A 5.0 mm diameter proton beam carries a total current of 1.5mA. The current density in the proton beam, which increases with distance from the center, is given by J = Jedge(r/R), where R is the radius of the beam and Jedge is the current density at the edge. I calculated that 9.375 x 10^15 protons are delivered by the proton beam each second, but I do not know where to start in calculating the value of Jedge.