New Experiment Solves a Nuclear Mystery
When an atomic nucleus is struck by a neutron, it has a chance of capturing that neutron and becoming a heavier isotope. In 2019, physicists studying this process in zirconium-88 (88Zr) made a surprising discovery: The isotope’s neutron-capture cross section for low-energy (thermal) neutrons was tens of thousands of times larger than theory predicted [1]. Extrapolating from that result, they calculated 88Zr’s neutron-capture cross section across all neutron energies—its so-called resonance integral—and found it to be 2 orders of magnitude larger than any previously measured. Now Thanos Stamatopoulos at Los Alamos National Laboratory in New Mexico and colleagues have tested the validity of that extrapolation [2a, 3]. Taking energy-resolved measurements of 88Zr’s neutron-capture cross section over a broad energy range, the team derived a thermal-neutron-capture cross section roughly in line with the surprise result from 2019, but the resonance integral turned out to be nearly 200 times smaller.
Conventionally, neutron-capture cross sections are measured by bombarding a sample with neutrons and then measuring the gamma-ray signature produced when some of those neutrons are captured. Using this method for a radioactive isotope such as 88Zr is difficult because the detector can be overwhelmed by gamma radiation from the decaying isotope itself. Instead, Stamatopoulos and colleagues fired a broad-spectrum neutron beam at a 88Zr target and measured the flux of neutrons that traversed the sample to reach a detector a few meters beyond. Since higher-energy neutrons arrived at the detectors sooner, the values obtained for the neutron-capture cross section were intrinsically energy resolved. The measurements revealed that 88Zr’s capture cross section is dominated by a single large peak at low energies, and then it plummets at high energies. Extrapolating this low-energy capture efficiency yielded the previous high resonance integral.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.
References
- J. A. Shusterman et al., “The surprisingly large neutron capture cross-section of 88Zr,” Nature 565, 328 (2019).
- A. Stamatopoulos et al., “Origin of the enormous 88Zr neutron-capture cross section and quantifying its impact on applications,” Phys. Rev. Lett. 134, 112702 (2025); .
- A. Stamatopoulos et al., “First study of 88Zr + n at DICER at LANSCE at energies up to 500 eV and relevance to explosive environments,” Phys. Rev. C 111, 034613 (2025).