  We have measured the threshold center-of-mass kinetic energy for knocking out
a single carbon atom from C$_{60}^-$ in collisions with He. Combining this
experimental result with classical molecular dynamics simulations, we determine
a semi-empirical value of 24.1$\pm$0.5~eV for the threshold displacement
energy, the energy needed to remove a single carbon atom from the C$_{60}$
cage. We report the first observation of an endohedral complex with an odd
number of carbon atoms, He@C$_{59}^-$, and discuss its formation and decay
mechanisms.
