Im trying to find the number of delta rays though a material and am having some trouble with the units, can anyone help? The number of delta rays through a material is given by N=epsilon(1/E1 - 1/Emax), where epsilon=[2*Pi*A^2*e^4*ne*x]/[m*c^2], where A is unitless, [ne]=cm^-3, and the denominator is in MeV's. I think I want epsilon in MeV's because N should be unitless, so it would cancel out the MeV from the rest of the equation. My problem is the factor of e. I know it can't be in coulombs, so I need to somehow convert it to the right units. From what Im seeing it needs to be in cm*MeV to have epsilon end up in MeV's. I know Coulombs/(4*Pi*epsilon not) is Joules*Meters, but Im not sure how to use that correctly without introducing a factor of (4*Pi*epsilon not) that isnt part of the equation.