doi:10.1016/j.scriptamat.2026.117250engMery, MarioAguirre, Myriam H.Nuñez, Jorge M.Esaulov, Vladimir A.García, CarlosValdés, Jorge E.On the use of transmitted-ion energy losses to infer implanted helium densities in materialsART-2026-148678Helium accumulation can severely degrade materials, making accurate quantification of implanted He and its effects on material structure essential. Here, we report the first exploration of using in situ transmission energy-loss measurements to infer helium densities during ion irradiation. Free-standing nanometric Ta₂O₅ films were irradiated with 9.85 keV He⁺ ions while monitoring the energy loss of transmitted ions, complemented by post-irradiation TEM. The measured energy loss evolves nonlinearly with fluence, reflecting the combined effects of helium incorporation and preferential oxygen sputtering induced oxide reduction. Post-irradiation TEM reveals bubbles with average diameters of ∼11 nm, confirming significant helium accumulation. A first-order phenomenological model is employed to describe stopping-power evolution and its relation to helium density, explicitly accounting for beam-induced material changes.2026http://zaguan.unizar.es/record/17010510.1016/j.scriptamat.2026.117250http://zaguan.unizar.es/record/170105oai:zaguan.unizar.es:170105info:eu-repo/grantAgreement/EC/H2020/101007825/EU/ULtra ThIn MAgneto Thermal sEnsor-Ing/ULTIMATE-IThis project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No H2020 101007825-ULTIMATE-IScripta Materialia 277 (2026), 117250 [6 pp.]by-nc-ndhttps://creativecommons.org/licenses/by-nc-nd/4.0/deed.esinfo:eu-repo/semantics/embargoedAccess