Partially absorbing boundary condition for a diffusion
= Partially absorbing boundary condition for a diffusion
A partially absorbing boundary removes a particle at finite surface reactivity $\kappa$ and reflects it otherwise. Its forward <Robin boundary condition> is $J\mathbin\cdot n=\kappa p$. If the normal diffusion coefficient is $b/2$, integration by parts gives the backward condition
$$
\frac b2\partial_nu=-\kappa u.
$$
= Radiation boundary condition
{synonym}