[en] Developed is a numerical method of calculating the diffusion coefficient D of β-radioactive elements for the case when the diffusion occurs from a prompt source to semiinfinite medium. A computer program has been prepared, using which tabulated is the dependence of the (D.T.) value on the ratio of the integral activity value I(Xsub(n),t)/I(0,t)=0.95-0.05 with the spacing of 0.02 (I(Xsub(n),t) is the value of the integral activity of a sample after removing the thickness layer Xsub(n) at time moment t), and values of Xsub(n)=0.0005sub(n)-0.0500cm with the spacing of 0.0005cm for X=0.0005-0.0100cm, and 0.0010cm for Xsub(n)=0.0110-O.0500cm for β-radioactive elements with the absorption coefficient of μ=400I/cm