Estimate the thickness of iron through which a beam of neutrinos with energy 200 GeV must travel if 1 in 10⁹ of them is to interact. Assume that at high energies the neutrino–nucleon total cross-section is given approximately by σ_ν ≈ 10⁻³⁸E_ν cm² , where E_ν is given in GeV. The density of iron is ρ = 7.9 g/cm³.