatives to boost evaluation manpower in this field. For example, at ORNLwe are creating a new computational infrastructure for nuclear astrophysics data. Thissuite of computer codes will expedite the incorporation of nuclear physics informationinto astrophysical simulation codes. Available on-line through a web browser, a simplepoint-and-click interface will guide users to convert input nuclear structure and reactioninformation – the products of evaluation activities – into thermonuclear reaction ratesin a variety of popular formats, including that of the widely-used REACLIB library[5]which contains over 60000 rates. The interface will also enable users to easily access andmanage databases – for example, to insert a new reaction rate into an online reaction ratelibrary, as well as to create, merge, store, document, and share custom rate libraries. Thisfunctionality will hopefully make it possible for the community to replace multiple, pro-prietary versions of REACLIB that each have different, partial reaction rate updates withfrequently updated public releases– making the intercomparison of results from differentastrophysics simulations much easier.

The infrastructure will also enable users to easily visualize rate libraries with Rateplot-ter, the first easy-to-use, interactive, platform-independent, graphical user interface toREACLIB-format rate libraries. This program, viewable through a web browser or asa stand-alone application, enables users to plot multiple rates, access rate parameters,add new rates and plot them, and create rate versus temperature tables, all througha point-and-click graphical user interface based on the chart of the nuclides. To hostthis new infrastructure for nuclear astrophysics data, a new website has been launched:

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www.nucastrodata.org. In addition to the components described above, this site fea-tures an extensive list of nuclear datasets (over 60 so far) important for nuclear astro-physics studies available from around the world. It is designed to help users navigatethrough these datasets, as well as to publicize them to the research community. Thissite and its new infrastructure have a strong potential to become a valuable asset for thenuclear astrophysics research community.

Even with the new computational infrastructure discussed above, more manpower willstill be needed for evaluations. Some of this may come from appeals for volunteer workfrom the nuclear astrophysics research community. Exploiting the overlap between thenuclear data and nuclear astrophysics communities[6] is another approach to increasingevaluation manpower. To make these approaches work, communication is crucial. First,the benefits of evaluations need to be clearly elucidated to enlist additional evaluators.Next, strong lines of communication between evaluators are needed, both nationally andinternationally, to share expertise and to help avoid unnecessary duplications of effort.Third, a robust dialogue between evaluators and astrophysical modelers, the end users ofthe nuclear data, is vital to ensure that evaluations are focused on the most importantreactions and nuclei.

Such enhanced communications, as well as other data activities, would be greatly fa-cilitated by the establishment of a Nuclear Astrophysics Data Coordinator, whose dutieswould include: maintaining and updating a central WWW site linking relevant datasets;modifying datasets for compatibility with astrophysical codes; and improving data acces-sibility via the creation of indices, search capabilities, graphical interfaces, bibliographies,error checking, plotting tools, and other enhancements. Other activities could includeencouraging and helping coordinate evaluation activities; establishing and maintaining anuclear astrophysics email distribution list; publicizing new nuclear astrophysics meetings,experimental results, and publications; and establishing and maintaining a priority listof important nuclear reactions and properties that require further study. It would alsobe beneficial for the Coordinator to maintain an active research program using nuclearastrophysics data to ensure the data activities truly fulfill the needs of data users. The es-tablishment of a Coordinator would have a strong positive impact on nuclear astrophysicsresearch efforts worldwide with only a modest investment.

ORNL is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contractDE-AC05-00OR22725.

REFERENCES

1. Stracener, D.W. Nucl. Inst. Meth. B204 (2003) 42.2. Blackmon, J.C. et al., Nucl. Phys. A718 (2003) 127.3. Shu, N. et al., Chin. Phys. Lett. 20 (2003) 1470.4. Coc, A. et al., Astron. Astrophys. 357 (2000) 561.5. F.-K. Thielemann et al., Adv. Nuclear Astrophysics 525 (1987) 1;

http://quasar.physik.unibas.ch/∼tommy/adndt.html#reaclib.6. M.S. Smith et al., U.S. Nuclear Data Program Astrophysics Task Force Report,

unpublished (1995);http://www.phy.ornl.gov/astrophysics/data/task/taskforce report.html.

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