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Quantitative Large Amplitude Shape Dynamics: fission and heavy ion fusion
(INT Program September 23 - November 15, 2013)

Reported by A.N. Andreyev, G.F. Bertsch, W. Loveland, W. Nazarewicz
  Date posted November 15, 2013

The INT program on Quantitative Large Amplitude Shape Dynamics: fission and heavy ion fusion was held in Seattle, WA from 23 September to 15 November, 2013. The organizers were: A.N. Andreyev (University of York), G.F. Bertsch (Institute for Nuclear Theory), W. Loveland (Oregon State University), and W. Nazarewicz (University of Tennessee/ORNL). The intent of this program was to: (i) bring together theorists working on predictive theories of the underlying shape dynamics to compare various approaches and computational methodologies; (ii) actively foster collaborations between the major actors in the field, including collaborations between experiment and theory; (iii) identify critical experimental data that will inform theoretical developments; and (iv) identify strategies and resources to break computational barriers in this area. We believe that excellent progress has been made in all four areas.

The program was well attended, with 65 total participants. Week 4 of the program was organized as predominantly experimental workshop, which dealt largely with new data on fission and fusion to challenge nuclear theory. Week 7 of the program contained a workshop on Reactor Antineutrinos.

The program needs to be put into the perspective of the intellectual development that occurred since the last INT program on large-amplitude dynamics, INT-95-3 (organized by Bertsch and Nazarewicz). As presented in the talks, we now have accurate descriptions (< 1 MeV) of fission barriers based on self-consistent mean-field theory, with the beginnings of systematic error assessments of the theory. Quantitative results are now available the calculation of heavy-ion fusion cross sections by TDHF. The calculations of spontaneous fission lifetimes have become much more sophisticated in the context of the WKB barrier penetration model: more collective degrees of freedom are taken into account in setting the fission path; the path may now be determined by minimizing the action; the inertia term in the action integral may be treated in a way consistent with the instanton formulation of barrier penetration. We also saw a major advance in the theory of fission mass distributions: many of the details can be reproduced by a statistical treatment with diffusive dynamics. We expect that new experimental efforts discussed during the program will not only greatly increase the database for which current theory can be applied but will also provide its crucial tests.

  • We expect that there will be a number of new collaborations or at least new theoretical studies that will have genesis in INT13-3.

  • An initial goal of the program, to reevaluate the basic assumption in the theory of nuclear fission, is being realized by a position paper being drafted to investigate a new approach to making a theory that is both broad enough to treat the adiabatic and the dissipative limits of fission dynamics, as well as being computationally tractable.

  • A reporting guide is being prepared for theorists to make it easier to compare different methods and to assess the reliability of theory for applications. The guide includes the list of measured/evaluated quantities (spontaneous fission half-lives, fission barrier heights, fission fragment mass distributions, and excitation energies of fission isomers) that can be used to benchmark various methodologies and optimize models. The guide also gives recommendations on error analysis of the theory and how to present the uncertainties.

  • Finally, a prioritized set of measurements related to reactor antineutrinos will be published.

We think that the embedded experimental workshops greatly enriched the program as they brought contact between theory and experiment to the forefront. The discussions between the two groups enhanced the understanding of both groups and led to productive collaborations. We strongly recommend having these opportunities to be part of future INT programs.