Distance of closest approach (head-on α on nucleus) (d_min = k (2e)(Ze) / Ek)

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d_min = k (2e)(Ze) / Ek

What each symbol means

SymbolWhat it stands for
ZTarget atomic number
eElementary charge (C)
k1/(4πε₀) (N·m²/C²)
E_kInitial α kinetic energy (J)
d_minClosest separation (m)

When to use this

Point nuclei, non-relativistic, Coulomb only until d ~ nuclear size.