  We measure the thickness of the heavy water layer trapped under the stress
corrosion fracture surface of silica using neutron reflectivity experiments. We
show that the penetration depth is 65-85 \aa ngstr\"{o}ms, suggesting the
presence of a damaged zone of $\approx$ 100 \aa ngstr\"{o}ms extending ahead of
the crack tip during its propagation. This estimate of the size of the damaged
zone is compatible with other recent results.
