Gold Foils and Supercomputing Put Ordinary Concrete’s Neutron Shielding to the Test

Ordinary concrete is one of the most unglamorous materials in nuclear engineering, yet it quietly stands between the people of the world and some of the most intense radiation environments humanity has ever built. Now a team of Indian researchers has put this humble workhorse through one of its most rigorous examinations to date, irradiating a carefully characterized concrete block in a mixed-energy neutron field and comparing the results, measurement by measurement, against sophisticated Monte Carlo simulations. The findings, published in Results in Physics, offer a fresh experimental benchmark for the computer models that underpin the design of biological shields for advanced fission and fusion reactors.

The study, led by Sachin Shet and Ankan Charan of the Manipal Institute of Technology along with colleagues including S.V. Suryanarayana, S. Ganesan, Sabu Thomas, Srinivas Shenoy Heckadka, Bharath K Bhat and Sudha Kamath, focused on a 10 by 10 by 10 centimeter block of Ordinary Portland Cement grade 43 concrete with a measured density of 2.41 grams per cubic centimeter. The block was placed in the horizontal irradiation channel of a heavily shielded concrete bunker housing a sealed 16 curie americium-beryllium neutron source, an installation that has served the Manipal facility since its calibration in January 2007 and whose decay-corrected activity stood at 15.51 curies, or 573.87 gigabecquerels, as of May 2026.

The americium-beryllium source generates neutrons through the beryllium-9 (alpha, n) carbon-12 reaction, in which alpha particles from the decay of americium-241 strike a beryllium target. The result is a broad, continuous neutron spectrum extending up to roughly 11 megaelectronvolts, with a characteristic emission peak near 4 to 5 megaelectronvolts and a coincident 4.438 megaelectronvolt gamma ray from the first excited state of the carbon-12 nucleus. Critically, because the source sits inside a concrete bunker, neutrons scatter repeatedly from the surrounding walls before reaching the sample, introducing a substantial thermal component that is entirely absent from the bare-source spectrum described by the ISO 8529-1 standard used in the simulations.

To quantify how many neutrons survive the journey through the concrete, the researchers attached thin foils of high-purity gold-197, more than 99.99 percent pure, to the front and rear faces of the block. Gold is a classical neutron flux monitor because its capture cross-section for the gold-197 (n, gamma) gold-198 reaction is large and well characterized, with a thermal value of 98.6 barns and a resonance integral of roughly 1550 barns. The team irradiated the assembly for 19 hours, then counted the characteristic 411 kiloelectronvolt gamma ray emitted by the decaying gold-198 using a high-purity germanium spectrometer calibrated with a europium-152 source, applying careful corrections for true coincidence summing at the extremely close 5 millimeter source-to-detector distance.

The measured spectrum-integrated capture reaction rates were 9.25 times ten to the minus nineteenth per atom per second at the front face and 5.42 times ten to the minus nineteenth at the rear face, each with combined relative uncertainties below 4 percent. The OpenMC Monte Carlo code, driven by the ENDF/B-VII.1 nuclear data library and 80 million neutron histories, predicted 9.884 and 5.710 times ten to the minus nineteenth respectively. The differences, 6.85 percent at the front and 5.39 percent at the rear, correspond to approximately 1.9 and 1.1 combined standard deviations, an agreement the authors attribute in part to localized density and compositional variations in the real block relative to the nominal model.

Even more striking was the agreement in the transmission ratio. The experimental ratio of rear-to-front reaction rates came out at 0.586, while the simulation gave 0.578, a difference of just 1.37 percent. Because systematic uncertainties largely cancel in a ratio, this figure represents the cleanest validation of the computational model. The researchers derived a transmission factor of 0.594, meaning the 10 centimeter block reduces the detectable neutron flux by roughly 41 percent beyond the geometric attenuation already expected from distance and environmental scattering. Simulations also confirmed that fast neutrons are attenuated more efficiently than thermal ones, consistent with hydrogen in the concrete moderating and capturing fast neutrons, and that room-return scattering keeps the flux at 20 centimeters at 51.33 percent of the 10 centimeter value, far above the 25 percent predicted by a simple inverse-square law.

The elemental composition of the concrete, drawn from the PNNL-15870 Rev. 2 compendium and consistent with Bureau of Indian Standards specifications, lists silicon, oxygen, calcium, aluminum, potassium, iron, sodium and trace manganese among its key constituents. Energy-dispersive X-ray spectroscopy confirmed the presence of the principal medium-Z elements on the actual block surface, with an elevated calcium-to-silicon ratio reflecting analysis of cement-paste-rich zones rather than any discrepancy with the bulk reference composition used in transport calculations.

Beyond shielding, the study probed what the neutron field does to the concrete itself. Gamma-ray spectrometry of the irradiated block, begun two hours after the 19-hour exposure, revealed prominent photopeaks from sodium-24 and manganese-56. Sodium-24 arises mainly from thermal capture on the concrete’s sodium content, with a secondary fast-neutron contribution from the aluminum-27 (n, alpha) reaction, while manganese-56 is produced by thermal capture on trace manganese impurities, present at only 100 to 500 parts per million but amplified by manganese-55’s large 13.3 barn capture cross-section, and by the iron-56 (n, p) pathway. A peak at 1460.8 kiloelectronvolts was traced to naturally radioactive potassium-40 rather than activation. These findings underscore that even trace constituents can dominate the induced radioactivity of concrete shielding, a consideration of real consequence for decommissioning and waste management.

The authors are candid about the scope of their work. Measurements were taken at only two positions, sufficient to determine a bulk transmission factor but not to resolve the spatial attenuation profile inside the block, and no secondary gamma-ray dose calculations were performed despite the significant induced activity from sodium-24 and manganese-56. Both limitations, they note, define the agenda for future campaigns at the Manipal facility, where multiple foil positions and dose assessments could extend the benchmark.

For a field racing toward compact reactors, fusion pilot plants and novel shielding composites doped with boron, heavy minerals and fibers, the message of this study is quietly reassuring: ordinary concrete behaves largely as the simulations say it should, and the open-source OpenMC framework paired with ENDF/B-VII.1 data is a trustworthy guide to its performance. As new materials are proposed to replace or augment it, this experimentally validated baseline provides the yardstick against which they will be judged.

Subject of Research: Experimental and Monte Carlo benchmarking of neutron shielding and activation in ordinary concrete using an AmBe neutron source

Article Title: Neutron shielding assessment of ordinary concrete in an AmBe mixed-energy neutron field

Article References: Shet, S., Charan, A., Suryanarayana, S., Ganesan, S., Thomas, S., Heckadka, S. S., Bhat, B. K., & Kamath, S. (2026). Neutron shielding assessment of ordinary concrete in an AmBe mixed-energy neutron field. Results in Physics, 89, Article 108753. https://doi.org/10.1016/j.rinp.2026.108753

Image Credits: AI Generated

DOI: 10.1016/j.rinp.2026.108753

Keywords: neutron shielding, ordinary concrete, americium-beryllium source, OpenMC, Monte Carlo simulation, gold foil activation, neutron transmission, HPGe spectrometry, induced radioactivity, sodium-24, manganese-56, nuclear reactor shielding