Accumulation and diffusion of radiogenic helium in beryllium

Accumulation of radiogenic helium isotope ⁴He, its diffusion characteristics, and microstructure of beryllium have been studied after irradiation at 70 and 200°C by neutrons with an energy of E > 0.1 MeV to fluences of (1.0–13.3) × 10²² cm^?2. Experimental results concerning the concentration of radiogenic helium in beryllium after irradiation and subsequent isothermal annealings for 1 h in a temperature range of 700–1200°C have been obtained. The accumulation of helium corresponds to a linear dependence up to a neutron fluence of ～6 × 10²² cm^?2 (E > 0.1 MeV); at fluences from 6 × 10²² to 13 × 10²² cm^?2 (E > 0.1 MeV), a deviation from the linear dependence toward the decrease in the rate of accumulation is observed. The diffusion of radiogenic helium in beryllium at annealing temperatures of 70–200°C is low; intense diffusion and degassing of helium from beryllium start at an annealing temperature of 700°C. At annealing temperatures of 1100–1200°C, virtually all helium escapes from the irradiated beryllium.