The iron core stars end their life in a supernova explosion, if their mass is large enough. The atomic electrons are absorbed by nuclei by the process e + Z→(Z - 1) + v_e. The star core implodes and its density grows enormously. Assume an iron core with density ρ = 100 000 t mm^-3. Consider the neutrino energy E_v = 10 MeV and the cross-section for iron σ ≃ 3 x 10^-46 m². Find the neutrino mean free path. [A_Fe = 56.]