THE DIFFUSION OF CESIUM IN THE GRAPHITIC MATRIX A3-3 IN THE PRESENCE OF HELIUM AT PRESSURES UP TO 10(7)-PA

The migration of Cs-137 in the graphitic matrix A3-3 was studied at he lium pressures of 4 X 10(6) and 10(7) Pa by use of the thin film techn ique with vapor deposited carrier-free Cs-137. Th, penetration profile s did not satisfy Fick's second law, but we applied successfully the s olution of the diffusion equation with terms for trapping and re-emiss ion derived by Gaus. Compared to Cs-137 migration in a vacuum a high h elium pressure lead to a decrease in the coefficient D for diffusion o f mobile Cs-137 and an increase in the activation energies for diffusi on and trapping. D and the trapping coefficient mu are given by D = 10 .3 exp(-1.76/kT) cm(2) s(-1) and mu = 2.2 X 10(3) exp(-1.63/kT) s(-1) in the temperature range 1073-1273 K at a helium pressure of 4 X 10(6) Pa, where k = 8.618 X 10(-5) eV/K.