The Coulomb barrier, named after Coulomb's law, which is in turn named after physicist Charles-Augustin de Coulomb, is the energy barrier caused by electrostatic interaction that two nuclei need to overcome so they can get close enough to undergo a nuclear reaction such as nuclear fusion.
Potential energy barrier
This energy barrier is given by the electric potential energy:
:<math>U_\text{coulomb} = {1\over 4\pi\varepsilon_0} {q_1 q_2\over r}</math>
where
:ε<sub>0</sub> is the permittivity of free space;
:q<sub>1</sub>, q<sub>2</sub> are the charges of the interacting particles;
:r is the interaction radius.
A positive value of U is due to a repulsive force, so interacting particles are at higher energy levels as they get closer. A negative potential energy indicates a bound state (due to an attractive force).
The Coulomb barrier increases with the atomic numbers (i.e. the number of protons) of the colliding nuclei:
:<math>U_\text{coulomb} =