Helium migration in alpha-irradiated beryllium

Mass-spectrometric investigation of the kinetics of helium emission from beryllium specimens has been carried on. Helium was introduced by means of specimen irradiation with alpha particles at the Van de Graaff accelerator. An equation of gaseous diffusion to vacuum from a plate with a limited and localized quantity of diffusing substance has been solved. It is shown by means of a model calculation using an electronic computer that the whole gas emission occurs practically through a surface which are the nearest to a saturation layer. Parameters o a temperature dependence of helium diffusion in beryllium in a wide temperature range (650-1150 deg) have been calculated from kicetic curves. It is concluded on the basis of a low value of helium diffusion activation energy in beryllium at annealing temperatures below 900 deg C that there is helium migration over interstitials or in the form of helium atom complexes with vacancies. Considerable increase of helium yield at temperatures above 900 deg C is attributed to increase in the yield surface and reduction of a diffusion path at the expense of ''irreversible'' structural changes in beryllium