Nuclear data evaluation of the 5^He system in fusion reactions

The Role of the ⁵He System in Fusion Reaction
A research team from the Institute of Applied Physics and Computation Mathematics and Tsinghua University has performed a systematic nuclear data evaluation of the five-nucleon ⁵He system, which serves as a key intermediate in fusion reactions. By adopting the Generalized Reduced R-matrix framework, the study enables a unified and physically consistent analysis of multiple reaction channels within a single theoretical model.

A Key Link Between Fundamental Fusion Reactions
The ⁵He system connects several reactions of fundamental importance, including T(d,n)⁴He, ⁴He(n,d)T, and others. Among these, the T(d,n)⁴He reaction is the dominant neutron source in thermonuclear fusion and a cornerstone process in nuclear astrophysics. Although evaluated cross sections are available in major nuclear data libraries, the associated uncertainty and covariance information have remained incomplete, limiting their use in high-precision applications.

Global Multi-Channel Analysis with R-Matrix Methods
To address this issue, the authors employed the R-matrix Analysis Code (RAC) developed by Prof. Chen Zhenpeng at Tsinghua University. The code integrates the Generalized Reduced R-matrix framework with covariance analysis techniques, enabling highly automated optimization of R-matrix parameters and simultaneous fitting of large experimental datasets.

Low-Energy Angular Distribution and Resonance Effects
Special attention was devoted to the low-energy region between 0.01 and 0.1 MeV, where detailed angular distribution analyses were conducted. The results indicate that the observed forward-angle behavior is primarily governed by the 3/2⁺ resonance of the ⁵He system, with additional contributions arising from S- and P-wave interference associated with the 3/2^- state. These mechanisms were quantitatively characterized using partial-wave analysis and Legendre polynomial decomposition.

Extending the Energy Range and Benchmarking with Libraries
The evaluated energy range was further extended to 46 MeV for neutron-induced reactions and 30 MeV for deuteron-induced reactions, resulting in a consistent and self-contained set of evaluated cross sections. While the evaluated central values remain in good agreement with major nuclear data libraries such as ENDF/B-VIII.0, the present work offers a substantial improvement in the description of uncertainties and correlations.

Toward High Precision Nuclear Data for Fusion Applications
The evaluated dataset provides reliable nuclear data for DT fusion reactor design, neutron transport simulations, and radiation shielding analyses. More broadly, this work represents an important step toward establishing an independent and systematic nuclear data evaluation framework for light nuclear systems. Future studies will extend this methodology to other light nuclei, including ⁵Li, ⁶He, and ¹²C, further strengthening the nuclear data foundation for fusion energy development and nuclear astrophysical research.

The complete study is via by DOI: https://doi.org/10.1007/s41365-025-01874-2