Capture Cross Sections for Isotopes Far from Stability

Jutta Escher (15-ERD-069)

Abstract

Neutron-induced reactions on short-lived fission products play a major role in the transmutation of elements in astrophysical bodies, nuclear explosions, and nuclear reactors. We propose a theoretical and experimental effort to determine the feasibility of obtaining neutron-capture cross sections for reactions on isotopes far from stability ("exotic nuclei") by taking advantage of data collected on the reverse process of neutron emission following beta decay. Predictions of cross sections for nuclei away from stability rely on extrapolations, and become increasingly uncertain. These cross sections are needed to understand the synthesis of heavy elements by supernova nucleosynthesis (creation of new atomic nuclei), and to model fission-fragment burn up relevant to radiochemistry. Our project addresses the need for a reliable, data-guided theory by seizing upon a unique opportunity to gain access to high-quality nuclear data well ahead of the scheduled operation of the DOE's Facility for Rare Isotope Beams at Michigan State University in East Lansing, and by establishing the connection between this data and capture cross sections.

We plan to combine an innovative experimental technique (measuring beta decay of trapped exotic nuclei) with state-of-the-art theory to obtain data-constrained reaction descriptions for nuclei far from stability. We will illustrate the approach using the xenon-136 isotope to obtain a cross-section estimate for the ¹³⁶xenon(n, gamma) reaction. Based on new data and theory results, we will formulate recommendations for capture calculations in the region around xenon-137 and outline a strategy for measurements that can significantly constrain the theory that predicts capture cross sections for tin and other exotic nuclei. The successful proof-of principle application to the selected benchmark case will demonstrate a new strategy for determining capture cross sections that play crucial roles in nuclear astrophysics and radiochemistry. Most immediately, this will pave the way for obtaining capture cross sections for tin nuclei, which are important for understanding the origin of heavy elements. More generally, because this cross-section information is currently not obtainable by other methods, we expect our work to guide future theoretical and experimental efforts in this area.

Mission Relevance

This research will develop a new approach to reliably determine, for the first time, capture cross sections for isotopes far from stability. The cross sections are needed to increase the fidelity of the physics models that calculate fission-product burn up and are used to understand supernovae nucleosynthesis, supporting the Laboratory's core competency of nuclear, chemical, and isotopic science and technology relevant to operation of nuclear weapons, and thus important for stockpile stewardship science efforts.

FY15 Accomplishments and Results

In FY15 we have focused on beta-delayed gamma emission. Specifically, we (1) extracted quantities relevant to neutron-capture calculations and compared them to results from alternative experiments; (2) submitted a proposal for a beta-decay experiment at Michigan State University; (3) determined the feasibility of obtaining neutron-capture cross sections for nuclei far from stability using an innovative experimental technique; (4) determined, using beta-decay data, to what extent nuclear-level densities can be constrained from such measurements, and produced a cross-section estimate for ¹³⁶xenon(n, gamma); and (5) compared our findings to known results and formulated a strategy for constraining capture cross sections in xenon––tin.

Schematic comparison of neutron capture on krypton-86 (⁸⁶Kr) and the "reverse" process of beta-delayed neutron and gamma-ray emission. Population of states above the neutron separation energy (S_n) provide information on the competition of neutron and gamma-ray emission, while below S_n, information on the gamma-ray strength function can be obtained. These quantities are crucial for calculations of neutron-capture cross sections.

Publications and Presentations

• Escher, J. E., et al., Compound-nuclear reactions with unstable isotopes: Constraining theory through innovative experimental approaches. (2015). LLNL-PRES-678475.
• Escher, J. E., et al., Towards predictive theories of nuclear reactions across the isotopic chart. (2015). LLNL-PROP-674898.
• Scielzo, N. D., et al., Determination of the ⁹²Sr(n,gamma) cross section and fission product burn up. (2015). LLNL-PROP-668706.
• Tonchev, A., et al., Measuring neutron capture cross sections on s-process radioactive nuclei. (2015). LLNL-PRES-670315.
• Tonchev, A. P., et al., Neutron capture cross sections for radioactive nuclei. (2015). LLNL-PRES-678627.