As we know that any positively charged particle like the alpha particle is positively charged , when approaches the nucleus, it gets repelled by the nucleus. This repulsive force between the approaching positively charged particle and the nucleus is known as the Nuclear Columbic Barrier.
To calculate the Potential energy of repulsion following formulae is used:
V = [(Ze)1 (Ze)2]/4πε₀ d =[ (Z)1 (Z)2 e2]/4πε₀ d
Where, Z is the atomic number
E is the unit of protonic charge = 1.601 X 10 -19 coulomb
D is the distance between the positively charged particle and the nucleus.