  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.
