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TORUS: Theory of Reactions for Unstable iSotopes
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Research Proposal

We propose to develop new methods that will advance nuclear reaction theory for unstable isotopes by using three-body techniques to improve direct-reaction calculations and by developing a new partial-fusion theory to integrate descriptions of direct and compound-nucleus reactions. This multi-institution collaborative effort is directly relevant to three areas of interest identified in the solicitation: (b) properties of nuclei far from stability; (c) microscopic studies of nuclear input parameters for astrophysics and (e) microscopic nuclear reaction theory.

Advances in reaction theory are crucial for science programs at present and upcoming rare isotope facilities, such as FRIB, the DOE's Facility for Rare Isotopes Beams, and T-REX, the Texas A&M Reaccelerated Exotics facility which will become fully operative in 2010. An important goal of these facilities is measuring properties and reactions of unstable isotopes that contribute to the nucleosynthesis of the heavy elements in the astrophysical s, r, and rp processes.

Deuteron-induced one-nucleon transfer reactions provide a unique tool for extracting relevant nuclear physics information; (d,p) and (d,n) reactions can be employed to study the structure of exotic nuclei and for determining neutron and proton capture rates which cannot be measured directly due to the short life time of the target nuclei involved.

Our work will provide for more accurate and reliable calculations of neutron and proton capture rates, make it possible to extract relevant structure information from one-nucleon transfer reactions, and advance methods for determining important reaction cross sections from indirect measurements.

The first component of this project aims at improving the descriptions of direct reactions. In particular, we will employ few-body techniques to develop an advanced treatment of breakup channels during transfer reactions, especially for transfers of nucleons to weakly-bound or unbound (continuum) states. The best present methods make approximations to the three-body dynamics which render inaccurate the transfer cross sections to just those states which are expected to be important for neutron capture reactions: weakly-bound states have a long tail which encourages capture of low energy neutrons, and unbound states (including resonances) that strongly enhance the capture rates.

We will carry out in-depth comparisons with exact few-body models in order to isolate and improve the methods of direct reaction theory, so that elastic scattering, transfers, breakup and continuum final states may all be included on an equal footing.

The second component of the proposed research will integrate descriptions of direct and compound-nucleus reactions. Current theories describe reactions that are either direct or statistical, but do not provide an accurate description of reactions that combine them. Understanding the transition from one class of reaction to the other, and developing quantitative descriptions of intermediate mechanisms is crucial for a variety of applications, both in the laboratory and the cosmos.

We will use the new theory of partial fusion being developed at Livermore, in order to describe reactions that involve both the disintegration of the projectile and the subsequent fusion of one or both fragments with the target nucleus. This will be important for nuclear-structure studies with low-energy (d,p) and (d,n) reactions, for extracting compound-nucleus capture cross sections from indirect measurements, for describing knock-out reactions, and for resolving controversies related to the fusion of halo nuclei. We will also examine the limits of statistical reaction descriptions, and the evolution and relative importance of direct, semidirect, and compound capture as one moves from the valley of stability to very neutron-rich nuclei.

This project relies on the collaboration of six experts in nuclear reaction theory. Joining forces will enable us to develop the advanced nuclear reaction methods needed for planning and interpreting experiments at modern rare isotope facilities. Post-doctoral scientists and students will play key roles in the development and implementation of the project and, in turn, receive valuable training in state-of-the-art techniques for modeling nuclear reactions and in modern computational science. These skills will position them well to compete for career positions at both national laboratories & universities.


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