The diffusion of cesium in the graphitic matrix A3-3 in the presence of helium at pressures up to 10 7 Pa

Abstract

The migration of 137CS in the graphitic matrix A3-3 was studied at helium pressures of 4 × 10 6 and 10 7 Pa by use of the thin film technique with vapor deposited carrier-free 137Cs. The penetration profiles did not satisfy Fick's second law, but we applied successfully the solution of the diffusion equation with terms for trapping and re-emission derived by Gaus. Compared to 137Cs migration in a vacuum a high helium pressure lead to a decrease in the coefficient D for diffusion of mobile 137Cs and an increase in the activation energies for diffusion and trapping. D and the trapping coefficient μ are given by D = 10.3 exp (−1.76/kT) cm 2s −1 and μ = 2.2 × 10 3 exp(−1.63/kT) s −1 in the temperature range 1073–1273 K at a helium pressure of 4 × 10 6 Pa, where k = 8.618 × 10 −5 eV/K.