Diffusion et precipitation de l'helium dans l'oxyde de beryllium irradie et recuit

With the help of several experimental methods we have shown that it is possible to determine from moment to moment the balance of helium in irradiated and annealed BeO. The specific heat at very low temperatures (near the λ-point of helium) establishes the mass of helium which has been precipitated; chromatography gives the amount of helium desorbed during the anneal and electron microscopy determines the radius and mean separation of helium bubbles in BeO. In this way the process of diffusion and precipitation of helium in BeO can be followed accurately. Precipitation of helium begins during irradiation; bubble nucleation begins from 600 °C and peaks at 1200 °C. The diffusion coefficient of helium in BeO satisfies the relation: D = 2.5 × 10−3exp (−A/kT), where A = 3.45 eV between 1080 and 1400 °C.