Microscopic Insights into Nuclear Fission Process

1. Microscopic description of the fission process Witold Nazarewicz University of Tennessee

2. Problem • Need capability to: • Predict fission probabilities • Neutron-induced cross section • Ground state • Excited states • Predict prompt fission fragment products • Mass distribution • Kinetic energy • Excitation energy • Predict emission from fission fragments • Neutron reactions with prompt fission fragments

3. Why Focus on a Microscopic Nuclear Theories? • The nuclear problem is very complex, computationally difficult • Much of the progress in the past 50 years has been based on empirical models (most with microscopic degrees of freedom) tuned to experimental data • This highly limits our predictive capability • And it is difficult to estimate the uncertainties • How can we do Quantification of Margins and Uncertainties? • The physics of nuclei impacts the programs because nuclei are the source of the energy and they are important diagnostics • Fission • Thermonuclear reactions • Neutron-induced reactions With a fundamental picture of nuclei based on the correct microphysics, we can remove the empiricism inherent today, thereby giving us greater confidence in the science we deliver to the programs

4. Powerful phenomenology exists… • … but no satisfactory microscopic understanding of: • Barriers • Fission half-lives and mass/energy splits • Fission dynamics • Cross sections • … • What is needed? • Expertise in nuclear theory • Collaborative efforts focused on programmatic needs • Microscopic many-body techniques • Computational algorithms • Hardware

5. Our SSAA Fission Program

6. http://www.phys.utk.edu/witek/fission/fission.html Funded 2002

7. People University of Tennessee Team Witold Nazarewicz UT/ORNL Arthur Kerman UT/MIT Jordan McDonnell Graduate Student, UT, Krell I. Fellow Nikolai Nikolov Graduate Student, UT Nicolas Schunck UT/ORNL External Collaborators , Team Members Andrzej Baran Maria Sklodowska-Curie University, Lublin, Poland Andrzej Staszczak Maria Sklodowska-Curie University, Lublin, Poland Jacek Dobaczewski University of Warsaw, Warsaw, Poland Javid Sheikh University of Kashmir, Srinagar, India Janusz Skalski Soltan Institute for Nuclear Studies, Warsaw, Poland Other External Collaborators Walid Younes LLNL Daniel Gogny LLNL Erich Ormand LLNL Patrick Talou LANL Yaron Danon Rensselaer Polytechnic Institute poster poster

8. Annual LACM/Fission Workshops 2004 Annual Workshop Theoretical Description of the Nuclear Large Amplitude Collective Motion (with a focus on fission) March 17-19, 2004, Joint Institute for Heavy Ion Research, Oak Ridge Attended by 25 participants 2005 Annual Workshop The Second International Workshop on the Theoretical Description of the Nuclear Large Amplitude Collective Motion March 30 - 31, 2005, Joint Institute for Heavy Ion Research, Oak Ridge. Attended by 21 participants 2007 Annual Workshop The Joint JUSTIPEN-LACM Meeting March 5-8, 2007, Joint Institute for Heavy Ion Research, Oak Ridge Attended by 60 participants 2008 Annual Workshop Joint LACM-EFES-JUSTIPEN Meeting January 23-25, 2008, Joint Institute for Heavy Ion Research, Oak Ridge Attended by 70 participants 2009 Annual Workshop The 3rd LACM-EFES-JUSTIPEN Meeting February 23-25, 2009, Joint Institute for Heavy Ion Research, Oak Ridge Attended by 90 participants All meetings involved participants from NNSA/DP Laboratories (LANL, LANL, NNSA), as well as many students and post-docs.

9. Recent Scientific Activities

10. Microscopic description of complex nuclear decay: Multimodal fission • Staszczak, A.Baran, J. Dobaczewski, W.N., Phys. Rev. C 80, 014309 (2009)

11. Microscopic description of spontaneous fission Two-dimensional total energy surface for 258Fm in the plane of two collective coordinates: elongation, Q20, and reflection-asymmetry, Q30. Dashed lines show the fission pathways: symmetric compact fragments (sCFs) and asymmetric elongated fragments (aEFs). Nuclear shapes are shown as three-dimensional images that correspond to calculated nucleon densities. Two-dimensional total energy surface for 258Fm in the plane of two collective coordinates: elongation, Q20, and necking, Q40. Summary of fission pathway results obtained in this study. Nuclei around 252Cf are predicted to fission along the asymmetric path aEF; those around 262No along the symmetric pathway sCF. These two regions are separated by the bimodal symmetric fission (sCF + sEF) around 258Fm. In a number of the Rf, Sg, and Hs nuclei, all three fission modes are likely (aEF + sCF + sEF; trimodal fission). In some cases, labeled by two-tone shading with one tone dominant, calculations predict coexistence of two decay scenarios with a preference for one. Typical nuclear shapes corresponding to the calculated nucleon densities are marked. Calculated fission half lives of even-even fermium isotopes, with 242 ≤ A ≤ 260, compared with experimental data. A. Staszczak et al. Phys. Rev. C 80, 014309 (2009)

12. Fission barriers of compound nuclei • J. Pei, J. Sheikh, WN, A. Kerman, Phys. Rev. Lett. 102, 192501 (2009) • J. Sheikh, WN, J. Pei, Phys. Rev. C 80, 011302(R) (2009) • J.D. McDonnell, WN, and J.A. Sheikh, Proc. Fourth International Workshop On Nuclear Fission And Fission Product Spectroscopy, Cadarache, France, May, 13-16, 2009. AIP Conference Proceedings 1175, 371, (2009). Unique HFB solvers made this work possible: HFBAX: J.C. Pei et al., Phys. Rev. C 78, 064306 (2008) HFODD: J. Dobaczewski and P. Olbratowski, Comput. Phys. Commun. 158, 158 (2004); ibid.167, 214 (2005)

15. Systematic Study of Fission Barriers of Excited Superheavy Nuclei • J. Sheikh, WN, J. Pei, Phys. Rev. C 80, 011302(R) (2009) • Focus on: • Mirror asymmetry and triaxiality at high temperatures • Systematic analysis of barrier damping

17. J.D. McDonnell, WN, J.A. Sheikh (Poster) Potential energy surfaces are calculated for the spontaneous fission of light actinides with (finite-temperature) HF+BCS theory. The fission path favors more symmetric scission configurations as excitation energy increases.

18. Surface Symmetry Energy and NEDF N. Nikolov et al., Poster by N. Schunck Leading terms in the leptodermous expansion Surface-symmetry term poorly determined in standard DFT, but crucial role in neutron-rich nuclei Mass A ~ 190 Deformation properties of LD driven by surface-symmetry term in neutron-rich nuclei ! Fission properties of neutron-rich U isotopes affected by poorly known assym Actinides Surface Surface-symmetry Curvature Coulomb (HF) 236U 248U 260U 270U 298U

19. Research goals, mid-term (1) • Applications of modern adiabatic and time-dependent theories • Use modern NEDF optimized for deformation effects • Investigate excitation energy dependence • Perform action minimization on many-dimensional meshes • Develop symmetry restoration schemes for DFT • Increase the number of collective coordinates in GCM, including pairing channel • Develop time-dependent framework for fission from excited states (T>0): ATDHFB and TDHFB

20. Collective inertia B(q) and ZPE Various prescriptions for collective inertia and ZPE exist: GOA of the GCMRing and P. Schuck, The Nuclear Many-Body Problem, 1980 ATDHF+Cranking Giannoni and Quentin, Phys. Rev. C21, 2060 (1980); Warda et al., Phys. Rev. C66, 014310 (2002) Goutte et al., Phys. Rev. C71, 024316 (2005) A.Baran et al., nucl-th/0610092 HFODD+BCS+Skyrme Recently, a nice progress with full ATDHFB inertia….

21. Research goals, mid-term (2) • Tests of non-adiabatic approaches • Non adiabatic tunneling, properly accounts for level crossings and symmetry breaking effects (collective path strongly influenced by level crossings). ATDHF not adequate • Evolution in an imaginary time • The lifetime is expressed by the sum of bounces • Difficulty in solving the periodic mean-field equations (fission bounce equations) • Important role of pairing correlations (restore adiabaticity) • Unclear how to restore broken symmetries bounce trajectory governing fission J. Skalski: Phys. Rev. C 77, 064610 (2008) Nuclear fission with mean-field instantons ATDHFB inertia

22. Fission was chosen as a principal research directions at workshop for computing at the extreme scale, see the summary report: http://extremecomputing.labworks.org/nuclearphysics/PNNL_18739_onlineversion_opt.pdf

23. Priority Research Direction: Fission Theory Scientific and computational challenges Summary of research direction Scientific: Tunneling and time-dependent dynamics of complex heavy nuclei. Computational: Implementation of highly nonlinear, nonlocal, multi-dimensional system of time-dependent integro-differential equations. Develop efficient solvers for the time-dependent (deterministic and stochastic) nuclear superfluid density functional theory (DFT). Solve the multi-dimensional optimization problem for collective action. Develop algorithms to compute bounce trajectories in imaginary time. Uncertainty quantification for complex nuclear processes. Potential impact on Nuclear Physics Problems that arise in NNSA/ASC? Expected Scientific and Computational Outcomes Validate and predict fission cross sections, fragment mass and energy distributions. Provide microscopic guidance (extrapolation capability) for phenomenological models of fission. Microscopic input to materials damage simulations. Neutron and photon spectra for stockpile simulation validation and nonproliferation. Scientific: Microscopic description, based on realistic internucleon interactions, of nuclear fission process. Explanation of tunneling motion of a many-body system. Computational: New fast multiresolution methods for the nuclear DFT. Implementation of new Importance Sampling Techniques for the time-dependent stochastic evolution equations.

24. Microscopic description of nuclear fission process Drivers: Improved understanding, validate and predict fission cross sections , fragment mass and energy distributions; neutron and photon spectra prediction; Uncertainty Quantification for complex nuclear processes Applications: Advanced Reactors, Stockpile Stewardship, Nuclear Forensics Prediction, Diagnostics Validation, Verification Present Exascale Peta

25. SciDAC Review, submitted

26. SUMMARY • There are fundamental problems in fission that cry to be solved • Basic science (nuclear structure, nuclear astrophysics) • Programmatic needs • Fission is a perfect problem for the extreme scale • Quantum many-body tunneling is tough • We are developing a microscopic model that will be predictive • Fission probabilities • Properties of fission fragments • Cross sections • We want to do Quantification of Margins and Uncertainties • The future is bright!