# How to calculate total cross section from differential cross section

svletana
I was doing the calculations for this: http://fermi.la.asu.edu/PHY531/cylinder/index.html
But I can't figure out how to go from $\frac{d\sigma}{d\phi}$ to the total cross section. My guess was that you did the integral from $\phi=0$ to $\phi=2\pi$, but that's not helping since I can't tell either how they got the absolute value squared inside the sum..

Thanks for anyone who listens :)