We study a branching random walk on \r with an absorbing barrier. The
position of the barrier depends on the generation. In each generation, only the
individuals born below the barrier survive and reproduce. Given a reproduction
law, Biggins et al. \cite{BLSW91} determined whether a linear barrier allows
the process to survive. In this paper, we refine their result: in the boundary
case in which the speed of the barrier matches the speed of the minimal
position of a particle in a given generation, we add a second order term an1/3 to the position of the barrier for the nth generation and
find an explicit critical value ac such that the process dies when aac.
We also obtain the rate of extinction when a<ac and a lower bound on the
surviving population when a>ac