Investigation by STEM-EELS of helium density in nanobubbles formed in aged palladium tritides

•Helium nanobubbles in palladium tritide are generated by tritium radioactive decay.•Helium-3 density mapping was realized using Electron Energy Loss Spectroscopy.•Nanobubbles pressures were determined in tritide powder aged up to 8.5 years.•A methodology to process spectroscopy data is provided. [Display omitted] 3He nanobubbles created by radioactive decay of tritium in palladium tritide are investigated after several years of aging. Scanning Transmission Electron Microscopy – Electron Energy-Loss Spectroscopy (STEM-EELS) has been used to measure helium density from the helium K-edge around 23 eV. Helium densities were found between 20 and 140 (±30) He/nm3 and the corresponding nanobubble pressures range between 0.1 and 3 (±0.2) GPa. Measuring helium density and mapping He atoms by STEM-EELS enables to differentiate bubbles from empty cavities in the palladium tritide matrix.