  We use particle-transport simulations to show that secondary pions play a
crucial role for the development of the hadronic cascade and therefore for the
production of neutrons and photons from thick spallation targets. In
particular, for the n_TOF lead spallation target, irradiated with 20 GeV/c
protons, neutral pions are involved in the production of ~90% of the
high-energy photons; charged pions participate in ~40% of the integral neutron
yield. Nevertheless, photon and neutron yields are shown to be relatively
insensitive to large changes of the average pion multiplicity in the individual
spallation reactions. We characterize this robustness as a peculiar property of
hadronic cascades in thick targets.
