where ε ε 0 is the vacuum electric permittivity, e the electron charge, z = 2 is the charge number of the alpha particle and Z the charge number of the nucleus (Z - z after emitting the particle). The integration limits are then r 2 = z ( Z − − z ) e 2 4 π π ε ε 0 E , where we assume the nuclear potential energy is still relatively small, and r 1 , which is where the nuclear negative potential energy is large enough so that the overall potential is smaller than E. Thus, the argument of the exponent in λ is: